Final estimate of a heavy Higgs signal

 All different decay channels of a heavy Higgs particle have been analysed and it can be concluded that the only realistic channels available for a Higgs mass of 1 TeV are the H $\rightarrow$ W ⁺W ^- $\rightarrow$ l$\nu$jj , H $\rightarrow$ ZZ $\rightarrow$ l ⁺l ^-$\nu$$\bar{\nu}$ and H $\rightarrow$ ZZ $\rightarrow$ l ⁺l ^-jj channels: the channels with more jets will drown in the QCD-jet background and the H $\rightarrow$ ZZ $\rightarrow$ l ⁺l ^-l ⁺l ^- channel has a too low rate.

A pessimistic estimate on the significance and the signal to background ratio after one year at high luminosity of the different channels is given in table 8.12. Following the discussion in section 8.8 the jW and jZ backgrounds were increased by 50% above the PYTHIA predicted cross section and the Higgs cross section modified to account for the cancellation between the different vector boson scattering diagrams (section 8.8.3). Only the H $\rightarrow$ W ⁺W ^- $\rightarrow$ l$\nu$jj will thus for sure be visible after one year at high luminosity while the visibility of the H $\rightarrow$ ZZ channels will depend on the level of the jZ background. In any case the two HZZ channels will be able to confirm a discovery in the H $\rightarrow$ W ⁺W ^- $\rightarrow$ l$\nu$jj channel.

 

Signal Background

$$\sqrt{B}$$ Decay # / 100 fb ^{- 1} # / 100 fb ^{- 1} S/B

$$\rightarrow \rightarrow \nu$$ 47 21 10.3 2.2

$$\rightarrow \rightarrow \nu \bar{\nu}$$ 7.0 < 4 3.5 1.8

$$\rightarrow \rightarrow$$ 8.3 8.9 2.8 0.9

 

In table 8.11 it can be seen that with a low mass Higgs particle (below a few hundred GeV) the scattering of longitudinal vector bosons is insignificant. Thus the discovery of a Higgs boson in the at the moment most favoured mass region around 115 GeV (see section 2.4), would make the measurement of longitudinal polarised vector boson scattering important as it will confirm that the Higgs particle stabilises the electroweak sector of the Standard Model. Assuming that the jW background can be properly calibrated, an upper limit of 25% of the expected cross section with a 1 TeV Higgs can be set with 95% confidence level after one year at high luminosity. This corresponds to a cross section for longitudinal polarised W pair production of 12 fb.

A measurement of the relative branching ratios of H $\rightarrow$ W ⁺W ^- and H $\rightarrow$ ZZ is interesting as it probes the SU(2) symmetry of the electroweak theory in the Higgs sector. The theory as seen in (2.60) and (2.60) predicts the H $\rightarrow$ W ⁺W ^- branching ratio to be exactly twice the H $\rightarrow$ ZZ branching ratio in the limit of a heavy Higgs particle. To measure this factor with reasonable accuracy will obviously take several years of data to get sufficient statistics in the H $\rightarrow$ ZZ channel.

In [82] some effort was devoted to the study of the mass peak from a 1 TeV Higgs particle. In the H $\rightarrow$ W ⁺W ^- $\rightarrow$ l$\nu$jj the mass can be reconstructed (with a two-fold ambiguity) from constraining the pair of the lepton and the missing transverse energy vector to the W mass. Without a simulation model that at the same time can treat the mass dependent width and the cancellations between the different VV $\rightarrow$ VV diagrams the use of the mass peak in the estimation of the significance of the Higgs signal seems speculative. For lower Higgs masses down to around 600 GeV where the H $\rightarrow$ W ⁺W ^- $\rightarrow$ l$\nu$jj channel would still be observable such a study makes more sense.