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Murgui, C., Peñuelas, A., Jung, M., & Pich, A. (2019). Global fit to b -> c tau nu transitions. J. High Energy Phys., 09(9), 103–45pp.
Abstract: We perform a general model-independent analysis of b -> c tau(nu) over bar (tau) transitions, including measurements of R-D, R-D*, their q(2) differential distributions, the recently measured longitudinal D* polarization F-L(D)*, and constraints from the B-c -> tau(nu) over bar (tau) lifetime, each of which has significant impact on the fit. A global fit to a general set of Wilson coefficients of an effective low-energy Hamiltonian is presented, the solutions of which are interpreted in terms of hypothetical new-physics mediators. From the obtained results we predict selected b -> c tau(nu) over bar (tau) observables, such as the baryonic transition Lambda(b) -> Lambda(c)tau(nu) over bar (tau), the ratio R-J/psi, the forward-backward asymmetries A(FB)(D()*()), the tau polarization asymmetries P-tau(D()*()), and the longitudinal D* polarization fraction F-L(D)*. The latter shows presently a slight tension with any new-physics model, such that an improved measurement could have an important impact. We also discuss the potential change due the very recently announced preliminary R-D(*) measurement by the Belle collaboration.
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Debastiani, V. R., Sakai, S., & Oset, E. (2019). Considerations on the Schmid theorem for triangle singularities. Eur. Phys. J. C, 79(1), 69–13pp.
Abstract: We investigate the Schmid theorem, which states that if one has a tree level mechanism with a particle decaying to two particles and one of them decaying posteriorly to two other particles, the possible triangle singularity developed by the mechanism of elastic rescattering of two of the three decay particles does not change the cross section provided by the tree level. We investigate the process in terms of the width of the unstable particle produced in the first decay and determine the limits of validity and violation of the theorem. One of the conclusions is that the theorem holds in the strict limit of zero width of that resonance, in which case the strength of the triangle diagram becomes negligible compared to the tree level. Another conclusion, on the practical side, is that for realistic values of the width, the triangle singularity can provide a strength comparable or even bigger than the tree level, which indicates that invoking the Schmid theorem to neglect the triangle diagram stemming from elastic rescattering of the tree level should not be done. Even then, we observe that the realistic case keeps some memory of the Schmid theorem, which is visible in a peculiar interference pattern with the tree level.
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Fontoura, C. E., Krein, G., Valcarce, A., & Vijande, J. (2019). Production of exotic tetraquarks QQ(q)over-bar (q)over-bar in heavy-ion collisions at the LHC. Phys. Rev. D, 99(9), 094037–8pp.
Abstract: We investigate the production of exotic tetraquarks, QQ (q) over bar (q) over bar T-QQ (Q = c or b and q = u or d), in relativistic heavy-ion collisions using the quark coalescence model. The T-QQ yield is given by the overlap of the density matrix of the constituents in the emission source with the Wigner function of the produced tetraquark. The tetraquark wave function is obtained from exact solutions of the four-body problem using realistic constituent models. The production yields are typically one order of magnitude smaller than previous estimations based on simplified wave functions for the tetraquarks. We also evaluate the consequences of the partial restoration of chiral symmetry at the hadronization temperature on the coalescence probability. Such effects, in addition to increasing the stability of the tetraquarks, lead to an enhancement of the production yields, pointing towards an excellent discovery potential in forthcoming experiments. We discuss further consequences of our findings for the search of exotic tetraquarks in central Pb + Pb collisions at the LHC.
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Fileviez Perez, P., Golias, E., Li, R. H., Murgui, C., & Plascencia, A. D. (2019). Anomaly-free dark matter models. Phys. Rev. D, 100(1), 015017–15pp.
Abstract: We investigate the predictions of anomaly-free dark matter models for direct and indirect detection experiments. We focus on gauge theories where the existence of a fermionic dark matter candidate is predicted by anomaly cancellation, its mass is defined by the new symmetry breaking scale, and its stability is guaranteed by a remnant symmetry after the breaking of the gauge symmetry. We find an upper bound on the symmetry breaking scale by applying the relic density and perturbative constraints. The anomaly-free property of the theories allows us to perform a full study of the gamma lines from dark matter annihilation. We investigate the correlation between predictions for final-state radiation processes and gamma lines. Furthermore, we demonstrate that the latter can be distinguished from the continuum gamma-ray spectrum.
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Bjorkeroth, F., de Medeiros Varzielas, I., Lopez-Ibañez, M. L., Melis, A., & Vives, O. (2019). Leptogenesis in Delta(27) with a universal texture zero. J. High Energy Phys., 09(9), 050–24pp.
Abstract: We investigate the possibility of viable leptogenesis in an appealing Delta(27) model with a universal texture zero in the (1,1) entry. The model accommodates the mass spectrum, mixing and CP phases for both quarks and leptons and allows for grand unification. Flavoured Boltzmann equations for the lepton asymmetries are solved numerically, taking into account both N-1 and N-2 right-handed neutrino decays. The N-1-dominated scenario is successful and the most natural option for the model, with M-1 is an element of [10(9), 10(12)] GeV, and M-1/M-2 is an element of [0.002, 0.1], which constrains the parameter space of the underlying model and yields lower bounds on the respective Yukawa couplings. Viable leptogenesis is also possible in the N-2-dominated scenario, with the asymmetry in the electron flavour protected from N-1 washout by the texture zero. However, this occurs in a region of parameter space which has a stronger mass hierarchy M-1/M-2< 0.002, and M-2 relatively close to M-3, which is not a natural expectation of the Delta(27) model.
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