RHIP Group at UT Austin

Relativistic Heavy Ion Physics

Research Interests

Members of the UT Relativistic Heavy Ion Physics group are active in the STAR experiment (Solenoidal Tracker at RHIC) at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in New York and at the ALICE experiment at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. We conduct resonance, heavy-flavor and correlation data analysis from both experiments as well as hardware construction and firmware/software development projects in support of detector upgrade efforts which are essential to our physics goals.


In broad terms our research at STAR and ALICE is intended to expand the knowledge of (1) the initial-state high gluon density accessible in relativistic heavy-ion collisions, (2) the heavy-flavor parton distribution functions (PDF) in nuclei, (3) heavy-flavor interactions in a dense partonic medium, (4) the nature and properties of the partonic medium (quark gluon plasma or QGP) generated in collisions, and (5) novel QCD mechanisms which are active in heavy-ion collisions. We analyze data from the STAR and ALICE experiments to measure hadronic resonance production and attenuation in the medium, heavy-flavor mesons (D and B mesons), heavy flavor resonances (D*), and correlations between light-flavor hadrons and between heavy-flavor mesons and non-identified hadrons. We also perform detector hardware and electronics development and construction in support of these physics goals.

Hadronic Resonances in the dense, hot medium and Chiral Symmetry

Hadron resonances are excited states of strong-interaction particles which only live for a very brief time before decaying into lighter mass particles. Their lifetimes are so brief that they only travel a few fermi (10^{-15} meters) before decaying, even when moving at speeds near that of light. Nevertheless, these fleeting objects provide ideal probes of the short-lived partonic matter produced in heavy-ion collisions because their lifetimes are comparable to that of the QGP phase of the collision system. The resonances selected for analysis are produced, propagate and decay within the medium and may therefore convey unique information about its properties. The most significant such property involves the possibility that the fundamental chiral symmetry of Quantum Chromo-dynamics (QCD) is temporarily restored in this medium causing the masses of resonances to reduce and their lifetimes to be even briefer. These changes in mass and lifetimes will produce very clear signals in the decay particles and can be measured at both STAR and ALICE.
Resonance production associated with jets may be the optimum method to search for evidence of chiral symmetry restoration. A resonance produced on the away-side relative to an identified jet (a focused spray of particles) or a high transverse momentum particle would be more likely to travel through the medium, due to surface bias of the selected jet or particle, therefore maximizing the exposure of the resonance to the medium. Our analysis has focused on the phi(1020)-meson (mass equals 1020 MeV/c^2), omega-meson, K*(892) and Lambda(1520) baryon resonance. So far we have not found evidence of chiral symmetry restoration in the data from STAR, but the ALICE jet-resonance program is just getting underway.

Heavy-Flavor hadron production and resonances

Heavy flavor (HF) quark (charm and bottom) production in relativistic heavy-ion collisions provides another unique probe of the partonic matter produced in these collisions because: (1) the energy scale (∼3 GeV for charm production) is sufficiently high that the production mechanisms can be calculated using perturbation expansions of QCD and are in principle known, (2) HF hadrons are sensitive to low energy scales (few hundred MeV) characteristic of the partonic medium because the HF hadron binding energies are of order a few hundred MeV, and (3) the short formation time (e.g. 1.6 fm/c for a 10 GeV/c D-meson) and long decay time (100 μm/c ) ensures that heavy-flavor mesons sample the medium throughout their entire lifetime during the QGP phase. We can therefore study medium induced modification of the HF quark's fragmentation into stable particles, the dissociation (break-up) of HF mesons, and both radiative and collisional energy loss mechanisms.
Heavy quarks (charm and bottom) and their anti-quark partners are either liberated from the initial-state wave functions of the colliding nuclei or are produced in the collision via gluon fusion. Once produced they either combine with the other HF quark (hidden heavy flavor) or with another light-flavor quark, such as up, down or strange, in the so-called "open flavor" mode, to form long-lived (relative to the lifetime of the heavy-ion collision system) HF particles. Our research focuses on open-flavor channels, specifically the D0-meson, D* resonance and B-meson. These open heavy-flavor particles may be detected via hadronic decays such as D0 -> kaon + pion or by way of the high momentum lepton (electrons and muons) from the semi-leptonic decay modes. Our analysis measures the production yields and attenuation of those yields with respect to experimentally controlled parameters. We are also studying the angular correlations between these HF mesons and the other particles produced in the collision in order to gain access to the dynamical interactions between HF mesons and the predominant gluon and light-flavor quarks of the medium.

Light-Flavor Two-particle correlations

Most of our past research at STAR has involved the measurement of two-particle angular correlations among non-identified particles and among identified light-flavor hadrons from Au+Au collisions. A major source of angular correlations among the particles produced in these collisions is due symmetries, i.e. conservation laws such as the conservation of momentum, charge, flavor (in the strong interaction), and baryon number. Other important sources include jet fragmentation, quantum interference, resonance decays and possible collective motion (flows) of final-state particles. Systematic study of the evolution of the correlation structures in going from low to high collision energies, and from "elementary" collision systems such as proton + proton and proton + nucleus to head-on (central) nucleus + nucleus collisions provides an essential tool for understanding the dense gluonic initial-state and the partonic medium produced in heavy-ion collisions. Unexpected correlations such as the long-range rapidity correlation known as the "ridge," were discovered by studying the systematic evolution of correlation structures. Our results offer some of the tightest constraints on theoretical models of the dense, hot medium (e.g. hydrodynamics, jets with perturbative QCD modifications, color-glass condensate flux tubes, to name a few).

