Bulletin of the American Physical Society
2019 Fall Meeting of the APS Division of Nuclear Physics
Volume 64, Number 12
Monday–Thursday, October 14–17, 2019; Crystal City, Virginia
Session GM: QCD Theory |
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Chair: Wally Melnitchouk, Thomas Jefferson National Accelerator Facility Room: Salon J |
Tuesday, October 15, 2019 2:00PM - 2:12PM |
GM.00001: Electric Polarizability of Hadrons from Lattice QCD Hossein Niyazi, Andrei Alexandru, Frank Lee Electric and magnetic polarizabilities are two of the fundamental properties of hadrons which help us understand the distribution of charge and currents inside hadrons and how they respond to external electromagnetic fields. For nucleons, these values are determined experimentally from Compton scattering. For charged pions, the experiments are more challenging since no free pion target is available and the results are less precise, but a number of experiments are planned that will improve the accuracy. Lattice QCD can be used to compute hadron properties as determined by quark and gluon dynamics, providing results that are complementary to other theoretical approaches. In this talk I will review the lattice QCD methods used to compute hadron polarizabilities, focusing on electric polarizability, and present our results. [Preview Abstract] |
Tuesday, October 15, 2019 2:12PM - 2:24PM |
GM.00002: More Gluons in the Pion Patrick Barry, Nobuo Sato, Wally Melnitchouk, Chueng Ji Looking at the QCD-motivated picture of the pion, we can use parton distribution functions (PDFs) to describe the probability of finding a quark, antiquark, or gluon in the pion at a certain momentum fraction, $x$, and energy scale, $Q^{2}$. Because PDFs are universal in observables in which factorization occurs, we may use data from multiple QCD processes to shape them. In the case of pions, people have traditionally used Drell-Yan (DY) data to fit pion PDFs, but DY data only exist at large-$x$. In this region, the valence quark distributions are known to dominate. Any PDFs determined from DY data alone in the small$x$ region are mere extrapolations and cannot be trusted. More recently, we have used Leading Neutron (LN) data to shape the PDFs at small$x$. The determinations of these PDFs using both DY and LN datasets show that gluons contribute to 30{\%} of the total momentum of the pion at the input scale compared with 10{\%} from determinations using strictly DY data. Current work includes soft gluon resummation, which improves perturbative calculations in the DY process. Soft gluon radiation from quark lines contribute nontrivially to the cross-section at large-$x$, which helps to constrain the valence quark distribution as $x$ goes to 1. [Preview Abstract] |
Tuesday, October 15, 2019 2:24PM - 2:36PM |
GM.00003: Collinear Factorization in Wide-Angle Hadron Pair Production in $e^+e^-$ Annihilation Eric Moffat, Ted Rogers, Andrea Signori, Nobuo Sato We compute the inclusive unpolarized dihadron production cross section in the far from back-to-back region of $e^+ e^-$ annihilation within leading order pQCD and using standard collinear factorization. We compare with event generator predictions from PYTHIA, and examine how the degree of agreement varies with the center-of-mass energy. While we find reasonable agreement at large center-of-mass energies, at moderate energies ($\sim 12$~GeV) we find order-of-magnitude or larger disagreement reminiscent of discrepancies between theoretical calculation and experimental measurements of large transverse momentum recently observed in semi-inclusive deep inelastic and Drell-Yan scattering. In combination with these previously observed disagreements, we argue that our results support a conclusion of general tension between cross sections computed in the large transverse momentum limit and measurements with moderate hard scales, and motivate further phenomenological studies of the application of collinear factorization at moderate hard scales. [Preview Abstract] |
Tuesday, October 15, 2019 2:36PM - 2:48PM |
GM.00004: Baryon self-energies in relativistic chiral SU(3) effective theory Marston Copeland, Chueng-Ryong Ji, Wally Melnitchouk We calculate the self-energies of the flavor SU(3) octet and decuplet baryons, using a relativistic chiral effective theory framework consistent with Lorentz and gauge invariance. The results are compared using several different regularization prescriptions, including finite-range regularization, Pauli-Villars, and dimensional regularization, which are shown to yield the same leading nonanalytic behaviors in the chiral limit, as expected in QCD. Using the same chiral effective theory framework, we also compute the pseudoscalar meson loop contributions to flavor asymmetries in parton distributions, such as the $\bar d - \bar u$ and $s-\bar s$ quark asymmetries in the proton. [Preview Abstract] |
Tuesday, October 15, 2019 2:48PM - 3:00PM |
GM.00005: Fermion-mass and charge renormalization using relativistic, time-dependent quantum mechanics Timothy Kutnink, Athanasios Petridis The time-dependent electromagnetically self-coupled Dirac equation is solved numerically by means of the staggered-leap-frog algorithm with reflecting boundary conditions. The stability region of the method versus the interaction strength and the spatial-grid size over time-step ratio is established. The expectation values of several dynamic operators are then evaluated as functions of time. These include the fermion and electromagnetic energies and the fermion dynamic mass. There is a characteristic time-dependence leading to asymptotic constants of these expectation values. In the case of the fermion mass and charge this amounts to renormalization. The dependence of the expectation values on the spatial-grid size is evaluated in detail and result in finite mass and charge. The contribution of positive and negative energy states to the asymptotic values and the gauge fields is analyzed. A statistical method, employing a canonical ensemble whose temperature is the inverse of the spatial-grid size, is used to remove the momentum-dependence and produce a finite result for each spatial-grid size value. The continuum limit is then taken to calculate both the fermion mass and charge. The renormalization mass correction is about 10\% while the charge correction is about 30\%. [Preview Abstract] |
Tuesday, October 15, 2019 3:00PM - 3:12PM |
GM.00006: Phenomenological modeling of first-order phase transition in QCD Thomas Welle, Christopher Plumberg, Joseph Kapusta We present a method for parametrizing the equation of state of QCD in multiple phases. This method involves the use of a switching function, taking values between 0 and 1, which interpolates between the equations of state for two phases as a function of temperature T and baryon chemical potential \textmu . As per the conjectured QCD phase structure, this function is constructed to be smooth for all T and \textmu except along a line of first-order phase transition which extends from some critical point to the T$=$0 axis. We use this method to model the transition between the hadron resonance gas and quark-gluon plasma phases of nuclear matter. These results are compared to results of lattice calculations for the case of 3 colors and 3 flavors. [Preview Abstract] |
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