Search for the pentaquark states in lepton-nucleon scattering at HERMES

in der HELMHOLTZ-GEMEINSCHAFT

DESY

DESY-THESIS-2006-014

July 2006

Search for the Pentaquark States

in Lepton-Nucleon Scattering

at HERMES

by

L. Rub´

aˇ

cek

ISSN 1435-8085

Search for the Pentaquark States

in Lepton-Nucleon Scattering

at HERMES

Inaugural - Dissertation

zur Erlangung des Doktorgrades der Naturwissens haften der Justus Liebig Universitat Giessen

Fa hberei h 07 (Mathematikund Informatik, Physik, Geographie)

vorgelegt von

Luk´

aˇ

s Rub´

aˇ

cek

ausNa hod

II. Physikalis hesInstitut der Justus Liebig Universitat Giessen

Contents

Zusammenfassung

1

1 Introduction

4

2 Search for Pentaquark

6

2.1 Why Sear h forthePentaquark? . . . 6

2.2 Quark Models . . . 7

2.2.1 Pentaquark Models . . . 10

2.2.2 Diakonov, Petrov&Polyakov Predi tions . . . 11

2.3 Experimental Results . . . 13

2.3.1 Eviden e forthe + . . . 13

2.3.2 Non-Eviden e for the + . . . 16

2.3.3 The Sear h forOtherCandidates . . . 18

2.4 Current Status . . . 19

3 The HERMES Experiment

20

3.1 The HERAA elerator . . . 20

3.2 The HERMESTarget . . . 21

3.3 Tra king Dete tors . . . 23

3.4 Parti le Identi ation . . . 25

3.5 BeamMonitoring . . . 29

3.6 Triggerand DataA quisition . . . 29

4 Analysis of HERMES Data

32

4.1 HERMES DataSele tion . . . 33

4.2 The  State . . . 40

4.3 Four Tra k Events Analysis . . . 42

4.4 MonteCarlo. . . 45

4.5 Width andAngular Distribution ofthe  + . . . 46

4.6 Karliner - Lipkin S heme . . . 49

4.6.1 (1520) and  (1520) . . . 49

4.6.2 Appli ation of K.-L. S heme. . . 52

4.7.2  0

(1690). . . 61

4.8 Summaryof thePentaquark Analysis . . . 64

5 The Recoil Project

66

5.1 Generalized Parton Distributions . . . 66

5.2 The Dete tor . . . 68

5.3 SFTFrontendEle troni s . . . 73

5.3.1 Signal Handlingin thePFM . . . 76

5.3.2 Testing andSetup ofthe PFMsforReadout ofPMT Signals . . . . 78

5.4 GSITestBeam . . . 83

5.4.1 Experimental Setup . . . 84

5.4.2 Digitization and DAQ . . . 85

5.4.3 Results of theTestbeam . . . 87

6 Conclusion

92

List of Figures

94

List of Tables

98

Bibliography

99

Zusammenfassung

AllebekannteHadronenkonnenalsSystemvonzweioderdreiQuarksbes hrieben werden. Modelle der Hadronenphysik erlauben zwar Zustande, deren Quantenzahlen man nur mit mehr als drei Quarks erklaren kann. Sol he Teil hen wurden aber in der Ver-gangenheit ni ht entde kt. Diakonov, Petrov undPolyakov, die ein Quark-Soliton Modell benutzten, sagten Pentaquark als s hmale Resonanz mit einer Masse von 1530MeV, einer Breite 15MeV und dem ZerfallskanalpK

S

voraus. Der vorausgesagte Zustand wurde erst-mals im Jahr 2002 dur h dasLEPS Experiment beoba htet. Mehrere Experimente haben diese Beoba htung bestatigt inklusive des HERMES Experiments. Die Su he na h dem vorausgesagten Teil hen in den HERMES Daten stellen einen S hwerpunkt dieser Arbeit dar.

Da eine gute Teil henidentikati o n fur die Analyse erforderli h ist, wurden nur Daten, die in denJahren 1998, 1999 und2000 mitdem HERMES Ring Image Cherenkov gemessen wurden, ausgewahlt. Aus dem Datensatz werden die Ereignisse ausgewahlt, die mindestens drei Hadronenspuren enthalten, davon zwei gegensatzli h geladene Pionen und ein Proton. Die Auswahlkriterien der Pionen werden so optimiert, dass der Unter-grundimK

0 S

Massenspektrummogli hst geringist. AllerdingsdurfendieseKriterienkeine kunstli he Strukturimp

+

 Massenspektrumerzeugen. DeswegenwerdenEreignisse,die mogli herweise aus einem (1116)-Zerfall stammen, verworfen. Im resultierendem Spek-trumwirdeinPeakderMasse15282:6(stat)2:1(syst)MeV undeineBreite =19MeV beoba htet. DieSignikanzdesbeoba htetenPeakentspri htungefahr4,abhangigdavon, wel he Funktion zurBes hreibung des Untergrund benutztwird.

Es wurden drei vers hiedene Verfahren benutzt, um den Untergrund besser zu verstehen. Erstens wurde ein Polynom drittes Grades angepasst, zweitens, es wurden die Impulse des K

0 S

Meth-ode wurden PYTHIA6 Simulationen dur hgefuhrt undmit gemessenen Daten vergli hen. Die Ergebnisse aus den PYTHIA6 Simulationen und der gemis hten Ereignisse stimmen 

uberein. Damit derFit desUntergrundsgutangepasstwerdenkann,wurden inPYTHIA6 diefehlenden  Resonanzenaddiert.

WeiterhinwirdinderAnalyse dieKinematik derZerfallsprodukteuntersu ht um etwasuber denProduktionme hanismuszulernen. werden. Diebishervorhandeneniedrige Statistik lasst allerdings keine eindeutigeS hlussfolgerung ausdenErgebnissen zu.

Da mehrereExperimente, die ebenfalls na h  +

Zustanden gesu ht haben, diese trotzguter Statistik ni ht beoba hteten, stellt si h die Frage, ob der 

+

Zustand wirkli h existiert. DerStandpunkt,dassdas

+

ni htexistiert, wirddur h diegeringeStatistikder Experimente, die es beoba hten, unterstutzt. Als Quelle des Peaks wurden kinematis he Re ektionen diskutiert. Zustande, die in K

S

 zerfallen, konnen dur h die bes hrankte Akzeptanz bei HERMES als pK

S

na hgewiesen worden sein. In dieser Arbeit wird die Mogli hkeit einen Peak dur h ni ht vollstandig na hgewiesene Endzustande zu erzeugen simuliert. Es wurde gezeigt, dass der Zerfall bekannter Zustande keinen in Masse, Breite undIntensitat glei hen Peakproduzierenkann.

Derzweite S hwerpunkt dieserArbeitliegtimBeitrag zumBaudesRe oil Dete -tor, derru kgestreute ProtoneninHERMESna hweisensoll. Diesesind besonders wi htig bei der Untersu hung der Tief-Virtuellen Compton-Streuung (DVCS), wobei im Endzus-tandeinProtonvon
-Resonanzenunters hiedenwerdensoll. DergesamteRe oilDete tor ist ausdrei vers hiedenen Detektorenaufgebaut,wobei inGiessen einDetektoraus szintil-lierenden Fasern gebautwurde.

Die Arbeit befasst si h mit dem Aufbau der Ausleseelektronik fur den Detektor ausszintillierenden Fasern. UmLi htsignale na hzuweisen,werden64-KanalMulti-Anoden Hamamatsu Photomultiplier benutzt. Die Ausleseelektronik wurde aus dem RICH De-tektor desHADES Experiments ubernommen. Jeder Photomultiplier wird mit F rontend-Modulen (FM) mit 64 Verstarkerkanalen und einer A/D-Wandlungseinheit mit Spei her fur 120 Ereignisse verbunden. Als ladungsempdli h Verstarker wird der GASSIPLEX-Chipbenutzt. Die AuslesedesFM sowieeineweitere Datenspei herung erfolgtuber einem Readout-Controller (RC) mitVME-Busans hluss.

Damiteine Auslese des Photomultipliers mitder HADES-Elektronik mogli h ist, wurdederEinbaueineskapazitiveKopplungskreisesfurdesAnalogsignalnotwendig.

Auer-in vers hiedenen Anordnungen untersu ht. Die Tests umfassen dabei von einem Pulser generierte elektris he Signaleam GASSIPLEX-Analogeingang bishin zuTestexperimenten mit einem gemis hten Proton- und Pionstrahl. Die letztgenannte Test wurde an der GSI in Darmstadt dur hgefuhrt. Die erzielte Ergebnisse zeigen dabei gute Teil henidentika -tionsfahigkeite ndesFaserndetektors,dieau hfurdieweitereUntersu hungdesPentaquarks beiHERMESwi htig sind.

