Fourth Generation Model - YR2 calculations.

• Higgs production and decay in the fourth generation model with SM-like couplings.
• M_D4=M_L4=600GeV and M_U4 - M_D4 = [ 1 + 1/5*ln(M_H/115) ] * 50 GeV ).
• Assumes the same NNLO QCD corrections as SM for ggF.
• QCD scale and PDF+α_s uncertainties are assumed to be the same as SM for ggF, i.e. +-15-20%.
• Old SM4 numbers with no EW radiative corrections.

NLO EW radiative corrections in the fourth generation model

• The NLO electroweak (EW) radiative corrections in the fourth generation model have been calculated recently for gg→H production (arXiv:1108.2025) and for Higgs decay in W→WW/ZZ→4f (Prophecy4f version 2.0).
• The NLO EW radiative corrections for Higgs production via heavy-quark loop in gg→H are positive for a light Higgs boson, being +12% at MH=120GeV with current (see figure) parameter setting. Corrections are relatively small around the ttbar-threshold, start to become negative at 450GeV, to become huge and negative towards the heavy-fermion threshold, but still remains -13% at MH=600GeV. If one assumes QCD/EW factorization this is, practically, the whole effect. Note here the breakdown of perturbation theory, i.e. 50% means that 3-loop corrections are not negligible.
• The NLO EW radiative corrections in Higgs boson decay-width for H→WW/ZZ→4f turned out to be very large and negative, -70% for MH<200GeV compared to the SM. Missing higher order corrections induce and uncertainty of 15%. Using HDECAY for the other decay channels (no NLO EW corrections for H→γγ included), the branching ratios to H→WW and H→ZZ compared to the SM, BR(SM4)/BR(SM), are suppressed to be 0.216 (0.570 without NLO EW) and 0.201 (0.582 without NLO EW) respectively, at MH=120GeV (see figure). The ratio BR(SM4)/BR(SM) regains to be nearly equal to 1 toward MH=200GeV and above. The large corrections to the partial decay widths largely cancel for MH>>2MW, because the BR's are completely dominated by H→WW and to a lesser extent by H→ZZ. Since H→WW/ZZ receive almost the same large corrections, the BR's are not modified much for large MH. Once MH>2Mtop it is expected that the BR for H→tt gets enhanced because of the large negative corrections to H→WW/ZZ. One should note that the corrections from SM4 NLO EW are huge for H→γγ with presence of heavy-quarks and W gauge boson in the loop. Therefore Higgs decay in H→bb, ττ are also expected to play the key role at low Higgs mass exclusion.
• (Comments by G. Passarino, S. Dittmaier, compiled by R.Tanaka, Nov., 2011.)

Figure 1: (left) NLO EW correction to the ggF Higgs production in SM4. (right) Ratio of BR in SM4/SM calculated with Prophecy4f and HDECAY .

ggF Higgs cross sections

The entries are:

• Higgs mass (in GeV)
• SM4 ggF cross section

Branching Ratios for H->ff

Branching Ratios for H → VV

• Important note: For BR(H → γγ) in big table below (Table 51-52 in YR2), NLO EW corrections are omitted.
• For BR with NLO EW radiative corrections, mumbers in small table below (Table 56 in YR2) should be used.

Branching Ratios for H->4f

-- ChiaraMariotti - 07-Nov-2011