Axial form factors of the octet baryons in a covariant quark model

Abstract

We study the weak interaction axial form factors of the octet baryons, within the covariant spectator quark model, focusing on the dependence of four-momentum transfer squared, Q 2 . In our model the axial form factors GA(Q 2 ) (axial-vector form factor) and GP (Q 2 ) (induced pseudoscalar form factor), are calculated based on the constituent quark axial form factors and the octet baryon wave functions. The quark axial current is parametrized by the two constituent quark form factors, the axial-vector form factor g q A (Q 2 ), and the induced pseudoscalar form factor g q P (Q 2 ). The baryon wave functions are composed of a dominant S-state and a P -state mixture for the relative angular momentum of the quarks. First, we study in detail the nucleon case. We assume that the quark axial-vector form factor g q A (Q 2 ) has the same function form as that of the quark electromagnetic isovector form factor. The remaining parameters of the model, the P -state mixture and the Q 2 -dependence of g q P (Q 2 ), are determined by a fit to the nucleon axial form factor data obtained by lattice QCD simulations with large pion masses. In this lattice QCD regime the meson cloud effects are small, and the physics associated with the valence quarks can be better calibrated. Once the valence quark model is calibrated, we extend the model to the physical regime, and use the low Q 2 experimental data to estimate the meson cloud contributions for GA(Q 2 ) and GP (Q 2 ). Using the calibrated quark axial form factors, and the generalization of the nucleon wave function for the other octet baryon members, we make predictions for all the possible weak interaction axial form factors GA(Q 2 ) and GP (Q 2 ) of the octet baryons. The results are compared with the corresponding experimental data for GA(0), and with the estimates of baryon-meson models based on SU (6) symmetry.

Semirelativistic approximation to the γ*N→N(1520) and γ*NRamalho1 2017Phys. Rev. DSelf Cite192120View full textAdd to dashboardCite

“…The covariant spectator quark model was already applied to the nucleon [22,23,[36][37][38], several nucleon resonances [8-10, 25, 27], ∆ resonances [10,34,35,[39][40][41][42], and other transitions between baryon states [24,30,[43][44][45].…”

“…The wave functions of the nucleon, N (1520) and N (1535) are discussed in Refs. [8,9,22].The covariant spectator quark model was already applied to the nucleon [22,23,[36][37][38], several nucleon resonances [8-10, 25, 27], ∆ resonances [10,34,35,[39][40][41][42], and other transitions between baryon states [24,30,[43][44][45].When the baryon wave functions are represented in terms of the single quark and quark-pair states, one can write the transition current in a relativistic impulse approximation as [22][23][24] where j µ q is the quark current operator and Γ labels the scalar diquark and vectorial diquark (projections Λ = 0, ±) polarizations. The factor 3 takes account of the contributions of all the quark-pairs by symmetry.…”

γ*N→N*(1520) form factors in the timelike regime

“…This model is based on the covariant spectator theory (CST) [19]. In a relatively successful and unifying way, our approach pictures a large variety of baryons as a superposition of a core of three valence quarks and meson cloud components [6][7][8][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. After a series of applications of the model to the electromagnetic excitation of baryons in the spacelike regime, we analyzed also the impact of the ∆(1232) resonance in the timelike reactions [6,7].…”

Holographic estimate of the meson cloud contribution to nucleon axial form factor

“…The weak structure of the nucleon is characterized by the axial form factor, G A , and the induced pseudoscalar form factor, G P . The study of the nucleon axial structure is important because it provides complementary information on the well known electromagnetic structure and also because involves both strong and weak interactions [26]. The nucleon axial form factors can be measured in quasielastic neutrino/antineutrino scattering with proton targets, by charged pion electroproduction on nucleons and also in the process of muon capture by protons [27][28][29].…”

“…The nucleon axial form factor has been calculated using different frameworks [26,. Recently, also lattice QCD simulations of the nucleon axial form factors became available for several pion masses (m π ), in the range m π ¼ 0.2-0.6 GeV [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69].…”