Trigger and background study for the LHCf experiment

We studied the effects of background in the LHCf experiment by MC simulation. An important source of background is given by particles colliding with the inner wall of the beam pipe after they are generated in a proton-proton collision. Because the energies of these particles are less than 100 GeV, they can be reduced at the trigger level. The MC simulations have shown that a signal-tonoise ratio of 1.2% can be achieved. These number can be reduced further in the analysis. Another source of the background is the particles generated in collisions between the beam and the residual gas in the beam pipe. Because the particles are emitted only in the direction of the beam, these backgrounds could be reduced with the coincidence of detectors at both sides of the interaction point. As the detection efficiency of a single calorimeter is only ∼ 15%, the coincidence condition reduces the efficiency down to ∼ 3%. It is the 20% of events by single Arm calorimeter. To solve this problem, we developed additional detectors that have wide aperture and high detection efficiency, called Front Counter. By using the Front Counter signal in the coincidence condition, we can keep ∼ 60% of the single side events but reduce the beam-gas background at the negligible level.