Abstract
Since the discovery in 2012 of the Higgs boson at the LHC, as the last missing piece of the Standard Model of particle physics, any hint of new physics has been intensively searched for, with no confirmation to date. There are however slight deviations from the SM that are worth investigating. The CMS collaboration has reported, in a search for heavy resonances decaying in t overline{t} with a 13-TeV center-of-mass energy and a luminosity of 35.9 fb−1, deviations from the SM predictions at the 3.5σ level locally (1.9σ after the look-elsewhere effect). In addition, in the ditau final state search performed by the ATLAS collaboration at sqrt{s} = 13 TeV and mathcal{L} = 139 fb−1, deviations from the SM at the 2σ level have been also observed. Interestingly, both slight excesses are compatible with a new pseudoscalar boson with a mass around 400 GeV that couples at least to fermions of the third generation and gluons. Starting from a purely phenomenological perspective, we inspect the possibility that a 400-GeV pseudoscalar can account for these deviations and at the same time satisfy the constraints on the rest of the channels that it gives contributions to and that are analyzed by the ATLAS and CMS experiments. After obtaining the range of effective couplings compatible with all experimental measurements, we study the gauge invariant UV completions that can give rise to this type of pseudoscalar resonance, which can be accommodated in an SO(6)/SO(5) model with consistency at the 1σ level and in a SO(5) × U(1)P × U(1)X/SO(4) × U(1)X at the 2σ level, while exceedingly large quartic couplings would be necessary to account for it in a general two Higgs doublet model.
Highlights
Results from CMS in the ttfinal state as well as the results from ATLAS in the τ +τ − channel, favor a pseudoscalar neutral particle that is produced via gluon fusion or in association with bottom-quarks
Scanning over the parameter space of the most general CP-invariant interaction Lagrangian linear in the pseudoscalar state a that can be written up to dimension-5 operators, we have found regions in which the new physics hints, as well as all experimental constraints, can be satisfied and in which the pseudoscalar coupling to gluons can be induced by the SM top quark, that gives by far the dominant contribution, but there can be room for contributions from heavy new states
Though our low-energy phenomenological effective model satisfies all current experimental bounds and provides an explanation for the hints in ditop and ditau final states for the regions of parameter space considered, one may wonder in which of these and/or other channels one could expect to find a signal for the presence of the pseudoscalar in future measurements
Summary
In this work we consider initially from a pure phenomenological bottom-up approach the possibility that a pseudoscalar with a mass of 400 GeV is responsible simultaneously for these observed hints and at the same time satisfies the constraints coming from other final states that it could contribute to and that are search for at the LHC. An idea in this direction was considered in [15].
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