Na h einer kurzenEinleitung, wird im zweiten Kapitel eine 

Ubersi ht uber theo-retis he Modelle die dasPentaquark bes hreiben,gegeben unddie experimentellen Ergeb-nisse uber das Pentaquark zusammengefasst. Das dritte Kapitel bes hreibt den Aufbau des HERMES Spektrometer am DESY in Hamburg. Das vierte Kapitel befasst si h mit der Analyse desPentaquark ausdenHERMES-Daten. Imfunften Kapitel werden Aufbau und Test des Ausleseelektronik furdas Re oil Dete tor Projekt bes hrieben. Im se hsten Kapitel wird eineZusammenfassung gegeben.

Chapter 1

Introduction

One of theunexplained phenomena in Hadroni Physi s is an existen e of states whi h an be interpreted only as a ombination of morethan three valen e quarks. Su h states have been dis ussed sin e the earliest quark models but have never been observed experimentally. The developments in the past few years in the eld of Hadroni Models postulate a new predi tion of a possible exoti strange pentaquark with a narrow width whi hwas afterwards onrmedbyobservations inseveralexperiments. Sin enoneofthese experiments wereoptimized forsu h asear h, the resultsexhibit a low signi an e of the observed parti le. Later on, the situation was muddied further when several experiments reported non-eviden e ofa su h pentaquark state.

Sin e there exist several dierent theoreti al models supporting both, existen e and non-existen eofthestrangeexoti pentaquark, thereisthe hallenge forexperimental physi s to prove whi h of those models are orre t. Although the HERMES experiment was designed for the measurement of the proton spin stru ture, the data re orded during several years of running was used in order to ontribute to theeort of sear hing for the strangeexoti pentaquark.

This thesis des ribes the sear h for the exoti pentaquark  +

at the HERMES experiment. The rst se tion is a short introdu tion to the theoreti al predi tions of the pentaquarkandthe urrentstatusofexperimentalresults. These ondse tiondes ribesthe HERMESexperimentatDESY.Thethirdse tiondes ribestheanalysisofthedataobtained by HERMES. The study of the pK

S

system published by the HERMES ollaboration in [18 ℄ has been reanalyzed. Furtheranalyses and MonteCarlo studies are fo used on topi s

pentaquark andidates. Kinemati studies of the (1520), whi h is related to the  +

due to its similar mass and similar de ay hannel, are performed. The fourth topi is that of simulations whi h are fo used to study potential fake sour es of the pentaquark peak -known as \kinemati al re e tions".

Thelast se tionpresentstheauthor's ontributionto theRe oil Dete tor proje t at HERMES. The Re oil Proje t isaimed to extend the HERMES a eptan eto measure slow parti les under wide angles. The study of the proton stru ture by means of Deeply VirtualComptonS atteringisthemaintaskfortheRe oilDete tor. Theauthor'sworkhas beenfo usedon thebuilding oftheread-outele troni s fortheS intillating FiberTra ker. The naldesign of theanalog ele troni s is presented and theresultsof a test experiment at GSI. Apart from thestudy of the internal stru ture of proton, the Re oil Dete tor an also playa signi ant role inthesear h fortheexoti 

+

Chapter 2

Search for Pentaquark

2.1

Why Search for the Pentaquark?

Thenature ofmaterialsubstan esintheuniversehasfas inated people sin etime immemorial. One of themost popular ideas of thean ient world was that of theUniverse as omposed of four innitely dividable elements - Earth, Fire, Water and Air. In the fth entury BC, Demokritos opposed it and suggested that everything was built from elementswhi h an notbedivided anyfurther. He named them\a-thomos",whi hmeans \indivisible".

First andidatesforDemokritos'atomswerefoundmorethantwothousandyears afterhisdeath,whenDaltonassignedtoea h hemi alelementonekindofatom. Mendeleyev lassied elements a ording their properties into the Periodi Table. The fa t that this table has an internal stru ture was a rst hint that Dalton's atoms are not \a-thomos" in the sense of Demokritos' idea. At the turn of the 20

th

entury, sub omponents of the atomswere found. In 1897, Thomson[1℄ dis overed ele trons in athode-rays and in1911 Rutherford [2 ℄ foundthat the majority of the atomi mass is on entrated in a positively hargednu leus whi h isve ordersof magnitude smallerthanthe entire atom.

Laterinthe entury,evennu leiweresplitintotheirownsub omponents-protons and neutrons. These two types ofparti le, in addition to the ele tron, were viewed as the fundamentalparti les. Ele trons atteringexperimentsdis overedanonpoint-likebehavior ofprotonsandneutrons. Withthedevelopmentofparti lephysi sinthe1950s,hundredsof newparti lesweredis overed. Thiswasthegenesisofhadronspe tros opy,whi hdeveloped

be onsideredasa ombinationoftwoorthreequarks,butthereisnotheorywhi hex ludes theexisten e of parti les omposed byfour,ve oreven morequarks.

The earliestquark models dis ussed the possibility of su h tetra- orpenta-quark states, however no su h state has been observed experimentally. In their 1995 work [25 ℄, Diakonov, Petrovand Polyakov predi ted anarrowexoti pentaquark state. These predi -tionsgalvanized experimental eortsandtherst observationofthis predi ted pentaquark wasreportedbyLEPS ollaboration[31 ℄. Severalexperiments onrmedthisresult,among themtheHERMESexperimentatDESY.However,theinitialenthusiasmovertheexisten e of the pentaquark has sin e been dampened by several experiments whi h found no evi-den eof su ha resonan e. Current opinionisdivided overtheexisten e ofthepentaquark - one fra tion supportingthe existen e oftheresonan e, theother denying it.

2.2

Quark Models

Initial models of the hadroni spe tra des ribed the hadrons using the following quantum numbers: Charge Q, Baryon Number B, Z-Component of the Isospin T, and Hyper harge Y. Gell-Mann [3 , 4 ℄ and Ne'emann [5 ℄ lassied mesons and baryons in a s heme whi h they alled\The Eightfold Way". Mesonsaredes ribedbysinglet ando tet presentationsofanSU(3)Liegroupandbaryonsbysinglet,o tetandde upletpresentations of su h a group. In this s heme baryons and mesons an be onsidered as a omposition of onstituents alledquarks and anti-quarks. The mesons arebuiltfrom qqpairswhereas baryonsare ombinationsofthreequarksqqq. Intheearliestquarkmodelsallhadronswere built from ombinations of only three types of quark known as ` avours': Up `u', Down `d' and Strange `s'. The multiplets of the mesons and baryons onsisting of these three lightestquarks areshown inFig. 2.1. Throughthedis overy ofnewparti leslike theJ= and , thequark`family' was given two new avours Charm` ' and Beauty `b'. Themost re entlyobservedquarkhas avorTop`t'. Currentknowledge ofquarks andtheirquantum numbers are summarized in Table 2.1. These quantum numbers are onserved in strong and ele tromagneti pro esses. Weakintera tion may hangethe avor ofthe quarks.

The quark-idea as obtained from the hadron spe tros opy be ame of more im-portan e after the dis overies in the s attering experiments of high energy ele trons on protons in the 1970s [6℄. These measurements showed results that were similar to those

Figure 2.1: The multiplets of the hadrons in the plane given by strangeness and z- omponents of the isospin. The left diagram shows the SU(3) o tet of the mesons with and a whi h belongs to the SU(3) singlet. The plot in middle shows the SU(3) baryon o tetand the rightmost thebaryon de uplet.

Quark d u s b t Q- ele tri harge -1/3 2/3 -1/3 2/3 -1/3 2/3 t z - isospin -1/2 1/2 0 0 0 0 s- strangeness 0 0 -1 0 0 0 - harm 0 0 0 1 0 0 b- beauty 0 0 0 0 1 0 t- topness 0 0 0 0 0 1

violated theexpe tation for s attering on adiusely distributed harge inside the proton. The \s aling" properties of the measured distributions suggested that there are lo alized s attering enters within a proton that are point like obje ts without any substru ture. This harged-s attering enters were named \partons". The partons behave like a

spin-1 2 obje t with2/3 or1/3 oftheelementary harge. They ould beidentied withthequarks thatwerefoundin hadronspe tros opy.

With the in reased pre ision of Deep Inelasti S attering (DIS), a violation of the observed s aling behavior was noti ed. Theoreti al xes a ounting for the s aling violations involving intera tions between thequarks predi ted theexisten e of anew type of parton. Thisele tri ally neutral, spin-1parton (nameda "gluon")plays therole of the gauge boson in the eld theory of the strongintera tions. However, the s aling violation was only anindire t hint aboutexisten e ofgluons. Thegluon was moredire tly observed in1979ine

+

e ollidingexperimentsatthePETRAa eleratoratDESY[7 ℄. Theobserved oplanar, three-hadroni jet events were explained as being generated by quark-antiquark pairsa ompanied bya hardnon- ollinear gluon.

