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Production of neutral pions in Pb+Au collisions at 158 AGeV-c [Elektronische Ressource] / presented by Rachik Soualah

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162 Pages
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Dissertationsubmitted to theCombined Faculties for the Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural Sciencespresented byDipl. Phys. Rachik Soualahborn in Biskra, AlgeriaOral examination: 24 June 2009Production of Neutral Pions in Pb+Au collisionsat 158 AGeV/cReferees: Prof. Dr. Johanna StachelProf. Dr. Norbert HerrmannAbstractThe direct photons are a particularly useful probe to search for an evidence ofthe Quark-Gluon Plasma formation in ultra-relativistic heavy-ion collisions. Directphotons can be extracted experimentally by measuring the large background from0π and η meson decays. This thesis work represents the production of the neutralpionmesonsmeasuredwiththeCERES/NA45experimentatthetopSPSenergyin0158 AGeV/c Pb-Au collisions. The π −→γγ (98.8%) is the decay channel used in0the reconstruction scheme. The π measurement is based on the data taken in theyear 2000. The CERES experiment can measure the photons that convert shortly+ −before the TPC by measuring the e e pairs in the TPC. The RICH2 mirror is themain converter used for this analysis. The presented analysis method describes in+ −more details the selection of the e and e tracks using only the TPC informationto reconstruct the converted photon. A Secondary vertex technique was developedand used to select the photons converted in the RICH2 mirror area.

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Published 01 January 2009
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Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl. Phys. Rachik Soualah
born in Biskra, Algeria
Oral examination: 24 June 2009Production of Neutral Pions in Pb+Au collisions
at 158 AGeV/c
Referees: Prof. Dr. Johanna Stachel
Prof. Dr. Norbert HerrmannAbstract
The direct photons are a particularly useful probe to search for an evidence of
the Quark-Gluon Plasma formation in ultra-relativistic heavy-ion collisions. Direct
photons can be extracted experimentally by measuring the large background from
0π and η meson decays. This thesis work represents the production of the neutral
pionmesonsmeasuredwiththeCERES/NA45experimentatthetopSPSenergyin
0158 AGeV/c Pb-Au collisions. The π −→γγ (98.8%) is the decay channel used in
0the reconstruction scheme. The π measurement is based on the data taken in the
year 2000. The CERES experiment can measure the photons that convert shortly
+ −before the TPC by measuring the e e pairs in the TPC. The RICH2 mirror is the
main converter used for this analysis. The presented analysis method describes in
+ −more details the selection of the e and e tracks using only the TPC information
to reconstruct the converted photon. A Secondary vertex technique was developed
and used to select the photons converted in the RICH2 mirror area. The extraction
0of the π needs a careful study of the combinatorial background determined using
the mixed event technique. The obtained invariant mass distribution of the two
0photons defined well the π peak. The neutral pion transverse spectra compared to
phenomenological models and other experiments validate our analysis method.
Direkte Photonen sind ein besonders gute Probe zum Nachweis eines Quark-
Gluon-Plasmas in ultrarelativistischen Kollisionen von Schwerionen. Sie k¨onnen
experimentell nach der Messung des erheblichen Untergrundes aus Zerf¨allen von
0π - und η-Mesonen extrahiert werden. Die vorliegende Arbeit stellt die Erzeugung
von neutralen Pionen vor, die innerhalb des CERES/NA5-Experimentes bei der
topSPS-Energie von 158 AGeV/c Pb-Au-Kollisionen gemessen wurden. Die Anal-
yse basiert auf der Datennahme des Jahres 2000. Zur Rekonstruktion wurde der
0π −→ γγ-Zerfallskanal (98.8%) verwendet. Das CERES-Experiment kann Photo-
+nen, die kurz vor der TPC konvertieren, durch die Messung von e e-Paaren nach-
weisen. DerindieserArbeitverwendeteHauptkonverteristderRICH2-Spiegel. Die
+hiervorgestellteRekonstruktionsmethodebeschreibtdetailliertdieAuswahldere e-
Spuren, wobei ausschliesslich TPC-Informationen zur Rekonstruktion konvertierter
Photonenbenutztwurden. EineSekund¨ar-Vertex-Technikwurdeentwickeltundzur
AuswahlvonkonvertiertenPhotonenimBereichdesRICH2-Spiegelsverwendet. Zur
Extraktrion der neutralen Pionen ist eine sorgf¨altige Studie des kombinatorischen
Untergrundes erforderlich, welcher mittels Mixed-Event-Technik bestimmt wurde.Die Verteilung der invarianten Massen von Photonpaaren zeigt den Peak bei der
0π -Masse sehr deutlich. Der Vergleich des hier ermittelten transversalen Spektrums
