Nuclear Theory·nucl-th

Friday·August 14, 2015

11 papers—4 primary·7 cross-listed

05
Can we decide whether QCD is confining or not at high temperature?
L. Ya. Glozman🇦🇹
At high temperature measurements of the Polyakov loop suggest a deconfinement transition to the (strongly interacting) quark-gluon plasma. At the same time at the infinitely large temperature the four-dimensional QCD is reduced to the three-dimensional QCD that is confining. The Polyakov loop and related Z_3 symmetry are strict order parameters only for infinitely heavy quarks. In such a situation the SU(2)_CS and SU(4) symmetries of confinement in the light quark sector could be helpful to distinguish between the confining and deconfining phase in a regime where SU(2)_L \times SU(2)_R and U(1)_A symmetries are manifest. In order to reveal a presence or absence of these symmetries one needs to measure and compare correlation functions related by these symmetry transformations.
↳ hep-lathep-phhep-thnucl-th1 citation
06
Light Pseudoscalar Mesons in Bethe-Salpeter Equation with Instantaneous Interaction
Wolfgang Lucha🇦🇹 · Franz F. Schöberl🇦🇹
The light pseudoscalar mesons play a twofold role: they may or have to be regarded both as low-lying bound states of the fundamental degrees of freedom of quantum chromodynamics as well as the (pseudo-) Goldstone bosons of the spontaneously broken chiral symmetries of quantum chromodynamics. We interrelate these aspects in a single quantum-field-theoretic approach relying on the Bethe-Salpeter formalism in instantaneous approximation by very simple means: the shape of the pseudoscalar-meson Bethe-Salpeter wave function dictated by chiral symmetry is used in Bethe-Salpeter equations for bound states of vanishing mass, in order to deduce analytically the interactions which govern the bound states under study. In this way, we obtain exact Bethe-Salpeter solutions for pseudoscalar mesons, in the sense of establishing the rigorous relationship between, on the one hand, the relevant interactions and, on the other hand, the Bethe-Salpeter amplitudes that characterize the bound states.
↳ hep-phnucl-thPRD(2015)·11 citations
07
Nonequilibrium fixed points in longitudinally expanding scalar theories: infrared cascade, Bose condensation and a challenge for kinetic theory
J. Berges🇩🇪 · K. Boguslavski🇩🇪 · S. Schlichting🇺🇸 · R. Venugopalan🇺🇸
In Phys. Rev. Lett. 114 (2015) 6, 061601, we reported on a new universality class for longitudinally expanding systems, encompassing strongly correlated non-Abelian plasmas and $N$ -component self-interacting scalar field theories. Using classical-statistical methods, we showed that these systems share the same self-similar scaling properties for a wide range of momenta in a limit where particles are weakly coupled but their occupancy is high. Here we significantly expand on our previous work and delineate two further self-similar regimes. One of these occurs in the deep infrared (IR) regime of very high occupancies, where the nonequilibrium dynamics leads to the formation of a Bose-Einstein Condensate. The universal IR scaling exponents and the spectral index characterizing the isotropic IR distributions are described by an effective theory derived from a systematic large- $N$ expansion at next-to-leading order. Remarkably, this effective theory can be cast as a vertex-resummed kinetic theory. The other novel self-similar regime occurs close to the hard physical scale of the theory, and sets in only at later times. We argue that the important role of the infrared dynamics ensures that key features of our results for scalar and gauge theories cannot be reproduced consistently in conventional kinetic theory frameworks.
↳ hep-phnucl-thPRD(2015)·55 citations
08
Exclusive $ρ^{0}$ Meson Photoproduction with a Leading Neutron at HERA
H1 Collaboration
A first measurement is presented of exclusive photoproduction of $ρ^{0}$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years $2006$ and $2007$ at a centre-of-mass energy of $s = 319$ GeV and correspond to an integrated luminosity of $1.16$ pb $^{- 1}$ . The $ρ^{0}$ mesons with transverse momenta $p_{T} < 1$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $x_{L} > 0.35$ , are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality $Q^{2} < 2$ GeV $^{2}$ , the total energy of the photon-proton system $20 < W_{γ p} < 100$ GeV and the polar angle of the leading neutron $θ_{n} < 0.75$ mrad. The cross section of the reaction $γ p \to ρ^{0} n π^{+}$ is measured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a $ρ^{0}$ meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, $σ^{el} (γ π^{+} \to ρ^{0} π^{+})$ , is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction $γ p \to ρ^{0} n π^{+}$ .
