Three-jet final states and measuring the $γγ$ width of the Higgs at a photon linear collider

The identification of the intermediate-mass Higgs process $γγ\to H \to b \bar b$ will be one of the most important goals of a future photon linear collider. Potentially important backgrounds from the continuum $γγ\to c \bar c, b \bar b$ leading-order processes can be suppressed by a factor $m_q^2/s$ by using polarized photon beams in the $J_z=0$ initial-state configuration. We show that the same $m_q^2/s$ suppressions do not necessarily apply to the radiative processes $γγ\to c \bar c g, b \bar b g$. These processes can mimic the two-jet topology of the Higgs signal when two of the three partons are collinear, or when one of the partons is soft or directed down the beam pipe. We calculate the contribution of these processes to the two-jet background in the $J_z=0$ channel. We investigate the effect of imposing additional event shape, jet width and secondary vertex cuts on both signal and background, and show that with reasonable detector capabilities it should be possible to reduce the background to a manageable level.

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