Contributions to the Development of Microstrip Gas Chambers (MSGC) for the HERA-B Experiment. Dissertation

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Contributions to the Development of Microstrip Gas Chambers

(MSGC) for the

HERA-B Experiment

Dissertation

zur

Erlangung der naturwissenschaftlichen Doktorw ¨urde (Dr. sc. nat.)

vorgelegt der

Mathematisch-naturwissenschaftlichen Fakult¨at der

Universit¨at Z ¨urich

von

Thomas M. Walter

von Gr ¨uningen ZH

Begutachtet von Prof. Dr. Peter Tru¨ol Prof. Dr. Ulrich Straumann

Z ¨urich 2001

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mann als Dissertation angenommen.

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Zusammenfassung

Zur Zeit wird eine neue Generation von grossen Teilchenphysik Experimenten aufgebaut (HERA-B) oder geplant (LHC Experimente). Ziele dieser Experimente sind die Messung der CP-Verletzung, die Entdeckung des Higgs-Bosons und die Top-, Beauty- und Tau- Physik. Da die Experimente f ür hohe Luminositäten, d.h. sehr viele Teilchenkollisionen pro Sekunde, ausgelegt sind, f ührt es zu sehr hohen Teilchenfl üssen. Darum m üssen alle benutzten Detektortechnologien, insbesondere die Spurkammern, viel strahlungshärter sein als alle herk ömmlichen.

Die Mikrostreifen-Gas-Kammern (MSGC) sind eine speziell f ür diese rauhen Bedingun- gen entwickelte Detektortechnologie. In Zusammenarbeit mit den Universitäten Heidel- berg und Siegen entwickelten und testeten wir MSGC-Spurkammern f ür den inneren Teil des HERA-B Experiments. Zur Qualitätskontrolle wurde ein Mikroskop mit einem grossen Objekttisch zur halbautomatischen, optischen Überpr üfung der MSGC-Platten gebaut. Zusätzlich entwickelten wir einen elektrischen, computergesteuerten Tester zur schnellen Kontrolle der Platten direkt nach der Produktion.

In den Strahltests am Paul Scherrer Institut (PSI) entdeckten wir, dass MSGCs durch Ent- ladungen bzw. Funken zerst ört werden. Daher wurden ausgedehnte Studien durch- gef ührt, wie diese Durchschläge verhindert werden k önnten, bzw. um eine neue strah- lenhärtere Technologie zu finden. Als L ösung wurde ein Gas–Elektronen–Vorverstärker in die MSGCs eingebaut, eine sogenannte GEM-Folie.

Selbstverständlich mussten diese GEM-MSGCs wieder ausgiebig, zusammen mit der ge- samten HERA-B-Auslesekette, getestet werden. Zusätzlich wurde die Strahlungshärte der MSGC-Plattenbeschichtung überpr üft. In einem weiteren Strahltest am PSI wurden die Kammereigenschaften über die gesamte aktive Fläche gemessen, um lokale Lei- stungsunterschiede einzugrenzen.

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Presently, a new generation of large scale high energy physics experiments is under construction (HERA-B) or in the development stage (LHC experiments). The goals of these experiments are the measurement of the CP violation parameters in the B system, the hunt for the Higgs boson as well as the study of top, beauty, and tau physics. The experiments are designed for high luminosity, i.e. for many particle collisions per second. This leads to very high particle fluxes. Thus all detector technologies, especially the tracking devices, have to be much more resistant against radiation than the technologies used up to now.

Microstrip gas chambers (MSGC) are detectors particularly designed for these harsh conditions. In a collaboration with the universities of Heidelberg and Siegen, we developed and tested MSGC tracking devices for the inner part of the HERA-B experiment. For the quality control of the substrates we built a microscope with a large object table for semi- automated scanning of the MSGC-wafers. For a very fast inspection of the wafers right after their production, we developed an electrical, computer controlled testing apparatus.

During beam tests at the Paul Scherrer Institute (PSI), Switzerland, we found that MSGCs are destroyed by discharges. Thus extensive studies have been carried out how to prevent these sparks or to find an alternative technology. The solution found was the insertion of agas electron multiplier(GEM) into the MSGC.

Naturally these GEM-MSGCs had to be tested again together with the whole electronic readout chain foreseen for the HERA-B experiment. We investigated the radiation hard- ness of the MSGC-wafers thoroughly. In a further beam test at the PSI, we measured the properties of the GEM-MSGC over the whole active detector area, to get a first idea on the performance variations over the whole chamber.

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Chapter 1 B physics and CP violation

One of the most important concepts in physics to understand nature is to find symmetries and conservation laws. Examples are time translation symmetry, meaning that the fundamental physics law stay the same in the past and the future, and energy conservation, meaning that the total energy of a closed system remains constant.

A very crucial role play the three symmetries C, P, and T. Most of the time particles and antiparticles behave the same way, except for their charge, which is described by the charge conjugation, the C symmetry. In a similar way, the fundamental interactions stay the same whether we observe them directly or through a mirror or if we view the processes in a movie in forward or backward direction. Here the parity symmetry P, and the time reversal symmetry T play their appropriate role.

1.1 CP violation

In the following chapter we follow chapter 1.2 of [1]. All existing field theories are based on the hypothesis that the product of the three symmetries C×^P×T is an exact symmetry of the equation of motion. In weak interaction processes, the charge conjugation C, which relates particles to their antiparticles and the parity P, which relates the left-handed to the corresponding right-handed (mirror images) particles, are known to be always violated.

