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Lepton-Flavor-Violating Decays of the Higgs Boson

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At present there is broad consensus in the high-energy physics (HEP) community that the 125-GeV scalar particle discovered at the LHC behaves as the Higgs boson of the standard model of particle physics (SM) should. One of the most difficult unsolved challenges in the field is to disentangle whether there ...

At present there is broad consensus in the high-energy physics (HEP) community that the 125-GeV scalar particle discovered at the LHC behaves as the Higgs boson of the standard model of particle physics (SM) should. One of the most difficult unsolved challenges in the field is to disentangle whether there exists new physics beyond the SM (BSM) that could be hidden in the Higgs sector. For example, the possibility of flavor violation through Higgs decays into two fermions of different flavor. This class of processes represents a unique window into new physics, owing to the strong suppression of flavor violation in the SM where flavor mixing is exclusively induced by Yukawa couplings of the corresponding fermionic sector. This is especially interesting in the leptonic sector, in which flavor mixing is hugely suppressed by extremely small Yukawa couplings. Therefore, the discovery of lepton-flavor-violating (LFV) processes would be a clear sign of new physics.

The field of Lepton-Flavor-Violating Higgs Decays (LFVHD) is actively being explored at the LHC. The first LFVHD direct searches were performed by the CMS and ATLAS collaborations, setting upper limits at the percent level. Subsequently, various analyses have been reported by the ATLAS, CMS, and LHCb experiments looking for LFV in the decays of Higgs boson and heavier resonances. The future prospects for LFVHD searches are encouraging due to the expected high statistics of Higgs events at future hadronic and leptonic colliders. For instance, the LHC will run at 14 TeV in the future, with a total integrated luminosity of first 300/fb and later 3000/fb, for which we expect the production of about 25 and 250 million Higgs events, respectively; compare this to the 1 million Higgs events that the LHC produced after its first run. These large numbers forecast an improvement in the long-term sensitivities to LFVHD of at least two orders of magnitude with respect to the present sensitivities. Similarly, at the planned lepton colliders, the expectations are for about 1 and 2 million Higgs events respectively, with much lower backgrounds due to the cleaner environment, which will also allow for a large improvement in LFVHD searches with respect to the current sensitivities. These experimental numbers must be compared with the theoretical predictions that have been long-studied within the framework of effective field theories (EFT) and a plethora of BSM models. Prominent examples include: models with a seesaw mechanism for neutrino mass generation, two-Higgs-doublet models (2HDM), and supersymmetry (SUSY). Additionally, search strategy proposals for LFVHD channels at the LHC are another area of intense current research.

In summary, the theoretical and phenomenological study of LFVHD has gained ground since the LHC tune-up in 2010. Since then, the research into these exotic processes has greatly developed, to the point of being an area of intrinsic interest and inherent weight today, giving rise to the proliferation of both phenomenological and experimental studies .


Keywords: Higgs physics, flavor physics, BSM phenomenology, collider physics, LHC


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