Nuclear Theory Seminars at Texas A&M: Spring 2016

Location: Cyclotron Conference Room (CYCL-300)


January 25, 4:00pm
      C.Y. Wong (Oak Ridge)
   
   
"The Hadron p_T Distribution in High-Energy pp Collisions and its Implications "
   
Abstract

Transverse momentum distribution of jets and hadrons provide useful information on the collision mechanisms and their subsequent dynamics. It was found recently that the hadron spectra spanning over 14 decades of magnitude from the lowest of 0.5 GeV/c to the highest $p_T$ of a few hundred GeV/c at central rapidity in pp collisions at LHC can be adequately described by a single Tsallis distribution with only three apparent degrees of freedom [1]. The simplicity of the p_T spectrum suggests that a single mechanism dominates over a large pT domain at central rapidity in these high-energy collisions. As the high-$p_T$ region is known to arise from the relativistic hard-scattering process at high pT, one is led to the suggestion that the hard-scattering process dominates over a very large pT domain in these high-energy pp collisions. We shall explore the implications of the pT distribution on many related topics of the diminishing role of the competing flux-tube fragmentation [2] and the initial conditions for the momentum kick model of the near-side ridge in pp collisions [3].

[1] C.Y.Wong and G.Wilk, Acta Phys. Pol. {B43}, 2047 (2012); C.Y.Wong and G.Wilk, Phys. Rev. {D87},114007 (2013); C. Y. Wong, G. Wilk, L. J. L. Cirto and C. Tsallis, Phys. Rev. {D91}, 114027 (2015).
[2] C.Y.Wong, Phys.Rev. {D92}, 074007 (2015).
[3] C.Y.Wong, Phys.Rev. {C84}, 024901 (2011).


January 27, 11:00am (CYCL-228)
      W.C. Chen (Florida State)
   
   
"Relativistic Mean Field Models for Finite Nuclei and Neutron Stars"
   
Abstract

Relativistic mean field (RMF) theory provides a natural framework to describe nuclear systems, ranging from finite nuclei, nuclear matter, to neutron stars, in a single unified way. In this talk I will introduce a calibration scheme to build RMF models using only real physical observables; properties of nuclear matter, such as incompressibility and symmetry energy, become genuine model predictions. Moreover, I will demonstrate how a follow-up covariance analysis can extract information content of the models, including theoretical uncertainties and correlations. This calibration scheme has been applied to study the elusive isovector sector and related physics. Comparing with experimental and theoretical constraints, we find that the neutron-skin thickness in 208Pb should be (0.16 ± 0.1) fm, implying a soft symmetry energy with J = ( 30.92 ± 0.47 ) MeV and L = (51.0 ± 1.5) MeV, respectively. The optimal model can also correctly locate the neutron drip line of oxygen at 24O and predicts that of calcium to be at 60Ca. Then, I will turn to discuss the recent tension between dense matter theory and the observation of neutron stars with very small radius (less than 10.6 km). We find that in order to support two-solar-mass neutron stars while preserving causality, the typical stellar radius must be greater than 10.7 km—barely consistent with recent analyses.


February 1, 4:00pm
      S. Cho (Kangwon National University, Korea)
   
   
"Reduction of the K* meson abundance and freeze-out conditions in heavy ion collisions"
   
Abstract

We discuss the reduction of the K* meson abundance during the hadronic stage in heavy ion collisions. We also investigate the freeze-out conditions of a particle in order to understand the productions of resonances, hadronic molecules and light nuclei in heavy ion collisions. Applying the kinetic freeze-out condition to the daughter particles of K* mesons, we find that the larger suppression of the yield ratio of K*/K at LHC than at RHIC compared to the expectations from the statistical hadronization model reflects the lower kinetic freeze-out temperature at LHC than at RHIC. Furthermore, we argue that for the light nuclei or hadronic molecules that are bound, the yields are affected by the freeze-out condition of the respective particle in the hadronic matter, which leads to the observation that the deuteron production yields are independent of the size of deuteron, and depend only on the number of ground state constituents.


February 3, 4:00pm
      S.H. Lee (Yonsei University, Korea)
   
   
"Few recent results on D meson in matter and dibaryons in constituent quark model"
   
Abstract

We probe effects of the partial chiral symmetry restoration to the mass of heavy-light mesons in a constituent quark model by changing the constituent quark mass of the light quark. Due to the competing effect between the quark mass and the linearly rising potential, whose contribution to the energy increases as the quark mass decreases, the heavy-light meson mass has a minimum value near the constituent quark mass typically used in the vacuum. Hence, the meson mass increases as one decreases the constituent quark mass consistent with recent QCD sum rule analyses, which show an increasing $D$ meson mass as the chiral order parameter decreases. We will also present some recent result on dibaryons in a constituent quark model.


