Meson Charge Research Articles

Nuclear medium meson structures from the Schwinger proper-time Nambu–Jona-Lasinio model

In this paper, we report the results of our study on the nuclear medium modifications of the meson electromagnetic form factors in the framework of the Nambu–Jona-Lasinio model with the help of the Schwinger proper-time regularization scheme to tame the loop divergence and simulate the effect of QCD confinement. In our current approach, the meson structure and nuclear medium are constructed in the same Nambu–Jona-Lasinio model at the quark level. We examine the free-space and in-medium charge radii of kaon and pion, the spacelike elastic electromagnetic form factors of kaon and pion, and their quark-sector form factors, which reflect their internal structures. By comparing our result to experimental data, we found that the free-space elastic electromagnetic form factors of the mesons are consistent with the data, while the in-medium elastic electromagnetic form factors of the mesons are found to decrease as the nuclear matter density increases, leading to a rise of meson charge radius, which is consistent with the prediction of other theoretical calculations. We also predict the axial nucleon coupling constant gA in nuclear medium computed via the Goldberger-Treiman relation, which is crucial for searching the neutrinoless double beta decay (0νββ). Published by the American Physical Society 2024

Relativistic, model-independent determination of electromagnetic finite-size effects beyond the pointlike approximation

We present a relativistic and model-independent method to derive structure-dependent electromagnetic finite-size effects. This is a systematic procedure, particularly well-suited for automation, which works at arbitrarily high orders in the large-volume expansion. Structure-dependent coefficients appear as zero-momentum derivatives of physical form factors which can be obtained through experimental measurements or auxiliary lattice calculations. As an application we derive the electromagnetic finite-size effects on the pseudoscalar meson mass and leptonic decay amplitude, through orders $\mathcal{O}(1/{L}^{3})$ and $\mathcal{O}(1/{L}^{2})$, respectively. The structure dependence appears at this order through the meson charge radius and the real radiative leptonic amplitude, which are known experimentally.

New perspective for the magnetic corrections to π - π scattering lengths in the linear sigma model

In this article, a new perspective for obtaining the magnetic evolution of $\pi-\pi $ scattering lengths in the frame of the linear sigma model is presented. When computing the relevant one-loop diagrams that contribute to these parameters, the sum over Landau levels --emerging from the expansion of the Schwinger propagator-- is handled in a novel way that could also be applied to the calculation of other magnetic-type corrections. Essentially, we have obtained an expansion in terms of Hurwitz Zeta functions. It is necessary to regularize our expressions by an appropriate physical subtraction when $|qB| \rightarrow 0$ ($q$ the meson charge and $B$ the magnetic field strength). In this way, we are able to interpolate between the very high magnetic field strength region, usually handled in terms of the Lowest Landau Level (LLA) approximation, and the weak field region, discussed in a previous paper by some of us, which is based on an appropriate expansion of the Schwinger propagator up to order $|qB|^{2}$. Our results for the scattering lengths parameters produce a soft evolution in a wide region of magnetic field strengths, reducing to the previously found expressions in both limits.

Pion and kaon elastic form factors in a refined light-front model

Within the framework of light-front field theory, we reassess the electromagnetic form factors of the pion and kaon. Comparison with experiment is made for the full range of momentum transfer, q^2<0, including recent data. The light-front model's single regulator mass, m_R, of the \bar qq bound-state vertex function is initially adjusted to reproduce the weak decay constants, f_\pi\ and f_K, and both meson's charge radii, r_\pi\ and r_K. We study the behavior of these observables under variation of the quark masses and find an optimized parameter set, m_u=m_d, m_s and m_R, for which they are in sensibly better agreement with experiment than in a previous analysis; a feature also observed for the elastic form factors, in particular at small q^2. This model refinement is important in view of an extension to vector and heavy-light mesons.

Production of exotic Λ hypernuclei via mesonic beams

We study the production of neutron-rich hypernuclei Λ 12 Be, Λ 16 C, and Λ 10 Li by the (π −, K +) and (K −, π +) reactions in flight and treat two different mechanisms of production. The first mechanism is a two-step process with meson charge exchange (e.g., π − p → π 0 n, π 0 p → K +Λ). The other mechanism is one-step production (π − p → K +Σ−) proceeding via a small Σ− component, arising in Λ hypernuclei due to ΛN–ΣN coupling, as a doorway state. Typically, the two-step mechanism is more productive. The forward differential cross section of the 10B(π −,K +) reaction is about 70 nb/sr at an incident momentum of 1.05 GeV/c. On the other hand, the one-step process can serve as a direct measurement of the Σ admixture if the two-step contribution is suppressed by a suitable choice of the reaction kinematics.

