Detection of the heavy Higgs boson at $γγ$ colliders

Abstract

We consider the possibility of detecting a heavy Higgs boson ($m_H>2m_Z$) in proposed $γγ$ colliders through the semi-leptonic mode $γγ\rightarrow H \rightarrow ZZ \rightarrow q\bar q \ell^+\ell^-$. We show that due to the non-monochromatic nature of the photon beams produced by the laser-backscattering method, the resultant cross section for Higgs production is much smaller than the on-resonance cross section and generally {\it decreases} with increasing collider energy. Although continuum $ZZ$ production is expected to be negligible, we demonstrate the presence of and calculate sizeable backgrounds from $γγ\rightarrow \ell^+\ell^-Z,\,q\bar qZ$, with $Z\rightarrow q\bar q,\,\ell^+\ell^-$, respectively, and $γγ\rightarrow t\bar t\rightarrow b\bar b\ell^+\ell^-ν\barν$. This channel may be used to detect a Higgs of mass $m_H$ up to around 350~GeV at a 0.5~TeV $e^+e^-$ collider, assuming a nominal yearly luminosity of 10--20~fb$^{-1}$.