↳ hep-exhep-phnucl-exnucl-thEPJC(2016)·25 citations
09
Anisotropic distributions in a multi-phase transport model
You Zhou🇩🇰 · Kai Xiao🇨🇳 · Zhao Feng🇨🇳 · Feng Liu🇨🇳 · Raimond Snellings🇳🇱
With A Multi-Phase Transport (AMPT) model we investigate the relation between the magnitude, fluctuations and correlations of the initial state spatial anisotropy $ε_{n}$ and the final state anisotropic flow coefficients $v_{n}$ in Au+Au collisions at $s_{_{NN}} =$ 200 GeV. It is found that the relative eccentricity fluctuations in AMPT account for the observed elliptic flow fluctuations, in agreement with measurements of the STAR collaboration. In addition, the studies based on 2- and multi-particle correlations and event-by-event distributions of the anisotropies suggest that the Elliptic-Power function is a promising candidate of the underlying probability density function of the event-by-event distributions of $ε_{n}$ as well as $v_{n}$ . Furthermore, the correlations between different order symmetry planes and harmonics in the initial coordinate space and final state momentum space are presented. Non-zero values of these correlations have been observed. The comparison between our calculations and data will, in the future, shed new insight into the nature of the fluctuations of the Quark-Gluon Plasma produced in heavy ion collisions.
↳ nucl-exnucl-thPRC(2016)·44 citations
10
Thermal equation of state of polarized fermions in one dimension via complex chemical potentials
A. C. Loheac🇺🇸 · J. Braun🇩🇪 · J. E. Drut🇺🇸 · D. Roscher🇩🇪
We present a nonperturbative computation of the equation of state of polarized, attractively interacting, nonrelativistic fermions in one spatial dimension at finite temperature. We show results for the density, spin magnetization, magnetic susceptibility, and Tan's contact. We compare with the second-order virial expansion, a next-to-leading-order lattice perturbation theory calculation, and interpret our results in terms of pairing correlations. Our lattice Monte Carlo calculations implement an imaginary chemical potential difference to avoid the sign problem. The thermodynamic results on the imaginary side are analytically continued to obtain results on the real axis. We focus on an intermediate- to strong-coupling regime, and cover a wide range of temperatures and spin imbalances.
↳ cond-mat.quant-gashep-latnucl-thPRA(2015)·15 citations
11
A New Dynamical Picture for the Production and Decay of the $X Y Z$ Mesons
Richard F. Lebed🇺🇸
I introduce an entirely new dynamical description for exotic charmoniumlike hadrons, based upon the competing effects of the strong attraction between quarks in a diquark, and the inability of the diquark to hadronize on its own due to being a color nonsinglet. This mechanism naturally explains, for example, the strong preference of the $Z (4475)$ to decay to $ψ (2 S)$ rather than the $J / ψ$ , the existence of a state $X (4630)$ that decays to $Λ_{c}$ baryon pairs, and why some but not all exotics lie near hadronic thresholds. Owing to high-energy constituent counting rules, the four-quark nature of the states produces major changes to both the high- $s$ scaling of cross sections for producing such states and to the potency of the cusp effect of attracting resonances to pair-production thresholds. The recently observed $P_{c}^{+}$ pentaquark candidates are seen to fit naturally into this scheme.
↳ hep-phnucl-th8 citations

Affiliations

first authorsco-authorsvia INSPIRE

Can we decide whether QCD is confining or not at high temperature?

Light Pseudoscalar Mesons in Bethe-Salpeter Equation with Instantaneous Interaction

Nonequilibrium fixed points in longitudinally expanding scalar theories: infrared cascade, Bose condensation and a challenge for kinetic theory

Exclusive ρ0 Meson Photoproduction with a Leading Neutron at HERA

Anisotropic distributions in a multi-phase transport model

Thermal equation of state of polarized fermions in one dimension via complex chemical potentials

A New Dynamical Picture for the Production and Decay of the XYZ Mesons

Affiliations

Exclusive $ρ^{0}$ Meson Photoproduction with a Leading Neutron at HERA

A New Dynamical Picture for the Production and Decay of the $X Y Z$ Mesons