However, the product CP, and thus T, is violated only in a very small fraction of the weak decays. CP violation was discovered experimentally in K⁰ K⁰ meson decays in 1964.

Since then, numerous experiments have followed to measure CP violation in the Kaon system.

In the Standard Model, CP violation is introduced through a single phase remaining in the Cabibbo–Kobayashi–Maskawa (CKM) matrix.V_CKMrepresents the weak coupling in the “three quark family” Standard Model, expressed by the basis of quark mass eigenstates. The CKM matrix has to be unitary if the three quark generations are the complete

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theory:

V_CKMV_CKM^† =V_CKM^† V_CKM=1 (1.1) Any unitary 3×3 matrix can be parameterized by four independent parameters whereof at least one has to be imaginary. Such parameterization of the CKM matrix, given in equation1.2, was proposed by Wolfenstein. Its foundation is the experimental observation, that the diagonal elements (coupling between the same generation) are of the order of 1, the coupling between the first and the second generation is suppressed by the fac- torλ'⁰.22(=sinθ_C), between the second and third generation byλ²and between the first and third generation byλ³. Terms of the order of O(λ⁴) or higher are neglected. All parametersλ,A, ρandηare real numbers.

V_CKM=





V_ud V_us V_ub V_cd V_cs V_cb V_td Vts V_tb



=







1− ^λ₂² λ λ³A(ρ−ⁱη)

−λ 1− ^λ₂² λ²A λ³A(1−ρ−ⁱη₎ −λ²A 1





 (1.2)

The unitarity of the CKM matrix can be used to define relations between the CKM parameters. This allows us to give a geometrical representation of amplitudes and phases of the CKM matrix. These can be constrained by experimental data. The most interesting unitarity relation is

V_udV_ub^∗ +V_cdV_cb^∗ +V_tdV_tb^∗ =0. (1.3) Using equation1.2, the three terms in the last equation can be re-expressed as:

V_cdV_cb^∗ =−^Aλ³ (1.4)

V_udV_ub^∗ = Aλ³(ρ+iη) (1.5) V_tdV_tb^∗ =Aλ³₍₁−(ρ+iη₎₎ _(1.6)

Equations1.4–1.6, rescaled byAλ³, form the sides of an unitarity triangle in the complex plane (see Figure1.1). The lengthsCAand BAof the rescaled unitarity triangle can be expressed in terms of the CKM parameters as follows:

CA=^pρ²+η²= |^VudV_ub^∗ |

|^VcdV_cb^∗| =

1+λ² 2

1 λ

|^Vub|

|^Vcb| ^(1.7)

CB=^p(1−ρ)²+η²= |^VtdV_tb^∗|

|^VcdV_cb^∗| = ¹ λ

|^Vtd|

|^Vcb| ^(1.8)

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1.2 B⁰–B⁰mixing 9

γ β

α

C = (0,0)

1 A = (ρ,η)

B = (1,0) η

ρ

− ρ − η i ρ +

η

i

Figure 1.1: The unitarity triangle scaled by Aλ³in theρ–η plane of the Wolfenstein parameterization.

and the angles are given by

α = arg

−^V^td^V

tb∗

V_udV_ub^∗

(1.9) β = arg

−^V^cd^V

cb∗

V_tdV_tb^∗

≈ −^argVtd (1.10)

γ = arg

−^V^ud^V

ub∗

V_cdV_cb^∗

≈ −^argVub. (1.11)

The triangle depicted in Figure1.1combined with|^Vus|^and|^Vcd|gives a full description of the CKM matrix.

1.2 B

⁰

–B

⁰

mixing

Neutral mesons, in this particular case B₀, mix via an intermediate state. Any arbitrary state is a superposition of the flavor eigenstates (B⁰,B⁰), which obey the time dependent Schr ¨odinger equation

id dt

a b

= H a

b

=

M−ⁱ

2Γ a

b

. (1.12)

The eigenvectors are the mass eigenstates. H stands for the heavy and L for the light meson.

|^BHi = p|^B⁰i −^q|^B⁰i ^(1.13)

|^BLi = p|^B⁰i+q|^B⁰i ^(1.14)

|^p²|+|^q²| = 1 (1.15)

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B

-

0

d b

W W

t

+ +

t b

d

B⁰

(a)

W

W +

- B

-

0

d t b

t

b d

B⁰

(b)

Figure 1.2: Two possibilities, (a) and (b), of B⁰–B⁰mixing in the standard model over an intermediate state.

Solving this equation for the flavor eigenstates gives

|^B⁰i = ¹

2p |^BHi+|^BLi ^(1.16)

|^B⁰i = ¹

−^2q |^BHi − |^BLi ^(1.17) The time evolution of the mass eigenstates is given by equation1.12

|^BH(t)i = e⁻^iM^H^te⁻¹²^Γ^H^t|^BHi ^(1.18)

|^BL(t)i = e⁻^iM^L^te⁻¹²^Γ^L^t|^BLi ^(1.19) whereMx−₂ⁱ^Γxare the eigenvalues. In contrast to theK⁰system, which includes a large life time difference, the width difference ∆Γ_B between the physical states is negligible compared to the mass difference∆m_B:

|∆ΓB|

∆m_B < 10⁻² (1.20)

From this relation follows that one can setΓ_H =Γ_L≡^ΓB, while the masses differ by∆m_B, so that M_H =m_B+¹₂∆m_B andM_L=m_B−¹₂^∆mB. With this notation, the time evolution ofB⁰is (1.18and1.19in1.13):

|^B⁰^(t)i = ¹

2pe⁻^im^B^te¹²^Γ^B^th

e⁻²ⁱ^∆m^B^t|^BHi+e⁺²ⁱ^∆m^B^t|^BLiⁱ ^(1.21) The time evolution ofB⁰is determined in a similar way.

