Chapter 1 Introduction

1.1 Standard model and Quantum Chromodynamics

The Standard Model of particle physics is a theory concerning the electromagnetic,weak，and strong nuclear interactions, which explains how the basic building blocks ofmatter interact. The Standard Model includes members of several classes of elementaryparticles: 12 elementary particles of spin 1/2 known as fermions, 4 gauge bosons ofspin 1 who carry the force of fundamental interactions and Higgs boson of spin 0 whichinteracts with elementary particles and gives them mass. The 12 fermions includes 6quarks (up (m), down (d)，strange ⑶，charm (c), top (t), bottom (b)), and 6 leptons(electron (e)，muon (fx), tau (r) and the corresponding neutrinos). The 4 gauge bosonsare gluon (g), photon (7)，W and Z bosons, which are classified by the fundamentalforce they carry. Gluons mediate the strong interactions between color charged particles(quarks). Gluons also carry color charge，so they can also interact with themselves.The strong interaction are described by the theory of quantum chromodynamics. Theelectric charged particles interact through the electromagnetic force mediated by photonswhich is well-described by the theory of quantum electrodynamics. The weak force iscarried by W and Z bosons. They are grouped with photons, as collectively mediationthe electroweak interaction. Figure 1.1 shows the particles generations and interactionsbetween them. Fermions interact with each other through the fundamental force carriedby gauge bosons and form the matter world. On 14 March 2013, the Higgs boson wastentatively confirmed to exist.

……….

1.2 Dilepton production in high energy heavy ion collisions

The heavy ion collision is believed to be the best way to study properties of QCDmatter in laboratory, since increasing the mass number of the incident particles is moreefficient than increasing beam energy. During the past 20 years, world wide efforts havebeen dedicated in this region. Several large-scale experiments have been conducted.Relativistic heavy ion collider (RHIC) built in Brookhaven National Laboratory is thefirst accelerator-collider dedicated to heavy ion collisions. During the first few yearsof its operation，plenty measurements support the existence of a new matter form: astrongly coupling Quark Gluon Plasma (sQGP). Currently the physics program at RHICwas changing to studying the property of strongly interacting matter created in highenergy heavy ion collisions and searching for phase boundary and critical point of theQGP phase diagram. At CERN, the Large Hadron Collider (LHC) also contributes in theheavy ion collisions program. It is pushing the colliding energy up to 5.5 TeV, where theenergy density and temperature is much higher than the requirement of QGP formation.Probes explored in experiments are mostly hadrons which have been used to demon?strate the formation of a strongly-coupled Quark Gluon Plasma (sQGP) in high energyheavy ion collision at RHIC and LHC. Dileptons as an electromagnetic probe, escape theinteracting system without suffering further strong interactions after production. In ad?dition, dilepton can be produced on the various stages of entire system evolution. Theyare therefore expected to be an outstanding probes to study the property of the mediumcreated in high energy heavy ion collisions.

……….

Chapter 2 Experiment set-up and detectors

2.1 The Relativistic Heavy Ion Collider

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven Nation Laboratory(BNL) was built at year 1999, after nine years construction. It is deigned to accelerateand collide heavy ions and polarized protons at relativistic energy. RHIC has capabil?ities to deliver beams ranged from proton to uranium with high luminosity. The topcenter-of-mass collision energy is 200 GeV per nucleon pair for heavy ion collisionsand 500 GeV for polarized p+p collisions. The basic design parameters of the colliderare listed in Table 2.1, The main physics goal of RHIC is to investigate the phase tran?sition from hadronic phase to QGP phase and to study the formation and property ofQGR RHIC also provides polarized p+p collision witii collision energy up to 500 GeVto expand the scientific objective of RHIC to include vigorous spin physics program.Figure 2,1 shows the layout of RHIC complex with the injector chain and the ringtunnel. The RHIC complex contains the Tandem van de GraafF pre-accelerator，a lin?ear proton accelerator，the Booster Synchrotron, the Alternating Gradient Synchrotron(AGS) and and ultimately the RHIC synchrotron ring. The acceleration scenario forgold ion beams is shown in Fig, 2.2. Negatively charged (Qt = —1) Au ions is injectedinto the Tandem Van de Graaff from the Pulsed Sputter Ion Source. They are partiallystripped of their electrons and accelerated inside the Tandem Van de Graaff and exit withthe energy of 1 MeV/nucIeon and charge state of Qt = +32. The ions are deliveredto the Booster Synchrotron and accelerated to 95 MeV/nucleon and further stripped toQt = +77 at the exit. Then they are transferred into the AGS，where they are accel?erated to 8,86 GeV/nucleon and sorted into four final bunches.

