ρ0(770) vector meson production and elliptic flow measurement in Cu+Cu and Au+Au collisions at √sNN = 200 GeV in STAR at RHIC Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy by Prabhat R. Pujahari (Roll No. 07412601) Supervisor Prof. Basanta K. Nandi Department of Physics INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY, INDIA 2012 Dedicated To My Parents & My Late Grandmother ii Declaration I declare that this written submission represents my ideas in my own words and where others’ ideas or words have been included, I have adequately cited and referenced the original sources. I also declare that I have adhered to all princi- ples of academic honesty and integrity and have not misrepresented or fabricated or falsified any idea/data/fact/source in my submission. I understand that any violation of the above will be cause disciplinary action by the Institute and can also evoke penal action from the sources which have thus not been properly cited or from whom proper permission has not been taken when needed. Prabhat Pujahari Roll No. 07412601 Date : ——————— Place : ———————- iii Abstract The Relativistic Heavy Ion community at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL), New York, aims to study nu- clear matter under extreme condition of temperature and density. This is sup- ported by lattice QCD prediction. The prediction is that at sufficiently high tem- perature and/or nuclear density, colored quarks and gluons, which are confined in a nucleon become de-confined resulting in a new phase of matter, called the Quark Gluon Plasma (QGP). In a QGP phase, quarks and gluons are able to move over regions larger than a hadronic length. Additionally, QGP is believed to have existed during the first few microseconds after the Big Bang. Understanding the properties of QGP could provide valuable insights on the evolution of our uni- verse. The experimental investigation at RHIC and intensive theoretical calcu- lations with state-of-the-art computers have led to the refinement of the physics goals. The task of the RHIC heavy-ion programme, therefore, will be to investi- gate the properties of the de-confined matter in much greater details. Hadronic resonance states have extremely short lifetimes ( few fm/c) which ∼ are comparable to or smaller than the lifetime of the system formed in relativis- tic heavy-ion collisions. Due to their short lifetimes, hadronic resonances can be used to investigate the freeze-out mechanisms after hadronization. In order to understand the properties of matter formed in relativistic nucleus-nucleus colli- sions, it is important to understand the production and absorption processes of resonant states created in those collisions. This is attributed to the resonance daughter particles’ re-scattering and re-generation effects. Thus, a systematic measurement of the resonances properties such as mass modification and/or width broadening is essential to study the hadronic in-medium effects in heavy- ion collisions. Again, the elliptic flow measurement will extend the sensitivity iv of resonance yields to the partonic state through additional information on con- stituent quark scaling. So far, RHIC has produced a great amount of data on various resonances. The 0 2 2 ρ vector meson (mρ0 = 775.5 MeV/c , Γρ0 = 150 MeV/c ) is one among such res- onances through which various properties of the hot and dense medium created in such heavy- ion (i.e. Cu+Cu and Au+Au) collisions can be studied. Because of the broad width, the ρ0 vector meson is expected to decay, re-scatter, and re- generate all the way from the chemical freeze-out to the kinetic freeze-out. In the context of statistical models, the measured ρ0 vector meson yield should reflect conditions at kinetic freeze-out rather than at chemical freeze-out. In p+p and d+Au collisions, the ρ0 vector meson is expected to be produced predominantly by string fragmentation. Therefore, the measurement of the ρ0 vector meson in p+p and heavy-ions, such as Cu+Cu and Au+Au, collisions at the same nucleon- nucleon center of mass energy can provide insight for the understanding of the dynamics of these systems. In addition, in-medium modification of the ρ0 vector meson mass and/or width due to the effects of increasing temperature and density has ben proposed as a possible signal of the phase transition of nuclear matter to a de-confined plasma of quarks and gluons, which is expected to be accompanied by the restora- tion of chiral symmetry. Therefore, the production of the ρ0 vector meson is stud- ied and compared with other resonances to investigate the evolution of the fire- ball. The other important physics goal of this thesis is to study and discuss the 0 results of elliptic flow (v2) measurement of the ρ vector meson. The number of 0 