Les Houches

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Responsible: Jose Miguel No

Participants/[the dream team]: Sophie W, Benj, Jérémie Quevillon, José Zurita, ... 


The starting point is the review/compendium work on exotic Higgs decays from Curtin, Gori, Shelton, Strassler... [[https://arxiv.org/abs/1312.4992|1312.4992]], which delineates a programme to cover as many exotic decays of the 125 GeV Higgs (and provide motivation!) as possible. While the list of possible decays in the review is quite exhaustive, several of these decays have not been studied in detail. Two such cases are h -> Z a (with a pseudoscalar a that decays visibly, eg. into gammagamma) and semi-visible decays like h-> gamma + E_T^miss, h-> gamma gamma + E_T^miss, h-> Z + E_T^miss (e.g. h -> Z a with a -> E_T^miss).    

**Visible decays:** Search for axion-like particles (ALPs) via h -> Z a (a -> gamma gamma). Preliminary study from Bauer, Neubert, Thamm [[https://arxiv.org/abs/1708.00443|1708.00443]]. They put limits on very light ALPs (m_a < 47 MeV) from constraints on h -> Z gamma, since for m_a < 47 MeV the two photons from the ALP decay are very collimated and seen as one in the detector. For not-so-light ALPs, they compute just number of signal events. 

Goal could be to provide a better estimate of LHC sensitivity to h -> Z a, a -> gamma gamma resolved (including some sort of background study?).

**Semi-visible decays:** 


__
h-> gamma + E_T^miss__

Initial pheno study by Riccardo et al [[https://arxiv.org/abs/1203.4563|1203.4563]], but seems not to work for current LHC triggers? (check!). Check coverage by ATLAS/CMS searches for gamma + E_T^miss + X. 


__h-> Z + E_T^miss__

Can be e.g. motivated by h -> Z a (with a -> DM DM, pseudo-scalar portal to DM), by h -> Z Z' (again, with a dark Z'), by h -> chi1 chi2 (with fermion/scalar chi1 invisible). Challenging since if the Higgs is not produced in association with another object, this is just Z + small E_T^miss (~ 20 GeV). SM Higgs decay h -> Z Z* -> Z vv constitutes possible sensitivity floor, corresponding to a BR(h->Z+E_T^miss) = 0.4 %.

Possible to probe the decay via p p -> Z h -> 4l + E_T^miss? Quite small rate ~ 4 fb times BR(h->Z+E_T^miss). SM background ZZ -> 4l cross section is ~ 35 fb, so maybe possible. Alternative option to probe same physics in mono-Higgs: p p -> Z* -> h + E_T^miss.
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