fuer neutrale Pionen mit ph¨anomenologischen Modellen und anderen Experimenten
verifizieren diese Analysemethode.To my grandmother ...Contents
Overview 1
1 Introduction 3
1.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 The Strong interactions. . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 The QCD Lagrangian. . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2 Asymptotic freedom . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.3 The quarks confinement . . . . . . . . . . . . . . . . . . . . . 8
1.2.4 Deconfinement and the Quark Gluon Plasma . . . . . . . . . . 9
1.2.5 The Lattice QCD . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3 Ultra relativistic heavy ion collisions . . . . . . . . . . . . . . . . . . 12
1.3.1 Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.3.2 The evolution of the QGP: Scenario of Bjorken . . . . . . . . 14
1.3.3 The Geometry of the collision . . . . . . . . . . . . . . . . . . 15
1.3.4 The experimental observations of the QGP . . . . . . . . . . . 17
1.3.5 Electromagnetic probes . . . . . . . . . . . . . . . . . . . . . . 17
2 The CERES Experiment 19
2.1 Experimental setup overview . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 The target region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.3 The trigger system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4 The Silicon Drift Detectors . . . . . . . . . . . . . . . . . . . . . . . . 23
2.5 The RICH Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.6 The CERES Time Projection Chamber . . . . . . . . . . . . . . . . . 27
2.6.1 Mechanical layout . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.6.2 Electric field . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.6.3 Magnetic field . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.6.4 Counting gas and drift velocity . . . . . . . . . . . . . . . . . 30
2.6.5 Readout chambers . . . . . . . . . . . . . . . . . . . . . . . . 31
2.7 TPC track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 32
2.7.1 TPC hit finding . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.7.2 The Track finding . . . . . . . . . . . . . . . . . . . . . . . . . 36
I2.7.3 TPC Track fitting . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.7.4 The coordinate system . . . . . . . . . . . . . . . . . . . . . . 39
2.8 Particle identification using specific energy loss (dE/dx) . . . . . . . . 40
2.9 Photon interactions in matter . . . . . . . . . . . . . . . . . . . . . . 40
+ −2.9.1 Photon conversions (γZ −→e e Z) . . . . . . . . . . . . . . 41
2.9.2 Radiation length . . . . . . . . . . . . . . . . . . . . . . . . . 42
3 The Data Analysis 43
3.1 Event data sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2 The reconstruction chain . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.3 The Electron and Positron selection . . . . . . . . . . . . . . . . . . . 45
3.4 The standard quality cut . . . . . . . . . . . . . . . . . . . . . . . . . 46
+ − + + − −3.5 The study of the unlike (e e ) and like sign e e ,e e pairs . . . . . 47
3.6 The photon reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 53
3.6.1 Map of reconstructed photon conversions . . . . . . . . . . . . 53
3.7 The Secondary Vertex (SV) fit algorithm . . . . . . . . . . . . . . . . 54
3.7.1 The mathematics of the Secondary Vertex fit . . . . . . . . . . 55
3.7.2 Application of the SV method to the reconstruction of the
conversion point . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.7.3 Selection of γ conversions in the RICH2 mirror . . . . . . . . 64
03.8 The π reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.9 The mixing event method . . . . . . . . . . . . . . . . . . . . . . . . 70
3.9.1 Invariant mass analysis . . . . . . . . . . . . . . . . . . . . . . 72
4 Monte Carlo Simulations 89
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.2 The physics event generator . . . . . . . . . . . . . . . . . . . . . . . 90
04.2.1 Expected number of π mesons . . . . . . . . . . . . . . . . . 92
4.3 The detector simulation . . . . . . . . . . . . . . . . . . . . . . . . . 95
4.3.1 The Conversion from Step2 to Step3c . . . . . . . . . . . . . . 96
4.3.2 The reconstructed tracks comparison . . . . . . . . . . . . . . 98
4.4 The unlike/like sign pairs comparison . . . . . . . . . . . . . . . . . . 100
4.5 The Photon mapping . . . . . . . . . . . . . . . . . . . . 104
4.6 The Secondary Vertex Comparison . . . . . . . . . . . . . . . . . . . 105
4.7 The γγ invariant mass distributions . . . . . . . . . . . . . . . . . . . 108
4.8 Acceptance and efficiency evaluation . . . . . . . . . . . . . . . . . . 114
5 Results 117
05.1 The total π yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.2 The Corrected Neutral Pion spectra . . . . . . . . . . . . . . . . . . . 119
II