Nuclear Theory·nucl-th

Friday·September 11, 2020

10 papers—5 primary·5 cross-listed

01
Gauge independence of pion masses in a magnetic field within the Nambu--Jona-Lasinio model
Jianing Li🇨🇳 · Gaoqing Cao🇨🇳 · Lianyi He🇨🇳
We investigate the properties of neutral and charged pions in a constant background magnetic field mainly at zero temperature within the Nambu--Jona-Lasinio model. In the previous calculations, the Ritus method, involving Schwinger phases in a fixed gauge, was employed within the momentum-space random phase approximation (RPA)~[Phys. Lett. B $782$ , 155-161 (2018)]. However, gauge invariance of the charged pion masses has not yet been examined. In this work, by adopting the linear response theory based on the imaginary-time path integral formalism, we derive the correlation functions for pions in the coordinate space, where the corresponding Schwinger phases show up automatically. At sufficiently large imaginary time $τ$ , the meson correlation function approaches an exponential form $\sim exp (- E_{G} τ)$ , where $E_{G}$ is the ground-state energy of the one-meson state and hence determined as the meson mass. Furthermore, we show that the mass of the charged pions is gauge independent, i.e., independent of the choice of the vector potential for the magnetic field. Actually, we also find that the momentum-space RPA is equivalent to the imaginary-time method used here.
nucl-thhep-phPRD(2021)·29 citations
02
Natural orbitals for many-body expansion methods
J. Hoppe · A. Tichai · M. Heinz · K. Hebeler · A. Schwenk
The nuclear many-body problem for medium-mass systems is commonly addressed using wave-function expansion methods that build upon a second-quantized representation of many-body operators with respect to a chosen computational basis. While various options for the computational basis are available, perturbatively constructed natural orbitals recently have been shown to lead to significant improvement in many-body applications yielding faster model-space convergence and lower sensitivity to basis set parameters in large-scale no-core shell model diagonalizations. This work provides a detailed comparison of single-particle basis sets and a systematic benchmark of natural orbitals in nonperturbative many-body calculations using the in-medium similarity renormalization group approach. As a key outcome we find that the construction of natural orbitals in a large single-particle basis enables for performing the many-body calculation in a reduced space of much lower dimension, thus offering significant computational savings in practice that help extend the reach of ab initio methods towards heavier masses and higher accuracy.
nucl-thPRC(2021)·44 citations
03
Constraints on the nuclear symmetry energy from asymmetric-matter calculations with chiral NN and 3N interactions
R. Somasundaram🇫🇷 · C. Drischler🇺🇸 · I. Tews🇺🇸 · J. Margueron🇫🇷
The nuclear symmetry energy is a key quantity in nuclear (astro)physics. It describes the isospin dependence of the nuclear equation of state (EOS), which is commonly assumed to be almost quadratic. In this work, we confront this standard quadratic expansion of the EOS with explicit asymmetric nuclear-matter calculations based on a set of commonly used Hamiltonians including two- and three-nucleon forces derived from chiral effective field theory. We study, in particular, the importance of non-quadratic contributions to the symmetry energy, including the non-analytic logarithmic term introduced by Kaiser [Phys.~Rev.~C \textbf{91}, 065201 (2015)]. Our results suggest that the quartic contribution to the symmetry energy can be robustly determined from the various Hamiltonians employed, and we obtain 1.00(8) MeV (or 0.55(8) MeV for the potential part) at saturation density, while the logarithmic contribution to the symmetry energy is relatively small and model-dependent. We finally employ the meta-model approach to study the impact of the higher-order contributions on the neutron-star crust-core transition density, and find a small 5\% correction.
nucl-thPRC(2021)·73 citations
04
Global polarization effect and spin-orbit coupling in strong interaction
Jian-Hua Gao🇨🇳 · Zuo-Tang Liang🇨🇳 · Qun Wang🇨🇳 · Xin-Nian Wang🇨🇳
In non-central high energy heavy ion collisions the colliding system posses a huge orbital angular momentum in the direction opposite to the normal of the reaction plane. Due to the spin-orbit coupling in strong interaction, such huge orbital angular momentum leads to the polarization of quarks and anti-quarks in the same direction. This effect, known as the global polarization effect, has been recently observed by STAR Collaboration at RHIC that confirms the theoretical prediction made more than ten years ago. The discovery has attracted much attention on the study of spin effects in heavy ion collision. It opens a new window to study properties of QGP and a new direction in high energy heavy ion physics -- Spin Physics in Heavy Ion Collisions. In this chapter, we review the original ideas and calculations that lead to the predictions. We emphasize the role played by spin-orbit coupling in high energy spin physics and discuss the new opportunities and challenges in this connection.
nucl-thhep-phLect.Notes Phys.(2021)·39 citations
05
Probing the multi-scale dynamical interaction between heavy quarks and the QGP using JETSCAPE
W. Fan🇺🇸 · G. Vujanovic🇺🇸 · A. Angerami · S. A. Bass · S. Cao · Y. Chen · J. Coleman · L. Cunqueiro · T. Dai · L. Du · R. Ehlers · H. Elfner and 38 other authors
