J. Phys. G: Nucl. Part. Phys.30(2004) S61–S73 PII: S0954-3899(04)64197-1
An update from STAR—using strangeness to probe relativistic heavy ion collisions
Helen Caines
1(for the STAR Collaboration)
J Adams
2, C Adler
3, Z Ahammed
4, J Amonett
5, B D Anderson
5, M Anderson
6, D Arkhipkin
7, G S Averichev
8, J Balewski
9,
O Barannikova
4, L S Barnby
5, J Baudot
10, S Bekele
11, V V Belaga
8, R Bellwied
12, J Berger
3, H Bichsel
13, A Billmeier
12, L C Bland
14, C O Blyth
2, B E Bonner
15, A Boucham
16, A Brandin
17, A Bravar
14, R V Cadman
18, X Z Cai
19, M Calder´on de la Barca S´anchez
14, A Cardenas
14, J Carroll
20, J Castillo
20, M Castro
12, D Cebra
6, P Chaloupka
21, S Chattopadhyay
12, H F Chen
22, Y Chen
23,
S P Chernenko
8, M Cherney
24, A Chikanian
1, B Choi
25, W Christie
14, J P Coffin
10, T M Cormier
12, M M Corral
26, J G Cramer
13,
H J Crawford
27, A A Derevschikov
28, L Didenko
14, T Dietel
3, X Dong
22, J E Draper
6, V B Dunin
8, J C Dunlop
1, V Eckardt
26, L G Efimov
8, V Emelianov
17, J Engelage
27, G Eppley
15, B Erazmus
16, P Fachini
14, V Faine
14, J Faivre
10, R Fatemi
9, K Filimonov
20, P Filip
21, E Finch
1, Y Fisyak
14, D Flierl
3, K J Foley
14, J Fu
20,22, C A Gagliardi
29,
N Gagunashvili
8, J Gans
1, L Gaudichet
16, M Germain
10, F Geurts
15, V Ghazikhanian
23, J E Gonzalez
23, O Grachov
12, V Grigoriev
17, D Grosnick
30, M Guedon
10, S M Guertin
23, E Gushin
17, T J Hallman
14, D Hardtke
20, J W Harris
1, M Heinz
1, T W Henry
30, S Heppelmann
31, T Herston
4, B Hippolyte
1, A Hirsch
4, E Hjort
20, G W Hoffmann
25, M Horsley
1, H Z Huang
23, S L Huang
14,1, T J Humanic
11, G Igo
23, A Ishihara
25, P Jacobs
20, W W Jacobs
9, M Janik
32, I Johnson
20, P G Jones
2, E G Judd
27, S Kabana
1, M Kaneta
20, M Kaplan
33, D Keane
5, J Kiryluk
23, A Kisiel
32, J Klay
20, S R Klein
20, A Klyachko
9, D D Koetke
30, T Kollegger
3, A S Konstantinov
28, M Kopytine
5, L Kotchenda
17, A D Kovalenko
8, M Kramer
34, P Kravtsov
17, K Krueger
18, C Kuhn
10, A I Kulikov
8, G J Kunde
1, C L Kunz
35, R Kh Kutuev
7, A A Kuznetsov
8, M A C Lamont
2, J M Landgraf
14, S Lange
3, C P Lansdell
25, B Lasiuk
1, F Laue
14, J Lauret
14, A Lebedev
14, R Lednick´y
8, V M Leontiev
28, M J LeVine
14, C Li
11,24, Q Li
12,
S J Lindenbaum
34, M A Lisa
11, F Liu
35, L Liu
35, Z Liu
35, Q J Liu
13, T Ljubicic
14, W J Llope
15, H Long
23, R S Longacre
14, M Lopez-Noriega
11, W A Love
14, T Ludlam
14, D Lynn
14, J Ma
23, Y G Ma
19, D Magestro
11, R Majka
1, R Manweiler
30, S Margetis
5, C Markert
1, L Martin
16, J Marx
20, H S Matis
20, Yu A Matulenko
28, T S McShane
24,
F Meissner
20, Yu Melnick
28, A Meschanin
28, M Messer
14, M L Miller
1, Z Milosevich
33, N G Minaev
28, J Mitchell
15, C F Moore
25, V Morozov
20, M M de Moura
12, M G Munhoz
36, J M Nelson
2, P Nevski
14,
V A Nikitin
7, L V Nogach
28, B Norman
5, S B Nurushev
28, G Odyniec
20,
0954-3899/04/010061+13$30.00 © 2004 IOP Publishing Ltd Printed in the UK S61
A Ogawa
14, V Okorokov
17, M Oldenburg
20, D Olson
20, G Paic
11,
S U Pandey
12, Y Panebratsev
8, S Y Panitkin
14, A I Pavlinov
12, T Pawlak
32, V Perevoztchikov
14, W Peryt
32, V A Petrov
7, J Pluta
32, N Porile
4,
J Porter
14, A M Poskanzer
20, E Potrebenikova
8, D Prindle