Baryonsarefermions,thustheirtotalwavefun tionhastobeantisymmetri . This fa t aused problemsinthe quarkmodel forstateslike ,

++

and
,wherequantum numberslead to thefollowing quark ontent:

j i=j"sij"sij"si; (2.1)

j
++

i=j"uij"uij"ui; (2.2)

j
i=j"dij"dij"di: (2.3)

In order to obey Fermi statisti s, the quarks must dierat least in one quantum number. The problem was solved by the introdu tion of a new quantum number named \ olor". Coloristhe hargeofthestrongintera tion andhasthreedegreesoffreedom. Ea h valen e quark inside thebaryon hasa dierent olor, whilst theentire baryon behaveslike a olor neutral obje t. The olors of quarks are onventionall y denoted as red, green and blue. The anti-quarks are onsidered to arrythe orresponding anti- olors.

Themodels whi h explain baryons interms ofthree quarks aretoday onsidered as a simplied view of the internal hadron stru ture. They are su essful in des ribing olle tive propertiesof hadronslike massormagneti moment. However, these modelsare no longer suÆ ient to explain results of experiments whi h are investigating the internal

intera tion evolved and today is known as Quantum Chromodynami s (QCD). The most important step in developing this theory was done by D.J. Gross, H.D. Politzer and F. Wil zek [8 , 9℄ who studied non-Abelian gauge theories and found that su h theories are renormalizable and have an asymptoti free behavior. The asymptoti freedom results from thefa t thatgluons, whi h mediate the for ebetween quarks, are arrying olor and an intera t with ea h other. The onsequen e of this feature of strong intera tion is that thequarks inhigh-energy ollisionsbehave like "free" parti les, while at lowenergies they arestrongly boundand onned in hadronsor mesons.

QCDallows the existen e of any ombination of quarks for whi h the total olor is\white". Thesimplestwaythis mightbe a hieved is bythe ombination ofthreequarks qqq inthe aseofbaryonsorbythe ombination quark-anitquarkqqinthe aseofmesons. In addition, the ombination of a olor neutral qqpair with a olor neutral qqq system is also possible. The wave fun tion of a baryon an be expressed in terms of a Fo k state expansion: jBi =jqqqi(a 0 +a 1 jqqi+a 2 jqqq qi +:::+b 1 jqqgi+:::); (2.4)

where higher order terms represent "sea quarks" whi h are reated by QCD va uum po-larization. As QCD is avor blind, naively one expe ts a avor-symmetri sea. Re ent measurements [11 , 12 ℄ have shown that avor symmetry inthe quark seais strongly bro-ken.

Forall knownbaryonsthevalen e quark ontent isredu ibletothree quarks. For example, in ombination of udd with uu the nal state has quark ontent uuudd. The 

u an annihilate with u. Nevertheless, there are ombinations, where the quark ontent an notbe redu edto three and subsets of these exist, wherethegluons ontribute to the ground state quantum numbers - known as an \exoti states" (E.g. uudds). Due to the presen eofthestrangeantiquark,thereisnopossibilityofannihilation. Anunderstanding of whether these exoti quark states exist and, if they do exist, what their properties are, isan important pie e inthepuzzleof understandingQCD inthenon-perturbative regime.

2.2.1

Pentaquark Models

TheMITbagmodelhasbeendevelopedtoexplainthenon-existen eoffreequarks. Thebasi ideaistotreatthe onnementpropertiesofnu leonsinaphenomenologi alway.

surfa e. In the internal spa e quarks are massless and free, in ontrast to the external spa ewheretheirmassin reasestoinnity. Ontheseparation surfa e, alled onnement, the quarks experien e the for es of strong intera tion. In general, the model provides a satisfa tory framework for treating hadrons as a system of onned quarks with residual QCD intera tion that needs only be onsidered in lowest order. Where the details of the onnement me hanism is important however, the models shows some signs of being unsatisfa tory.

Forthebagmodel,theexisten eofmulti-quarkstateshasbeendis ussedelsewhere [19 , 20 , 21 ℄. The parametersof these models were t to the masses of N,
, and ! and resultedinamassspe trum onsistentwithobservan es. Throughsymmetry onditionsthe onstru tedgroundstatesq

2  q 2 andq 4 

qstatesaremembersofthe avournonet. Themodel predi tedthatthesestateswouldbeinanon-exoti groundstateandmaybemis-identied as normal qqor q

3

. Exoti s areheavier - theirmass hasto beabove de ay threshold into (qq)(qq) or(qq)(q

3

). Forthis reason, the width has to be very broad if, indeed, they are resonant at all. Observation ofsu hstates will bediÆ ult.

The MIT bag model was the rst whi h dis ussed the existen e of pentaquark states. There were other models in whi h the dis ussion about possible pentaquark exis-ten e appeared. These models in lude, among others, the soliton model in the works of Kopeliovi h [22 ℄, Chemtob[23 ℄ andWalliser [24 ℄.

2.2.2

Diakonov, Petrov & Polyakov Predictions

Themostre ent experimentalsear h forthepentaquarkwas startedbya predi -tionofDiakanov,PetrovandPolyakov[25 ℄. Theypredi tedanarrowpentaquarkresonan e with amassof 1530MeV and width of about15MeV.

Thepredi tion arose through onsideration ofthe QuarkSoliton Model, whi h is based on the Skyrme idea [26 , 27 ℄, where nu leons an be interpreted as solitons of the pion eld. The lassi ation of the light baryons as the rotational states of the soliton is the strength of the model. Performing rotation in ordinary and in the SU(3) spa e (and its quantization), the lowest baryons are lassied as membersof the o tet (spin1/2) and de uplet (spin3/2).

The authors onsidered thenext rotational ex itation of solitons. The next ex i-tation in the three- avor ase should be an anti-de uplet with spin 1=2. They identied

known nu leon resonan e N(1710) as a member of the anti-de uplet. The s heme of the anti-de uplet is shown in Fig. 2.2, where the naive interpretation of quark ontents is

in-Figure 2.2: Anti-de uplet of baryons. The orners of this diagramare manifestlyexoti .

di ated by oordinate system T 3

;Y. The andidates for exoti pentaquarksare lo ated in the orners of triangle. Inthe top orner isthe 

+

with quark ontent uudds, in the left bottomthereis hyperon with ontent ddssuand inright ornerthehyperon

+ with uuss



d quarks. Theother members ofanti-de uplet arenot exoti .

Using N(1710) as a foundation on whi h to build, the massand width values of members laying on the anti-de uplet right side have been predi ted. The exoti member denoted as 

+

hasbeen predi ted with a mass of 1520MeV and a width of 15MeV. The se ond non-exoti memberhasbeenpredi tedwitha massof1890MeV andawidth about 70MeV. This resonan e an be identied as the observed (1880)P

11

Figure 2.3: The rst observation of the  +

by the LEPS experiment [31 ℄. Left panel a) shows the missing mass MM

K

+

spe trum for K +

K produ tion for the signal sample (solid histogram) and forevents whi h area ompanied witha proton hit. Right panel b) shows MM

K

for thesignal sample(solid histogram) and forevents a ompanied with a proton hit (dottedhistogram).

140MeV.Asapossible andidate,theobserved(2030) isproposed, howeverthequantum numbersof this state arenotwell established yet.

2.3

Experimental Results

2.3.1

Evidence for the

 +

In this subse tion, an overview is given that des ribes the urrent status of the experimental sear h forpentaquarks. For review reports about experimental sear hes for pentaquarkssee [28,29 , 30 ℄. The rst observation of the andidates ame from the LEPS ollaboration at theSPring-8 fa ility inJapan [31 ℄. The rea tion n !K

+

K n on a 12

C target has been investigated with a energy between 1.5 and 2.4GeV/

2

. An observed sharp resonan e in the re onstru ted hannel K

+

n exhibits a mass of 1.540.01GeV/ 2

and a width smaller than 25MeV/ 2

. The signi an e of the peak has been estimated at 4.6. The de ay hannel requires astrangeness quantum numberS =+1. The resonan e

observedresonan eisshowninFig. 2.3. There entpreliminary analyses oftheLEPSdata onrmed theexisten e ofthestate. Thepeak ontains 90entries in omparison to the19 publishedin therst report.

Therstobservationofanexoti narrowresonan eattheLEPSexperimentin ited an intensive sear h among other experiments. The andidates for the the 

+

have been observed in dierent rea tions hannels. The resultsare summarizedin the Table 2.2 and ploted inFig. 2.4.

These ond experiment whi h reported [32 ℄ positive eviden e of the +

state was DIANA at ITEP.The rea tion K

+

Xe! K 0

pXe 0

was analyzed. The K 0

is de aying into 

+

 pair and the tra ks of these, and of thep, were dete ted in anXebubble hamber. Theinvariant massoftheK

0

psystemwas re onstru ted. Theobserved narrow peakhasa massof 1.539GeV/

2

and awidth 9MeV/ 2 . The rea tion p ! nK 0 S K +

was measured at the SAPHIR experiment at ELSA [33 ℄. The liquid hydrogen target was bombarded with photons with energies between 0.87GeVand 2.63GeV.The K

0 S

was identied bythede ay hannel K 0 S

! +

 whereas neutron by a kinemati al t and momentum onservation laws, while the SAPHIR dete -tor ould only dete t harged parti les. A peak with a mass of 1540MeV/

2

and a width lowerthan25MeV/

2

wasfoundinthere onstru ted invariant massspe trumofthenK +

system.

