Nuclear Theory·nucl-th

Friday·July 21, 2023

7 papers—1 primary·6 cross-listed

02
Sensitivity of finite size effects to the boundary conditions and the vacuum term
Győző Kovács🇭🇺 · Péter Kovács🇭🇺 · Pok Man Lo🇵🇱 · Krzysztof Redlich🇵🇱 · György Wolf🇭🇺
Finite volume effects are studied both with low-momentum cutoff and with momentum discretization in the framework of an (axial)vector meson extended quark-meson model with Polyakov-loop variables. In the momentum cutoff scenario, the CEP moves to lower temperatures and larger quark chemical potentials as the characteristic system size is reduced, however, the treatment of the vacuum term significantly affects its trajectory. The size dependence of the baryon fluctuations is also studied by the kurtosis and the skewness, both of which show moderate dependence on temperature and some dependence on quark chemical potential. The order of the phase transition is also studied near the chiral limit at finite system size and found to be second-order only at vanishing explicit breaking. The implementation of the finite size effect with momentum discretization is more complicated and shows peculiar behavior due to the different modes dropping below the Fermi surface and strong dependence on the type of the boundary condition chosen. We found that both the different boundary conditions and the treatment of the vacuum term cause significant changes in the trajectory of the CEP as the characteristic system size is changed.
↳ hep-phnucl-thPRD(2023)·12 citations
03
The semileptonic decays $η^{(')} \to π^{0} ℓ^{+} ℓ^{-}$ and $η^{'} \to η ℓ^{+} ℓ^{-}$ in the standard model
Hannah Schäfer🇩🇪 · Marvin Zanke🇩🇪 · Yannis Korte🇩🇪 · Bastian Kubis🇩🇪
We perform a theoretical analysis of the semileptonic decays $η^{(')} \to π^{0} ℓ^{+} ℓ^{-}$ and $η^{'} \to η ℓ^{+} ℓ^{-}$ , where $ℓ = e, μ$ , via a charge-conjugation-conserving two-photon mechanism. The underlying form factors are modeled using vector-meson dominance, phenomenological input, and $U (3)$ flavor symmetry. We consider both a monopole and a dipole model, the latter tailored such that the expected high-energy behavior is ensured. Furthermore, we benchmark the effect of $S$ -wave rescattering contributions to the decays. We infer significant effects of the form factors neglected in the literature so far, still finding branching ratios of the various decays well below the current experimental upper limits.
↳ hep-phhep-exnucl-thPRD(2023)·23 citations
04
Two-pole nature of the $Λ (1405)$ from lattice QCD
John Bulava🇩🇪 · Bárbara Cid-Mora🇩🇪 · Andrew D. Hanlon🇺🇸 · Ben Hörz🇩🇪 · Daniel Mohler🇩🇪 · Colin Morningstar🇺🇸 · Joseph Moscoso🇺🇸 · Amy Nicholson🇺🇸 · Fernando Romero-López🇺🇸 · Sarah Skinner🇺🇸 · André Walker-Loud🇺🇸
This letter presents the first lattice QCD computation of the coupled channel $π Σ - K ˉ N$ scattering amplitudes at energies near $1405 MeV$ . These amplitudes contain the resonance $Λ (1405)$ with strangeness $S = - 1$ and isospin, spin, and parity quantum numbers $I (J^{P}) = 0 (1/ 2^{-})$ . However, whether there is a single resonance or two nearby resonance poles in this region is controversial theoretically and experimentally. Using single-baryon and meson-baryon operators to extract the finite-volume stationary-state energies to obtain the scattering amplitudes at slightly unphysical quark masses corresponding to $m_{π} \approx 200$ MeV and $m_{K} \approx 487$ MeV, this study finds the amplitudes exhibit a virtual bound state below the $π Σ$ threshold in addition to the established resonance pole just below the $K ˉ N$ threshold. Several parametrizations of the two-channel $K$ -matrix are employed to fit the lattice QCD results, all of which support the two-pole picture suggested by $S U (3)$ chiral symmetry and unitarity.
↳ hep-lathep-phnucl-exnucl-thPRL(2024)·77 citations
05
Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars
Michail Chabanov🇩🇪 · Luciano Rezzolla🇩🇪
In the violent post-merger of binary neutron-star mergers strong oscillations are present that impact the emitted gravitational-wave (GW) signal. The frequencies, temperatures and densities involved in these oscillations allow for violations of the chemical equilibrium promoted by weak-interactions, thus leading to a nonzero bulk viscosity that can impact dynamics and GW signals. We present the first simulations of binary neutron-star mergers employing the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by Müller, Israel and Stewart. With the spirit of obtaining a first assessment of the impact of bulk viscosity on the structure and radiative efficiency of the merger remnant we adopt a simplified but realistic approach for the viscosity, which we assume to be determined by direct and modified Urca reactions and hence to vary within the stars. At the same time, to compensate for the