13, C Pruneau
12, J Putschke
26, G Rai
20, G Rakness
9, O Ravel
16, R L Ray
25, S V Razin
8, D Reichhold
4, J G Reid
13, G Renault
16, F Retiere
20, A Ridiger
17, H G Ritter
20, J B Roberts
15, O V Rogachevski
8, J L Romero
6, A Rose
12, C Roy
16, L J Ruan
22, V Rykov
12, I Sakrejda
20, S Salur
1, J Sandweiss
1, I Savin
7, J Schambach
25, R P Scharenberg
4, N Schmitz
26,
L S Schroeder
20, A Sch ¨uttauf
26, K Schweda
20, J Seger
24, D Seliverstov
17, P Seyboth
26, E Shahaliev
8, M Shao
22, K E Shestermanov
28,
S S Shimanskii
8, F Simon
26, G Skoro
8, N Smirnov
1, R Snellings
37, P Sorensen
23, J Sowinski
35, H M Spinka
18, B Srivastava
4, S Stanislaus
30, E J Stephenson
9, R Stock
3, A Stolpovsky
12, M Strikhanov
17,
B Stringfellow
4, C Struck
3, A A P Suaide
12, E Sugarbaker
11, C Suire
14, M Sumbera ˇ
21, B Surrow
14, T J M Symons
20, A Szanto de Toledo
36, P Szarwas
32, A Tai
23, J Takahashi
36, A H Tang
20, D Thein
23, J H Thomas
20, V Tikhomirov
17, M Tokarev
8, M B Tonjes
38,
T A Trainor
13, S Trentalange
23, R E Tribble
29, V Trofimov
17, O Tsai
23, T Ullrich
14, D G Underwood
18, G Van Buren
14, A M VanderMolen
38, A N Vasiliev
28, M Vasiliev
29, S E Vigdor
9, S A Voloshin
12, F Wang
4, X L Wang
22, Z M Wang
22, H Ward
25, J W Watson
5, R Wells
11,
G D Westfall
38, C Whitten Jr
22, H Wieman
20, R Willson
11, S W Wissink
9, R Witt
1, J Wood
23, J Wu
22, N Xu
20, Z Xu
14, Z Z Xu
22, A E Yakutin
28, E Yamamoto
20, J Yang
23, P Yepes
15, V I Yurevich
8, Y V Zanevski
8, I Zborovsk´y
21, H Zhang
1, W M Zhang
5, Z P Zhang
11,24,
P A Zołnierczuk
9, R Zoulkarneev
7, J Zoulkarneeva
7and A N Zubarev
81Yale University, New Haven, CT 06520, USA
2University of Birmingham, Birmingham, UK
3University of Frankfurt, Frankfurt, Germany
4Purdue University, West Lafayette, IN 47907, USA
5Kent State University, Kent, OH 44242, USA
6University of California, Davis, CA 95616, USA
7Particle Physics Laboratory (JINR), Dubna, Russia
8Laboratory for High Energy (JINR), Dubna, Russia
9Indiana University, Bloomington, IN 47408, USA
10Institut de Recherches Subatomiques, Strasbourg, France
11Ohio State University, Columbus, OH 43210, USA
12Wayne State University, Detroit, MI 48201, USA
13University of Washington, Seattle, WA 98195, USA
14Brookhaven National Laboratory, Upton, NY 11973, USA
15Rice University, Houston, TX 77251, USA
16SUBATECH, Nantes, France
17Moscow Engineering Physics Institute, Moscow Russia
18Argonne National Laboratory, Argonne, IL 60439, USA
19Shanghai Institute of Nuclear Research, Shanghai 201800, People’s Republic of China
20Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
21Nuclear Physics Institute AS CR, ˇReˇz/Prague, Czech Republic
22University of Science and Technology of China, Anhui 230027, People’s Republic of China
23University of California, Los Angeles, CA 90095, USA
24Creighton University, Omaha, NE 68178, USA
25University of Texas, Austin, TX 78712, USA
26Max-Planck-Institut fuer Physik, Munich, Germany
27University of California, Berkeley, CA 94720, USA
28Institute of High Energy Physics, Protvino, Russia
29Texas A & M, College Station, TX 77843, USA
30Valparaiso University, Valparaiso, IN 46383, USA
31Pennsylvania State University, University Park, PA 16802, USA
