Upgrade of the ATLAS level-1 muon trigger for the high luminosity LHC

The luminosity upgrade for the LHC (phase-1 upgrade) in 2018 will be performed to enhance the physics potential. The transverse momentum (p_T) threshold for muon trigger needs to be kept below 20 GeV/c even after the upgrade in order to maximally pursue the physics program, in particular a measurement of the Higgs boson properties and a search for the super-symmetry particles. Immediate concern regarding the upgrade is a trigger rate of the level-1 trigger. The rate of the L1_-MU20 which is a single muon trigger with p_T threshold at 20 GeV/c will well exceed the upper limit of 25 kHz if we leave the current trigger system without any measures. Therefore a refurbishment of the trigger system, particularly the level-1 trigger is mandatory to reduce the trigger rate below the allowable upper limit. By examining the real events triggered with the L1_-MU20, we found that about 60% of them were fake triggers which did not contain any muons, and also found that satisfiable events which actually had muons of p_T > 20 GeV/c were about 4%. Then we found out, with a help of Monte Carlo simulation, the fake triggers were originated by the protons which were generated around the endcap toroidal magnets and shields for them. Therefore the trigger rate reduction must be achieved by eliminating these fakes and low p_T muon events without losing any targeted muon event with p_T > 20 GeV/c. Current level-1 muon trigger only utilize the trigger chambers in the middle station (Big Wheel) located after the magnetic field. By adding the information from the inner station (Small Wheel) in front of the magnetic field, the fake triggers are completely rejected and a large fraction of the low p_T muon events is also eliminated. During the phase-1 upgrade the New Small Wheel (replacement to the current Small Wheel) is planed to be installed not only for the triggers but also for precision muon measurements to coop with the luminosity increase. Using the information from the New Small Wheel we introduce a set of new cut parameters with which the events are ensured to have a particle originated at the interaction point. Thereby the fake triggers are effectively removed and also the p_T discrimination capability is significantly improved. In order to optimize the performance of the New Small Wheel, requirements on position and angular resolution particularly from the point of view of the hardware oriented level-1 trigger system were investigated. Then we found that the resolution of θ angle should be less than 8 [mrad] and that of R less than 0.004 in η and that of φ less than 22 [mrad]. The more rate reduction will be required at the further luminosity upgrade in 2022 (phase-2 upgrade). Introducing the same method as already used in the level-2 trigger for the calculation of p_T into the level-1 was considered. In order to accord with the method, the angular resolution of θ for the detectors in both the New Small Wheel and Big Wheel should be as good as a few milliradian. (author)