Collision induced background in the DT chambers

The background observed in the DT chambers has mainly the following two origins.

1. The so called "neutron gas": it is produced by colliding beams and fills the empty space between the detector and the cavern's ceiling. It is mainly responsible for background in the external stations (MB4) of the upper sectors.
2. The high particle multiplicity related to Pile Up in the forward regions: hadron punch through and shower leaks are the main cause of background in the internal stations (MB1) of external wheels.

An approach to background evaluation is measuring the rate of hits that do not belong to any in-time track segments. In this study, background hits were searched for requiring less than three hits recorded in the same superlayer within the total time window read out from the TDC's (1250 ns). For superlayers matching this condition the background rate was then determined, wire by wire, counting hits recorded outside the signal region, namely in the time window [0,250] ns.

Background rate, wire by wire, in the MB4 chambers of wheel +2. The rate decreases from top to bottom as it is expected for background produced by neutron gas.

Background rate, wire by wire, in the MB4 chambers of wheel -2. The rate decreases from top to bottom as it is expected for background produced by neutron gas. Overall the rate is lower than in wheel +2 due to differences in the cavern's geometry (e.g. the main shaft is located on the YB-2 side).

Background rate, wire by wire, in the MB1 chambers of the 5 wheels. The background in this station is symmetrical with respect to azimuth, therefore in this plot the 12 sectors have been summed up. The rate is higher in the external wheels, as it is expected for background produced by high multiplicity Pile Up events.

Background rate computed fill by fill and wheel by wheel as a function of beam luminosity, for the chambers of station MB4, sector 4.

The chambers of station MB4, sector 4 are those where the background is largest.

The dependency of background on beam luminosity is approximately linear and allows for extrapolations to future higher values of luminosity. From these figures we expect the maximum background rate not to exceed 5 Hz/cm**2 at L = 10**34 cm**-2/s**-1 and sqrt(S) = 8 TeV (no attempt has been done in this study to to take future energy upgrade into account).