32Warsaw University of Technology, Warsaw, Poland
33Carnegie Mellon University, Pittsburgh, PA 15213, USA
34City College of New York, New York City, NY 10031, USA
35Institute of Particle Physics, CCNU (HZNU), Wuhan, 430079 People’s Republic of China
36Universidade de Sao Paulo, Sao Paulo, Brazil
37NIKHEF, Amsterdam, The Netherlands
38Michigan State University, East Lansing, MI 48824, USA E-mail: helen.caines@yale.edu
Received 27 May 2003 Published 11 December 2003
Online at stacks.iop.org/JPhysG/30/S61 (
DOI: 10.1088/0954-3899/30/1/005) Abstract
An overview of the strangeness measurements made by the STAR collaboration at RHIC for Au–Au collisions at √ s
N N= 130 and 200 GeV plus p–p collisions at √ s
N N= 200 GeV is presented. A wealth of information has been generated on the kinematics and scale of strange particle production by this experiment.
When viewed in combination a picture emerges of particles demonstrating a surprisingly high degree of collective motion, suggestive of strong internal pressure within the source which builds up rapidly. The non-resonance yields are consistent with statistical hadron formation. Although there appears to be a rapid decoupling of the source, resonance particle measurements show signs of rescattering during the hadronic phase. Meanwhile the observed suppression of high momentum probes and their large azimuthal asymmetry indicate that this hot dense matter has significant interactions with particles moving through it.
1. Introduction
The production and dynamics of strange particles have long been deemed as worthy of study when attempting to understand relativistic heavy ion collisions (for an overview of previous experimental and theoretical results see [1, 2] and references therein). A major development occurred when, in 2000, RHIC commenced operation and STAR took data for Au–Au collisions at √ s
N N= 130 GeV. In 2002 the physics program continued with data collected at the top RHIC energy of √ s
N N= 200 GeV for the Au–Au and p–p collision systems. This order of magnitude increase in collision energy from SPS to RHIC meant that copious strange particle production occurred for the first time. This allows for a detailed study of strange particles and resonances as functions of both centrality and p
⊥.
1.1. The STAR detector
The design of the solenoidal tracker at RHIC (STAR) detector makes it ideal for strange particle
analysis due to its large acceptance and high tracking efficiency. The main tracking device is
50 100 150 200 250 300 0
2 4 6 8 10 12 14 16 18 20 22
dNh-/dη
dN/dy 0.5*K-
Λ Λ_ 2*Ξ− Ξ+ _
STAR Preliminary Au-Au 130 GeV
Ω−+Ω_+
Figure 1.Preliminary measured particle yields as a function of dNh−/dηfor Au–Au collisions at
√sN N =130 GeV. Error bars are statistical only. The systematic uncertainties are less than 10%.
a large time projection chamber (TPC) which provides particle identification and momentum information for charged particles at mid-rapidity. A central trigger barrel surrounding the TPC and two zero degree calorimeters positioned ±18 m from the interaction region provides trigger information for the heavy ion running. Two beam–beam counters, placed ±3.5 m from the interaction region, were used for the p–p triggering.
