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Coppola, M., Gomez Dumm, D., Noguera, S., & Scoccola, N. N. (2019). Pion-to-vacuum vector and axial vector amplitudes and weak decays of pions in a magnetic field. Phys. Rev. D, 99(5), 054031–18pp.
Abstract: We propose a model-independent parametrization for the one-pion-to-vacuum matrix elements of the vector and axial vector hadronic currents in the presence of an external uniform magnetic field. It is shown that, in general, these hadronic matrix elements can be written in terms of several gauge covariant Lorentz structures and form factors. Within this framework we obtain a general expression for the weak decay pi(- )-> l(nu)over bar(l) and discuss the corresponding limits of strong and weak external magnetic fields.
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Coppola, M., Gomez Dumm, D., Noguera, S., & Scoccola, N. N. (2019). Neutral and charged pion properties under strong magnetic fields in the NJL model. Phys. Rev. D, 100(5), 054014–17pp.
Abstract: In the framework of the Nambu-Jona-Lasino (NJL) model, we study the effect of an intense external uniform magnetic field on neutral and charged pion masses and decay form factors. In particular, the treatment of charged pions is carried out on the basis of the Ritus eigenfunction approach to magnetized relativistic systems. Our analysis shows that in the presence of the magnetic field three and four nonvanishing pion-to-vacuum hadronic form factors can be obtained for the case of the neutral and charged pions, respectively. As expected, it is seen that for nonzero magnetic field the pi(0) meson can still be treated as a pseudo Nambu-Goldstone boson, and consequently the corresponding form factors are shown to satisfy various chiral relations. For definite parametrizations of the model, numerical results for pi(0) and pi(+/-) masses and decay constants are obtained and compared with previous calculations given in the literature.
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Coppola, M., Gomez Dumm, D., Noguera, S., & Scoccola, N. N. (2020). Weak decays of magnetized charged pions in the symmetric gauge. Phys. Rev. D, 101(3), 034003–13pp.
Abstract: We consider the decay pi(-) -> l (nu) over bar (l) (l = e(-) , mu(-)) in the presence of an arbitrary large uniform magnetic field, using the symmetric gauge. The consequences of the axial symmetry of the problem and the issue of angular momentum conservation arc discussed in detail. In particular, we analyze the projection of both the canonical and the mechanical total angular momenta along the direction of the magnetic field. It is found that while the former is conserved in the symmetric gauge, the latter is not conserved in both the symmetric and Landau gauges. We derive an expression for the integrated pi(-) -> l (nu) over bar (l) width that coincides exactly with the one we previously found using the Landau gauge, providing an explicit test of the gauge independence of that result. Such an expression implies that for nonzero magnetic fields the decay width does not vanish in the limit in which the outgoing charged leptons can be considered as massless, i.e., it does not exhibit the helicity suppression found in the case of no external field.
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Coppola, M., Gomez Dumm, D., Noguera, S., & Scoccola, N. N. (2020). Magnetic field driven enhancement of the weak decay width of charged pions. J. High Energy Phys., 09(9), 058–19pp.
Abstract: We study the effect of a uniform magnetic field B on the decays pi- > l- nu_l bar, where l(-)=e(-), μ(-), carrying out a general analysis that includes four pi (-) decay constants. Taking the values of these constants from a chiral effective Nambu-Jona-Lasinio (NJL) model, it is seen that the total decay rate gets strongly increased with respect to the B = 0 case, with an enhancement factor ranging from similar to 10 for eB = 0.1 GeV2 up to similar to 10(3) for eB = 1 GeV2. The ratio between electronic and muonic decays gets also enhanced, reaching a value of about 1 : 2 for eB = 1 GeV2. In addition, we find that for large B the angular distribution of outgoing antineutrinos shows a significant suppression in the direction of the magnetic field.
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Coppola, M., Gomez Dumm, D., Noguera, S., & Scoccola, N. N. (2024). Masses of magnetized pseudoscalar and vector mesons in an extended NJL model: The role of axial vector mesons. Phys. Rev. D, 109(5), 054014–30pp.
Abstract: We study the mass spectrum of light pseudoscalar and vector mesons in the presence of an external uniform magnetic field B., considering the effects of the mixing with the axial-vector meson sector. The analysis is performed within a two-flavor NJL-like model which includes isoscalar and isovector couplings together with a flavor mixing 't Hooft-like term. The effect of the magnetic field on charged particles is taken into account by retaining the Schwinger phases carried by quark propagators, and expanding the corresponding meson fields in proper Ritus-like bases. The spin-isospin and spin-flavor decomposition of meson mass states is also analyzed. For neutral pion masses it is shown that the mixing with axial vector mesons improves previous theoretical results, leading to a monotonic decreasing behavior with B that is in good qualitative agreement with lattice QCD (LQCD) calculations, both for the case of constant or B-dependent couplings. Regarding charged pions, it is seen that the mixing softens the enhancement of their mass with B. As a consequence, the energy becomes lower than the one corresponding to a pointlike pion, improving the agreement with LQCD results. The agreement is also improved for the magnetic behavior of the lowest.thorn energy state, which does not vanish for the considered range of values of B-a fact that can be relevant in connection with the occurrence of meson condensation for strong magnetic fields.
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