Studying CP violation (see Section1.3), the ratio q/p is of interest. It depends on the off diagonal elements,M₁₂andΓ₁₂(see [2] Formula 182):

q p

= ^M

∗12−₂ⁱ^Γ^∗₁₂ M₁₂−₂ⁱΓ12

(1.22)

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1.3 Experimental observation of CP violation 11

From (1.20) follows that|^Γ12| |^M12|(see [2] Formula 188) and the ratio can be approxi- mated as

q p

' − ^M^∗¹²

|^M12|

1−¹ 2ImΓ₁₂

M₁₂

. (1.23)

In this approximation the rate ofB⁰–B⁰mixing is just a phase

q p

' ¹. (1.24)

The vertices in Figure1.2are proportional toV_tb^∗ andV_td: M₁₂'^VtbV_td^∗V_tbV_td^∗ ([2] Formula 190):

q p

'

s M^∗₁₂ M₁₂ =

s(V_tb^∗V_td)² (V_tb^∗V_td)² = ^V

tb∗V_td

V_tbV_td^∗ (1.25)

= ¹·^Aλ³(1−ρ−ⁱη)

1·^Aλ³(1−ρ+iη) = ^CB·^e⁻ⁱ^β

CB·^eⁱ^β =e⁻²ⁱ^β (1.26)

1.3 Experimental observation of CP violation

There are three manifestations of CP violation: direct,indirectandmixing inducedCP violation, which will be shortly described below.

1.3.1 Direct CP violation

Direct CP violation results from interference of decay amplitudes in the weak decays.

CP|^P(~p)i = eⁱ^φ^P|^P(−~p)i ^(1.27) PandPare the CP conjugate states. The phase is unmeasurable and arbitrary. Lets take a closer look at the decays of CP conjugated pseudoscalar meson states (e.g. B⁺andB⁻ or the neutralBmesons) in the CP conjugated final states f and f.

A = h^f|^H|^Pi=

∑

k

A_keⁱ^δ^ke^iΦ^k (1.28)

A¯ = h^f|^H|^Pi=eⁱ⁽^φ^P⁻^φ^f⁾

∑

k

A_keⁱ^δ^ke⁻^iΦ^k (1.29) The sum runs over all possible decay diagrams. The parameterδ_i appears in scattering due to strong interactions, and Φ_i is the weak phase that violates CP. The physically meaningful quantity is the absolute value of the ratio of the amplitudes:

A¯ A

6=₁ ⇒ direct CP violation (1.30)

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To avoid mixing, the easiest way to observe direct CP violation is in the decays of charged mesons. The measurable CP asymmetry is defined as

a_f = ^Γ(P

+→ ^f⁾−^Γ(^P⁻→ ^f⁾

Γ(P⁺→ ^f⁾+_Γ(P⁻→ ^f⁾ = ¹− |^A^¯/A|²

1+|^A^¯/A|². (1.31) If there is only one partial decay amplitude, i.e. only one summand in equation1.28, the phase is unmeasurable. Thus interference of at least two diagrams is required.

1.3.2 Indirect CP violation

Indirect CP violation arises from the mixing of neutral mesons. Because of mixing, P⁰ andP⁰form the mass eigenstates

|^P1⁰i = p|^P⁰i −^q|^P⁰i ^(1.32)

|^P₂⁰i = p|^P⁰i+q|^P⁰i ^(1.33)

|^p²|+|^q²| = 1 (1.34)

Ifp=q=1/√

2 there is no CP violation:

CP|^P1⁰i = |^P1⁰i ^(1.35)

CP|^P₂⁰i = −|^P₂⁰i ^(1.36)

The physical meaningful probe is:

q p

6=₁ ⇒ indirect CP violation (1.37)

Indirect CP violation corresponds to the CP asymmetry observed in the neutral Kaon system. It arises from the life time differences∆Γof the two mass eigenstates (see Sec- tion1.2).

1.3.3 Mixing induced CP violation

CP violation in the interference of direct decays and decays via mixing gives access to the phases of the quantities ¯A/Aandq/p. For the decay of neutral mesons into the same CP eigenstates we define the following quantities:

A ≡ h^fCP|H|^P⁰i ^(1.38)

A¯ ≡ h^fCP|^H|^P⁰i ^(1.39)

ξ ≡ ^q

p ·^A^¯

A (1.40)

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As we will show, the productξis physically meaningful.