2. Soft physics
The bulk of the particle production occurs at low momentum, the study of which allows us to probe the properties of the source at chemical freeze-out and the subsequent thermal properties of the various particles at kinetic freeze-out. Analysis of resonance particles, with their short lifetimes, yields information on the dynamics and timescale of the collision volume between hadronization and kinetic freeze-out. Figure 1 shows the measured yields for K
−, , , ¯
−, ¯
+and
−+ ¯
+as a function of dNh
−/dη, which is strongly correlated to centrality, for Au–Au collisions at 130 GeV. The lines are fits to the data and indicate that all measured strange particle yields appear to increase linearly with the measured negative charged hadron yields. The increased statistics of the 200 GeV run will allow STAR to make much more detailed studies of particle production and dynamics by extending the measured ranges and allowing for a finer binning of the measurements as a function of centrality. The large production yields mean statistically significant strange particle event-by-event measurements and correlation studies can be performed and compared to non-strange particles to try to observe differences between the behavior of various particle types.
2.1. Chemical freeze-out and re-scattering
Analysis of non-identical particles, including non-strange to strange species, shows that all
ratios are well described by statistical models [3]. These models suggest a chemical freeze-out
temperature parameter of ∼176 MeV and a baryon chemical potential of ∼40 MeV, driven by
the anti-baryon/baryon ratios. Statistical models do not try to explain particle production from
basic principles but use the ansatz that all species are produced statistically from a thermally
equilibrated source. The success of these models in replicating the data is taken as a strong
suggestion that the source is thermalized. Further evidence of this thermalization comes from
the and ¯ spectra which are nearly identical for all centralities [4, 5], as are those of the
K*/K
4 x Λ(1520)/Λ 2 x ρ/π STAR Preliminary
s=200GeV
√
Nch / dη
0 100 200 300 400 500 600
Ratio
0 0.05
0.1 0.15 0.2 0.25 0.3 0.35 0.4
Figure 2. Preliminary resonance to ‘stable’ particle ratios as a function of dNch/dηfor Au–Au collisions and p–p collisions at√sN N =200 GeV. The statistical and systematic uncertainties are added in quadrature.
multiply strange baryons,
−and ¯
+[6]. Considering the different contributions of valence quarks and pair production to the creation of these particles and anti-particles may indicate a high degree of rescattering in the collision region.
STAR has also measured an impressive number of resonance particles; details of the φ [7], K
∗[8],
∗(1520) [9], ρ and f
0[10] were presented at this conference. The small cτ of these particles is comparable to the expected lifetime of the source, hence a significant fraction of these particles is expected to decay within the fireball lifetime. It is then possible for the daughter particles to re-interact. These scatterings alter the momentum of the daughters as a result of which the original parent particle can no longer be identified via invariant mass analysis and it is ‘lost’ to the reconstruction. By a similar argument collisions of particles between hadronization and kinetic freeze-out create resonance particles, a phenomenon known as regeneration, and hence increase the measured yield of the resonance with respect to that existing at hadronization. The particle density of the fire ball, the lifetime of the resonance and its dominant decay mode are major factors in determining if the regeneration or loss term has the dominant effect on the measured yield at kinetic freeze-out. Hence the measurements of different resonances as a function of centrality are vital in determining the source properties.
With this in mind it is perhaps odd to note that the thermal models were successful in calculating the K
∗/K ratio at 130 GeV, the large error in the measurement should however not be ignored and a more detailed comparison at 200 GeV will be illuminating.