………..

2.2 STAR experiment

The Solenoidal Tracker at RHIC (STAR) is one of the two large detecter systemsconstructed at RHIC. Heavy ion collision at RHIC creates a nuclear environment of alarge produced particles (up to approximately 1000 per unit pseudo-rapidity) and highmomentum particles from hard parton-parton scattering. The main physics goal of theSTAR experiment is to measure many observables simultaneously to investigate tiiesignatures of a possible QGP phase transition and to understand the space-time evolutionof the collision process in ultra-relativistic heavy ion collisions. In order to accomplishthis, STAR was designed primarily for measurements ofhadron production over a largesolid angle. The STAR detector systems are very effective in high precision tracking,momentum analysis, and particle identification at the central rapidity region. With theinstallation of the Time Of Flight detector (TOF) in 2009，STAR gained the capabilityto identify electrons and positions.

……………

Chapter 3 Analysis.........  35

3.1 Data set and event selection.........    35

3.2 Centrality definition  ......... 36

3.3 Track selection and electron identification.........  37

3.4 Pair reconstruction and background.........  42

3.5 Efficiency and acceptance correction.........  49

3.6 Hadronic cocktails ......... 57

3.7 Systematic uncertainty ......... 59

3.8 Combine the Au+Au results from year 2010 and year 2011 ......... 62

Chapter 4 Result and discussion  .........  69

4.1 Dielectron production in 200 GeV p+p collisions at STAR......... 69

4.2 Dielectron production in 200 GeV Au+Au collisions at STAR.........  70

4.2.1 Dielectron invariant mass spectra.........  70

4.2.2 Comparison to models.........    71

4.2.3 px and centrality dependence  ......... 74

4.2.4 Correlated charm contributions.........  78

4.2.5 Low mass vector meson yields.........  80

4.2.6 rriT slope parameters ......... 83

4.3 Summary and outlook ......... 85

Chapter 4 Result and discussion

4.1 Dielectron production in 200 GeV p+p collisions at STAR

Figure 4.1 shows the dielectron invariant mass spectra from 200 GeV p+p collisionstaken in year 2012. The cocktail is taken from the STAR published result [1], and thecharm cross section is updated to 797 土 210 (stat.) 士認5 (sys,) fib with respect to thenewest published result from STAR [64]. The cocktail simulation can reproduce thenew preliminary result very well. With a full TOF coverage and more data taken, year2012 result has greatly improved statistics which is ?7 times more than STAR previouslypublished result [1]. The large statistics new results at p+p 200 GeV provide a betterbaseline for Au+Au collisions. The green band around unity indicates theuncertainties on the cocktail calculations, which are mainly determined by the uncertain?ties on the dN/dy and decay branching ratios for each individual source as discussedin Sec 3.6. We consider p mesons are strongly coupled with the medium in Au+Aucollisions, thus we don't include it in default hadronic cocktail calculations. We leftits contribution to the theory model calculations which will be discussed in followingsubsections. The correlated charm contribution as described in Sec 3.6 is taken fromPYTHIA simulation and scaled by the number of binary collisions.

…….

Conclusion

The measured mass position and thewidths of signal distribution are well reproduced by the full GEANT simulation. Themeasured  invariant spectrum through the dielectron decay channel is consistent withthe previous measurement through hadron decay channel. The uj spectrum canalso be well reproduced by the Tsallis Blast-Wave model prediction which uses the sameset of parameters from the simultaneous fit to all available light hadrons measurements.In the intermediate mass region, our understanding of the dielectronproduction is limited both statistically and systematically. We have little control oncontributions from the correlated charm decays which is dominant dielectron source inthis mass region. The data in minimum bias collisions can be well reproduced by theNbin scaled p+p contribution from PYTHIA calculation. However, when comparingthe mass spectra between minimum bias and the central collisions, the data shows adifference about 1,5a in slopes of exponential fit in mass region. This couldbe an indication of the possible modification of the correlated charm contribution orother contributing source from the medium in Au+Au collision. Due to the same reason，currently the data don't allow to disentangle the contributions from thermal radiation andcorrelated charm decay in rriT slope measurement.

…………

Reference (omitted)