lack of a precise knowledge about the strength of bulk viscosity, we explore the possible behaviours by considering three different scenarios of low, medium, and high bulk viscosity. In this way, we find that large values of the bulk viscosities damp the collision-and-bounce oscillations that characterize the dynamics of the stellar cores right after the merger. At the same time, large viscosities tend to preserve the $m = 2$ deformations in the remnant, thus leading to a comparatively more efficient GW emission and to changes in the post-merger spectrum that can be up to $100 Hz$ in the case of the most extreme configurations. Overall, our self-consistent results indicate that bulk viscosity increases the energy radiated in GWs soon after the merger by $≲ 2%$ in the (realistic) scenario of small viscosity, and by $≲ 30%$ in the (unrealistic) scenario of large viscosity.
↳ gr-qcastro-ph.HEnucl-thPRL(2025)·81 citations
06
Analytic Solution for the Revised Helicity Evolution at Small $x$ and Large $N_{c}$ : New Resummed Gluon-Gluon Polarized Anomalous Dimension and Intercept
Jeremy Borden🇺🇸
We construct an exact analytic solution of the revised small- $x$ helicity evolution equations derived recently. The equations we solve are obtained in the large- $N_{c}$ limit (with $N_{c}$ the number of quark colors) and are double-logarithmic (summing powers of $α_{s} ln^{2} (1/ x)$ with $α_{s}$ the strong coupling constant and $x$ the Bjorken $x$ variable). Our solution provides small- $x$ , large- $N_{c}$ expressions for the flavor-singlet quark and gluon helicity parton distribution functions (PDFs) and for the $g_{1}$ structure function, with their leading small- $x$ asymptotics given by \begin{align} \Delta \Sigma (x, Q^2) \sim \Delta G (x, Q^2) \sim g_1 (x, Q^2) \sim \left( \frac{1}{x} \right)^{\alpha_h} , \notag \end{align} where the exact analytic expression we obtain for the intercept $α_{h}$ can be approximated by $α_{h} = 3.66074 \frac{α _{s} N _{c}}{2 π}$ . Our solution also yields an all-order (in $α_{s}$ ) resummed small- $x$ anomalous dimension $Δ γ_{GG} (ω)$ which agrees with all the existing fixed-order calculations (to three loops). Notably, our anomalous dimension is different from that obtained in the infrared evolution equation framework developed earlier by Bartels, Ermolaev, and Ryskin (BER), with the disagreement starting at four loops. Despite the previously reported agreement at two decimal points based on the numerical solution of the same equations, the intercept of our large- $N_{c}$ helicity evolution and that of BER disagree beyond that precision, with the BER intercept at large $N_{c}$ given by a different analytic expression from ours with the numerical value of $α_{h}^{B E R} = 3.66394 \frac{α _{s} N _{c}}{2 π}$ . We speculate on the origin of this disagreement.
↳ hep-phnucl-th2 citations
07
Fluid dynamics from the Boltzmann equation using a maximum entropy distribution
Chandrodoy Chattopadhyay🇺🇸 · Ulrich Heinz🇺🇸 · Thomas Schaefer🇺🇸
Using the recently developed ``Maximum Entropy'' (or ``least biased'') distribution function to truncate the moment hierarchy arising from kinetic theory, we formulate a far-from-equilibrium macroscopic theory that provides the possibility of describing both free-streaming and hydrodynamic regimes of heavy-ion collisions within a single framework. Unlike traditional hydrodynamic theories that include viscous corrections to finite order, the present formulation incorporates contributions to all orders in shear and bulk inverse Reynolds numbers, allowing it to handle large dissipative fluxes. By considering flow profiles relevant for heavy-ion collisions (Bjorken and Gubser flows), we demonstrate that the present approach provides excellent agreement with underlying kinetic theory throughout the fluid's evolution and, especially, in far-off-equilibrium regimes where traditional hydrodynamics breaks down.
↳ hep-phnucl-thPRC(2023)·14 citations

Affiliations

first authorsco-authorsvia INSPIRE

Sensitivity of finite size effects to the boundary conditions and the vacuum term

The semileptonic decays η(′)→π0ℓ+ℓ− and η′→ηℓ+ℓ− in the standard model

Two-pole nature of the Λ(1405) from lattice QCD

Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars

Analytic Solution for the Revised Helicity Evolution at Small x and Large Nc​: New Resummed Gluon-Gluon Polarized Anomalous Dimension and Intercept

Fluid dynamics from the Boltzmann equation using a maximum entropy distribution

Affiliations

The semileptonic decays $η^{(')} \to π^{0} ℓ^{+} ℓ^{-}$ and $η^{'} \to η ℓ^{+} ℓ^{-}$ in the standard model

Two-pole nature of the $Λ (1405)$ from lattice QCD

Analytic Solution for the Revised Helicity Evolution at Small $x$ and Large $N_{c}$ : New Resummed Gluon-Gluon Polarized Anomalous Dimension and Intercept