Figure 2 shows the STAR measured ratios, (1520)/, K
∗/K and ρ/π as a function of
dN
ch/dη for Au–Au collisions at 200 GeV. These ratios for p–p collisions at the same energy
are also shown. The cτ are 13, 4 and 1.3 fm/c for the (1520), K
∗and ρ respectively. It
can be seen that both the K
∗and (1520) ratios show a significant decrease from p–p to
peripheral and central Au–Au collisions. This suggests that significant rescattering already
occurs in our most peripheral Au–Au collision triggers of 50–80% centrality. The ρ/π on the
other hand appears constant, although further measurements at higher centralities are needed
before a firm statement can be made. The measured decay channel is ρ → π π and hence
this constancy could be explained by the large cross-section of the π and the short lifetime
of the ρ combining such that the regeneration and decay rates compensate. These losses and
gains are so rapid that the final yield is only a measure of those ρ formed close to the kinetic
freeze-out, those ρ formed at hadronization are lost.
0.35 0.4 0.45 0.5 0.55 0.6 0.05
0.1 0.15 0.2
(π, K, p, Λ) (Ξ)
> (c) β
<
(GeV)foT
STAR Preliminary Au-Au 130 GeV
Figure 3.The kinetic freeze-out temperature versus transverse flow contours for hydrodynamical model for fits to the preliminary m⊥spectra from Au–Au collisions at 130 GeV. See text for futher details.
2.2. Kinetic freeze-out
As stated earlier, statistical model analyses of these collisions suggest a thermalization of the system. Equilibration means a resultant loss of information about how the particles were originally created. Further evidence of thermalization is given by studies of the transverse momenta of π, K, p and particles using a blastwave parametrization suggested by [16] with a flow velocity profile of β
⊥(r) = β
R(r/R)
0.5, where β is the transverse flow velocity and R is the maximum source radius. A combined fit to the m
⊥spectra of the above particles for the 130 GeV data suggests a common kinetic freeze-out temperature combined with a significant radial flow effect from a rapidly expanding source. The one, two and three sigma contours of this fit are shown as dashed lines in figure 3. The contours for a separate fit to the
−and ¯
+baryons are also shown as solid lines [6]. The diamonds indicate the best fit locations in both cases. It is evident that the prefer a different set of fit parameters to that of the lighter particles. The appear to freeze out with a significantly higher temperature parameter, T
f o= 182 ± 29 MeV and a lower β
⊥= 0.42 ± 0.06c. Under this parametrization the kinetic freeze-out for the occurs at approximately the same temperature as the chemical freeze-out. The still significant collective transverse flow of these particles can then be taken as an indication that a sizeable fraction of the radial flow is built up in the pre-hadronic phase.
The calculated p
⊥for the π
−, K
−, ¯p and φ particles as a function of centrality of Au–
Au and p–p collisions at 200 GeV is shown in figure 4. The φ is reconstructed through the φ → K
+K
−channel as described in [7]. The higher p
⊥for heavier mass particles and the increase for the K
−and ¯p as a function of centrality are all consistent with a picture of radial flow. The φ seems to show a different behavior. The mass of the φ is close to that of the p and hence one would expect the two particles to be affected by the transverse collective flow in a similar manner. However it appears that after an initial jump in p
⊥from p–p to Au–Au collisions the p
⊥of the φ remains constant while that of the ¯p shows a steady increase.
The hadronic cross-section of the φ is small and so, like the , this may be an indication
that there is significant flow before hadronization. If the φ were predominantly produced via
Number of Charged P articles 0 100 200 300 400 500 600 700 800
> (GeV/c)T<p
0 0.2 0.4 0.6 0.8 1 1.2 1.4
π
-K
-p
T>
<p φ Filled symbols:
STAR preliminary
Figure 4.Preliminaryp⊥forπ, K, p andφas a function of the measured mid-rapidity charged particles for Au–Au and p–p collisions at 200 GeV.—pp, —Minbias,—Central.
K
+K
−coalescence close to kinetic freeze-out it would be expected that the φ would have a p
⊥close to twice that of the kaon, which is clearly not the case.