The dynamics of shower development for a jet traveling through the QGP involves a variety of scales, one of them being the heavy quark mass. Even though the mass of the heavy quarks plays a subdominant role during the high virtuality portion of the jet evolution, it does affect longitudinal drag and diffusion, stimulating additional radiation from heavy quarks. These emissions partially compensate the reduction in radiation from the dead cone effect. In the lower virtuality part of the shower, when the mass is comparable to the transverse momenta of the partons, scattering and radiation processes off heavy quarks differ from those off light quarks. All these factors result in a different nuclear modification factor for heavy versus light flavors and thus for heavy-flavor tagged jets. In this study, the heavy quark shower evolution and the fluid dynamical medium are modeled on an event by event basis using the JETSCAPE Framework. We present a multi-stage calculation that explores the differences between various heavy quark energy-loss mechanisms within a realistically expanding quark-gluon plasma (QGP). Inside the QGP, the highly virtual and energetic portion of the shower is modeled using the MATTER generator, while the LBT generator models the showers induced by energetic and close-to-on-shell heavy quarks. Energy-momentum exchange with the medium, essential for the study of jet modification, proceeds using a weak coupling recoil approach. The JETSCAPE framework allows for transitions, on the level of individual partons, from one energy-loss prescription to the other depending on the parton's energy and virtuality and the local density. This allows us to explore the effect and interplay between the different regimes of energy loss on the propagation and radiation from hard heavy quarks in a dense medium.
nucl-thhep-exhep-phPoS(2021)·1 citation
06
Wave Functions of Pentadiagonal Matrices in the Weak Coupling Limit
Larry Zamick
We consider a pentadiagonal matrix which will be described in the text. We demonstrate practical methods for obtaining weak coupling expressions for the lowest eigenvector in terms of the parameters in the matrix, v and w. It is found that the expressions simplify if the wave function coefficients are put in the denominator.
↳ physics.gen-phnucl-thIJMPE(2020)·2 citations
07
Two-loop analysis of the pion-mass dependence of the $ρ$ meson
Malwin Niehus🇩🇪 · Martin Hoferichter🇨🇭 · Bastian Kubis🇩🇪 · Jacobo Ruiz de Elvira🇨🇭
Analyzing the pion-mass dependence of $π π$ scattering phase shifts beyond the low-energy region requires the unitarization of the amplitudes from chiral perturbation theory. In the two-flavor theory, unitarization via the inverse-amplitude method (IAM) can be justified from dispersion relations, which is therefore expected to provide reliable predictions for the pion-mass dependence of results from lattice QCD calculations. In this work, we provide compact analytic expression for the two-loop partial-wave amplitudes for $J = 0, 1, 2$ required for the IAM at subleading order. To analyze the pion-mass dependence of recent lattice QCD results for the $P$ -wave, we develop a fit strategy that for the first time allows us to perform stable two-loop IAM fits and assess the chiral convergence of the IAM approach. While the comparison of subsequent orders suggests a breakdown scale not much below the $ρ$ mass, a detailed understanding of the systematic uncertainties of lattice QCD data is critical to obtain acceptable fits, especially at larger pion masses.
↳ hep-phhep-latnucl-thPRL(2021)·52 citations
08
$J / ψ$ - $p$ Scattering Length from the Total and Differential Photoproduction Cross Sections
Lubomir Pentchev (Thomas Jefferson National Accelerator Facility, Newport News, Virginia, USA)🇺🇸 · Igor I. Strakovsky (The George Washington University, Washington, D.C., USA)🇺🇸
The $J / ψ$ - $p$ scattering length, $α_{J / ψ p}$ , can be extracted from the $J / ψ$ photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction $γ p \to J / ψ p$ to $J / ψ p \to J / ψ p$ . Such estimates based on experimental data result in values for $∣ α_{J / ψ p} ∣$ , which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample, such as the near threshold $J / ψ$ photoproduction dataset. We estimate a range for $∣ α_{J / ψ p} ∣$ of $0.003$ to $0.025$ ~fm as extracted from experimental data within the VMD model and discuss possible reasons for such lower values compared to the theoretical results.
↳ hep-phhep-exnucl-exnucl-thEPJA(2021)·37 citations
09
Exclusive $J / ψ$ photoproduction off deuteron in d+Au ultra-peripheral collisions at STAR
Zhoudunming Tu (for the STAR Collaboration)🇺🇸
Gluon density and its distributions inside nuclei and the parton modification of bounded nucleons inside a nucleus, are some of the main standing problems in nuclear and particle physics. In recent years, ultra-peripheral collisions (UPC) of heavy ions have provided a new way of probing the gluon density, which is based on coherent diffractive vector-meson productions, e.g., $J / ψ$ meson. For heavy ions, e.g., Pb, the gluon density is found to be significantly suppressed through the UPC $J / ψ$ measurement, suggesting a strong gluon shadowing effect in heavy nuclei. In this analysis, we aim to look at a unique set of data taken by the STAR experiment, where $J / ψ$ mesons are photoproduced off the deuteron target with no other particle produced, except for the deuteron or its breakup products. The Zero Degree Calorimeter response with respect to the deuteron dissociation by detecting a beam-rapidity neutron is also investigated and provides additional information about the underlying physics process. The cross section of $J / ψ$ photoproduction in the photon-deuteron system is measured at the photon-nucleon center-of-mass energy $W \sim 25 GeV$ , as well as the momentum transfer $t$ dependence cross section, $d σ / d t$ . Data suggests a wider gluon density distribution than the Hulthen charge density distribution in deuteron.
↳ nucl-exhep-exhep-phnucl-thPoS(2021)·3 citations
10
Early Time Dynamics and the Bulk
Bin Wu🇨🇭
Deciphering the origin of collective phenomena in small colliding systems is one of contemporary focuses in heavy-ion physics. It entails penetrating the barrier between two previously separated research topics: thermalization/hydrodynamization and phenomenological studies of collectivity. I first review some recent progress in understanding thermalization/hydrodynamization in large colliding systems, centralized on bottom-up thermalization. Then, using a simple kinetic theory I demonstrate how the investigation of hydrodynamization is intertwined with the study of flow in small colliding systems. Connections of these studies to "hard probes" are also commented where possible.
↳ hep-phhep-exnucl-exnucl-thPoS(2021)·1 citation

Affiliations

first authorsco-authorsvia INSPIRE