ξ 6=1 ⇒ CP violation (1.41)

In this case interference between partial decays is not necessary. Therefore hadronic uncertainties can be minimized. In an experiment the time dependent asymmetry between the numbers of B⁰andB⁰mesons decaying into the same CP eigenstate is measured:

a_f_CP(ˆt) = ^Γ(B

0(ˆt)→ ^fCP)−^Γ(B⁰^(ˆ^t)→ ^fCP)

Γ(B⁰(ˆt)→ ^fCP)+_Γ(B⁰_(ˆt)→ ^fCP) (1.42) The asymmetry is only visible after some time of mixing. At an arbitrary time ˆt, the flavor composition is given by equation1.21. The amplitude of the decay of Bto the final state

f_CPis:

h^fCP|^H|^B⁰^(ˆ^t)i = h^fCP|^H ¹

2pe⁻^im^B^te¹²^Γ^B^th

e⁻²ⁱ^∆m^B^t|^BHi+e⁺ⁱ²^∆m^B^t|^BHiⁱ ^(1.43)

= ¹

2pe⁻^im^B^te¹²^Γ^B^th^fCP|^H^h^e⁻²ⁱ^∆m^B^t^p|^B⁰i −^q|^B⁰i +e⁺²ⁱ^∆m^B^t

p|^B⁰i+q|^B⁰iⁱ ^(1.44)

= ¹

2pe⁻^im^B^te¹²^Γ^B^tp

e⁻²ⁱ^∆m^B^t+e⁺²ⁱ^∆m^B^t

h^fCP|H|^B⁰i + ¹

2pe⁻^im^B^te¹²^Γ^B^tq

e⁻²ⁱ^∆m^B^t−^e⁺²ⁱ^∆m^B^th^fCP|H|^B⁰i (1.45)

= e⁻^im^B^t^ˆe⁻^Γ²^B^t^ˆ

Acos ∆m_B

2 tˆ

+iA¯ q psin

∆m_B 2 ˆt

(1.46)

= Ae⁻^im^B^t^ˆe⁻^Γ²^B^t^ˆ

cos ∆m_B

2 tˆ

+iξsin ∆m_B

2 tˆ

(1.47) The width is:

Γ(B⁰(ˆt)→ ^fCP)

=

h^fCP|^H|^B⁰^(ˆ^t)i

2 (1.48)

= |^A|²^e⁻^Γ^B^t^ˆ

1+|ξ|²

2 +¹− |ξ|²

2 cos ∆m_Bˆt

−^Im(ξ) sin ∆mtˆ

(1.49) In a similar way the corresponding width forB⁰(ˆt) can be expressed as:

Γ(B⁰(ˆt)→ ^fCP)

= |^A|²^e⁻^Γ^B^t^ˆ

1+|ξ|²

2 −¹− |ξ|²

2 cos ∆m_Bˆt

+_Im(ξ_{) sin} _∆mtˆ

(1.50)

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Using the two latter equations, the measurable asymmetry (equation1.42) is calculated as:

a_f_CP(ˆt) = ⁽¹− |ξ|²^{) cos} ^∆mBtˆ

−^{2 Im(}ξ) sin ∆mtˆ

1+|ξ|² ^(1.51)

In the approximation|ξ|=1,a_f_CP is:

a_f_CP(ˆt) = −^Im(ξ) sin ∆mtˆ

(1.52) For many Bdecays ¯A/_A=_e^2iΦ _{as well as} _q/_p=_e⁻²ⁱ^β hold to the first approximation, which gives:

|ξ| = 1 (1.53)

Imξ = −^sin(2β−^2Φ). (1.54)

1.3.4 The Golden Decay

The decayB⁰→^J/ψK⁰_soffers a very clean measurement of CP violation for the following four reasons:

1. CP violation is expected to be large inB⁰–B⁰mixing.

2. The final state is a CP eigenstate.

3. The contamination from penguin diagrams is small.

4. It has a clear signature separating it from the high combinatorial background.

Because of these favorable conditions, the channel is named thegold platedmode for mea- suring sin 2β. The decay diagrams are depicted in Figure1.4. The amplitude of the tree diagram (a) is proportional to

A¯_tree = V_cbV_cs^∗ =Aλ²+O(λ⁴). (1.55) Three penguin diagrams 1.4(a)contribute to the decay: with a t-, c- or u-quark in the loop.

A¯_t₋_penguin = V_tbV_ts^∗ =−^Aλ²+O(λ⁴) (1.56) A¯_c₋_penguin = V_cbV_cs^∗ =Aλ²+O(λ⁴) (1.57)

A¯_u₋_penguin = V_ubV_us^∗ =O(λ⁴) (1.58)

Hence the penguin diagrams cancel each other to the order ofλ⁴∼10⁻³. The hadronic uncertainties are of the same order of magnitude.

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4

Main goal of HERA-B Experiment

p

µ^{− −},e

µ^{− −},e µ⁺

,e+ (37 GeV)

(16 GeV)

B^∗∗+

B−

D⁰ B⁰

(130 GeV)

920 GeV

π⁺

π⁻ K_S⁰

K⁻

Vertex charge tag (P = 0.16)

Kaon tag (P = 0.24) Lepton tag (P = 0.17)

B

⁰

→ J ψ K

_S⁰

The Golden Decay:

Signal B

tag (P = 0.19) 11 mm

1.2 m

Tagging B

Target Jψ

Figure 1.3: The Golden Decay: B⁰ →^J/ψ K⁰_s. The B mesons are produced in proton–nucleon collisions. The B⁰ decays in the signal path into J/ψ and K⁰_s. The simultaneously produced spectator B mesons (tagging B) are used to determine whether B⁰or B⁰decayed into aJ/ψand a K⁰_s.

B-0

d-

b c

W

c -

-

s J/ψ

K_S

(a)

B-0 W-

d-

b q

s c

c-

K_S

J/ψ

(b)

Figure 1.4: The Feynman diagrams of the Golden Decay. (a) The tree diagram and (b) the penguin diagrams. The quark q in the loop can either be a t-, c-, or u-quark.

