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Since the discovery in 2012 of the Higgs boson at the LHC, as the lastmissing piece of the Standard Model of particle physics, any hint of newphysics has been intensively searched for, with no confirmation to date. Thereare however slight deviations from the SM that are worth investigating. The CMScollaboration has reported, in a search for heavy resonances decaying in $t\bar t$ with a 13-TeV center-of-mass energy and a luminosity of 35.9 fb$^{-1}$,deviations from the SM predictions at the 3.5$\sigma$ level locally(1.9$\sigma$ after the look-elsewhere effect). In addition, in the ditau finalstate search performed by the ATLAS collaboration at $\sqrt{s}=13$ TeV and$\mathcal{L}=139$ fb$^{-1}$, deviations from the SM at the 2$\sigma$ level havebeen also observed. Interestingly, both slight excesses are compatible with anew pseudoscalar boson with a mass around 400 GeV that couples at least tofermions of the third generation and gluons. Starting from a purelyphenomenological perspective, we inspect the possibility that a 400-GeVpseudoscalar can account for these deviations and at the same time satisfy theconstraints on the rest of the channels that it gives contributions to and thatare analyzed by the ATLAS and CMS experiments. After obtaining the range ofeffective couplings compatible with all experimental measurements, we study thegauge invariant UV completions that can give rise to this type of pseudoscalarresonance, which can be accommodated in an $SO(6)/SO(5)$ model with consistencyat the 1$\sigma$ level and in a $SO(5)\times U(1)_{P}\timesU(1)_{X}/SO(4)\times U(1)_X$ at the 2$\sigma$ level, while exceedingly largequartic couplings would be necessary to account for it in a general two Higgsdoublet model.

Interpretation of LHC excesses in ditop and ditau channels as a 400-GeV pseudoscalar resonance