April 01, 4:00pm
      M. Kordell (Wayne State University)
   
   
"Jets in p(d)-A Collisions: Centrality Dependent Effects: Color Transparency or Energy Conservation?"
   
Abstract

The production of jets, and high momentum hadrons from jets, produced in deuteron (d)-Au collisions at the relativistic heavy-ion collider (RHIC) and proton (p)-Pb collisions at the large hadron collider (LHC) are studied as a function of centrality, a measure of the impact parameter of the collision. A modified version of the event generator PYTHIA, widely used to simulate p-p collisions, is used in conjunction with a nuclear Monte-Carlo event generator which simulates the locations of the nucleons within a large nucleus. We demonstrate how events with a hard jet may be simulated, in such a way that the parton distribution function of the projectile is frozen during its interaction with the extended nucleus. Using our approach, we demonstrate that the puzzling enhancement seen in peripheral events at RHIC and the LHC, as well as the suppression seen in central events at the LHC are mainly due to mis-binning of central and semi-central events, containing a jet, as peripheral events. This occurs due to the suppression of soft particle production away from the jet, caused by the depletion of energy available in a nucleon of the deuteron (in d-Au at RHIC) or in the proton (in p-Pb at LHC), after the production of a hard jet. We conclude that partonic correlations built out of simple energy conservation are mostly responsible for such an effect.


April 22, 4:00pm
      J. Noronha-Hostler (University of Houston)
   
   
"Event-by-event hydrodynamics + jet energy loss: A solution to the RAA × v2 puzzle"
   
Abstract

High pT > 10 GeV elliptic flow, which is experimentally measured via the correlation between soft and hard hadrons, receives competing contributions from event-by-event fluctuations of the low pT elliptic flow and event plane angle fluctuations in the soft sector. A proper account of these event-by-event fluctuations in the soft sector, modeled via viscous hydrodynamics, combined with a jet energy loss model reveal that the positive contribution from low pT v2 fluctuations overwhelms the negative contributions from event plane fluctuations, which leads to an enhancement of high pT > 10 GeV elliptic flow in comparison to previous calculations. This provides a natural solution to the decade long high pT RAA × v2 puzzle. We also present the first theoretical calculation of high pT v3, which is shown to be compatible with current LHC data. Furthermore, we discuss how short wavelength jet-medium physics can be deconvoluted from the physics of soft, bulk event-by-event flow observables using event shape engineering techniques.


April 29, 4:00pm
      T. Song (Frankfurt University / FIAS)
   
   
"Heavy flavor production in relativistic heavy-ion collisions"
   
Abstract

We study the production of heavy flavor in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial heavy quarks are produced by the Pythia event generator tuned to fit the transverse momentum spectrum and rapidity distribution of heavy quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced heavy quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM), which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross sections are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the heavy quarks are hadronized into heavy mesons through coalescence and fragmentation. The hadronized heavy mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. Finally, the nuclear modification factors and the elliptic flows of heavy mesons and of single electrons from the heavy mesons are compared with the experimental data from the Beam Energy Scan to LHC energies. We also discuss about the (anti)shadowing effect on heavy flavor production and the azimuthal angle correlations of heavy flavor pairs.


May 06, 4:00pm
      D. Molnar (Purdue University)
   
   
"Flow from Anisotropic Escape"
   
Abstract

Hydrodynamics is considered the dominant paradigm for describing heavy-ion collisions at RHIC and LHC energies, though its applicability to nuclear reactions is not very well understood. Open question remain about the mechanism of rapid thermalization, initial conditions, treatment of decoupling (conversion of the fluid to particles), finite system effects, and quantum corrections in very small systems, for example. In a recent work (arXiv:1502.05572) we showed that in the AMPT transport model elliptic flow is generated quite differently from hydrodynamics, mainly through anisotropic escape from the collision zone. I will demonstrate that this is, in fact, a general feature of kinetic theory, originating in the modest opacities \sim 4-5 not only in AMPT calculations but also in calculations that use MPC (Molnar's Parton Cascade). Identified particle flow and mass ordering will also be discussed.





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