BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., & Oyanguren, A. (2019). Extraction of form Factors from a Four-Dimensional Angular Analysis of (B)over-bar -> D*l(-)(nu)over-bar(l). Phys. Rev. Lett., 123(9), 091801–8pp.
Abstract: An angular analysis of the decay (B) over bar -> D*l(-)(nu) over bar (l), l is an element of {e, mu}, is reported using the full e(+) e(-) collision data set collected by the BABAR experiment at the Upsilon(4S) resonance. One B meson from the Upsilon(4S) -> B (B) over bar decay is fully reconstructed in a hadronic decay mode, which constrains the kinematics and provides a determination of the neutrino momentum vector. The kinematics of the semileptonic decay is described by the dilepton mass squared, q(2), and three angles. The first unbinned fit to the full four-dimensional decay rate in the standard model is performed in the so-called Boyd-Grinstein-Lebed approach, which employs a generic q(2) parametrization of the underlying form factors based on crossing symmetry, analyticity, and QCD dispersion relations for the amplitudes. A fit using the more model-dependent Caprini-Lellouch-Neubert (CLN) approach is performed as well. Our form factor shapes show deviations from previous fits based on the CLN parametrization. The latest form factors also provide an updated prediction for the branching fraction ratio R(D*) B((B) over bar -> D* tau(-)(nu) over bar (tau)) /B((B) over bar -> D*l(-)(nu) over bar (l)) = 0.253 +/- 0.005. Finally, using the well-measured branching fraction for the (B) over bar -> D*l(-)(nu) over bar (l) decay, a value of vertical bar V-cb vertical bar = (38.36 +/- 0.90) x 10(-3) is obtained that is consistent with the current world average for exclusive (B) over bar -> D(*)l(-)(nu) over bar (l) decays and remains in tension with the determination from inclusive semileptonic B decays to final states with charm.
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Anamiati, G., De Romeri, V., Hirsch, M., Ternes, C. A., & Tortola, M. (2019). Quasi-Dirac neutrino oscillations at DUNE and JUNO. Phys. Rev. D, 100(3), 035032–12pp.
Abstract: Quasi-Dirac neutrinos are obtained when the Lagrangian density of a neutrino mass model contains both Dirac and Majorana mass terms, and the Majorana terms are sufficiently small. This type of neutrino introduces new mixing angles and mass splittings into the Hamiltonian, which will modify the standard neutrino oscillation probabilities. In this paper, we focus on the case where the new mass splittings are too small to be measured, but new angles and phases are present. We perform a sensitivity study for this scenario for the upcoming experiments DUNE and JUNO, finding that they will improve current bounds on the relevant parameters. Finally, we also explore the discovery potential of both experiments, assuming that neutrinos are indeed quasi-Dirac particles.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Measurement of b hadron fractions in 13 TeV pp collisions. Phys. Rev. D, 100(3), 031102–11pp.
Abstract: The production fractions of (B) over bar (0)(s) and Lambda(0)(b) hadrons, normalized to the sum of B- and (B) over bar (0) fractions, arc measured in 13 TeV pp collisions using data collected by the LHCb experiment, corresponding to an integrated luminosity of 1.67 fb(-1). These ratios, averaged over the b hadron transverse momenta from 4 to 25 GeV and pseudorapidity from 2 to 5, are 0.122 +/- 0.006 for (B) over bar (0)(s) and 0.259 +/- 0.018 for Lambda(0)(b) where the uncertainties arise from both statistical and systematic sources. The Lambda(0)(b) ratio depends strongly on transverse momentum, while the (B) over bar (0)(s) ratio shows a mild dependence. Neither ratio shows variations with pseudorapidity. The measurements are made using semileptonic decays to minimize theoretical uncertainties. In addition, the ratio of D+ to D-0 mesons produced in the sum of (B) over bar (0) and B- semileptonic decays is determined as 0.359 +/- 0.006 +/- 0.009, where the uncertainties are statistical and systematic.
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Sobczyk, J. E., Rocco, N., & Nieves, J. (2019). Polarization of tau in quasielastic (anti)neutrino scattering: The role of spectral functions. Phys. Rev. C, 100(3), 035501–14pp.
Abstract: We present a study of the tau polarization in charged-current quasielastic (anti)neutrino-nucleus scattering. The spectral function formalism is used to compute the differential cross section and the polarization components for several kinematical setups, relevant for neutrino-oscillation experiments. The effects of the nuclear corrections in these observables are investigated by comparing the results obtained using two different realistic spectral functions, with those deduced from the relativistic global Fermi gas model, where only statistical correlations are accounted for. We show that the spectral functions, although they play an important role when predicting the differential cross sections, produce much less visible effects on the polarization components of the outgoing tau.
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Arbelaez, C., Helo, J. C., & Hirsch, M. (2019). Long-lived heavy particles in neutrino mass models. Phys. Rev. D, 100(5), 055001–15pp.
Abstract: All extensions of the standard model that generate Majorana neutrino masses at the electroweak scale introduce some heavy mediators, either fermions and/or scalars, weakly coupled to leptons. Here, by “heavy,” we mean implicitly the mass range between a few 100 GeV up to, say, roughly 2 TeV, such that these particles can be searched for at the LHC. We study decay widths of these mediators for several different tree-level neutrino mass models. The models we consider range from the simplest d = 5 seesaw up to d = 11 neutrino mass models. For each of the models, we identify the most interesting parts of the parameter space, where the heavy mediator fields are particularly long lived and can decay with experimentally measurable decay lengths. One has to distinguish two different scenarios, depending on whether fermions or scalars are the lighter of the heavy particles. For fermions, we find that the decay lengths correlate with the inverse of the overall neutrino mass scale. Thus, since no lower limit on the lightest neutrino mass exists, nearly arbitrarily long decay lengths can be obtained for the case in which fermions are the lighter of the heavy particles. For charged scalars, on the other hand, there exists a maximum value for the decay length in these models. This maximum value depends on the model and on the electric charge of the scalar under consideration but can at most be of the order of a few millimeters. Interestingly, independent of the model, this maximum occurs always in a region of parameter space, where leptonic and gauge boson final states have similar branching ratios, i.e., where the observation of lepton number-violating final states from scalar decays is possible.
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