Charm Production and in-Medium QCD Energy Loss with ALICE [thesis]

259 Pages·2003·6.09 MB·English

by  A. Dainese

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UNIVERSITA DEGLI STUDI DI PADOVA
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0 Sede Amministrativa: Universit`a degli Studi di Padova
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2 Dipartimento di Fisica “G. Galilei”

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DOTTORATO DI RICERCA IN FISICA
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CICLO XVI

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0 Charm production and in-medium QCD energy loss
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in nucleus–nucleus collisions with ALICE.
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/ A performance study.
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Coordinatore: Ch.mo Prof. Attilio Stella
Supervisore: Ch.mo Prof. Maurizio Morando
Dottorando: Andrea Dainese
31 Ottobre 2003

Charm production and in-medium QCD energy loss
in nucleus–nucleus collisions with ALICE.
A performance study.
Andrea Dainese
Universit`a degli Studi di Padova
October 31st, 2003

Table of Contents
Introduction 1
1 Heavy ion physics at the LHC: study of deconfined QCD matter 9
1.1 Phenomenology of hot and dense matter . . . . . . . . . . . . . . 11
1.1.1 The QCD phase diagram . . . . . . . . . . . . . . . . . . . 11
1.1.2 Lattice QCD results . . . . . . . . . . . . . . . . . . . . . 12
1.2 Evidence for deconfinement in heavy ion collisions: the SPS programme 14
1.3 RHIC: focus on new observables . . . . . . . . . . . . . . . . . . . 17
1.4 LHC: study of ‘deeply deconfined’ matter . . . . . . . . . . . . . . 19
1.4.1 Systems, energies and expected multiplicity . . . . . . . . 19
1.4.2 Why ‘deep deconfinement’? . . . . . . . . . . . . . . . . . 22
1.5 Novel aspects of heavy ion physics at the LHC . . . . . . . . . . . 23
1.5.1 Low-x parton distribution functions . . . . . . . . . . . . . 24
1.5.2 Hard partons: probes of the QGP medium . . . . . . . . . 30
2 Charm in heavy ion collisions 35
2.1 Heavy quark production in pQCD . . . . . . . . . . . . . . . . . . 36
2.2 Physics of open charm in heavy ion collisions . . . . . . . . . . . . 38
2.3 Parton energy loss . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.3.1 Medium-induced radiative energy loss . . . . . . . . . . . . 41
2.3.2 Quenching weights . . . . . . . . . . . . . . . . . . . . . . 44
2.3.3 Dead cone effect for heavy quarks . . . . . . . . . . . . . . 46
2.4 Pre-LHC measurements of open charm production in pA and AA 49
2.5 Probing the QGP with charm at the LHC . . . . . . . . . . . . . 53
2.5.1 Strategy for the exclusive reconstruction of D0 mesons with ALICE 53
2.5.2 Outline for the physics sensitivity studies . . . . . . . . . . 55
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3 Charm and beauty production at the LHC 59
3.1 Cross sections in nucleon–nucleon collisions . . . . . . . . . . . . . 59
3.2 Extrapolation to heavy ion collisions . . . . . . . . . . . . . . . . 63
3.2.1 Nucleus–nucleus collisions . . . . . . . . . . . . . . . . . . 63
3.2.2 Proton–nucleus collisions . . . . . . . . . . . . . . . . . . . 67
3.3 Heavy quark kinematical distributions . . . . . . . . . . . . . . . 69
3.4 Heavy quark production in Monte Carlo event generators . . . . . 72
3.5 Hadron yields and distributions . . . . . . . . . . . . . . . . . . . 76
4 The ALICE experiment at the LHC 81
4.1 The ALICE detector . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.1.1 Detector layout . . . . . . . . . . . . . . . . . . . . . . . . 81
4.1.2 Inner Tracking System (ITS) . . . . . . . . . . . . . . . . . 84
4.1.3 Time Projection Chamber (TPC) . . . . . . . . . . . . . . 86
4.1.4 Particle identification system . . . . . . . . . . . . . . . . 88
4.2 Event simulation and reconstruction . . . . . . . . . . . . . . . . 89
4.2.1 Event generators . . . . . . . . . . . . . . . . . . . . . . . 90
4.2.2 Simulation of the detector response . . . . . . . . . . . . . 91
4.2.3 Track reconstruction . . . . . . . . . . . . . . . . . . . . . 92
4.3 LHC beams and interaction region . . . . . . . . . . . . . . . . . 102
4.3.1 Luminosity and beam size . . . . . . . . . . . . . . . . . . 102
4.3.2 Interaction region in Pb–Pb collisions . . . . . . . . . . . . 103
4.3.3 Interaction region in pp collisions . . . . . . . . . . . . . . 104