The CP eigenstateK_s⁰is formed byK⁰−^K⁰mixing. This mixing factor adds to the value of ξ_J_/ψ_K0

s. The participating amplitudesV_csand V_cd^∗ have no CP violating phases in the Wolfenstein parameterization and hence do not contribute to the imaginary part ofξ_J_/ψ_K0

s. The ratio of the decay amplitudes depicted in Figure1.4

A¯tree

A_tree = ^V^cb^V

cs∗

V_cb^∗V_cs (1.59)

is also real, resulting inΦ=0. Using equation1.54and1.25gives Imξ_J_/ψ_K0

s = −^sin(2β−^2Φ)=−^sin(2β). (1.60)

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Therefore the detection of an asymmetry in the Golden Decay allows us to directly measure sin(2β) of the unitarity triangle.

1.3.5 Btagging

Because the measurement of the CP asymmetrya_f_CP(1.51) involvesB⁰–B⁰mixing, it must be known whether one has started with aB⁰or with aB⁰. The determination of the initial Bflavor is calledBtagging [3].

TheBmesons are produced in the strong interaction, hence they appear always in pairs, one with ab- and the other with a ¯b-quark. Onebdecays into the Golden Decay channel, the other one can be used to determine with which kind of B⁰ we have started. If the tagging Bwas aB⁻ we started with aB⁰, accordingly a B⁺leads to a B⁰. To determine the quark constitution of theB⁰s, HERA-B uses different tagging methods:

• The charge of the lepton in a semileptonic decay.

b→^c+l⁻+ν_l (1.61)

b¯ →^c^¯+l⁺+ν_l (1.62)

• The charge of the kaon as a decay product

B⁻(bu)¯ →^D⁰^(c^u)^¯ +. . . , D⁰(cu)¯ →^K⁻^(s^u)^¯ +. . . (1.63) B⁺(¯bu)→^D⁰^{( ¯}^cu)+. . . , D⁰( ¯cu)→^K⁺^{( ¯}^su)+. . . (1.64)

• The jet charge: The weighted charge in a jet or originating form a secondary vertex, where~_ais the unit vector along the jet axis.

Q_jet≡ ^∑^qⁱ|~p_i·~a|

∑|~p_i·~a| ^(1.65)

1.4 Experimental constraints on unitarity triangle

The constraints for the tip of the unitarity triangle that follow from|^Vub|^,^Bmixing, and the measurement of ¹ are shown in Figure 1.5. |^Vub| ^and |^Vub/V_cb| respectively, was measured in semileptonicBdecays likeb→^ulν_l at LEP and CLEO.

|^Vub/V_cb|=0.090±⁰.025 [4] (1.66) The constraints of sin 2β, not indicated in Figure1.5, are summarized in Table1.1.

1theCPviolating parameter of the neutral Kaon system

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1.4 Experimental constraints on unitarity triangle 17

B

_____∅M_Bs

∅M_Bd

_____∅M_Bs

∅M_Bd

—0.004 —0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.000

0.002 0.004 0.006

|

s₁₂V_cb

|

V_ub

∅M_Bd

V_ub

A A

C C

Figure 1.5: Constraints on the position of the vertex, A, of the unitarity triangle following from

|^Vub|, B mixing, and. A possible unitarity triangle is shown with A in the preferred region (from [4]).

Experiment sin 2β_ψ_K Reference

CDF 0.79±⁰.43 [5]

BaBar 0.12±⁰.37±⁰.09 [6]

Belle 0.45⁺₋⁰₀^._.⁴³₄₄⁺₋⁰₀^._.⁰⁷₀₉ [6]

gobal analysis 0.750⁺₋⁰₀^._.⁰⁵⁸₀₆₄ [7]

Table 1.1: Summary of published measurements ofsin 2β.

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HERA-B, an experiment to study CP-violation

HERA-B [3], [8] is located in the west-hall of the HERA, the electron proton storage ring at DESY, Hamburg. The main measurement of HERA-B is the CP violation in theGolden Decays of B⁰-mesons into J/ψ K⁰_s (see Section 1.3.4). Hence B-mesons have to be produced and their decay products (e^±,µ^±, K⁰_sandπ^±) have to be identified and measured.

Additionally the accompanying (charged) tagging B-meson must be observed (see Sec- tion1.3.5). The expected rates and errors of the CP violating parameter are shortly summarized in Table2.1.

nominal detector 20% reduced track eff. half inter. rate

bb/year 3.7×10⁸ 3.7×10⁸ 1.8×10⁸

B⁰s+B⁰s 7.4×10⁷ 7.4×10⁷ 3.7×10⁷

Golden Decays 148000 148000 74000

producedµ⁺µ⁻π⁺π⁻events 6130 6130 3060

producede⁺e⁻π⁺π⁻events 6130 6130 3060

reconstr.µ⁺µ⁻π⁺π⁻events 490 200 245

reconstr.e⁺e⁻π⁺π⁻events 233 95 116

backgr. fract. inµ⁺µ⁻channel 0 0 0

backgr. fract. ine⁺e⁻channel 0.1 0.1 0.1

tagging power 0.3 0.28 0.3

error on sin(2β) after 1 year 0.16 0.27 0.23

error on sin(2β) after 4 year 0.08 0.14 0.12

Table 2.1: The CP reach of HERA-B for different detector scenarios. For the case of a 20% reduced tracking efficiency the B meson reconstruction efficiency is reduced by0.8⁴and the tagging efficiencies for the lepton and the kaon tags are scaled by 80% (from [9]).

