Chiral heavy fermions in a two Higgs doublet model: 750 GeV resonance or not

TL;DR

This paper explores a two Higgs doublet model with a heavy chiral fourth generation, analyzing its compatibility with experimental data and its potential to explain the 750 GeV diphoton excess, predicting distinctive collider signatures.

Contribution

It introduces a specific 4G2HDM framework that can explain the 750 GeV resonance and predicts unique high jet-multiplicity signals and flavor-changing decays at the LHC.

Findings

The 750 GeV diphoton excess can be explained by the heavy CP-even scalar in the model.

The model predicts observable q'q' resonance signals with high jet multiplicity.

It accommodates the h→τμ decay hint within its parameter space.

Abstract

We revisit models where a heavy chiral 4th generation doublet of fermions is embedded in a class of two Higgs doublets models (2HDM) with a discrete $Z_2$ symmetry, which couples the "heavy" scalar doublet only to the 4th generation fermions and the "light" one to the Standard Model (SM) fermions - the so-called 4G2HDM introduced by us several years ago. We study the constraints imposed on the 4G2HDM from direct searches of heavy fermions, from precision electroweak data (PEWD) and from the measured production and decay signals of the 125 GeV scalar, which in the 4G2HDM corresponds to the lightest CP-even scalar h. We then show that the recently reported excess in the $\gamma\gamma$ spectrum around 750 GeV can be accommodated by the heavy CP-even scalar of the 4G2HDM, H, resulting in a unique choice of parameter space: negligible mixing (sin\alpha ~ O(0.001)) between the two CP-even scalars h,H and heavy 4th generation quark and lepton masses m_t',m_b' < 400 GeV and $m_{\nu'},m_{\tau'}$ > 900 GeV, respectively. Whether or not the 750 GeV \gamma \gamma resonance is confirmed, interesting phenomenology emerges in q' - Higgs systems (q'=t',b'), that can be searched for at the LHC. For example, the heavy scalar states of the model, S=H,A,H^+, may have BR(S -> q'q') ~ O(1), giving rise to observable q'q' signals on resonance, followed by the flavor changing q' decays t'->uh (u=u,c) and/or b'->dh (d=d,s,b). This leads to distinct high jet-multiplicity signatures, with or without charged leptons, of the form q'q' -> (nj + mb + lW)_S (j and b being light and b-quark jets, respectively), with n+m+l =6-8 and unique kinematic features. It is also shown that the 4G2HDM can easily accommodate the interesting recent indications of a percent-level branching ratio in the lepton-flavor-violating (LFV) decay $h \to \tau \mu$ of the 125 GeV Higgs, if confirmed.