5 Identification of heavy flavour decay vertices: experimental issues107
5.1 Primary vertex reconstruction in Pb–Pb . . . . . . . . . . . . . . 109
5.2 Track impact parameter resolution in Pb–Pb . . . . . . . . . . . . 110
5.2.1 Transverse momentum dependence . . . . . . . . . . . . . 110
5.2.2 Effect of missing and misassigned clusters . . . . . . . . . 111
5.2.3 Comparison of detailed and fast ITS simulation . . . . . . 113
5.2.4 Particle type dependence . . . . . . . . . . . . . . . . . . . 114
5.3 Primary vertex reconstruction in pp . . . . . . . . . . . . . . . . . 114
5.3.1 Outline of the method . . . . . . . . . . . . . . . . . . . . 114
5.3.2 Expected resolutions . . . . . . . . . . . . . . . . . . . . . 115
5.3.3 Vertex finding . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.3.4 Vertex fitting . . . . . . . . . . . . . . . . . . . . . . . . . 116
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5.4 Track impact parameter resolution in pp . . . . . . . . . . . . . . 124
5.5 Secondary vertex reconstruction . . . . . . . . . . . . . . . . . . . 127
6 Exclusive reconstruction of D0 particles 131
6.1 Feasibility study for Pb–Pb collisions . . . . . . . . . . . . . . . . 131
6.1.1 Background and signal generation . . . . . . . . . . . . . . 132
6.1.2 Detector simulation and event reconstruction . . . . . . . . 135
6.1.3 Particle identification in the TOF detector . . . . . . . . . 137
6.1.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
6.1.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.1.6 Results scaled to a lower-multiplicity scenario . . . . . . . 154
6.1.7 Feed-down from beauty . . . . . . . . . . . . . . . . . . . . 154
6.2 Feasibility study for pp collisions . . . . . . . . . . . . . . . . . . 159
6.2.1 Background and signal generation . . . . . . . . . . . . . . 159
6.2.2 Event reconstruction and particle identification . . . . . . 163
6.2.3 Analysis and results . . . . . . . . . . . . . . . . . . . . . 164
6.2.4 Feed-down from beauty . . . . . . . . . . . . . . . . . . . . 172
6.3 Expected results for p–Pb collisions . . . . . . . . . . . . . . . . . 174
6.4 Results at lower magnetic field: B = 0.2 T . . . . . . . . . . . . . 175
6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
7 Performance for the measurement of D0 production 177
7.1 Estimation of the statistical uncertainty . . . . . . . . . . . . . . 177
7.1.1 S/B and S/√S +B with optimized p -binning . . . . . . . 178
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7.1.2 Fit of the invariant mass distribution . . . . . . . . . . . . 181
7.2 Estimation of systematic uncertainties . . . . . . . . . . . . . . . 185
7.2.1 Correction for feed-down from beauty . . . . . . . . . . . . 186
7.2.2 Cross section normalization . . . . . . . . . . . . . . . . . 187
7.3 Errors on d2σ(D0)/dp dy and dσ(D0)/dy . . . . . . . . . . . . . . 189
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7.4 Comparison with pQCD predictions . . . . . . . . . . . . . . . . . 192
7.5 Energy extrapolation of the pp result . . . . . . . . . . . . . . . . 193
7.6 Perspectives for the measurement ofN(b B D0)/N(c D0) . 195
→ → →
8 Quenching of open charm mesons 201
8.1 Medium parameters: path length and transport coefficient . . . . 202
8.2 Charm energy loss with quenching weights . . . . . . . . . . . . . 209
8.3 Results (I): nuclear modification factor R for D mesons . . . . . 214
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8.4 Results (II): D/hadrons ratio . . . . . . . . . . . . . . . . . . . . 218
Conclusions 223
A Kinematics of the D0 K−π+ decay 225
→
B PYTHIA parameters used for heavy quark generation at LHC energies229
C Parameterization of the TPC tracking response: validation tests231
C.1 Description of the track parameters used in the TPC . . . . . . . 231
C.2 Validation tests and results . . . . . . . . . . . . . . . . . . . . . . 232
C.2.1 Tracking efficiency in the TPC and in the ITS . . . . . . . 232
C.2.2 Resolution on the track parameters in TPC–ITS . . . . . . 234
C.2.3 Effect on the required computing resources . . . . . . . . . 234
D Further studies on the impact parameter resolution 237
References 243
Acknowledgements 251
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Introduction
The search for quark–gluon plasma —the state of deconfined strongly interacting
matter which is thought to have constituted the 1-µs-old Universe— received a