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2.1 The HERA-B detector 19

The HERA-B Experiment at DESY

Ring Imaging Cherenkov Counter

250 mrad 220 mrad

160 mrad

Magnet

Si-Strip Vertex Detector

Calorimeter TRD

Muon Detector

Target Wires

0 m 5

10 15

20

Photon Detector

Planar Mirrors

Top View

Side View

Proton Beam

Electron Beam

Proton Beam

Electron Beam

Spherical Mirrors

Vertex Vessel Inner / Outer Tracker

Beam Pipe

C4 F10

Figure 2.1: The HERA-B detector.

2.1 The HERA-B detector

The B⁰mesons are produced by a wire target in the proton beam halo. To collect a suffi- cient number ofGolden Decays, four proton–nucleon interactions per bunch crossings are needed on average. In every bunch crossing there are about 200 tracks of charged particles in the detector. This leads to a large track density and to irradiation conditions only comparable to the ones in the forthcoming LHC experiments. Because HERA-B is a fixed target experiment, the whole detector is built as a forward spectrometer (see Figure2.1) with aperture of 220 mrad which is almost 4π in the rest frame of the proton–nucleon system. To ensure that the decay products originate from a B⁰, its decay vertex (= secondary vertex) i.e. its decay length, must be known precisely. Thus one can distinguish directly produced particles from decay products. This is done by the silicon vertex detector. The momentum of the charged particles is determined using the magnet and the tracking chambers. The decay particles – electrons, muons, kaons, and pions – are iden-

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tified with the help of the electro-magnetic calorimeter, the muon system, and the ring imaging ˇCerenkov counter.

2.1.1 The target

The target (see Figure 2.2) consists of two sets of four wires placed in the halo of the HERA proton beam. Each of them can be separately inserted and retracted in steps of 0.05 mm [10]. Several different materials are used as target wire. During the run 2000 the

proton beam

target wires steering

colimator

target wire

indensity

beam profile

no wire

with wire X

Figure 2.2: On the left side is a schematic drawing halo wire target, consisting of two sets of four wires (only one set is drawn). The right side shows how the reduces the beam tails.

following materials samples were investigated: Ti, Al, W and C. All wires have a cross section of 50×⁵⁰⁰µm²except for the carbon wire, which has a size of 50×¹⁰⁰⁰µm². The wire target has the effect of a collimator in the sense that it scrapes away beam tails, the protons on the border of the phase space, which would probably leave the beam anyhow.

Thus it effectively reduces the size of the beam. It does not deteriorate the beam quality for the electron–proton experiments H1 and Zeus and their luminosity stays (almost) the same.

2.1.2 The vertex detector

The vertex detector is composed of silicon micro strip detectors. Its main purpose is to separate the different “events” (proton target interaction) in every bunch crossing and to measure the decay length of the B⁰-meson. To minimize the dead material in front of it, it is operated in the evacuated vertex vessel where the vacuum is almost as good as in the beam tube. Only a seamless 200µm thin aluminium cap separates it from the HERA ring vacuum. Seven of the eight vertex detector layers are in the vessel and the last one is located right behind it. To minimize radiation damage, they are retracted during the injection of the proton beam. Therefore for every proton fill a new online alignment has to be performed.

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2.1.3 The magnet

The particle momenta are measured with a normal conducting magnet in conjunction with the tracking chambers. The integrated field amounts to 2.2 Tm. The distance between the magnet center and the target is 4.5 m. 85% of the K⁰_ss have decayed within this flight path and can therefore be identified and analysed.

2.1.4 The outer tracker

As the particle density decreases with 1/r² (r = distance from the beam axis) (see [4]

p. 188, formula 25.6), HERA-B is composed of two tracking systems. The inner tracker discussed in greater details in Section 2.2, covers the region 45×^{45 cm}² ^{to 50}×^{50 cm}² along the beam pipe. The outer tracker covers the remaining part of the acceptance region. It consists of honeycomb drift chambers with a cell size of 5 mm in the inner and 10 mm in the outer region. For an accurate momentum measurement the track resolution should be better in the bending plane than perpendicular to it. Hence there are no detectors with wires along the in horizontal direction. Only some chambers are tilted by a stereo angle of±5^◦to enable a relatively inaccurate measurement of the y-position.

Additionaly, detectors tilted by 90^◦would lead to a lot of ambiguities in the pattern recog- nition.

2.1.5 The RICH

Behind the magnet and the main part of the tracking system, there is a large ring imaging Cerenkov counter (RICH). The task of the RICH is to separate charged kaons from pions,ˇ electrons, and protons in a momentum region from a few GeV to about 50 GeV. The main part of the RICH consists of a big tank filled with C₄F₁₀ gas. When a charged particle passes the tank, it emits light, which is reflected against two mirrors and detected by a photomultiplier. From the radius of the ring of photon impact points the opening angle of the light cone and further the velocity of the particle is reconstructed. Together with the momentum from the tracking system this information leads to a mass determination.

2.1.6 The electromagnetic calorimeter

The main purpose of the ECAL is to produce the pretrigger (see Section2.1.9) for electrons from the Golden Decay. It is divided into an inner, a middle, and an outer section, with segmentations to ensure an occupancy of at most 10%. The calorimeter modules are sampling plastic scintillators absorber sandwiches. They are read out via optical fibers by photomultipliers. The absorber material in the inner region is tungsten to keep the Moli`ere radius low; in middle and outer region, where the track density is lower, lead is used.