Five dierent neutrino experiments have beenanalyzed bythe ITEP group [34 ℄. The A ! K

0

pX rea tion has been measured by means of bubble hambers, where A represents either hydrogen H, deuterium D, or neon Ne. The resulting peak shows 20 ountsaboveba kground(12 ounts)atamassof1533MeV.Thisgivesarealisti statisti al signi an e of3.5.

The ZEUS experiment at DESY measured the rea tion e + p ! e + K 0 pX at a enter-of-mass energy of about 300GeV. The observation of a peak near to a value of 1522MeV hasbeen reported [35 ℄. The peak is seen for events with 4-momentum transfer Q

2

>20GeV 2

withstatisti al signi an eof5. Inthe aseofasamplewhere4-momenta transferQ

2

>1GeV 2

,nopeakis visible. Good eviden e for the 

+

baryon has been reported by the COSY-TOF exper-iment [36 ℄ . Using the ex lusive hadroni rea tion pp ! K

0 p

+

from two dierent runs with slightly dierent energies, the measurement of 

+

inthe nal state an a t as a tool +

Rea tion Mass FWHM 's Group Ref. [MeV℄ [MeV℄ C! K + K X 154010 <25 4.6 LEPS [31 ℄ K + Xe!K 0 pX 15392 <9 4.4 DIANA [32 ℄ d!K + K 0 (n) 15406 <25 4.8 SAPHIR [33 ℄ A!K 0 pX 1533 5 <20 6.7 ITEP [34 ℄ e + d!K 0 pX 15282.6 139 5 HERMES [18 ℄ e + p!e + K 0 pX 15223 84 5 ZEUS [35 ℄ pp!K 0 p + 15305 <18 4-6 COSY-TOF [36 ℄ pA!K 0 pX 15265 <24 5.6 SVD [37 ℄

Table2.2: Published experimentswith eviden e for + resonan e.

Mass [MeV]

1520

1530

1540

1550

Mass [MeV]

1520

1530

1540

1550

FWHM [MeV]

0

10

20

30

40

50

60

FWHM [MeV]

0

10

20

30

40

50

60

LEPS

DIANA

SAPHIR

ITEP

HERMES

ZEUS

COSY-TOF

SVD

Figure2.4: Compilationoftheworld dataobsereved  +

of about 1.53GeV,with astatisti al signi an e ofbetween 4and 5. Thelastreportedobservationofthe

+

baryon omesfromtheSVD ollaboration at IHEP.In themostre ent report [38 ℄,reanalyzed data [37 ℄ fortherea tion pA!K

0 pX usingprotonsof70 GeVispresented. The observed peakinthepKsystemexhibitsamass of about 1.523MeV. This result shows the highest signi an e amongst all the reported observations ofthe 

+

baryon.

2.3.2

Non-Evidence for the

 +

Lowstatisti sareoneofthe ommonfeaturesofanexperimentreturningapositive signal for 

+

. Eviden e for the \observed" peak is very lose to that for a statisti al u tuation. Proof for the existen e of 

+

rests on an experiment with higher statisti al signi an e. Experimentswithhigherrates ofparti le produ tionswould, one wouldhope, may throw more light on the situation. However, most su h experiments them reported negative results. An overview of published non-observation of the 

+

baryon is shown in Table 2.3.

Non-observan e hasbeen reported by the e +

e ollider experiments BaBar[39 ℄ , Belle[40℄andALEPH[41 ℄ whi hperformedsear hbyre onstru tionofthepK

0 S

systemsin thenal states. The BES ollaboration investigated thee

+

e ! J= ! 

rea tion [42 ℄. Thelevelofstatisti s obtainedinthelepton ollidingexperimentsisanorderofmagnitude higherthanintheexperimentswithpositivetestresults. Thesimplestexplanationwouldbe thatthelowstatisti sexperimentsarewronginthis ase. Thesituationismore ompli ated ifwe assume thatthey are, in fa t, orre t. Onesolution isoeredin aBaBar report[39 ℄. They found that hadron produ tion rates de rease smoothly as the mass in reases. For baryons, this fall issteeper thanformesons. This baryon ratesuppression maybenaively explainedinthefollowingway. Ine

+

e ollisionsapairqqisprodu edinitiallywhi histhen hadronized. Inorder to produ e a baryon, two other pairs of quarks have to be produ ed from the va uum, ratherthan the single pair needed for mesonprodu tion. The question of whether the suppression fa tor in reases in theprodu tionof parti les ontaining more quarks isavalid one. Unfortunately there isno theorywhi h an give lear guidan e. The experiments BaBar,Belle and LEP give uslimits fortheprodu tion, butthey annotrule outtheexisten e of the

+ .

Rea tion Limit Group Ref. e + e !(4S)!pK 0 X <1:010 4

Br

BaBar [39 ℄ e + e !B 0  B 0 !ppK 0 X <2:310 7

Br

Belle [40 ℄ e + e !Z !pK 0 X <6:210 4

Br

LEP [41 ℄ e + e !J= !   <1:110 5

Br

BES [42 ℄ pA!K 0 pX <0:02  HERA-B [43 ℄ pCu!K 0 pX <0:3%K 0 p HyperCP [44 ℄ pp!K 0 pX <0:03  CDF [45 ℄ pC !K 0  + X <0:1  SPHINX [46 ℄ +Si!K 0 pX <0:02  Belle [47 ℄ e+Be!pK 0

+X not given BaBar [48 ℄

d+Au!K n X not given PHENIX [49 ℄

p!  K 0 K + n <0:002  CLAS [50 ℄ d!K +

K pn not given CLAS [51 ℄

 A!K 0

pX

Br



0

<1:8b WA89 [52 ℄

Table 2.3: Published experiments withnon observation ofthe  +

resonan e.

thanthee +

e experiments,that laimnon-observationofthe +

: HERA-Bwitha920GeV proton beam on a arbon target [43 ℄. The HyperCP experiment at Fermilab is designed to measure CP violation in as ade () and anti- as ade de ays. They sought for 

+

in rea tions of protons and  +

with tungsten ollimators [44 ℄. The in iden e momenta of protons or pions is in the range 100-250GeV/ . The CDF experiments studying pp ollisions atan energy of2TeV published null results as well [45 ℄.

Contradi tory to the SVD,the SPHINX ollaboration reports no eviden e of the 

+

resonan e de aying into pK 0

system [46 ℄. The experiment is using the same 70GeV proton beamon anu lear target.

Parallel to theanalyses of thee +

e rea tion, Belle and BaBar analyzedrea tions oming from theintera tion of thebeamwith surroundingmaterial. Belle usedse ondary s atteringofmesonsintheirsili on vertexdete tor[47 ℄,while BaBaranalyzedintera tions of positrons withthebeampipe [48℄. Both experiments reported null results.

The last non-observation of  +

was reported by the PHENIX ollaboration [49 ℄ throughanalysis ofthe rea tiond+Au!K nX at

p s

NN

=200GeV.

TheCLAS ollaboration attheThomas JeersonNational A eleratorFa ilityin therstreportobservedanarrowbaryonpeakintheex lusive rea tion d!K

+

K pn[53 ℄. The neutron in the nal state was re onstru ted by missing mass te hniques sin e the in ident photon energy was known from the tagging system. Theresulting invariant mass spe trumoftheK

+

nsystemshowed asharppeakatthemass1.542GeV/ 2

withaFWHM value of 21MeV/

2

. Signi an e of thepeak was estimated at 5.2. Further eviden e for the 

+

was reported in [54 ℄ in the p !  +

K +

K (n) hannel. In the last year, CLAS has performed a dedi ated run aimed to sear h for 

+

. The reported results [50 ℄ show non-observation of a resonan e in the range between 1.525 to 1.555MeV for the p ! 

+ K

+

K (n)re onstru ted hannel. The preliminary results[51 ℄ show thesame for d ! K

+

K pn hannel.

2.3.3

The Search for Other Candidates

Thepredi ted  +

baryon resonan eis assumedto bean iso-singlet. Howeverthe sear hhasbeenperformedforiso-ve tor statewith hargeQ=+2aswell. Noexperimental observationof

++

hasbeenreportedex eptbytheSTAR ollaborationatRHIC[55 ℄. The hannelpK

+

wasinvestigated inadAu ollisionsample. Theobservedpeakofamassabout 1530MeVexhibitssigni an eof5. Assumingthat

++

isreal,the +

mustalsobeseen. Intheirpublishedresults,theSTAR ollaboration showsasmallpeakwithlowsigni an e inthe pK

S

invariant massspe trum,shifted byabout10MeV/ 2

to highermass.

As mentioned above, the anti-de uplet orners in Fig. 2.2 represent the exoti states. The sear h forthe 

5

has been performedby several experiments [45 , 57 , 58 , 59 , 60 , 61 , 62 ℄ without positive results. Up to now, there is only one reported andidate for  by NA49[63 ℄, whi h makestheexisten e ofa with areported massof1862MeV doubtable.

