Bulletin of the American Physical Society
APS Ohio Section Fall 2020
Volume 65, Number 15
Friday–Saturday, October 16–17, 2020; VIRTUAL
Session F01: Ultra-High Energy Nuclear Physics - Theory |
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Chair: Joey Adams, Ohio State University |
Saturday, October 17, 2020 11:15AM - 11:30AM |
F01.00001: Small-x Helicity Phenomenology Daniel Adamiak, Yuri Kovchegov, Wally Melnitchouk, Daniel Pitonyak, Nobuo Sato, Matthew Sievert One of the key components to solving the proton spin problem is understanding the small-x asymptotics of the helicity parton distribution functions (hPDFs). Several years ago, novel, small-x evolution equations were derived using the shock-wave/Wilson line formalism, designed for calculating the x-dependence of the quark and gluon hPDFs and the proton g1 structure function. These equations can be used to predict the contribution to the spin of the proton coming from the helicities of the small-x quarks and gluons. In this talk we will present the first-ever attempt to describe the world data on the g1 structure function at small x using the evolution equations derived the novel evolution equations within the JAM global analysis framework. Our results serve as a prediction for future measurements at the EIC and can be used to estimate the net amount of quark spin at small-x, ultimately bringing us one step closer to understanding the proton spin. *This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Award Number DE-SC0004286. [Preview Abstract] |
Saturday, October 17, 2020 11:30AM - 11:45AM |
F01.00002: Correlations and dynamic fluctuations in high energy collisions. Zoulfekar Mazloum, Sean Gavin, George Moschelli, Ahmed Khubrani Is thermalization necessary for hydrodynamic flow in nuclear collisions? The discovery of flow-like azimuthal correlations in pA and high-multiplicity pp collisions raises profound questions about the onset of collective flow and its relation to hydrodynamics. We seek independent experimental information on the degree of thermalization in order to identify those hydrodynamic collision systems in which flow is sensitive to equilibrium QCD properties. \newline We aim to develop a protocol for identifying the degree of thermalization using a combination of momentum and multiplicity correlation. To study the effect of thermalization on these correlations, we use Boltzmann equation in the relaxation time approximation with Langevin noise. We derive a new non-equilibrium transport equation for the two-body distribution function that is consistent with the conservation laws obeyed by microscopic scattering processes. We find that transverse momentum fluctuations in peripheral PbPb collisions at LHC markedly deviate from equilibrium behavior. We propose new measurements that can provide more refined information.~~ [Preview Abstract] |
Saturday, October 17, 2020 11:45AM - 12:00PM |
F01.00003: Transverse Momentum Fluctuations in Longitudinal Asymmetric System. ahmed khubrani, sean gavin, george moschelli, zoulfekar mazloum Correlation measurements in nuclear collisions provide new information on the interplay of dissipation and fluctuations in the quark gluon plasma. Viscous diffusion and thermal noise can broaden the rapidity dependence of two-particle transverse momentum correlations. In this presentation we develop a new method for computing these correlations using the second order Israel-Stewart hydrodynamic with stochastic noise. We further expand this method invoking third order diffusion equations to enforce causality and generalization to asymmetric system. We compare new calculations to recent measurements by the ALICE collaboration at the LHC and the STAR collaboration at RHIC. [Preview Abstract] |
Saturday, October 17, 2020 12:00PM - 12:15PM |
F01.00004: An improved anisotropic hydrodynamics ansatz Huda Alalawi, Michael Strickland We introduce an improved form for the anisotropic hydrodynamics distribution function which explicitly takes into account the free-streaming and equilibrating contributions separately. We demonstrate that with this improvement one can better reproduce exact results available in the literature for the evolution of moments of the distribution function, in particular, for moments which contain no powers of the longitudinal momentum in their definition (m = 0 moments). Using the resulting dynamical equations, we extract the non-equilibrium attractor associated with our improved aHydro ansatz and demonstrate that the improvement also allows one to better reproduce the exact dynamical attractor obtained using kinetic theory in the relaxation time approximation, particularly at early rescaled times and for m = 0 moments. [Preview Abstract] |
Saturday, October 17, 2020 12:15PM - 12:30PM |
F01.00005: Non-equilibrium attractor in high-temperature QCD plasmas Dekrayat Almaalol, Aleksi Kurkela, Michael Strickland We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, we assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions. [Preview Abstract] |
Saturday, October 17, 2020 12:30PM - 12:45PM |
F01.00006: Helicity at Small x: Oscillations and LLA Corrections Yossathorn Tawabutr, Yuri Kovchegov Proton spin puzzle is a longstanding problem in high-energy and nuclear physics: how is the proton spin distributed between quarks and gluons in the proton? A missing piece of the puzzle is the amount of spin coming from the quarks inside the proton at small Bjorken-$x$. Integral equations which predict quark helicity distributions at small $x$ were derived only recently. In this work, we construct a numerical solution of these equations at large-$N_c \& N_f$, with the aim to establish the small-$x$ asymptotics of the quark helicity distribution. (Here $N_c$ and $N_f$ are the numbers of quark colors and flavors.) Our main result is the following: we find that the quark helicity distribution should oscillate as a function of $\ln(1/x)$. The oscillation period depends on $N_f$ and spans many units of rapidity. This result may relate to the sign variation of the strange quark helicity distribution with $x$ seen in phenomenology. Our solution provides a better constraint on the quark's helicity distribution at small $x$, contributing to the resolution of the proton spin puzzle. [Preview Abstract] |
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