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2.1.7 The TRD

To separate electrons from hadrons, a transition radiation detector is placed in front of the inner part of the ECAL. A charged particle passing the boundary between two media of different dielectric constants emits transition radiation. The rate strongly depends on the Lorentz factor γ, i.e. the lighter the particle the more light is emitted. In this way hadrons faking an electron from a Golden Decay can be strongly suppressed.

2.1.8 The muon system

The penetrating muons are separated from the hadrons by the use of a massive absorber.

Three iron/concrete absorber blocks alternate with the muon chambers. No absorber is between the two last chamber layers. Hits from these two layers are used asµ-pretrigger (see Section2.1.9).

2.1.9 The trigger

The event input rate of 10 MHz and the limited data logging rate of 50 Hz at HERA-B put some strong requirements on the background rejection factor, which the trigger has to achieve. The main trigger method is to look for J/ψ from the Golden Decay. This is done in a five stage process:

Pretrigger The pretrigger delivers the starting points (Regions of Interest = RoI) for the track search in the following trigger level. RoIs are large clusters in the ECAL or multiple hits in the last two muon chamber layers.

FLT The first level trigger starts looking for tracks at the RoIs. It moves upstream from tracking plane to tracking plane towards the magnet. From the position and the angle of these track the corresponding momentum is estimated assuming it comes from the nominal vertex. If the invariant mass fo two tracks combined lies in the region of the J/ψmass, the event is accepted. The FLT’s implementation consists of the track finding units (TFU), the track parameter units (TPU), and the track decision unit (TDU).

SLT In the second level trigger the tracks found in the first level are refitted using more exact data from the tracking chambers. The cut on the J/ψ is narrowed. If a secondary vertex is found, the event is directly sent to the fourth level trigger. Digital signal processors (Sharcs) are used as second level buffer and to take the trigger decision.

TLT A refined reconstruction with correct treatment of multiple scattering is done at the third trigger level. Secondary vertices are searched. Additionally physics channels that do not necessary contain a secondary vertex are selected at this level.

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4LT The fourth level trigger performs a full event reconstruction. The final event selec- tion and various monitoring tasks are carried out before the events are written to tape. The TLT and 4LT is implemented as a computing farm of 200 Linux PCs.

proton

TFU TFU TFU

ECAL Pre

TFU

TFU TFU

TFU

TFU TFU

TFU

TFU TFU

TFU

TPU

TFU TFU TFU

TFU TFU TFU TFU TFU TFU TFU

MUON Pre MUON

Pre

MUON Pre

TPU

TFU TFU TFU ECAL

Pre TFU TFU TFU MUON

Pre

ECAL Pre

TDU

TFU

beam µ

high P e

electromagnetic calorimeter

(ECAL) (ITR & OTR)inner & outer tracker magnet

target

pretrigger High-Pt ECAL

pretrigger pretrigger

MUON

muon tracker (MUON)

t

Figure 2.3: The principle of first level trigger. Tracks of decayingJ/ψare searched starting from the back towards the target. Hits in the muon system or the electromagnetic calorimeter are used as starting points (= pretrigger).

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2.2 The inner tracking system

At HERA-B the tracking system is, as already mentioned in Section2.1and2.1.4, divided into three main parts: the vertex detector (VDS), the outer tracker (OTR), and the inner tracker (ITR). Even though the area covered by the ITR is much smaller than the one of the outer tracker, it is absolutely vital for HERA-B. Due to the strong Lorentz boost the same number of particles pass the ITR and the OTR. The inner tracker was jointly developed by the Physikalisches Institut der Universität Heidelberg, the Gesamthochschule Siegen, and thePhysik–Institut der Universität Z ürich.

2.2.1 Requirements for the inner tracker

The following requirements are resulting from the physics demands and the construction boundaries of HERA-B:

• The ITR covers the area between 6 cm and 30 cm radially perpendicular to the beam.

• In this region particle densities of up to 2×¹⁰⁴/mm²s are expected.

• Due to these high particle densities the detectors are exposed to an irradiation of up to 1 Mrad/a. Therefore the detectors have to be very radiation hard to survive 3 to 5 years of operation.

• The hit resolution in the X-coordinate (bending plane of the magnet) should be less than 100µm, in the vertical (Y-) direction the resolution may be lower (≤^{1 mm),} and along the beam pipe it should be about 3 mm.

• The detectors have to be fast, because every 96 ns (= bunch separation) it has to be ready for a new event. Additionaly the ITR signals are used in the first trigger level.

• A single detector layer has to reach an efficiency of at least 94% to get a 90% efficient first level trigger (see Formula2.6).

• The occupancy (=number of channels with a hit/total number of channels) should be less than 10% at an interaction rate of 40 MHz. This results in a channel pitch of less then 350µm at a strip length of 25 cm.

• To get an efficient track reconstruction the detectors are distributed over 10 stations between the vertex detector and the electromagnetic calorimeter. Depending on the zposition of the station they consist of 8 to 32 chambers (2 to 8 layers of 4 chambers each) with different orientation angles inφof 0^◦ and±⁵^◦^.

• Four stations are operated in the magnet (B = 0.85 T). Hence every single compo- nent from the chamber to the readout electronics must function properly in a strong magnetic field. Additionally all magnetic material has to be avoided for the construction of the detector.

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2.2 The inner tracking system 25

• To minimize multiple scattering, which leads to deterioration of the momentum resolution, and to suppressγ conversion, which results in higher occupancy and a ECAL reponse, the radiation length has to be short. That means the detector, the electronic, and the support structure should be as light as possible.