Relativistic instant-form approach to the structure of two-body composite systems: Nonzero spin

The relativistic approach to electroweak properties of two-particle composite systems developed in previous work is generalized here to the case of nonzero spin. This approach is based on the use of the instant form of relativistic Hamiltonian dynamics. The generalization makes use of a special mathematical technique for the parametrization of matrix elements of electroweak current operators in terms of form factors. In this technique the parametrization is a realization of theWigner--Eckart theorem on the Poincar\'e group and form factors are reduced matrix elements. As in the case of zero spin the electroweak current matrix element satisfies the relativistic covariance conditions and in the case of electromagnetic current it also automatically satisfies the conservation law. Physical approximations such as, for example, the relativistic impulse approximation, are formulated in terms of reduced matrix elements. The electromagnetic structure of $\rho$ meson is calculated as an example of realization of the technique proposed.

One-loop calculation of meson charge radii in the static cavity: The large-basis model.

Large-basis meson wave functions, built from static cavity quarks and gluons, are used in conjunction with a simple plane-wave projection prescription to calculate meson charge radii with $O({g}^{2})$ corrections. After fitting to the light-meson mass spectrum the pion charge radius is used to constrain the remaining degree of freedom of the model parameters. The resulting large-basis model is found to give the following mass predictions in the light-quark sector: ${K}^{*}(892)=904.4\ifmmode\pm\else\textpm\fi{}0.1$ MeV, $\ensuremath{\varphi}(1020)=1040.6\ifmmode\pm\else\textpm\fi{}0.1$ MeV. For the kaon charge radius we obtain ${〈{r}^{2}〉}_{K}=0.35\ifmmode\pm\else\textpm\fi{}0.040$ ${\mathrm{fm}}^{2}$ (${〈{r}^{2}〉}_{K}^{\mathrm{expt}}=0.34\ifmmode\pm\else\textpm\fi{}0.05$ ${\mathrm{fm}}^{2}$).

Charge radii and PCAC without soft-current limit

Pseudoscalar meson charge radii calculated in the PCAC soft-current limit approximation at the triangle level are in good agreement with the data. The authors study whether or not the removal of the soft-current limit improves the comparison between theory and experiment. No clear conclusion can be drawn as yet, but a slight improvement in the data will decide the issue.

On the charge exchange of stopped negative pions on complex nuclei

It is shown that stopped π −-meson charge exchange on complex nuclei is sharply suppressed.

Vanishing of the renormalized charge in electrodynamics and in meson theory

The author discusses the solutions of the quantum electrodynamics equations for the particles with spin 1/2 obtained by various authors firstly with one « smearing out » parameter and subsequently with two parameters. He proves in Sect. 1 that the solutions obtained are correct for all values of the non-renormalized electric chargee1(e1≪ 1 as well ase1 ≫1), and that both theories lead to the conclusion that the renormalized chargee turns out to be zero. In Sect. 2 the author investigates if the same property holds also for the meson chargeg in the pseudoscalar theory. Using the two-limits technique developed by Abrikosov and Halatnikov, he comes to the conclusion that alsog turns out to be zero. In Sect. 3 the author considers in detail the problem of the vanishing of the renormalized meson charge examining the properties of the functionsG, D and Γ at arbitrary g12. The result of this analysis corresponds to the preceding conclusions, and confirms the vanishing ofg.

Proton Charge Density

The recent experiments on electron-proton scattering (1) carried out at Stanford have begun to give some insight into the structure of the charge distribution in a physical proton. This distribution of charge is, in the outer regions, probably produced by the cloud of virtual π mesons surrounding the proton. Near the core, however, heavier mesons will undoubtedly also contribute. Since a large number of phenomena involving π mesons can be described reasonably well in terms of the theory of Chew and Low(2), it is of interest to find the meson charge density predicted by this theory.