Electroweak properties of pions in a nuclear medium

Abstract

Kaon electroweak properties in symmetric nuclear matter are studied in the Nambu-Jona-Lasinio model using the proper-time regularization. The valence quark properties in symmetric nuclear matter are calculated in the quark-meson coupling model, and they are used as inputs for studying the in-medium kaon properties in the NJL model. We evaluate the kaon decay constant, kaon-quark coupling constant, and K + electromagnetic form factor by two different approaches. Namely, by two different ways of calculating the in-medium constituent quark masses of the light quarks. We predict that, in both approaches, the kaon decay constant and kaon-quark coupling constant decrease as nuclear density increases, while the K + charge radius increases by 20-25% at normal nuclear density. * parada.hutauruk@apctp.org † kazuo.tsushima@gmail.com following reasons; (i) the constituent quark masses in vacuum are the input parameters in Ref.[32], but it is preferable to calculate them dynamically, (ii) the vacuum in the light-front approach is generally believed to be "trivial", and there is no clearly defined quark chiral condensates in the light-front approach. Thus, the model does not have the dynamical chiral symmetry breaking mechanism nor does the model have direct connection with the emergence of (pseudo-)Goldstone bosons such as kaons, and (iii) the in-medium kaon decay constant as well as the kaon-quark coupling constant are assumed to be density-independent. The purpose of the present work is to improve further the work of Ref. [32]. We extend the approach used for studying the pion properties in symmetric nuclear matter in Ref. [33], and study the in-medium kaon properties using the Nambu-Jona-Lasinio (NJL) model [34,35], supplemented by the QMC model inputs. The NJL model is a powerful chiral effective quark theory of low-energy QCD. Importantly, the model describes the dynamical chiral symmetry breaking, the origin of the pseudoscalar Goldstone bosons such as pions and kaons. Furthermore, the model satisfies the chiral limit as QCD dictates. Thus, the NJL model, which has several improved aspects as addressed above, is suitable to study the in-medium kaon properties.The experimental evidences, such as the EMC effect [36,37] and the observed modifications of bound proton EMFFs at JLab [38], suggest that the internal structure of hadrons would be modified in a nuclear medium. The phenomena of inmedium modifications of hadron properties [39][40][41][42][43][44][45][46][47][48][49][50] are tightly connected with partial restoration of chiral symmetry [51][52][53][54]. The order parameters of chiral symmetry in QCD are the lightquark chiral condensates, and their changes are expected to be one of the most important driving forces for the change of hadron properties in a nuclear medium.Spontaneous breaking of chiral symmetry generates the nonet of massless pseudoscalar Goldstone bosons. But the explicit breaking of U(1) axial symmetry selectively shifts up the η 0 -singlet mass, leaving the SU(3) flavor octet of pions, K-mesons, and...

Valence-quark distributions of pions and kaons in a nuclear medium

“…Recently, the NJL model was applied for studying the kaon electromagnetic form factor and valence-quark distributions in vacuum [5], as well as the in-medium electroweak properties of pions [41]. By extending the works of Refs.…”

“…By extending the works of Refs. [5,41], we explore the valence-quark distributions of the π + and K + mesons, dynamical quark masses, and pion/kaon properties in symmetric nuclear matter. This paper is organized as follows.…”

A study of the elastic $πN$ scattering at finite baryon density

“…However, in this work, since we study the in-medium effects in the nucleon rest frame, the vector potential can be ignored for brevity. In the QMC model, the nuclear medium effects come from the self-consistent exchange of the scalar σ and vector ω meson fields which directly coupled to the confined quark in baryon [16][17][18]. The QMC effective Lagrangian in the symmetric nuclear matter (SNM) is expressed as [16,17]…”

“…We employ various experimental and theoretical information to determine the model parameters in vacuum (ρ B = 0), such as the coupling constants, full decay width, and cut-off masses for the form factors. In order to consider the medium modifications of the hadrons, we make use of the quark-meson coupling (QMC) model for the symmetric nuclear matter (SNM) [16][17][18]. In this model, the in-medium hadron masses (M * N,∆ ) are obtained from variation calculations using the effective Hamiltonian from the quark-meson interaction Lagrangians with the confinement bag pressure.…”

Gluon and valence quark distributions for the pion and kaon in nuclear matter

“…Aside from focusing on the pion and kaon gluon distributions in nuclear medium, we also compute the pion and kaon valence-quark distributions in nuclear medium as well. The NJL model has been successfully applied for various physics phenomena of low-energy nonperturbative QCD, namely, the transverse momentum dependent (TMD) [23], the fragmentation function (FF) [24], the pion and kaon properties in nuclear medium [25,26] and the properties of neutron star [27].…”