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Cepeda, M., No, J. M., Ramos, C., Sanda Seoane, R. M., & Zurita, J. (2025). Exotic h → Za Higgs decays into τ leptons. J. High Energy Phys., 07(7), 009–25pp.
Abstract: Exotic Higgs decays are among the most promising areas to be explored at the High-Luminosity LHC, given the unprecedentedly large amount (similar to 3 x 10(8)) of 125 GeV Higgs bosons that will be produced. In this context, we propose a new search channel for which the Higgs boson decays to a (leptonically decaying) Z boson and a light BSM pseudoscalar a, which subsequently decays to a pair of tau-leptons (h -> Za -> ll tau tau). After performing a validation of existing ATLAS and CMS exotic Higgs decay searches in related channels, we analyze the HL-LHC projected sensitivity of our a -> tau tau search, targeting the kinematic region where the exotic Higgs decay is two-body. We are able to probe pseudoscalar masses m(a) is an element of [5, 33] GeV by leveraging both leptonic and hadronic tau decays, and establish model-independent 95% C.L. sensitivity projections on the branching fraction BR(h -> Za) x BR(a -> tau tau). These a -> tau tau projections yield a competitive probe of light pseudoscalars, which depending on the model can become significantly more sensitive than projections from existing experimental searches in a -> μμand a -> gamma gamma final states. Finally, we explore the potential of our search to probe an Axion-Like-Particle (ALP) solution to the muon (g – 2) anomaly (when taken face-value), finding that our proposed h -> Za, a -> tau tau search can provide valuable constraints on such ALP scenario, in complementarity with existing h -> Za, a -> gamma gamma experimental searches.
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Fernandez-Martinez, E., Lopez-Pavon, J., No, J. M., Ota, T., & Rosauro-Alcaraz, S. (2023). nu Electroweak baryogenesis: the scalar singlet strikes back. Eur. Phys. J. C, 83(8), 715–23pp.
Abstract: We perform a comprehensive scan of the parameter space of a general singlet scalar extension of the Standard Model to identify the regions which can lead to a strong first-order phase transition, as required by the electroweak baryogenesis mechanism. We find that taking into account bubble nucleation is a fundamental constraint on the parameter space and present a conservative and fast estimate for it so as to enable efficient parameter space scanning. The allowed regions turn out to be already significantly probed by constraints on the scalar mixing from Higgs signal strength measurements. We also consider the addition of new neutrino singlet fields with Yukawa couplings to both scalars and forming heavy (pseudo)-Dirac pairs, as in the linear or inverse Seesaw mechanisms for neutrino mass generation. We find that their inclusion does not alter the allowed parameter space from early universe phenomenology in a significant way. Conversely, there are allowed regions of the parameter space where the presence of the neutrino singlets would remarkably modify the collider phenomenology, yielding interesting new signatures in Higgs and singlet scalar decays.
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Casas, J. A., Moreno, J. M., Rius, N., Ruiz de Austri, R., & Zaldivar, B. (2011). Fair scans of the seesaw. Consequences for predictions on LFV processes. J. High Energy Phys., 03(3), 034–22pp.
Abstract: We give a straightforward procedure to scan the seesaw parameter-space, using the common “R-parametrization”, in a complete way. This includes a very simple rule to incorporate the perturbativity requirement as a condition for the entries of the R-matrix. As a relevant application, we show that the somewhat propagated belief that BR(mu -> e, gamma) in supersymmetric seesaw models depends strongly on the value of theta(13) is an “optical effect” produced by incomplete scans, and does not hold after a careful analytical and numerical study. When the complete scan is done, BR(mu -> e, gamma) gets very insensitive to theta(13). This holds even if the right-handed neutrino masses are kept constant or under control (as is required for succesful leptogenesis). In most cases the values of BR(mu -> e, gamma) are larger than the experimental upper bound. Including (unflavoured) leptogenesis does not introduce any further dependence on theta(13), although decreases the typical value of BR(mu -> e, gamma).
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Casas, J. A., Gomez Vargas, G. A., Moreno, J. M., Quilis, J., & Ruiz de Austri, R. (2018). Extended Higgs-portal dark matter and the Fermi-LAT Galactic Center Excess. J. Cosmol. Astropart. Phys., 06(6), 031–16pp.
Abstract: In the present work, we show that the Galactic Center Excess (GCE) emission, as recently updated by the Fermi-LAT Collaboration, could be explained by a mixture of Fermi bubbles-like emission plus dark matter (DM) annihilation, in the context of a scalar-singlet Higgs portal scenario (SHP). In fact, the standard SHP, where the DM particle, S, only has renormalizable interactions with the Higgs, is non-operational due to strong constraints, especially from DM direct detection limits. Thus we consider the most economical extension, called ESHP (for extended SHP), which consists solely in the addition of a second (more massive) scalar singlet in the dark sector. The second scalar can be integrated-out, leaving a standard SHP plus a dimension-6 operator. Mainly, this model has only two relevant parameters (the DM mass and the coupling of the dim-6 operator). DM annihilation occurs mainly into two Higgs bosons, SS -> hh. We demonstrate that, despite its economy, the ESHP model provides an excellent fit to the GCE (with p-value similar to 0.6-0.7) for very reasonable values of the parameters, in particular, ms similar or equal to 130 GeV. This agreement of the DM candidate to the GCE properties does not clash with other observables and keep the S – particle relic density at the accepted value for the DM content in the universe.
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