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Kim, C. S., Lopez-Castro, G., Tostado, S. L., & Vicente, A. (2017). Remarks on the Standard Model predictions for R(D) and R(D*). Phys. Rev. D, 95(1), 013003–7pp.
Abstract: Semileptonic b -> c transitions, and in particular the ratios R(D-(*())) = Gamma(B -> D-(*())tau nu)/Gamma(B -> D-(*())l nu), can be used to test the universality of the weak interactions. In light of the recent discrepancies between the experimental measurements of these observables by the BABAR, Belle, and LHCb collaborations and the Standard Model predicted values, we study the robustness of the latter. Our analysis reveals that R(D) might be enhanced by lepton mass effects associated to the mostly unknown scalar form factor. In contrast, the Standard Model prediction for R(D*) is found to be more robust, because possible pollutions from B* contributions turn out to be negligibly small; this indicates that R(D) is a promising observable for searches of new physics.
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Celis, A., Fuentes-Martin, J., Vicente, A., & Virto, J. (2017). DsixTools: the standard model effective field theory toolkit. Eur. Phys. J. C, 77(6), 405–40pp.
Abstract: We present DsixTools, a Mathematica package for the handling of the dimension-six standard model effective field theory. Among other features, DsixTools allows the user to perform the full one-loop renormalization group evolution of the Wilson coefficients in the Warsaw basis. This is achieved thanks to the SMEFTrunner module, which implements the full one-loop anomalous dimension matrix previously derived in the literature. In addition, DsixTools also contains modules devoted to the matching to the Delta B = Delta S = 1, 2 and Delta B = Delta C = 1 operators of the Weak Effective Theory at the electroweak scale, and their QCD and QED Renormalization group evolution below the electroweak scale.
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Celis, A., Fuentes-Martin, J., Vicente, A., & Virto, J. (2017). Gauge-invariant implications of the LHCb measurements on lepton-flavor nonuniversality. Phys. Rev. D, 96(3), 035026–8pp.
Abstract: We study the implications of the recent measurements of R-K and R-K* by the LHCb Collaboration. We do that by adopting a model-independent approach based on the Standard Model effective field theory (SMEFT), with the dominant new physics (NP) effects encoded in the coefficients of dimension-6 operators respecting the full Standard Model (SM) gauge symmetry. After providing simplified expressions for R-K and R-K*, we determine the implications of the recent LHCb results for these observables on the coefficients of the SMEFT operators at low and high energies. We also take into account all b -> sll data, which combined lead to effective NP scenarios with SM pulls in excess of 5 sigma. Thus, the operators discussed in this paper would be the first dimension-6 terms in the SM Lagrangian to be detected experimentally. Indirect constraints on these operators are also discussed. The results of this paper transcend the singularity of the present situation and set a standard for future analyses in b -> s transitions when the NP is assumed to lie above the electroweak scale.
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Cai, Y., Herrero-Garcia, J., Schmidt, M. A., Vicente, A., & Volkas, R. R. (2017). From the Trees to the Forest: A Review of Radiative Neutrino Mass Models. Front. Physics, 5, 63–56pp.
Abstract: A plausible explanation for the lightness of neutrino masses is that neutrinos are massless at tree level, with their mass (typically Majorana) being generated radiatively at one or more loops. The new couplings, together with the suppression coming from the loop factors, imply that the new degrees of freedom cannot be too heavy (they are typically at the TeV scale). Therefore, in these models there are no large mass hierarchies and they can be tested using different searches, making their detailed phenomenological study very appealing. In particular, the new particles can be searched for at colliders and generically induce signals in lepton-flavor and lepton-number violating processes (in the case of Majorana neutrinos), which are not independent from reproducing correctly the neutrino masses and mixings. The main focus of the review is on Majorana neutrinos. We order the allowed theory space from three different perspectives: (i) using an effective operator approach to lepton number violation, (ii) by the number of loops at which the Weinberg operator is generated, (iii) within a given loop order, by the possible irreducible topologies. We also discuss in more detail some popular radiative models which involve qualitatively different features, revisiting their most important phenomenological implications. Finally, we list some promising avenues to pursue.
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Aristizabal Sierra, D., Herrero-Garcia, J., Restrepo, D., & Vicente, A. (2016). Diboson anomaly: Heavy Higgs resonance and QCD vectorlike exotics. Phys. Rev. D, 93(1), 015012–12pp.
Abstract: The ATLAS Collaboration (and also CMS) has recently reported an excess over Standard Model expectations for gauge boson pair production in the invariant mass region 1.8-2.2 TeV. In light of these results, we argue that such a signal might be the first manifestation of the production and further decay of a heavy CP-even Higgs resulting from a type-I two Higgs doublet model. We demonstrate that in the presence of colored vectorlike fermions, its gluon fusion production cross section is strongly enhanced, with the enhancement depending on the color representation of the new fermion states. Our findings show that barring the color triplet case, any QCD “exotic” representation can fit the ATLAS result in fairly large portions of the parameter space. We have found that if the diboson excess is confirmed and this mechanism is indeed responsible for it, then the LHC Run-2 should find (i) a CP-odd scalar with mass below similar to 2.3 TeV, (ii) new colored states with masses below similar to 2 TeV, (iii) no statistically significant diboson events in the W(+/-)Z channel, (iv) events in the triboson channels W(+/-)W(-/+)Z and ZZZ with invariant mass amounting to the mass of the CP-odd scalar.
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