The p
⊥is not a very sensitive measure of the freeze-out conditions as T
f oand β
⊥may be combined in many permutations to extract the same p
⊥. It is therefore of great interest to extend the blastwave study to the 200 GeV data as a function of centrality for all particles including the φ, and . The different quark contents of the various species may also allow us to determine how radial flow is transmitted at the quark level. If, as indicated by the and φ measurements, looking at the spectra of particles with different hadronic cross-section allows us to differentiate between transverse flow at the partonic and hadronic level a much deeper understanding of how heavy-ion collisions develop will emerge.
2.3. Elliptic flow
Elliptic flow (v
2) also allows us to study the early times of the collision. An initial anisotropic coordinate space generates a momentum space anisotropy through constituent interactions.
As the volume expands the spatial anisotropy reduces and the pressure gradients disperse;
hence this effect is self-quenching and measures only initial properties of the collision [17].
Elliptic flow measurements have been reported previously by STAR for inclusive charged hadrons [18–21] and identified particles [22, 23] at 130 GeV. Figure 5 shows preliminary v
2measurements as a function of p
⊥to p
⊥= 6 GeV/c for K
0s, + ¯ and charged hadrons.
The data are centrality inclusive from 0–80% of the collision cross-section. Curves from a hydrodynamical prediction [24] for different particle species are also shown. As with the 130 GeV data there is good agreement between the model and the data below p
⊥∼ 2 GeV/c.
The deviation from the hydrodynamical model for the appears to occur at higher p
⊥than
for the K
0s. The v
2for the different species then plateaus with the hyperon saturating at
a higher value. This may simply be due to the fact that the flow effects are stronger for the
heavier mass and hence they ‘feel’ the domination of hydrodynamical effects over other
physical processes to larger p
⊥. However it may also be due to a meson versus baryon affect
[25]. The measurement of v
2for the heavy φ meson would help resolve this question. The
Anisotropy Parameter v2
0 0 0.1 0.2 0.3
0
K
SΛ + Λ h
+2 +h
-Hydro calculations π
K p Λ
Transverse Momentum p
2 4 ⊥(GeV/c)
6STAR Preliminary Au-Au 200 GeV
Figure 5. Preliminary minimum bias v2 measurements versusp⊥ for+ ¯, K0s and charged hadrons in Au–Au collisions at 200 GeV. Errors are statistical only and the curves are hydrodynamical calculations.
0 0.2 0.4 0.6 0.8 1 1.2
0 1 2 3 4 5 6 7
π − K− K0s STAR Preliminary
Au-Au 200 GeV
Μ⊥ (GeV/c2) RInv (fm)
Figure 6. PreliminaryRInvHBT radii versusm⊥forπ−, K−and K0s from Au–Au collisions at 200 GeV.
centrality dependence of these measurements and further discussion of the possible causes of the high p
⊥saturation are discussed in more detail in [26].
2.4. Radii and timescales
To study the spacetime geometry of the source we use two particle intensity interferometry (HBT) [11]. Further dynamical information can be extracted by exploring these measurements as a function of p
⊥and particle species. In the presence of radial flow the measured radii are expected to drop as a function of m
⊥[12]. Figure 6 summarizes the measured STAR R
Invradii as a function of m
⊥[13]. While the π and charged K show the expected drop in radii with increasing m
⊥the neutral kaon measurement does not follow the apparent trend. This could be an indication that the K
0sis emitted over a longer time period than that of the other particles.
However, it should be remembered that R
Invis an integration over all three dimensions and
Figure 7.Preliminary balance function widths versus number of participants forπand K in p–p and Au–Au collisions at 200 GeV. The calculation forπfrom HIJING is also shown.
hence a more detailed study must be made, when more statistics become available, before firm statements can be made on the apparently large homogeneity length of the K
0s.