big boost in the 1990s with the acceleration of heavy ions in the Super Proton
Synchrotron at CERN. There, several fixed-target experiments gave results, on
different physical observables, indicating that a new state of matter with unusual
properties is formed in the early stage of the collisions. Heavy ion physics has now
entered the collider era. Results from experiments at the Relativistic Heavy Ion
Collider (RHIC) have provided further evidence for the long-sought quark–gluon
plasma and encourage the study of its properties at the Large Hadron Collider
(LHC), where energy densities of 100-600 times the density of atomic nuclei
∼
will be reached in the collisions of lead nuclei at 5.5 TeV per nucleon–nucleon
pair.
The recent results from RHIC suggest that it is possible to probe the dense
medium formed in nucleus–nucleus collisions through the reduction in the pro-
ductionof high-momentum particles. This effect may be,indeed, due to anenergy
loss, or quenching, of the partons as they propagate through the medium. If this
is the case, the new deconfined phase can be probed and investigated by means
of a ‘tomography’ with beams of energetic partons.
At the LHC the probes being used at RHIC, light quarks and gluons, will
extend their energy range by one order of magnitude and a new type of probe
will become available with fairly high cross sections: heavy quarks.
The large masses of the charm and beauty quarks make them qualitatively
different probes, since, on well-established quantum chromodynamics grounds,
in-medium energy loss off massive partons is expected to be significantly smaller
than off massless partons. Therefore, a comparative study of the attenuation of
massless (gluons and light quarks) and massive probes is a promising tool to
test the coherence of the interpretation of quenching effects as energy loss in a
deconfined medium and to further investigate the properties of such medium.
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In this work we focus on charm physics with ALICE1, the heavy ion ded-
icated experiment at the LHC. The aim is to study the ALICE capability to
measure charm production with good precision (small statistical errors) and ac-
curacy (small systematic errors) even in the high track-multiplicity environment
of central lead–lead collisions and to carry out the above-mentioned comparative
quenching studies.
The physics framework is outlined in the first part of the thesis (Chapters 1
and2),where we present thestatus of theexperimental study ofdeconfinement in
heavy ion collisions and the qualitative improvement expected in this field at the
LHC collider and we detail how charm particles can serve as probes of deconfined
matter.
The experimental framework, ALICE, is described in Chapter 4, in terms of
layout, main sub-systems and their expected performance.
The activity carried out for this thesis can be summarized in the following
four parts.
Definition of a baseline for heavy quarks production cross sections and kine-
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matical distributions. The HVQMNR computer program for perturbative
quantumchromodynamics calculationswasdeployed toobtainandcompare
results at different energies and for different colliding systems, taking into
account known nuclear collective effects. The Monte Carlo event generator
PYTHIA was tuned in order to reproduce such results. This item is covered
in Chapter 3.
Studyoftheexperimentalissuesrelatedtotheidentificationofthedisplaced
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decay vertices of charm mesons. Since charm particles have decay lengths
of few tenths of a millimeter, a precise reconstruction of the event topology
in the interaction region is mandatory for a high-quality charm physics
programme.TheALICE InnerTracking Systemwasdesigned toprovidethe
required precision. Using the latest detector geometry/response parameters
and track reconstruction algorithms, we carried out a systematic study of
the track impact parameter resolution for different particle species and in
differentmultiplicity environments, fromcentrallead–leadtoproton–proton
collisions.Forthelattercase,wedevelopedandtestedadedicatedalgorithm
for the reconstruction of the interaction vertex position in three dimensions.
These items are discussed in Chapter 5.
1A Large Ion Collider Experiment
2