Non-photonic electron production in proton-proton and gold-gold collisions at the center of mass energy = 200 GeV

Xin Li, Purdue University

Abstract

The focus of this thesis work is on studying the production of electrons from heavy flavor decays, i.e. non-photonic electrons, at both high pT and low pT in p+p collisions. The outcome of this work resolved the long standing discrepancy between STAR and PHENIX measurements and thus had a large impact in understanding the interactions between heavy quarks and the Quark Gluon Plasma produced in relativistic heavy-ion collisions. Nuclear and particle physics aims to understand nature in terms of the most fundamental ingredients and interactions. The most fundamental ingredients, so called elementary particles, include spin 1/2 fermions, which are the constituents of matter, and spin 1 gauge bosons, which are the force carriers. Except gravity, the other three most fundamental interactions can be well understood by quantum field theories. Quantum Chromo-dynamics (QCD), based on the SU (3) group, is the theory of the strong interactions of colored quarks and gluons. At high temperature or high energy density, the interaction between partons was expected to be significantly weakened enabling them to move around like a free gas and no longer confined inside the hadrons. This state of matter with de-confined partons is named as "Quark Gluon Plasma" (QGP) in analogy to the conventional plasma in atomic physics. The Relativistic Heavy Ion Collider (RHIC) was designed to collide all species of nucleus at high energy to produce QGP and study its property using a wide range of probes. Since it began operating in year 2000, RHIC has done systematic studies on a broad range of physics probes and discovered a new state of medium with unprecedented temperature and density. The properties of the new medium created at RHIC are more complicated than had been anticipated. It has very high density (~5GeV/fm3) and very high temperature (150- 180MeV), which is well above the predicted QCD threshold for the occurrence of de-confinement. However, instead of behaving like a free gas, the observed large hadron elliptic flow suggests it is more like a "perfect fluid" with the ratio of viscosity to entropy close to the quantum limit. Clearly the goal of future RHIC physics programs should focus on the detailed studies of the hot and dense matter to clarify its properties. Heavy quarks (charm and bottom) are rare probes and have not been studied in detail at RHIC. They are produced early in the collisions and interact with the medium very differently from light quarks because of their large mass. Therefore studying heavy quark production would provide crucial opportunities to reveal new properties of the medium. Heavy flavor hadrons were thought to be less suppressed due to their heavy mass. However, it was found by both the PHENIX and STAR experiments in 2005 that the production rate of non-photonic electrons was as strongly suppressed as the light hadrons. This observation posed a serious challenge to our theoretical understanding heavy quark energy loss in QGP and triggered concerted efforts in the field to investigate novel energy loss mechanisms. Although the STAR and PHENIX measurements of the non-photonic electron nuclear modification factor were consistent, the measured production rate by STAR was, however, twice that measured by PHENIX in both proton-proton (p+p) and gold-gold (Au+Au) collisions. This discrepancy had essentially halted further progress in the understanding of heavy flavor energy loss in QGP. This thesis presents analysis details in identifying and measuring non-photonic electrons with data recorded during the 2008 and 2009 p+p runs at = 200GeV. This work leads to the resolution of the STAR and PHENIX discrepancy and thus had a large impact in understanding the heavy quark production in QGP. The STAR non-photonic electron invariant cross sections in p+p collisions can be described by the Fixed-Order Next-to-Leading Logarithm (FONLL) perturbative QCD (pQCD) calculation within its theoretical uncertainties. The measurement of the nuclear modification factor in Au+Au collisions shows a strong suppression in non-photonic electron production at high pT and challenge the understanding of heavy quark energy loss mechanism in the field.

Degree

Ph.D.

Advisors

Xie, Purdue University.

Subject Area

Nuclear physics|Particle physics

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