The balance function hopes to study the time between hadronization and kinetic freeze- out [14]. The idea is that initially charge/anti-charge pairs are created close in spacetime, the longer the time between creation and final freeze-out the more the amount of scattering that occurs and the looser the remaining correlation in phase space, such as rapidity. The balance function measures the probability of finding a charged particle with rapidity y
1given that an oppositely charged particle has already been detected with rapidity y
2. Figure 7 shows the width of these correlation functions for π and charged kaons in p–p and Au–Au collisions at 200 GeV as a function of centrality [15]. A smooth reduction is seen in the widths as a function of multiplicity in agreement with theoretical calculation of delayed hadronization in central Au–Au collisions when compared to p–p. HIJING, which does not incorporate late hadronization in A–A collisions, shows no such narrowing as shown in figure 7.
3. Hard physics
The effects of hard–parton scattering and suppression of particles at large transverse momenta have been measured for the first time in heavy-ion collisions at RHIC [27, 28] opening a new regime for probing the validity of pQCD calculations.
Topological particle identification is possible to very high momentum where traditional hadron identification, via specific ionization in the detector gas or time-of-flight measurements, fails. Many of the produced strange particles are neutral and hence identification is accomplished predominantly via their decay into charge daughters. Therefore these strange particle measurements are available at high p
⊥and allow a unique test, at both the baryonic () and mesonic
K
s0levels, of theories on specific particle production rates and spectra that would otherwise be made on the charged particle ensemble.
3.1. High p
⊥suppression
Results on charged hadron production at high p
⊥have been shown to exhibit a depletion of
hadrons with p
⊥above 2 GeV/c relative to elementary p–p, or peripheral Au–Au, collisions.
periph)|〉binN〈/T(dN/dpcent)|〉binN〈/T(dN/dp 0 2
10-1 1
Scaling binary participant 0
K
SΛ + Λ
h+2+h-60-80%
0-5%
4 6
Transverse Momentum p⊥ (GeV/c)
STAR Preliminary
Figure 8.Preliminary Rcpfor+ ¯and K0sin Au–Au collisions at 200 GeV.
This depletion increases with the centrality of the collision and is believed to be indicative of parton energy loss in a dense medium. New results for + ¯ and K
0sin Au–Au collisions at 200 GeV are shown in figure 8 (presented in more detail in [31]), these results agree qualitatively with that presented by the PHENIX collaboration for the π
0and p [29, 30].
baryon production appears to scale with the number of binary collisions for moderate p
⊥while that of the K
0smeson is strongly supressed, a similar suppression is also observed by STAR for charged kaons [32]. However these results extend the reach in p
⊥from that of PHENIX and shows, for the baryon at least, that by p
⊥∼ 5 GeV/c the baryon and meson measurements result in a similar suppression. It is speculated that at these higher p
⊥‘standard’ fragmentation is being observed, whereas at lower p
⊥the effects of hydrodynamical flow and/or other physical processes are competing, the lighter meson being dominated by quenched pQCD effects earlier. Again measurements with the heavy φ meson would aid in the resolution of these questions.
4. p–p
The study of strange particle production in elementary p–p collisions is important not only as a baseline with which to compare A–A, but also in its own right. Without a detailed knowledge of particle production in elementary collisions we cannot hope to understand the more complex systems produced when heavy ions are collided.
4.1. Anti-baryon to baryon ratios
Hyperon production was previously measured in ¯p–p collisions at √
s = 200 GeV by UA5 [33], however this experiment has no magnetic field, and hence was not able to distinguish particle from anti-particle decays. STAR is therefore able to make the first anti-baryon/baryon ratio measurements, including multi-strange baryons, in p–p collisions at this energy (figure 9). The ratios for Au–Au collisions at √ s
N N= 130 and 200 GeV are also shown.
All ratios are corrected for absorption in the detector material but not for feed-down, it is expected that this correction is not greater than 5% for the ¯p/p and much less for the other ratios. The proximity to unity of these ratios indicates significant pair production even in p–p collisions. This is not the case at the SPS energies as reported at this conference [35].
Comparisons with theory [36] show that while NeXuS, UrQMD and HIJING can
reproduce the measured multi-strange baryon ratios in p–p collisions; only NeXuS 3.97
predicts the ¯p/p and ¯ / ratios correctly.
