Is the LHC Observing the Pseudo-scalar State of a Two-Higgs Doublet Model ?

TL;DR

This paper proposes that a two-Higgs doublet model with a light pseudo-scalar could explain the 125 GeV diphoton signal observed at the LHC, predicting specific future signals and aligning with models involving fermion condensation.

Contribution

It introduces a scenario where the lightest CP-odd scalar explains the diphoton signal, with heavier scalars decaying into it, and connects this to fermion condensation models of electroweak symmetry breaking.

Findings

The pseudo-scalar can account for the diphoton excess.

Future data should see increased diphoton signals and decreased diboson signals.

Heavier scalars decay into the light pseudo-scalar, avoiding current bounds.

Abstract

The ATLAS and CMS collaborations have recently shown data suggesting the presence of a Higgs boson in the vicinity of 125 GeV. We show that a two-Higgs doublet model spectrum, with the pseudo-scalar state being the lightest, could be responsible for the diphoton signal events. In this model, the other scalars are considerably heavier and are not excluded by the current LHC data. If this assumption is correct, future LHC data should show a strengthening of the $\gamma\gamma$ signal, while the signals in the $ZZ^{(*)}\to 4\ell $ and $WW^{(*)}\to 2\ell 2\nu$ channels should diminish and eventually disappear, due to the absence of diboson tree-level couplings of the CP-odd state. The heavier CP-even neutral scalars can now decay into channels involving the CP-odd light scalar which, together with their larger masses, allow them to avoid the existing bounds on Higgs searches. We suggest additional signals to confirm this scenario at the LHC, in the decay channels of the heavier scalars into $AA$ and $AZ$. Finally, this inverted two-Higgs doublet spectrum is characteristic in models where fermion condensation leads to electroweak symmetry breaking. We show that in these theories it is possible to obtain the observed diphoton signal at or somewhat above of the prediction for the standard model Higgs for the typical values of the parameters predicted.