Onlypentaquark andidateswithstrangequark ontenthavebeendis ussedsofar. Pentaquarksbuiltfromother avors arepossible aswell, howeverthesear hfor andidates with harm quark ontent has been performed in the last years by several experiments. Only H1 reported a signal orresponding to a 

0

(3100) [64 ℄. ZEUS and FOCUS laim in ompatibility of theirresultswith H1 ndings.

2.4

Current Status

There were several theoreti al works whi h sear hed for an explanation of the observedpeak. Inaquark-modelapproa hJaeandWil zek[65 ℄exploredthepossibilityof di-quarkattra tionstrongenoughtoformanewstablehadroni state. Twohigly orrelated ud-pairs are oupledto an anti-quark, withthelowest stateshaving J

P = 1 2 + . Ano tet of di-quark pentaquarks was predi ted to a ompany the antide uplet, and it was suggested that harm and bottomanalogs to the

+

might also be stableagainst strongde ay. Dierent example of an explanation omes from sin ea majority of experiments laimingobservationofthe

+

wereusing anu lear target. GalandFriedman[66 ℄ studied thepossibility offormation of

+

bymeansof theK +

- nu leus phenomenology.

No ontributionofthelatti e omunitybroughtanyenlightementon urrent situa-tion. Resultsof10groupsare ompiledin[67 ℄. Thestudiesshowthatthemost ompli ated problemforthelatti e al ulationisto distinguishasimpleKN ontinuums atteringstate from a 

+

pentaquark. About half of the results reported a pentaquark stru ture, while theothersdid not. The next dis repan iesto be onsidered will beregardedto estimation of thespin and parity.

It is diÆ ult to nd a orre t explanation for the ontradi tory results of the sear hesfor

+

. Inview ofthenon-observations inhigh energyhadroni experimentswith high statisti s one may simply on lude that 

+

doesn't exist. On the other hand, this on lusionassumesthatseverallowenergyexperimentsareallwrongaboutapeakatabout 1530MeV whi h wouldbe asurprising oin iden e. The solution to this onundrum ould be found in an examination of the possible produ tion methods for a 

+

. A step in this dire tionwas taken byTitovatal. [68 ℄. Using well-known highenergyphenomenologylike energy dependen e of the Regge traje tories and thes aling behavior of thehadroni am-plitudesthey foundthat the

+

produ tion ross se tion, when ompared to onventional three quarkhyperon, is(athigh energy pro esses) stronglysuppressed.

Although there is lot of eort to nd theorti al explanations of the situation, the ru ial question either this strange pentaquark exists ornot an only be answered by experiments.

Chapter 3

The HERMES Experiment

3.1

The HERA Accelerator

TheHERAa elerator atDESY, Hamburg, provides twobeams -one of 920GeV protons and one of 27.5GeV ele trons or positrons. In addition to HERMES, these are used for two ollider experiments ZEUS and H1. The HERMES experiment is lo ated in the East experimental hall, diametri ally opposite to the HERA-B hall. It is one of two xed target experiments. Collider experiments ZEUS and H1 are investigating nu leon stru turefun tionsbymeansofunpolarized deepinelasti s atteringoverawidekinemati region. ThegoalofHERA-BexperimentwastostudyCP-violationinB-mesonprodu tion inproton-proton ollisions.

HERMES(HERaMEasurement ofSpin) [69 ℄is designedforpre ise measurement of nu leon spin stru ture. This is a omplished by meansof analyzing semi-in lusive spin asymmetriesinpolarizedDeep-Inelasti lepton-nu leonS attering(pDIS).Beam urrentsof upto 50mAatthebeginningofa llhavebeena hieved. Duetoresidual gas intera tions, thebeam urrent de ays nearly exponentially. The average lifetime, whi h an be derived from thede ay onstant, isbetween 12 and 14hours. Usually the beamis dumpedearlier byinsertinghighdensitygasintheHERMES target ell inorderto obtainhighnumberof statisti s forunpolarized DIS(important forthestudy of nu lear ee ts).

The ele tron beam is distributed within 220 bun hes along the HERA storage ring's6.3km ir umferen e. Thepolarizationofthebeamisa hievedbytheSokolov-Ternov ee t, whi his ausedbyasmallasymmetry inthesyn hrotronradiation. Thetheoreti al

Beam

Direction

Polarimeter

Transverse

Polarimeter

Spin Rotator

Spin Rotator

p

e

Spin Rotator

Spin Rotator

Spin Rotator

Spin Rotator

Longitudinal

000

000

000

000

000

000

000

111

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111

111

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000

000

000

000

000

000

000

111

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111

111

111

Figure 3.1: Lo ation of HERMES at the HERA storage ring. The other experiments H1, HERA-B and ZEUS, the Spin rotators and polarimeters are superimposed. The setup of run2001-2007 isshown. Prior to 2001 there wereno rotators at H1 andZEUS.

minutes afterthe llstart.

Spin rotators have been installed upstream and downstream from the HERMES experiment (see Fig. 3.1),sin e forthe measurementof thebeamspin asymmetries, longi-tudinalpolarizationisrequiredin ontrasttopolarizationinthestorageringwhi hisinthe transverse dire tion. The spin rotators at H1 and ZEUS were installed in addition during theshutdownof 2001. The polarization ofthebeamis measuredby twolaser ba ks atter-ing polarimeters. The transverse polarimeter is lo ated in theWest Hall and longitudinal polarimeteris insidethe spin rotatorat theEastHall.

3.2

The HERMES Target

HERMESwasintendedtorunalongsideZEUSandH1without ausingsigni ant disruption to the beam lifetime, requiring a ll to remain for at least 10 hours. This immediatelyex luded anypossibilityof usingsolid materialforthetargetinHERMES. In

fa t, the target density is limited to 10 15

atoms/ m 2

. Thus a gaseous target ell intended to sit inline to thestoragering was designed foruseinthe experiment.

The HERMESexperiment uses both polarized and unpolarized targets. The po-larizedtarget onsistsofHydrogen, Deuteriumor

3

He,whilsttheunpolarizedgasesareone of H 2 , D 2 , 3 He, 4 He, N 2

, Ne, Kr, and Xe. A polarized 3

He target was used in 1995 but during1996 - 1997 alongitudinally polarized hydrogen target was used andin1998 - 2000 longitudinally polarized deuterium. In 2001, a transversally polarized target was installed duringa HERAupgrade.

Thepolarizedhydrogen(deuterium)beamisprodu edbymeansofanatomi beam sour e(ABS)[70 ℄. Thisdevi e onsistsofdisso iator,powerfuldierentialpumpingsystem, beamforming system, sextuplemagnet systemand adiabati high-frequen y transitions.

Mole ularhydrogen/deuteriumgasisdisso iatedbyaradiofrequen yof13.56MHz ina pyrex-type tube. The degree ofprodu eddisso iation isup to 80%. Atomi gas ows througha oni alnozzle withanopening ofdiameter2mm,whi his ooled to 100K.Five sextupole permanent magnets split this beaminto hyperne states. The parti ular polar-izationstateofinterestis sele tedby ombinationofstrongeldtransition(SFT),medium eld transition (MFT) and weak eld transition (WFT). The polarized atomi beam is inje ted into target ell with apressureofabout10

7

mbar. The target ell isan ellipti al tube with open ends whi h holds the gas at the lepton beam position. At the end of the target elltwo powerful turbo-pumps areinstalledinorderto prote tultrahighva uumin thea elerator ring.

Forpolarized gas,there aretwo instrumentsinstalledformonitoringa Breit-Rabi Polarimeter(BRP)andTargetGasAnalyzer (TGA).Theformermeasuresthepolarization of the gas and the latter gives an estimate of the degree of disso iation. For unpolarized gas, neithermeasurement makessense, sothegas islleddire tly into thetarget ell.

TheHERMESdete tor[69 ℄is onstru tedasaspe trometerwithadipolemagnet. Thesymmetryplanes oftheele tron andprotonbeampipesdividethedete tor intoupper and lower halves. The omplex system of dete tors that make up the spe trometer allow thedete tion ofpropertiesforanyregisteredparti le (seeFig. 3.2). Thelargest omponent of theHERMESspe trometer is alarge dipole magnetwithan integratedeld strength of 1.3Tm. A eptan e of thespe trometer is given bythe opening angleof thespe trometer magnets. Thisangle is170mrad inthehorizontal dire tionand 140mradin theverti al.

1

0

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m

LUMINOSITY

CHAMBERS

DRIFT

FC 1/2

TARGET

CELL

DVC

MC 1-3

HODOSCOPE H0

MONITOR

BC 1/2

BC 3/4

TRD

PROP.