Strangeness Content per Particle
0 1 2 3
Anti-Baryon/Baryon Ratio
0 0.2 0.4 0.6 0.8 1
1.2 STAR Au+Au 130 GeV STAR Au+Au 200 GeV
STAR p+p 200 GeV
STAR preliminary
Ω- +/ Ω Ξ- +/ Ξ Λ Λ/ /p p
Figure 9.Preliminary anti-baryon to baryon ratios for increasing strangeness content for Au–Au collisions at 130 and 200 GeV and p–p collisions at 200 GeV. Errors are statistical only, systematic uncertainties∼7–15% progressively.
Mass [GeV]
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
<p⊥> [GeV/c]
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1
STAR Preliminary p-p 200 GeV
Figure 10.Preliminaryp⊥versus mass for p–p collisions at√sN N =200 GeV. Error bars are statistical plus systematic uncertainties added in quadrature.
4.2. Mean p
⊥The high statistics of our measurements also allow us to probe the kinematic properties of
produced particles in much more detail than was previously possible. We have long interpreted
the dependence on the measured inverse slopes of the p
⊥spectra, or p
⊥, in A–A collisions
as evidence for radial flow [34]. Figure 10 shows the p
⊥as a function of particle mass as
extracted from power law fits to the measured p
⊥spectra [7–10, 37]; a clear mass dependence
is observed. It is not expected that these collisions should exhibit collective motion, although
this possibility cannot yet be completely eliminated, and hence this trend has been attributed
to multiple mini-jet production [38]. Also consistent with multiple mini-jet production is the
dependence of p
⊥on the dN
ch/dy observed by STAR for the K
0s[37]. These measurements
show that care needs to be taken when interpreting the Au–Au collision measurements,
especially for the most peripheral events.
5. Conclusion and outlook
These heavy ion collisions produce a large number of particles which appear to interact briefly but strongly resulting in a thermalized system before kinematic freeze-out. The resonance particle results suggest significant rescattering after hadronization but only over a short period of time. This is consistent with the balance function results that suggest a smaller period of time between hadronization and kinetic freeze-out for central Au–Au collisions than p–p.
Measurements at higher p
⊥show a suppression of particle production when compared to p–p collisions. A significant difference is observed in the p
⊥dependence of this suppression for K
0smesons when compared to baryons. Plateaus in the measured azimuthal anisotropy are also observed for both and K
0sat high p
⊥.
The p–p data are still being analysed but the spectra suggest multiple mini-jet production that creates p
⊥distributions suggestive of radial flow. The final analysis of yields and spectra as functions of multiplicity will help shed more light on particle production in elementary collisions. The current d–Au run has not yet finished being analysed, these data are vital for understanding the different physics contributions to the results reported here. By studying high momenta particle production in p–p, where jets are the predominant form of particle production, and d–Au collisions, where there is excited matter but no QGP is expected to be formed, we can compare to the more complicated A–A collisions. In this way we can study the effects on different particles as they interact with highly excited media.
The planned RHIC program, for 2003, of an extended top energy Au–Au is vital for the strangeness studies where statistics hungry analysis, such as the φ and , are required to enable us to deepen our understanding of heavy-ion collisions, especially at the early stages.
Acknowledgments
I would like to thank the STAR collaboration, in particular the Strangeness and Spectra physics working groups for providing me with the analysed data for these proceedings. STAR also thanks the RHIC Operations Group and the RHIC Computing Facility at Brookhaven National Laboratory, and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory for their support. This work was supported by the Division of Nuclear Physics and the Division of High Energy Physics of the Office of Science of the US Department of Energy, the United States National Science Foundation, the Bundesministerium fuer Bildung und Forschung of Germany, the Institut National de la Physique Nucleaire et de la Physique des Particules of France, the United Kingdom Engineering and Physical Sciences Research Council, Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, Brazil, the Russian Ministry of Science and Technology, the Ministry of Education of China the National Natural Science Foundation of China and the Swiss National Science Foundation.
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