Title: The Hadron pT Distribution in High-Energy pp Collisions and its Implications

Speaker: Cheuk-Yin Wong (ORNL)

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).

Title: The Quark Recombination Model and Applications in High Energy Collisions

Speaker: Rainer Fries

Abstract:

Quarks, usually captives of confinement, can be liberated in high energy collisions either at small distance scales

or at high temperatures. The process of re-hadronization of quarks in such collisions is a complicated problem in quantum
chromodynamics which is not fully solved. Since ultimately hadrons, or their decay products, are measured in detectors,
effective descriptions and hadronization models are usually used to to describe this important step in a collision. In this
talk I will argue why quark recombination or coalescence models have their place in our toolbox, and what they have
achieved specifically when describing collisions of nuclei at high energies.. I will end with a look at some recent
developments, including a project to describe the hadronization of QCD jets in the presence of quark gluon plasma.

Title: The chiral magnetic effect: from quark-gluon plasma to Dirac semimetals, and back

Speaker: Dimitri Kharzeev (SUNY Stony Brook)

Abstract:

Chirality (“handedness”) is an ubiquitous concept in modern science, from particle physics to biology. Recently it has been realized that chirality has dramatic implications for the macroscopic behavior of systems with chiral particles.

In particular, the imbalance between the densities of left- and right-handed fermions in the presence of magnetic field induces the non-dissipative transport of electric charge (“the Chiral Magnetic Effect”, CME) analogous to superconductivity. In quark-gluon plasma, this leads to the charge asymmetry studied in the experiments at Relativistic Heavy Ion Collider and the Large Hadron Collider.

Chirality defines the unique properties of recently discovered Dirac semimetals. I will report the observation of CME in a Dirac semimetal ZrTe5, and discuss the implications for heavy ion experiments.

Title: Transverse Geometry of the Hard and Soft pp and pA Collisions at the LHC
Speaker: Mark Strikman (PSU)

Abstract: Decades of studies of hard QCD phenomena provided a precision picture of the distribution of proton constituents (quarks and gluons) over the fraction of energy, x, which they carry in fast nucleons. More recently, studies of the hard exclusive processes provided first information about three dimensional structure of fast nucleons – the x-dependent impact parameter single parton distributions in the nucleon and led to the two scale geometric picture of the proton – proton interactions at the LHC. I will review how this picture explains a number of regularities of hadron production observed at the LHC, and reveals presence of the parton – parton correlations in nucleons. I will also present evidence for existence of significant fluctuations of gluon strength in nucleons as well as fluctuations of the overall strength of the interactions and discuss implications for pp and pA collisions.

Title: Imaging the proton with multi-particle correlations at the LHC

Speaker: Kevin Dusling (PRL)

Abstract: The Large Hadron Collider at CERN is able to study the structure of the proton at shorter time scales and higher resolution then ever before. The first year of operation brought an entirely unexpected discovery; in proton-proton collisions a peculiar correlation between particles traveling in opposite directions was uncovered in extremely rare events. More recently, a similar correlation was observed in high multiplicity proton-lead collisions. We discuss the implications of these measurements on our understanding of the proton at high energies.

Title: Dileptons and chiral symmetry restoration

Speaker: Paul Hohler (TAMU)

Abstract: A long standing goal in heavy ion physics is to identify an experimental signal for chiral symmetry restoration. Ideally this can be achieved by comparing the in-medium spectral functions of chiral partners, such as the light vector and axial-vector mesons (rho and a1). While the in-medium rho spectral function can be obtained from interpretations of dilepton data in heavy ion collisions, measuring the in-medium a1 spectral function remains elusive. This talk will thus discuss theoretical techniques which can be used to calculate the in-medium axial-vector spectral function given a rho spectral function which describes available dilepton data so that the question of chiral symmetry can be critically addressed. We will demonstrate that the combination of QCD and Weinberg sum rules provide a powerful selection method for the axial-vector spectral functions. Using this at different temperatures, we can obtain a temperature progression from the vacuum towards restoration, and show that experimental results of the vector channel suggest compatibility with restoration.  Results will also be presented from an effective theory approach which can calculate the spectral functions in both channels in vacuum and in-medium from a microscopic perspective. This serves as a foundation for future systematic studies in this area.

Title: Jet physics with heavy ions at the LHC

Speaker: Ivan Vitev (LANL)

Abstract: In the past several decades, advances in the theory of strong interactions can be traced through theoretical and experimental developments in understanding the physics of jets in high-energy collider experiments. Jets have been instrumental in establishing Quantum Chromodynamics (QCD) and the parton model. Jet physics is the frontier where nuclear and particle physics intersect. In this talk, I will describe the theory that underlays jet production in ultra-relativistic collisions of heavy nuclei at the Large Hadron Collider (LHC).  I will show how insightful choices of experimental observables can help elucidate the differences between vacuum and in-medium parton showers and pinpoint the properties of the quark-gluon plasma (QGP) produced in heavy ion reactions. Toward the end of the talk I will discuss a new effective theory of jet propagation in matter that is being developed with the goal of improving heavy ion phenomenology.