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

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.Comment: 13 pages, late

Collider signals of a composite Higgs in the Standard Model with four generations

Recent fits of electroweak precision data to the Standard Model (SM) with a 4th sequential family (SM4) point to a possible "three-prong composite solution": (1) the Higgs mass is at the TeV-scale, (2) the masses of the 4th family quarks t',b' are of O(500) GeV and (3) the mixing angle between the 4th and 3rd generation quarks is of the order of the Cabibbo angle, \theta_{34} ~ O(0.1). Such a manifestation of the SM4 is of particular interest as it may suggest that the Higgs is a composite state, predominantly of the 4th generation heavy quarks. Motivated by the above, we show that the three-prong composite solution to the SM4 can have interesting new implications for Higgs phenomenology. For example, the Higgs can decay to a single heavy 4th generation quark via the 3-body decays (through an off-shell t' or b') H -> t'(bar) t'* -> t'(bar) b W+ and H -> b'(bar) b'* -> b'(bar) t W-. These flavor diagonal decays can be dramatically enhanced at the LHC (by several orders of magnitudes) due to the large width effects of the resonating heavy Higgs in the processes gg -> H -> t'(bar) t'* -> t'(bar) b W+ and gg -> H -> b'(bar) b'* -> b'(bar) t W-, thus yielding a viable signal above the corresponding continuum QCD production rates. In addition, the Higgs can decay to a single t' and b' in the loop-generated flavor changing (FC) channels H -> b' b(bar), t' t(bar). These FC decays are essentially "GIM-free" and can, therefore, have branching ratios as large as 10^{-4} - 10^{-3}.Comment: As published in Phys.Lett.B688:195-201,201