CHAMBERS

FIELD CLAMPS

PRESHOWER (H2)

STEEL PLATE

CALORIMETER

DRIFT CHAMBERS

TRIGGER HODOSCOPE H1

0

1

2

3

4

5

6

7

8

9

10

RICH

270 mrad

270 mrad

MUON HODOSCOPE

WIDE ANGLE

FRONT

MUON

HODO

MAGNET

m

IRON WALL

e+

27.5 GeV

140 mrad

170 mrad

170 mrad

140 mrad

MUON HODOSCOPES

SILICON

Figure3.2: Side view ofthe urrent setup oftheHERMES spe trometer

byaseptum plate whi h limits a eptan efor small verti al s atteringangle to 40mrad.

3.3

Tracking Detectors

Thede e tion of the hargedparti les bymagneti eldis used formeasurement of theirmomenta. This is a omplishedby measuringthe parti les' tra kin themagneti eld region of the spe trometer. Tra king dete tors measure a full omplement of tra k information for any registered parti le: the s attering angle (), the azimuthal angle () and the vertex position. The tra king dete tor systemis depi ted in Fig. 3.2. In front of themagnetthere issetofdrift hambers. Twoare alled theFrontChambers(FC1/2) [71 ℄ and thethird is alled theDriftVertex Chamber(DVC).They areused fordetermination of the initial traje tory in thefront region. Four driftba k hambers (BC1-4) [72 ℄ behind the magnetprovide tra king after the de e tion of theparti le by meansof themagneti eld. Threeproportional hambers(MC1-3) [73 ℄lo ated withintheopening ofthemagnet areusedforthere onstru tionofthetra kswithlowmomentathatarede e tedoutofthe spe trometer a eptan e. Ea h spa e point determined by a tra king dete tor is dened bythree o-ordinates: horizontal(x)and two at stereoangles 30

Æ

Thedrift hambersare onstru tedwiththesameprin iplebuttheirsizein reases withdistan efromthetarget. Ea h hamberisassembledas amodulewithsixlayers. One layer onsistsofaplaneofalternatinganode/ athodewiresbetweenapairof athode foils. The athodesareatnegativevoltageandtheanodesare onne tedtoground. Thewiresare orientedverti ally intheX planeand atstereoangles 30

Æ

forplanes U and V. Oneblo k is ompound from pairof layers. Within theblo k, ea h layer is staggered withrespe tto theirpartner byhalf a ell size inorder to help resolve left-right ambiguities.

The hoi e of gas mixture for the drift hambers was governed by the serious problems presented when needing to ontrol a ammable gas in a tunnel environment. All drift hambers are operated with the same Ar(90%)=CO

2

(5%)=CF 4

(5%) gas mixture whi h is fast and non- ammable. Its drift velo ity is about 70m/ns at the average eld strength of E = 800V/ m. The readout of the drift hamber is a omplished by the Amplier-Shaper-Dis riminator ard (ASD) onne ted to the Fastbus Multi-hit Time-to-Digital-Converter (TDC)witha timeresolution of 0.5ns.

Forthemulti-wireproportional hamberswithinthemagnetopening,thesamegas mixtureasintheDriftChamberisused,butwiththemixratiore al ulatedforoptimization during MWPC operation: Ar(65%)=CO

2

(30%)=CF 4

(5%). The readout is based on the LeCroy PCOSIVsystem. An on- hamber ards provideampli ation, dis rimination and delayaswellaslat hingtotheevent trigger. TheMCsprovideaspatialresolutionofabout 700m.

In2001,asili on dete torsystemnamedthe\LambdaWheel"(LW)was installed in the front of the spe trometer magnet with the intent of in reasing the a eptan e of HERMESto pions from de ays.

Tra king re onstru tion uses a tree-sear h algorithm. The algorithm is based on theuse of apatterndatabase that ontains all possible parti le tra ks fora given dete tor system andresolution. The re orded hits from ea h dete tor are en odedina bit pattern. Aone dimensionalbitarrayis reatedand thesearraysare ombined into two-dimensional pi turesforall hamberswiththesame wireorientation. Thedete torpatternis ompared tothepatterndatabaseand ombinationsthat ouldnothavebeen ausedbystraighttra ks are ltered out. The iterative pro ess starts with a \2-bit dete tor" that dierentiates between a hit in the left or right side of the dete tor. In ea h step, the resolution of the bit pattern is doubled. After 11 iterations, the tra k is ompletely determined. The

spe trometer. Thepartialtra ksare ombinedinto tra ksgoingthroughthemagneti eld of the spe trometer. The Hermes Re onstru tion Program (HRC) uses several methods forthis. The momentum of the tra ks is determined from a look-up table,generated only on e during the initialization of the program, saving the omputing time to al ulate the momentumfromthede e tioninthemagnet. Themomentumresolutionoftheexperiment forpositronsis 0.7-1.25%. The un ertainty inthe s atteringangle isabout0.6mrad.

3.4

Particle Identification

Parti leIdenti ation(PID)isanimportantfa torinthemeasurementofDIS.The HERMESexperimentprovidesex ellenthadron-leptonseparation. Thisisa omplished by sub-system dete tors whi h arethelead-glass Ele tromagneti Calorimeter, thePreshower Dete tor, the Transition Radiation Dete tor (TRD) and the Ring Imaging



Cerenkov De-te tor(RICH).

Dierent types of intera tion take pla e as a parti le passes through matter. Charged parti les produ e mainly ionization or bremsstrahlung. The amount of energy lost bythesetwo pro esses dependson themomentum,massand harge oftheintera ting parti le and on thematerial throughwhi h theparti le passes.

Ele trons orpositronsunder thein uen eof theele tromagneti eld ofan atom radiate a highenergy bremsstrahlungphoton. Su h photonsintera t with ele tromagneti eldsinthematterinwhi h they were produ edand,inturn,produ e pairsof e

+

e . Itis possible to use this hain rea tion to reate ele tromagneti showers in amaterial. This is typi albehaviorforalepton,butheavierparti lesdonot ausesu hshowersasthepro ess isinhibitedduetotheirmass. Usingsu hamaterialto reateshowersallows lepton-hadron separation inside the alorimeter and preshowerdete tor.

TheHERMES alorimeter[74 ℄ measurestheenergyofin ident leptonorphotons. It is builtfrom 840 radiation resistant F101 lead-glass blo ks whi h arearranged into two 4210 arrays fortopand bottomdete torparts. Ea hblo k isofsize9950 m

3 . The length of the alorimeter blo ks orresponds to 18 radiation lengths. The hadron-lepton separationinthe alorimeterisbasedonthefa tthatele tronsdepositalltheirenergyinthe alorimeter. The ratio E=pis, for ele trons, equal to one. Hadroni showers develop more slowly thanele tromagneti ones, therfore theratio E=pis lessthanone. Neutralphotons

tra k in the dete tor, they are identied as a hit in the alorimeter and a orresponding missing hitin thehodos opesystem. The resolution ofthe HERMES alorimeter is

(E) E [%℄=1:5+ 5:1 p E[GeV℄ (3.1)

Similar pro esses to bremsstrahlung an o ur if a parti le passes the boundary betweentwomediawithdierentdiele tri onstants. Inthis ase, anele tromagneti wave isemitted. Thephotonsareusuallyemittedinthevisiblepartofthespe trumbutanX-ray maybeemittedaswell. Toin reasetheprobability oftheemissionofX-rayphotonsinthe Transition Radiation Dete tor (TRD), a high number of boundaries are introdu ed. The ideal asewould be a set of thin foils with anarrow uniform separation by va uum. Su h a onstru tion is te hni ally diÆ ult. In the HERMES TRD it is repla ed by pa kets of pseudo-randomly arranged polypropylene/polye thy l ene bers with diameters of 17-20m. Thebermaterialisheldinpla ewithanaluminumframeandstit hedtogethertomaintain theproperdensity. The radiatorblo ks are6.35 m thi k, 3.4mwide and 0.8m high. The dete tion of X-raysis a omplishedbyMulti-Wire Proportional Chambers(MWPC) with agasmixture 90%Xeand 10%CH

4

behind theradiator. TheHERMES TRD onsists of sixmodulesea h onsisting ofa radiatorblo k and aMWPC.

Thesele tionof leptonsfrom hadronsisdoneby ombining theinformation from allPIDdete tors. Theprobabilityfun tionsP

j i

(p;x)(wheretheparti le iwithmomentum p ausesa response xinthedete torj) are al ulated. Thisis a omplished by omparing dete torresponsetoso- alled\parentdistributions". Theparentdistributionsareobtained from data and Monte-Carlo simulations. By ombining the probability fun tions of the alorimeter and thepreshower, alikelihood (rPID3) is obtained:

rPID3=log 10 P e Cal P e Pre P h Cal P h Pre : (3.2)

The rPID3 isthe logarithmi likelihood that a ertainparti le hasbeenidentied bythe alorimeterandthepreshowerasalepton, ratherthanahadron. Thesame methodisused forthe al ulationofthevaluerPID5inthe aseoftheTRD,whereea hmoduleistreated as anindependent dete tor: rPID5=log 10 Q 6 i=1 P e TRD i Q 6 i=1 P h TRD i : (3.3)

Whentravelling throughamaterialwith aspeedhigherthanthespeedoflight in thatmaterial, hargedparti lesemitphotonsinapro ess knownas\



CerenkovRadiation". The minimal momentum p forthe



CerenkovRadiation is given by

p= m p (n 2 1) (3.4)

where m is the massof the parti le and n isthe dira tive index of the medium. This is used in threshold



Cerenkov ounters. In addition the 

Cerenkov angle,  C

, of the radiated photons(i.e. theangle withrespe t to thedire tionof theparti le) isgiven by:

os C = 1 n (3.5)

where n is the again dira tive index of the medium and  = v

( is the speed of light in va uum and v is speed of the parti le in the medium). In the RICH dete tors, the



Cerenkov oneisfo usedon thematrixofphotondete tors wherearingpatternis reated. The diameterof thering isproportional to the



Cerenkovangle  C

.

