Higgs-boson signals in superstring-inspired models at hadron supercolliders

In superstring-inspired ${\mathrm{E}}_6$ supergravity models, the mass of the lightest Higgs scalar ($H_L$) is expected to be below ∼170 GeV if ${\mathrm{E}}_6$ is broken directly to a rank-5 subgroup and below ∼210 GeV if intermediate scales of ∼${10}^{10}$ GeV are allowed. We show that if the mass of the additional neutral gauge boson $Z^{0\mathcal{’}}$ is less than about 600 GeV, then ∼${10}^4$–${10}^5$ $Z^{0\mathcal{’}}$→$Z^0$ $H_L$ pairs may be expected per ${10}^4$ ${\mathrm{pb}}^{\mathrm{-}1}$, the annual integrated luminosity anticipated at the proposed 40-TeV pp supercollider. This large rate allows us to use the clean $Z^0$→ll¯ decays as a trigger for the Higgs-boson search. We find that even if $H_L$→tt¯ decays dominate, a signal-to-background ratio substantially exceeding unity is expected for reasonable experimental resolutions on the measurement of $Z^0$tt¯ and tt¯ invariant masses. We further show that for a wide range of parameters, $H_L$ can decay into chargino and neutralino pairs. These decays lead to hadron-free $Z^0$+missing $p_T$ (p${?}_T$) events which may stand out over the $Z^0$ $Z^0$ background and also to spectacular $Z^0$+n lepton+p${?}_T$ events (n≤4) which are almost free of standard-model backgrounds.