Detector R & D

Over the years a major component of our group’s activities has been associated with construction projects which improve STAR’s experimental capabilities that have direct impact on our physics goals. We expect these kinds of activities to continue in the future at both the STAR and ALICE experiments. At STAR we continue to play major roles in (1) the Time-of-Flight (TOF) detector which provides particle identification capability and is essential for our HF, resonance, and light-flavor correlation programs, (2) the Muon Telescope detector (MTD) which is essential for the hidden HF program via di-muon decay channels, and (3) the Heavy Flavor Tracker (HFT) which enables precise identification of open HF meson decay vertices which typically occur of order 100 μm from the main collision point in the detector (primary vertex). At ALICE we plan to participate in the Barrel Tracking Upgrade (BTU) which will enable the main tracking detector (time projection chamber - TPC) and vertex tracking system to handle a 100x increased data rate following the Long Shutdown 2. The higher data rate is essential for studies of rare processes such as HF production and high energy jets with associated semi-leptonic resonance decays discussed above.


The TOF subsystem utilizes Multigap Resistive Plate Chamber (MRPC) technology and has greatly extended STAR’s particle identification (PID) reach. The TOF subsystem is comprised of 120 “trays,” each with 32 MRPCs containing 6 “pads” of dimensions 3.5 cm x 6.1 cm. Pseudorapidity coverage is |eta| < 1, with full 2pi azimuthal coverage . TOF’s intrinsic time resolution of ∼ 80 ps leads to particle identification for pions, electrons, and kaons up to ∼ 2 GeV/c and protons up to ∼ 3 GeV/c. With TOF we can identify resonance decay particles out to 3 GeV/c with a purity of 95% while suppressing the background from particle misidentification by up to a factor of 10. The MRPCs were built by our colleagues in China (Tsinghua and USTC) and sent to the University of Texas (UT). Our machine shop at UT manufactured most of the parts for the “trays” in which the MRPCs are mounted. Members of our group built, assembled, tested, and delivered all 120 “trays” that now comprise the finished TOF subsystem. We are now responsible for maintaining the TOF configuration data base, TOF firmware upgrades, slow controls software, and online controls software as this subsystem continues to be used by the STAR collaboration.


The MTD is located at mid-rapidity and extends the reach of the STAR physics program by allowing for the detection of di-muon pairs from QGP thermal radiation, light vector mesons and quarkonia, as well as single muons from semi-leptonic decays of heavy flavor hadrons. The MTD subsystem contains 122 “trays,” each containing a large area MRPC (12 pads of dimensions 3.8 cm x 87 cm). Pseudorapidity coverage is | eta| < 0.5 and azimuthal coverage is about 50% continuous. Members of our group fabricated, assembled, tested, and delivered the 122 trays to STAR at BNL. In addition, we were responsible for the FPGA/MCU programming of the electronics, the delivery of the THUBs needed for readout, and related electronics and DAQ tasks. Now that the MTD has been installed and commissioned we remain responsible for maintaining the MTD configuration data base, MTD firmware upgrades, slow controls software, and online controls software. We also maintain a fully equipped laboratory at UT for repairing MTD trays when problems arise (such as leaks).


The HFT is a state-of-the-art micro-vertex detector which utilizes active pixel sensors and silicon strip technology. It significantly extends the physics reach of the STAR experiment for precision measurement of yields and spectra of particles containing heavy quarks. This is accomplished through topological identification of D mesons by reconstruction of displaced decay vertices with a precision of approximately 50 μm for p+p, d+A, and A+A collisions. The Pixel Subsystem (PXL) has 2 layers (at radial positions 2.5 cm and 8.0 cm) containing 40 ladders, each ladder with 10 sensors. Pseudorapidity coverage is |eta | < 1 with full 2pi azimuthal coverage. The spatial resolution is ∼ 18 μm. Members of our group refined the design of the HFT PXL readout electronics and we were responsible during the construction phase of this project for the Readout firmware design and implementation, Pixel Slow Controls and configuration software, and Pixel Online monitoring software. We will continue to make improvements in the PXL firmware, program the online software used for PXL operation, provide software for online display for real-time quality assurance, provide slow controls software maintenance, and provide on-sight support at BNL.


During the LHC Long Shutdown 2 the ALICE detector will be upgraded to allow high precision measurements of rare probes at low momentum while sampling the full 50 kHz Pb+Pb interaction rate. This data rate is a factor of 100 more than the present limit. The project contains two major hardware components and a software online-offline component to deal with the higher data-rates. The hardware upgrades proposed are: 1) a novel readout scheme for the ALICE Time Projection Chamber (TPC), and 2) an entirely new seven layer Inner Tracking System (ITS) based on silicon pixel technology. Our group's participation in this project will include system design and firmware development of a common read-out unit (CRU) to be used in all detector subsystems at ALICE. We will also develop custom firmware for the ITS and will collaborate with LBNL in the development of FPGA based readout systems for the different designs and for the testing tasks for this project. We will assume leadership of the development of readout firmware and software for these systems.
A complete probe station setup will be developed which will initially be used at LBNL for testing various architecture designs. We are also planning to develop a cosmic ray test set-up for long term tests of completed sub-assemblies of intermediate layers of the ITS. This setup will provide opportunities for students and post-docs to learn about and help with the trigger setup, and to perform data analysis including cluster size measurement and cosmic ray track reconstruction.
The completed intermediate ITS layers will be delivered during the long LHC shutdown in 2018. We will then participate in the installation, integration, and commissioning of these layers into the complete ITS detector at ALICE.