Search

Since the 1960s, it has been known that the protons and neutrons in nuclei are composed of more fundamental particles called quarks. To date, there is no evidence that quarks have a substructure. Experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab, on Long Island, New York, study the particle emerging from very energetic collisions between heavy nuclei like gold (each made up of 197 protons and neutrons), and offer a unique means for investigating the properties of quarks and the particles associated with the forces between quarks. The most high-profile scientific breakthrough so far at the RHIC accelerator was the discovery of quark-gluon plasma (QGP). The existence of this new phase of matter had been predicted on theoretical grounds, and it is believed to have existed briefly during the first microsecond after the Big Bang. Its discovery was further confirmation of the Standard Model – a unified description of matter in terms of quarks, particles in the electron family (leptons), and the fields mediating their interactions. Until recently, the available evidence always pointed to a smooth transition between ordinary matter and QGP, unlike the familiar discontinuous change of state (phase transition) when ice turns to liquid water, or when liquid water turns to steam. Changes of phase like in water are called first-order phase transitions. Theorists had anticipated that the normal maximum energy of the RHIC accelerator is too high to observe a first- order phase transition, and they predicted that it might be seen at a lower energy. This insight prompted a multi-year effort at RHIC, known as the Beam Energy Scan (BES), to investigate a wide range of beam energies and search for such phenomena. One of the challenges that arise in interpreting BES measurements is to investigate if a bombarding energy exists where a maximum occurs in the nuclear compression during the early stages of the collision. Naively, one might expect compression to keep increasing indefinitely as the beam energy increases, but nuclei, even if they collide exactly head-on, will transparently pass through each other without stopping at the highest energies, and this counteracts the above- mentioned increase in nuclear compression. Therefore, there probably exists an optimum beam energy where compression is a maximum. Nuclear compression during a nucleus-nucleus collision cannot be measured experimentally, and a model calculation is needed. This thesis involves study of the nuclear transport model AMPT (A Multi-Phase Transport). More specifically, the goal is to study nuclear compression as a function of time during the collision at a given collision energy, extract the maximum compression, then study this maximum as a function of collision energy over the range of energies being scanned at RHIC. The basic computer code to implement the AMPT model is freely available for investigation by any interested user, but various modifications and pieces of additional code need to be developed in order to probe the specific questions of relevance to the scientific objectives explained above.

Quantum Chromodynamics (QCD), the theory of the strong interaction between quarks and gluons, predicts that at extreme conditions of high temperature and/or density, quarks and gluons are no longer confined within individual hadrons. This new deconfined state of quarks and gluons is called Quark-Gluon Plasma (QGP). The Universe was in this QGP state a few microseconds after the Big Bang. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) on Long Island, NY was built to create and study the properties of QGP.Due to their heavy masses, quarks with heavy flavor (charm and bottom) are mainly created during the early, energetic stages of the collisions. Heavy flavor is considered to be a unique probe for QGP studies, since it propagates through all phases of a collision, and is affected by the hot and dense medium throughout its evolution. Initial studies, via indirect reconstruction of heavy flavor using their decay electrons, indicated a much higher energy loss by these quarks compared to model predictions, with a magnitude comparable to that of light quarks. Mesons such as D0 could provide information about the interaction of heavy quarks with the surrounding medium through measurements such as elliptic flow. Such data help constrain the transport parameters of the QGP medium and reveal its degree of thermalization.Because heavy hadrons have a low production yield and short lifetime (e.g. ct = 120µm for D0), it is very challenging to obtain accurate measurements of open heavy flavor in heavy-ion collisions, especially since the collisions also produce large quantities of light-flavor particles. Also due to their short lifetime, it is difficult to distinguish heavy-flavor decay vertices from the primary collision vertex; one needs a very high precision vertex detector in order to separate and reconstruct the decay of the heavy flavor particles in the presence of thousands of other particles produced in each collision.The STAR collaboration built a new micro-vertex detector and installed it in the experiment in 2014. This state-of-the-art silicon pixel technology is named the Heavy Flavor Tracker (HFT). The HFT was designed in order to perform direct topological reconstruction of the weak decay products from hadrons that include a heavy quark. The HFT consists of four layers of silicon, and it improves the track pointing resolution of the STAR experiment from a few mm to around 30 µm for charged pions at a momentum of 1 GeV/c.In this dissertation, I focus on one of the main goals of the HFT detector, which is to study the elliptic flow v2 (a type of azimuthal anisotropy) for D0 mesons in Au+Au collisions at vsNN = 200 GeV. My analysis is based on the 2014 data set (about 1.2 billion collisions covering all impact parameters) that include data from the HFT detector. There are two new and unique analysis elements used in this dissertation. First, I performed the analysis using a Kalman filter algorithm to reconstruct the charmed-meson candidates. The standard reconstruction is via a simple helix-swim method. The advantage of using the Kalman algorithm is in the use of the full error matrix of each track in the vertex estimation and reconstruction of the properties of the heavy-flavor parent particle. Second, I also used the Tool for Multivariate Analysis (TMVA), a ROOT-environment tool, to its full potential for signal significance optimization, instead of the previous approach based on a set of fixed cuts for separating signal from background.This dissertation presents the elliptic component (v2) of azimuthal anisotropy of D0 mesons as a function of transverse momentum, pT . The centrality (impact parameter) dependence of D0 v2(pT) is also studied. Results are compared with similar studies involving light quarks, and with the predictions of several theoretical models.