Figure 2.4: A HERA-B event: The eight wire targets are drawn at the bottom left. The first station of the silicon vertex detector (VDS) follows right behind them. Hits found are indicated by thin lines in the corresponding detector plane. The first station of the inner tracker MS01 in the top right corner is located just behind the last VDS station. The tracks found in the vertex detector are displayed as fat lines.

A crude approximation of the required detector efficiency can be made as follows: where e_d is the double layer efficiency, e₁₄ the efficiency of trigger station 14, e_t the track efficiency, e_f the FLT efficiency, and es the single layer efficiency, respectively. The single

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layer efficiency can then be written as:

e₁₄,15 = e_d³ (2.1)

e₁₀,13 = e_d⁴ (2.2)

e_t = e₁₀×ê13×ê14×ê15=e_d¹⁴ (2.3)

e_f = e_t²=e_d²⁸=90% (2.4)

e_d = ²⁸^pe_f = ²⁸√

0.9=0.9962 (2.5)

e_s = 1−^p¹−^ed=93.9% (2.6)

2.2.2 The ITR detector technology

In the Proposal [3] and the Design Report[8] it was intended to use MSGCs as detector technology for the inner tracker. At the time these documents were written test results by other groups indicated that MSGCs would fulfill the requirements of the HERA-B ITR.

In march 1996 we started our own tests at PSI, the first one in a high intensity pion and proton beam and we discovered the phenomenon of induced discharges [11], [12].

At pion rates of some 10³mm⁻²s⁻¹ and a gas gain of 3000 the chambers suffered from anode–cathode discharges. These discharges could destroy the anode structure and the MSGCs slowly became ineffective in the hadronic beam. That’s why MSGC could not be used at HERA-B.

This observation stimulated extensive investigations on alternative detector technologies.

A scintillating fiber detector [13] and the GEM-MSGCs (see Section3.2) were studied in detail. Studies showed that a fiber detector would work at HERA-B. However, after an accumulated irradiation comparable to one year of HERA-B operation we expected that the light yield drops significantly. Consequently the efficiency decreases over the time of operation.

GEM-MSGCs were proposed in 1996 by Fabio Sauli [14], [15]. In this new kind of detector an additional gas electron multiplier (GEM) foil (see Section3.3) is used as a preamplifier.

The gas amplification is divided into two steps. Therefore the MSGC cathode voltage can be reduced to avoid anode–cathode sparks. In the middle of 1997, the decision was made to use GEM-MSGC at HERA-B for the inner tracking detector.

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Chapter 3 The microstrip gas chamber

The microstrip gas chamber(MSGC) may be seen, from the application point of view, as intermediate step between a silicon microstrip detector and a drift chamber or proportional chamber. A typical wire chamber has outer dimension of the order of meters and a wire pitch of millimeters. In contrast, a typical silicon micro strip detector has a pitch of 50µm and a typical length of 50 mm.

drift chamber MSGC silicon microstrip largest silicon microstrip 2000 mm×^{2 mm} ^{250 mm}×⁰.3 mm 50 mm×⁵⁰µm 120 mm×¹⁰⁰µm

Table 3.1: Typical dimensions (overall length× strip or wire separation) of position sensitive detectors used in particle physics experiments.

In principle, the MSGC can be considered as a special type of drift chamber where the signal and potential wires are replaced by micro structures on a suitable substrate. Thin glass is most commonly used as substrate. The microstructures are produced in industry by a photolithographic procedure, which allows structures of a few micrometers. Because the anode and cathode strips are fixed to the substrate, they cannot develop mechanical instabilities as the wires of a drift and proportional chambers do if the electrical field be- comes too high. Therefore the granularity of the detector can be much smaller compared to that of wire chambers. This kind of production by industry permits a cost effective manufacturing of MSGCs compared to Si microstrip detectors.

Lower costs, larger areas and only moderately lower resolution as compared to silicon microstrip detectors, made MSGCs a prime choice for high rate, high track density experiments at existing and future colliders.

3.1 Short historical review on the development of MSGCs

The principal advantages of MSGCs could not be realized under experimental conditions.

The history of MSGC development is troubled by problems such as a lack of longterm

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stability, gain fluctuations, and aging.

3.1.1 The genuine MSGC design

The first MSGC was presented by A. Oed [16]. The structure used (see Figure 3.1(a)) was deduced from drift chambers. The anodes (black strips in Figure3.1(a)) act as signal wires. An anode width (10µm) corresponding to the smallest diameter of wires in drift chambers was used. The cathode (hatched area) corresponds to the potential wire. How- ever the biggest difference as compared to the conventional wire chambers is the fact that anodes and cathodes are fixed to a non conducting substrate and the rectangular, flat shape (see Figure 3.1(b)). The dielectricum strongly influences the field configuration, and therefore the operating properties of the chamber.

(a)Top view (b)Cross-section

Figure 3.1: Electrode structure of the first MSGC built by A. Oed [16]. The equipotential lines (solid, 0. . . 10) and electron drift directions (dashed) are shown in the right Figure. However the influence of the substrate () is neglected.

The first and major problem of MSGCs is that ions are deposited on the substrate. The surface of the insulator will be charged up until the field of the surface charge prevents further deposition. This leads to a distortion of the electrical field and therefore to a gain variation, which strongly depends on the counting rate. A. Oed already observed this

Contributions to the Development of Microstrip Gas Chambers (MSGC) for the HERA-B Experiment. Dissertation