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Hernandez, E., Nieves, J., Sanchez, F., & Sobczyk, J. E. (2022). Tau longitudinal and transverse polarizations from visible kinematics in (anti-)neutrino nucleus scattering. Phys. Lett. B, 829, 137046–9pp.
Abstract: Since the upsilon(tau) ((upsilon) over bar (tau))A(Z) -> tau(-/+) X reaction is notoriously difficult to be directly measured, the information on the dynamics of this nuclear process should be extracted from the analysis of the energy and angular distributions of the tau decay visible products. These distributions depend, in addition to d(2)sigma / (dE(tau)d cos theta(tau)), on the components of the tau-polarization vector. We give, for the first time, the general expression for the outgoing hadron (pion or rho meson) energy and angular differential cross section for the sequential upsilon(tau) A(Z) -> tau(-) (pi(-)upsilon(tau,) rho(-)upsilon(tau))X and (upsilon) over bar (tau)A(Z) -> tau(+) (pi(+)(upsilon) over bar (tau), rho(+)(upsilon) over bar (tau)) X reactions. Though all possible nuclear reaction mechanisms contribute to the distribution, it may be possible to isolate/enhance one of them by implementing appropriate selection criteria. For the case of the quasi-elastic reaction off oxygen and neutrino energies below 6 GeV, we show that the pion distributions are quite sensitive to the details of the tau-polarization components. We find significant differences between the full calculation, where the longitudinal and transverse components of the tau polarization vector vary with the energy and the scattering angle of the produced tau, and the simplified scheme in which the polarizations are set to one and zero, being the latter their respective asymptotic values reached in the high energy regime. In addition to its potential impact on neutrino oscillation analyses, this result can be used to further test different nuclear models, since these observables provide complementary information to that obtained by means of the inclusive nuclear weak charged-current differential cross section. We also study the effects on the cross section of the W-4 and W-5 nuclear structure functions, which contributions are proportional to the charged lepton mass, and therefore difficult to constrain in muon and electron neutrino experiments.
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Hernandez, E., & Nieves, J. (2017). Neutrino-induced one-pion production revisited: The nu(mu)n -> mu(-)n pi(+) channel. Phys. Rev. D, 95(5), 053007–18pp.
Abstract: Understanding single pion production reactions on free nucleons is the first step towards a correct description of these processes in nuclei, which are important for signal and background contributions in current and near future accelerator neutrino oscillation experiments. In this work, we reanalyze our previous studies of neutrino-induced one-pion production on nucleons for outgoing pi N invariant masses below 1.4 GeV. Our motivation is to get a better description of the nu(mu)n -> mu(-)n pi(+) cross section, for which current theoretical models give values significantly below data. This channel is very sensitive to the crossed Delta(1232) contribution and thus, to spin 1/2 components in the Rarita-Schwinger Delta propagator. We show how these spin 1/2 components are nonpropagating and give rise to contact interactions. In this context, we point out that the discrepancy with experiment might be corrected by the addition of appropriate extra contact terms and argue that this procedure will provide a natural solution to the nu(mu)n -> mu(-)n pi(+) puzzle. To keep our model simple, in this work, we propose to change the strength of the spin 1/2 components in the. propagator and use the nu(mu)n -> mu(-)n pi(+) data to constraint its value. With this modification, we now find a good reproduction of the nu(mu)n -> mu(-)n pi(+) cross section without affecting the good results previously obtained for the other channels. We also explore how this change in the. propagator affects our predictions for pion photoproduction and find also a better agreement with experiment than with the previous model.
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Hernandez, E., & Nieves, J. (2011). Study of the strong Sigma(b) -> Lambda(b)pi and Sigma*(b) -> Lambda(b)pi in a nonrelativistic quark model. Phys. Rev. D, 84(5), 057902–5pp.
Abstract: We present results for the strong widths corresponding to the Sigma(b) -> Lambda(b)pi and Sigma*(b) -> Lambda(b)pi decays. We apply our model from Phys. Rev. D 72, 094022 (2005), where we previously studied the corresponding transitions in the charmed sector. Our nonrelativistic constituent quark model uses wave functions that take advantage of the constraints imposed by heavy quark symmetry. The partial conservation of axial current hypothesis allows us to determine the strong vertices from an analysis of the axial current matrix elements.
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Kaskulov, M., Hernandez, E., & Oset, E. (2010). On the background in the gamma p -> omega(pi(0)gamma)p reaction and mixed event simulation. Eur. Phys. J. A, 46(2), 223–230.
Abstract: In this paper we evaluate sources of background of the gamma p -> omega p reaction, with the omega detected through its pi(0)gamma decay channel, to compare with the experiment carried out at ELSA. We find background from gamma p -> pi(0)pi(0)p followed by decay of a pi(0) into two gamma, recombining one pi(0) and one gamma, and from the gamma p -> pi(0)eta p reaction with subsequent decay of the eta into two photons. This background accounts for the data at pi(0)gamma invariant masses beyond 700 MeV, but strength is missing at lower invariant masses which was attributed to photon misidentification events, which we simulate to get a good reproduction of the experimental background. Once this is done, we perform an event mixing simulation to reproduce the calculated background and we find that the method provides a good description of the background. A closer look reveals this is accidental. We show that the mixed event generated background in the region of the omega mass and beyond is completely tied to the events at low pi(0)gamma invariant masses where the d sigma/dM(pi 0 gamma) distribution is much larger. This has as a consequence that the mixed event method produces the same background at high invariant masses independently of the actual background in that region, as a consequence of which, the method is unsuited to give the background at energies around the peak of the omega and beyond.
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Penalva, N., Hernandez, E., & Nieves, J. (2020). Hadron and lepton tensors in semileptonic decays including new physics. Phys. Rev. D, 101(11), 113004–24pp.
Abstract: We extend our general framework for semileptonic decay, originally introduced in N. Penalva et al. [Phys. Rev. D 100, 113007 (2019)], with the addition of new physics (NP) tensor terms. In this way, all the NP effective Hamiltonians that are considered in lepton flavor universality violation (LFUV) studies have now been included. Those are left and right vector and scalar NP Hamiltonians and the NP tensor one. Besides, we now also give general expressions that allow for complex Wilson coefficients. The scheme developed is totally general and it can be applied to any charged current semileptonic decay, involving any quark flavors or initial and final hadron states. We show that all the hadronic input, including NP effects, can be parametrized in terms of 16 Lorentz scalar structure functions, constructed out of the NP complex Wilson coefficients and the genuine hadronic responses, with the latter determined by the matrix elements of the involved hadron operators. In the second part of this work, we use this formalism to obtain the complete NP effects in the Ab Acr(/ semileptonic decay, where LFUV, if finally confirmed, is also expected to be seen. We- stress the relevance of the center of mass (CM) d2F/ (dwd cos 0i) and laboratory (LAB) d2F/(dwdE,) differential decay widths, with (o the product of the hadron four-velocities, Oe the angle made by the three -momenta of the charged lepton and the final hadron in the 11/- CM frame and the charged lepton energy in the decaying hadron rest frame. While models with very different strengths in the NP terms give the same differential d17 do) and total decay widths for this decay, they predict very different numerical results for some of the cos (.),, and E coefficient -functions that determine the above two distributions. Thus, the combined analysis of the CM d2F1(dcodcos0,,) and LAB d21'/(doidE,.) differential decay widths will help clarifying what kind of NP is a better candidate in order to explain LFUV.
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