PDF《CERN-PH-EP-2011-228》

7 TeV. Applying beam, detector and data quality requirements resulted in a total integrated luminosity of 1.94 fb?1 . The integrated luminosity has a relative uncertainty of 3.7% [15, 16]. Signal Monte Carlo (MC) samples were generated us- ing PYTHIA [17, 18] to simulate gluon fusion produc- tion (gg → h0 ) and decay of the Higgs (h0 → πvπv). Four samples were generated: mh0 =

120 and

140 GeV and for each mh0 two πv masses of

20 and

40 GeV. The predicted Higgs production cross sections [19] are: σ(mh0 =120 GeV) = 16.6+3.3 ?2.5 pb and σ(mh0 =140 GeV) = 12.1+2.3 ?1.8 pb, and the branching ra- tio for h0 → πvπv is assumed to be 100%. The response of the ATLAS detector was modeled with GEANT4 [20, 21]. The e?ect of multiple pp collisions occurring during the same bunch crossing (pileup) was simulated by superim- posing several minimum bias events on the signal event. The MC events were weighted so that the pileup in the simulation agrees with pileup conditions found in data. ATLAS is a multipurpose detector [22] consisting of an inner tracking detector (ID) surrounded by a super- conducting solenoid that provides a

2 T ?eld, electromag- netic and hadronic calorimeters and a MS with a toroidal magnetic ?eld. The ID, consisting of silicon pixel and strip detectors and a straw tube tracker, provides pre- cision tracking of charged particles for | η | ≤ 2.5. The calorimeter system covers | η | ≤ 4.9 and has 9.7 inter- action lengths at η = 0. The MS consists of a barrel and two forward spectrometers, each with

16 φ sectors instrumented with detectors for ?rst level triggering and precision tracking detectors for muon momentum mea- surement. Each spectrometer has three stations along the muon ?ight path: inner, middle, and outer. In the barrel, the stations are located at radii of ?4.5 m,

7 m and

10 m, while in the forward MS, they are located at |z| ? 7.5 m,

2 14 m and

20 m. This analysis uses muon tracking for | η | ≤ 2.4, where each station is instrumented with two multilayers of precision tracking chambers, Monitored Drift Tubes (MDTs). It also utilizes Level

1 [23] (L1) muon triggering in the barrel MS (| η | ≤ 1). The trigger chambers are located in the middle and outer stations. The L1 muon trigger requires hits in the middle station to create a low pT muon Region of Interest (RoI) or hits in both the middle and outer stations for a high pT RoI. The muon RoIs have a spacial extent of 0.2*0.2 in ?η * ?φ and are limited to two RoIs per sector. A dedicated, signature-driven trigger, the muon RoI cluster trigger [13], was developed to trigger on events with a πv decaying in the MS. It selects events with a cluster of three or more muon RoIs in a ?R = 0.4 cone in the MS barrel trigger chambers. This trigger con- ?guration implies that one πv must decay in the bar- rel spectrometer, while the second πv may decay either in the barrel or the forward spectrometer. With this trigger, it is possible to trigger on πv decays at the outer radius of the hadronic calorimeter and in the MS with high e?ciency. The backgrounds of punch-through jets [24] and muon bremsstrahlung are suppressed by re- quiring no calorimeter jets with ET ≥

30 GeV in a cone of ?R = 0.7 and no ID tracks with pT ≥

5 GeV within a region of ?η * ?φ = 0.2*0.2 around the RoI cluster center. These isolation criteria result in a negligible loss in the simulated signal while signi?cantly reducing the backgrounds. As depicted in Fig. 1(a)[25], MC studies show the RoI cluster trigger is ?30 ? 50% e?cient in the region from