The harged hadrons in the HERMES experiment have momenta in the range 2-20GeV/ . In order to a hieve reasonable PID over the full range of momenta, a dual radiator RICH[75 ℄ was designed. Therst radiator is awall made from aerogeltiles. The wall is built from 425 tiles of size 11111:0 m. The refra tive index of the aerogel is 1.0303,thustheaerogelwall an overPIDwithinthelowerpartofthea eptedmomentum range. The se ond radiator is a C

4 F

10

gas with refra tive index1.00137. The gas llsthe spa eof theRICHhousingbetweentheaerogel wall andthemirror. Themirror(radius of urvature of 220 m) re e ts and fo uses



Cerenkovlight on to thePMT plane. The PMT plane ontains 1934 Philips XP1911 type PMTswhi h arereadoutbythePCOS4.

TheRICHparti le identi ation is basedonre onstru tionofthe 

Cerenkovangle andinformationaboutparti lemomentum. Asmentionedabove,theparti lemomentumis estimated bytra kre onstru tion inBCand FCand de e tion radiusinthespe trometer magnet. The



Cerenkov photons aredete ted by the RICHPMTs and all this information together an be usedto re onstru t the



Cerenkov one.

Dire tand Indire tRayTra ingMethods(DRTandIRT)areusedfor re onstru -tion of the



Cerenkov rings. For ea h hit in the PMTs matrix, the point of emission is estimated by means of the re onstru ted tra k in the BC. Using information about the position of the mirror enter, the emission point, the point of light dete tion and the

by the tra ing methods, likelihood algorithms are applied. They take into a ount both the



Cerenkov angles and the numberof red PMTs. The likelihood is al ulated for ea h hypotheses that light was emitted by, K, or p. The diameter of the imagined ringsfor ertain parti les depends dire tly on the angle of the



Cerenkov one. Dependen e of the 

Cerenkovangle C

on thehadronmomentum fortheHERMESRICHisshown inFig. 3.3. The type of parti le identied isthat whosehypotheses is mostprobable. The quality

pa-Figure3.3: 

Cerenkovanglesversusmomentumforaerogel andC 4

F 10

gas. Theupper urve shows anglesof the



Cerenkovlight one omingfromtheaerogelradiator. Thelower urve orrespondsto theC

4 F

10 gas.

rameterQp is al ulatedas adieren eoflogarithmsofthehighestand these ond highest likelihoods. Qp=log 10 L 1 L 2 (3.6)

3.5

Beam Monitoring

Pre ise knowledge of the luminosity play an important role in DIS studies. The relative luminosity is requiredforestimation of ross-se tion asymmetriesinDIS with dif-ferenttargetorbeamspinstates. Theabsoluteluminosityisne essaryforthemeasurement ofabsolute stru turefun tions orstudiesof unpolarized semi-in lusive hadronprodu tion. The luminosity measurement is based on theobservation of elasti s attering of thebeam positrons o the target gas ele trons e

+

e ! e

+

e (Bhabha s attering) and their anni-hilation into photon pairs e

+

e ! . When an ele tron beam is used, ele tron-ele tron elasti s atteringe e !e e (Moller s attering)ismeasured. Sin e the ross-se tionsof these pro esses are al ulable using Quantum Ele trodynami (QED) te hniques and the ele tron density in the target is the same as the nu leon density, the luminosity an be extra ted fromits measurement. Bymeasuringevent ratesR ,theluminosityLisgiven as:

L= R R
(d=d) (3.7)

where  is dete tion eÆ ien y and the integration is performed over an a eptan e angle
. The forwards attered ele trons passing an opening inthe septumplate aredete ted byaluminosity dete tor [76 ℄.

Theratesareestimatedinthefollowingway. Theseptumplateopeningdetermines the a eptan e of the luminosity monitor. The luminosity monitor onsists of a pair of ele tromagneti alorimeters. Ea hoftheseis onstru tedasamatrixof4x3radiationhard NaBi(WO 4 ) 2 (NBW) 

Cerenkov rystals ea h of the size22mm 2

. Crystals are oupledto photomultipliers and readout is performedbyLeCroy ADCs.

Be ause the beam position has signi ant in uen e on the luminosity monitor a eptan e, it is measuredby a set of beam position monitors [77 ℄. For physi s analyses, the measuredrates are orre ted with respe tto thebeam position at themoment of the measurement [78 ℄.

3.6

Trigger and Data Acquisition

TheaimoftheHERMESexperimentistomeasurepropertiesofprotonsbymeans ofDIS.Thetriggersetupisoptimizedtosele teventswithDISele trons. Thetriggersignal

The Hodos ope H0 sits in front of the spe trometer magnet. Due to the small area of the a eptan e, it onsists of two blo ks of plasti s intillator, one above the beam pipe, one below. Ea h blo kis readout by meansof two photomultipliers from whi h time and amplitude information is re orded. A parti le traveling at the speed of light passes the distan ebetweenH0andH1withinapproximately18ns. Thustimeinformationallowsthe analyzertodistinguishbetweenforward-andba kward-travell ing parti les. Thiseliminates ba kground ausedbyproton beamshowers.

The hodos opes H1 and H2 are mounted behind the HERMES magnet, just in front of the TRD (H1) and just behind (H2). The ounters are omposed of 84 verti al s intillator modules, split evenly between the upper and lower parts. Modules are built from fast s intillating material (BC-412 from Bi ron Co.). Ea h module (9:3911 m) is read out by means of a photomultiplier tube oupled via light guides to the outside ends of the s intillator. A 3mmoverlap between themodulesensures full overage of the a eptan e. The average energy deposition is about 2MeV for H1. The ele trons deposit about20MeVin H2due to an11mmthi k Pbradiator infront of thes intillator.

The main DIS trigger is formed by the oin iden e inall upperorlower parts of thehodos opesandthe alorimeter. Thethresholdfordeposited energyinthe alorimeter requires a minimum energy of 1.4GeV, whi h may vary dependent on run ondition. In-formation on themultipli ities ofH1, H2,LUMIhits, ba k hamber, magnet hamber and muon hodos ope signals an also bereadoutin order todene additional typesof trigger, ifrequired.

Theinformationthatswit hestriggertypesisprodu edbyProgrammableLookup Units(PLU).ThedierenttriggertypesaredenedandloadedintoPLUs. HERMESrelies on those to distinguish between two types of trigger. \Physi al" triggers are tied to the HERA- lo k. Thenon-physi al triggersarearbitraryintime. Themainphysi altrigger 21 orrespondstoahitinallofthehodos opesanda alorimeterhitabovea ertainthreshold. Adedi atedtriggerhasbeenproposedforthepentaquarksear h. IntheHERMES experiment, a hannel pK

0 S

isanalyzedwheretheK 0 S

isde aysintotwo oppositely harged pions. Forfull re onstru tion ofsu h events, threehadron tra ks have to be dete ted. The \pentaquark trigger" requires two tra ks in the upper halfof the dete tor and one in the bottom half, or vi e versa. A single hadron tra k is dened by a one hit in H0, H1 and H2 andadditionally ahitintheba k hambers. The\pentaquark trigger" hasbeeninuse

TheHERMESdataa quisitionsystemisbasedonaFast-Busba kbone. TheF ast-Bus TDCs (Time-to-Digital Convertor) and ADCs (Analog-to-Digital Convertor) perform the readout. The drift hambers are read out by TDCs. The magnet hambers are read outbya PCOSIVsystemthat isrestri ted to a single bitper hannel.

Thedataa quisitionstream isseparatedinto runs. Ea h run onsistsofadataset of events. During a ll of the positron ma hine, the data is written onto hard dis s in the online ma hine. Parallel to the data stream, slow ontrol data is stored on the dis . Betweenthells,thedataistransferedtoataperobotontheDESYmainsiteandaba kup is writtento lo al DLTtapes.