A new form of matter with de-confined quarks and gluons, named the "Quark Gluon Plasma" (QGP), is predicted by Lattice Qutantum Chromodynamics to exist at high temperatures and/or high baryon density regions in the QCD phase diagram. Experimental evidence indicates that the QCD matter created in high energy Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven National Lab is a strongly-coupled Quark-Gluon Plasma. One of the central goals of heavy ion physics is to understand the QGP through quantitative comparisons between theoretical calculations and experimental measurements. Heavy flavor quarks are believed to be a unique probe for this task, because they are dominantly produced in the initial hard scatterings, where the production rate can be well calculated by perturbative-QCD(pQCD). When heavy favor quarks traverse the QGP medium, they bear the imprints of the medium via their interactions with the medium. Dynamical models have been developed to calculate the interactions between heavy quarks and the QGP medium to quantitatively extract the medium properties.This dissertation presents a series of experimental studies with the electrons produced in the semi-leptonic decays of heavy flavor quarks in Au+Au and p+p collisions, which serve as the proxies for heavy flavor quarks. These electrons are referred to as non-photonic electrons (NPE), to be differentiated from the main background of photonic electrons. The production of NPE at high pT is found to be highly suppressed in central and semi-central Au+Au collisions, compared to binary-collision scaled production in p+p collisions. The azimuthal anisotropy of NPE is found to be finite at high pT, and the azimuthal correlation between high pT NPE and low pT hadrons exhibits a broadening in the away-side, both of which strengthen the evidence for a strong coupling between heavy flavor quarks and the QGP medium. In addition, the bottom quark production cross-section in p+p collisions is obtained based on the measured spectrum of NPE and the ratio of bottom/charm decay electrons.

This Dissertation project involved using the Endcap Electro-Magnetic Calorimeter (EEMC) to try to measure charm quark production in proton-proton (p+p) collisions at √s=200 GeV. This was performed by identifying electrons produced in the decay of charm D0 mesons. This project included testing and calibration of elements used in the installation of the EEMC on the Solenoidal Tracker At RHIC (STAR) detector on the Relativistic Heavy Ion Collider (RHIC) accelerator at Brookhaven National Laboratory (BNL). These measurements provided an upper limit for non-photonic electron production at pseudo-rapidities (η) between 1.1 and 1.5 in p+p collisions at √s=200 GeV using the EEMC as the main detector in conjunction with the Time Projection Chamber (TPC) in STAR. Non-photonic electrons are the electrons from the semi-leptonic decay of charm D0 mesons. Ionization energy loss, dE/dx, of a charged particle in the TPC and the associated momentum (p) of the particle are two important observables that help identify electrons from hadrons, mainly in the mid-rapidity region. In the forward rapidity region, where the TPC tracking resolution is poor, energy deposit (E) information in the pre-shower, post-shower and shower maximum detectors (SMD) in the EEMC provide additional power in distinguishing electrons from hadrons. A combination of dE/dx and p/E cuts provide electrons with purity >92% and an acceptance efficiency of ~52% for electrons incident upon the EEMC. The hadron-corrected inclusive electron spectrum is contaminated by photonic background electrons. The major sources of this photonic background are electrons from photon conversions and π0 Dalitz decays. We make use of the invariant mass technique to reconstruct photonic background electrons, taking advantage of the fact that electron-positron pairs from conversion photons or π0 Dalitz decays have a small invariant mass and small opening angles in φ and θ, while there is no such correlation for non-photonic electrons. Reconstruction efficiency of photonic electrons is obtained from simulations. Our calculations provide an upper limit for the non-photonic electron yield and consequently an upper limit for the total quark charm production. In this dissertation, we present the methods and procedures adopted to obtain these results.

A large area time of flight (TOF) detector based on multi-gap resistive plate chamber (MRPC) technology has been developed for the STAR (Solenoidal Tracker at RHIC) experiment at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory, New York. The TOF detector replaces STAR's Central Trigger Barrel detector with 120 trays, each with 32 MRPCs. Each MRPC has 6 channels. The TOF detector improves by a factor of about 2 STAR's particle identification reach in transverse momenta and enhances STARs physics research program.