constituent quark scaling of v2 for the ρ vector meson will potentially provide information about the ρ0 production mechanism. The data used for the analysis in this thesis were taken with the Solenoidal Tracker at RHIC (STAR) detector. Measurement of the ρ0 vector meson, through 0 + the hadronic decay channel ρ π + π−, in peripheral Cu+Cu and Au+Au → collisions and minimum bias p+p and d+Au collisions are presented. The invari- ant mass spectra of the ρ0 vector meson are reconstructed using a combinatorial v technique, and the same-event like-sign technique is applied to estimate the un- correlated background. A line shape analysis is carried out to extract the various information for the ρ0 vector meson. A much complicated hadronic cocktail func- tion is fit to the background subtracted invariant mass spectra to extract the mass 0 and the uncorrected yield for the ρ vector meson. The corrected pT spectra, in- verse slope parameters, and yields of the ρ0 vector meson in the mid-rapidity 0 (i.e. -0.5 < y < 0.5) is studied. The average pT (< pT >) of the ρ vector meson is compared with other particles to investigate effects of radial flow and particle 0 production mechanism. The ρ /π− ratio is studied and compared with K∗/K− ratio to understand the regeneration vs. re-scattering effects. 0 In addition, significant amount of the ρ vector meson elliptic flow, (v2(pT )), has been measured in Au+Au collisions at √sNN = 200 GeV. It has been observed in peripheral, i.e. 40% - 80% Au+Au collisions that in the intermediate pT (1.5 0 < pT < 5 GeV/c), the ρ vector meson elliptic flow coefficient (v2) followed the number of constituent quark, n=2, meson-scaling. This is a strong evidence for the partonic collectivity of the medium created in the collisions. Keywords : QGP, Resonances, Vector meson, Chiral symmetry, Freeze-out, Re- scattering, Re-generation, Cocktail fit, Elliptic flow, Quark scaling, Partonic col- lectivity. vi Contents 1 Introduction 1 1.1 TheStandardModel ........................... 2 1.2 ConfinementandAsymptoticfreedom. 3 1.2.1 TheQuarkGluonPlasma . 4 1.3 LatticeQCDandPhaseDiagram . 5 1.4 RelativisticHeavy-IonCollisions . ... 7 1.5 ExperimentalObservables . 10 1.5.1 ParticleSpectraandRatios . 10 1.5.2 AzimuthalAnisotropy(CollectiveFlow) . 13 1.6 ρ0 vectormesontostudythemediumcreatedatRHIC. 22 1.7 ScopeandOrganisationofthethesis . 26 2 The Experimental Facilities 29 2.1 TheRelativisticHeavy-IonCollider. ... 29 2.2 TheSTARDetector ............................ 31 2.2.1 TheTimeProjectionChamber(TPC) . 33 2.2.2 ParticleIdentification(PID) . 39 2.2.3 Time-Of-FlightDetector . 40 2.2.4 SiliconVertexTracker . 41 2.2.5 SiliconStripDetector. 42 2.2.6 BarrelElectro-MagneticCalorimeter . 42 2.2.7 EndcapElectromagneticCalorimeter . 43 2.2.8 Forward Time Projection Chambers (FTPC) . 44 2.2.9 PhotonMultiplicityDetector . 45 2.3 TheSTARTrigger ............................. 45 2.3.1 TheSTARDAQ.......................... 47 2.4 STARComputingFacilities . 49 vii CONTENTS CONTENTS 3 ρ0 Production in Cu+Cu and Au+Au Collisions at √sNN = 200 GeV 51 3.1 TriggerforData .............................. 52 3.2 EventSelection .............................. 52 3.3 TrackSelection............................... 53 3.4 ρ0 InvariantMassReconstruction . 58 3.4.1 Mixed-eventTechnique . 58 3.5 HadronicCocktail............................. 63 3.6 ρ0 MassandYieldExtraction . 67 3.7 OtherPossibleWaysofCocktailFit . 70 3.8 EfficiencyCorrection . .. .. .. .. .. .. .. 75 0 4 ρ Elliptic Flow (v2) Measurement in Au+Au Collisions at √sNN =200GeV 85 4.1 DataAnalysis ............................... 86 4.1.1 EventandTrackSelection . 86 4.2 ReactionPlaneEstimation . 89 4.3 RemovalofAuto-correlations . 93 4.4 Extracting ρ0 EllipticFlow ........................ 94 4.4.1 The Event Plane (φ Ψ ) binMethod ............. 95 − 2 4.4.2 The v2 vs. minv Method ..................... 96 4.5 EventPlaneResolutionCorrection . 101 5 Results and Discussion 113 5.1 ρ0 massvs.transversemomentum . 113 5.1.1 In-MediumEffectsandMassModification . 117 5.2 TransverseMomentumSpectra . 118 5.3 Mean Transverse Momentum (<pT >).................121 5.4 ParticleRatio................................ 124 5.4.1 Re-Scattering and Re-Generation Effects and Evolution Prop- erties ................................126 5.5 Azimuthal Anisotropy (v2) ........................128 5.5.1 MethodsComparison . 129 5.5.2 v2 vs. Transverse Momentum pT ................132 5.5.3 Number of Constituent Quark Scaling of v2 ..........137 6 Summary and Outlook 139 6.1 Summary..................................139 viii CONTENTS CONTENTS 6.2 FutureProspective .. .. .. .. .. .. .. .. 141 A 143 A.1 RapidityandPseudo-rapidity . 143 A.2 EnergyDensity .............................. 144 B 145 B.1 Systematic Uncertainty