Chapter 4

Analysis of HERMES Data

Anexperiment sear hingforanewparti le hasto fulllseveral onditions. Ithas to have suÆ ient momentum resolution in order to re onstru t the mass and width. In addition to the momentum resolution, parti le identi ation (PID) and vertex re onstru -tion apability are needed. Su h features an help to redu e ba kground originating from dierent rea tions. Finally, a suÆ ient a eptan e in the explored kinemati region and de ay hannel isne essary. Theabilityofa spe trometer to dete t ertain parti les an be studied by means of Monte Carlo (MC) simulations. The ross he k of quality of MC is usually performedby theanalysisof well established parti les.

In the sear h for Pentaquark  +

, two basi s hemes were applied among exper-iments. In lusive re onstru tion s hemes re onstru t the parti le mass by the sum of 4-momenta ve tors of thede ay produ ts and their squares. Only de ay produ ts are taken into a ount. Theba kgroundisredu edbysele ting eventswhi h have good PIDandthe tra k of ea h produ t an be onsideredas originating from a vertex ommon to allof the de ay produ ts.

Experiments laiming an ex lusive measurement must dete t all produ ts. The nal state an be re onstru ted in the same way as in the in lusive s heme, where the a ompanied parti le is used as a tag for a ertain produ tion hannel ora missing mass te hnique is applied. This te hnique is useful for situations where one of the sought-for parti le's de ay produ t is neutral and thus a momentum measurement is diÆ ult. The 4-momentum in the input hannel is known as well as the 4-momenta of parti les a - ompanying the investigated de ay hannel. Using the 4-momentum onservation law, an

invariant massofthe de ay hannel of interest anthus be established.

4.1

HERMES Data Selection

The analysishen eforth des ribed was performedusing the in lusive method de-ned in the previous se tion of this hapter. The predi ted de ay hannel [25 ℄ of the 

+

into KN was investigated in HERMES data. Sin e lean PID is needed forall the de ay produ ts, the only data sele ted is that whi h has been measured with the RICH. This data was olle ted in the years 1998, 1999 and 2000 and has an integrated luminosity of L=

R

L

dt=295:7pb 1

. Onlydata on deuterium targets, bothpolarized and unpolarized, hasbeenusedforthis analysis.

The rst step was to sele t events whi h ontain three ormore hadron tra ks in thedu ialvolumeofthespe trometer. Thetra ksmustbelongtra ks,be auseonlythese allow PID by means of the RICH. The tra k is dened as a long tra k when tra k parts are re onstru ted in both sets of tra king hambers,FC and BC. The hadronshave been sele ted a ording to PID values in theDSTstable:

g1Tra k:rPID3+g1Tra k:rPID5<0 (4.1)

Asthenext riteria, hargeinformationandRICHparti letypeinformationhavebeenused to ndevents. Two tra ks have to be identied as oppositely harged pions and one tra k as aproton. This orrespondsto type values obtained by RICH:

smR ICH:iType=3( +

; ) (4.2)

smR ICH:iType=5(p) (4.3)

Sin ethereliabilityoftheRICHPIDdiersfor ertainmomentaregionsandparti le types, limits have beenintrodu ed forpionmomenta.

1GeV <P 

<15GeV (4.4)

and forproton momenta

4GeV <P p

<9GeV (4.5)

forpions and to

smR ICH:rQp>1:5 (4.7)

forprotons.

Thenextstepintheanalysiswastore onstru tK S

fromthesele tedeventsample. Thefour-momentaoftwo piontra kswasaddedandtheinvariant massofboth al ulated. The invariant mass spe trum of the 

+

 is shown in Fig. 4.1. The resulting spe trum

)[GeV]

-π

+

π

M(

0.4

0.45

0.5

0.55

0.6

(2 MeV)

⁄

Events

0

200

400

600

800

1000

1200

1400

p)[GeV]

-π

+

π

M(

1.45

1.5

1.55

1.6

1.65

1.7

(8 MeV)

⁄

Events

0

500

1000

1500

2000

2500

3000

Figure 4.1:  +

 invariant mass spe trum (left) and  +

 p invariant mass spe trum (right). No uts are applied on sele ted protons and pions. The 

+

 spe trum shows lear K

S

peakwhile nostru ture isseen inthe +

 p spe trum.

exhibits a lear K S

peak at 497MeV, while the average value a ording to the PDG [79 ℄ is 497.2720.031MeV. In order to improve the K

S

signal-to-ba kground ratio, additional uts have beenintrodu ed. The uts have beenbased ontheevent topology shown inFig. 4.2. The position of the de ay vertex B and the distan e of the losest approa h (DCA) of thepiontra kshave been al ulated. The tra ks ofparti les whi h areprodu edbythe de ay of the same parent parti le are supposedto ome from the same vertex. This is, in

S



S



ᶎᶐᶍᶒᶍᶌ

ᶘᴾᶁᶍ

ᶍ

ᶐᶂᶇᶌ

ᵿ

ᶒᶃ

ᵢ

ᶇ ᶑᶒᵿ

ᶌ

ᶁᶃᴾᶀᶃᶒᶕ

ᶃᶃᶌ

ᴾ

ᵐᴾ

S

ᶒᶐᵿᶁᶉᶑᴾᵆᵚᵏᶁᶋᵇ

ᵩ

ᵎ

ᵱ

ᶋᶍᶋᶃᶌᶒᶓᶋ

ᵢ

ᶇ ᶑᶒᵿ

ᶌ

ᶁᶃᴾᶀᶃᶒᶕ

ᶃᶃᶌ

ᴾᵩ

ᵎ

ᵱ

ᵿᶌᶂᴾᶎᶐᶍᶒᶍᶌᴾᶒᶐᵿᶁᶉᴾ

ᵆᵚᵔᶋᶋᵇ

ᵢ

ᶇ ᶑᶒᵿ

ᶌ

ᶁᶃᴾᶀᶃᶒᶕ

ᶃᶃᶌ

ᴾᵩ

ᵎ

ᵱ

ᶂᶃᶁᵿᶗᴾᵆᵠᵇᴾ

ᵿᶌᶂᴾᶎᶐᶍᶂᶓᶁᶒᶇᶍᶌᴾᶔᶃᶐᶒᶃᶖᴾᵆᵡᵇᴾᵆᵜᵕᶁᶋᵇ

ᵢ

ᶇ ᶑᶒᵿᶌ

ᶁᶃᴾᶀᶃᶒᶕᶃᶃᶌ

ᴾᶀᶃᵿᶋᴾᵿᶌ

ᶂᴾᶎ

ᶐᶍ

ᶂᶓ

ᶁᶒᶇᶍ

ᶌ ᴾ

ᶔᶃᶐᶒᶃᶖᴾᵆᵰᵿᶂᶇᵿᶊᴾᶁᶓᶒᵘᴾᵰᵚᵒᶋᶋᵇ

ᶚᶐᵴᶃᶐᶒᵸ

ᵽ

ᶎᵩ

_ᵱ

ᶚᵚᵏᵖᶁᶋ

ᵠ

ᵡ

Figure4.2: Topology ofre onstru ted three-tra kevents

DCA 

+ 

<1 m: (4.8)

Thenexttopology ututilizesknowledgeofthemeanlifeoftheK S

(8:9ns). ForaK S

with a momentum of 1GeV/ , the mean life orresponds to a ight distan e of approximately 10 m. In order to estimatede ay length of potential K

S

, its tra kand produ tion vertex have been re onstru ted. The dire tion of the K

S

tra k is given by the sum of the two pions' three-momenta ve tors. The K

S

produ tion vertex C has been re onstru ted using the assumption that the K

S

and theproton ome from the same produ tion vertex. The distan e betweentwo verti es gives ade ay length of theK

S

,whi h is requiredto be:

jB Cj>7 m (4.9)

Mass spe tra of boththe  +

 and the  +

 p systems areshown inFig.4.3. The signal to ba kground ratio of theK

S

peak in +

 spe trum isimproved whereas no signi ant +

)[GeV]

-π

+

π

M(

0.4

0.45

0.5

0.55

0.6

(2 MeV)

⁄

Events

0

100

200

300

400

500

p)[GeV]

-π

+

π

M(

1.45

1.5

1.55

1.6

1.65

1.7

(8 MeV)

⁄

Events

0

100

200

300

400

500

600

Figure4.3: The +

 invariantmassspe trumleftandthe +

 pinvariantmassspe trum right. TheDCAandthede aylength utsareappliedonsele tedpions inordertoimprove theK

S

signal.

Up to now the shown  +

 p invariant spe tra has been onstru ted without onstraints on the 

+

 mass. In order to satisfy the ondition that the  +

 oming from K

S

de ay, the events where the  +

 invariant mass agree within 2 of the mean valueof theK

S

peak have beensele ted.

jM 

+ 

497jMeV <12MeV: (4.10)

Theregionofsele ted K S

isdepi tedbythelledareainFig. 4.4. Further,a ausality ut has been applied forthese plots- the position of the K

S

de ay vertex hasto appear after theposition of the K

S

-protonprodu tionvertex with respe tto theK S

momentum. Due to the HERMES a eptan e limitations in the forward dire tion, this ut an be simply redu edto a uton z- oordinates.

B z C z >0 m: (4.11) +