1. Inefficiencies in the feet region 40 GeV muons selection efficiency   Barrel – End Cap transition 10th International Conference on Advanced Technology and Particle Physics (ICATPP) 8-12 October 2007 Villa Olmo, Como, Italy = Trigger Element = Feature Extraction Algorithm = Feature = Hypothesis Algorithm A trigger chain example High p T muons are important for many known processes, that can be used for monitoring and calibration (Z   ) and for several new phenomena predicted at the LHC energy (Higgs, SUSY), therefore the muon trigger system is of primary importance. The ATLAS trigger system is designed to keep high efficiency for interesting events, while rejecting standard model physics low p T events, with a suppression factor of the order of 10 7, reaching the ~200 Hz data storage capability of the DAQ system. The Muon Vertical Slice consists of three main trigger steps, one hardware, level 1 (LVL1) and two software, level 2 (LVL2) and event filter (EF). Last two compose the High Level Trigger (HLT). Decision on each event is based on reduced-granularity detector data for interesting region at LVL1, full-granularity and precision, but only for same LVL1 regions, at LVL2, and full event data, as in offline, at EF. Implementation and performance of the ATLAS Trigger Muon “Vertical Slice” Toroid LVL1 trigger uses RPC in the barrel (|  |<1) and TGC in the endcaps (1<|  |<2.4) MUONROI muFast MU6MOORE MuIdSA MuIdCB MU6’ MU6’’MU6’’’ MU6’’’’ Vtx close to IP  production point p T, ,  Minimum p T  Comb  Fast B Physics processor  Tile TrigDiMuon  Isol  Comb ~2 kHz 10 ms ~ 200 Hz 2 s LVL1 Selection LVL2 Selection EF Selection RPC TGC  CTPI CTP 2.5  s barrel endcap  30% of all inefficiencies are due to feet and elevator sector  No RPC hit Low-p T plane missing High-p T plane missing Level 2 Trigger Level 1 Trigger Core algorithm is muFast, that confirm/reject LVL1 result and refine muon p T evaluation, using MDT precision measurements. muFast resolutions in the barrel and in the endcap for low p T and high p T muons To refine the muFast p T, muComb uses Inner Detector (ID) data, allowing to sharpen the threshold at low p T. Following steps are to be achieved within the 10 msec latency time: “Global Pattern Recognition” involving trigger chambers and positions of MDT tubes (no use of drift time); “Global Pattern Recognition” involving trigger chambers and positions of MDT tubes (no use of drift time); “Track fit” involving drift time measurements, performed for each MDT chamber; “Track fit” involving drift time measurements, performed for each MDT chamber; Fast “p T estimate” via a Look-Up-Table with no use of time consuming fit methods. Fast “p T estimate” via a Look-Up-Table with no use of time consuming fit methods. Event Filter MOORE and MuId are two object oriented offline packages written in C++, already integrated in the ATLAS software framework: ATHENA. Imported in the trigger environment, they become TrigMOORE. The Muon Event Filter consists of three algorithms: MOORE, MuId StandAlone and MuId Combined. The role of MuId StandAlone is to extrapolate MS tracks to the production point using calorimetric information. Muon momenta are measured with a resolution  p T /p T < 10% up to 1 TeV. TrigMOORE can work in two different modes, wrapped, scanning full detector data, and seeded, starting from RoI seeded by LVL2 (full trigger chain) or alternatively by LVL1 (for trigger studies). In-flight decays of pions and kaons are the main source of LVL1 trigger rate at low p T. One goal of the muon HLT is to reject such fake muons while having high selection efficiency on prompt muons. Trigger software works with objects called Trigger Elements (TE). Feature Extraction Algorithms (FEX) are activated by input TE produced by previous trigger levels. FEX are able to access the detector data and compute physical quantities, Features, that are then attached to the output TE. Selection is done in Hypothesis Algorithms, that can validate or reject TE that do not satisfy trigger requirements. Sergio Grancagnolo (University of Salento & INFN Lecce - Italy) on behalf of the ATLAS Muon HLT group Tracks are reconstructed also in the ID by a dedicated software (newTracking). MuId Combined uses these informations, thus enhancing momentum resolution for p T up to 50 GeV and allowing good discrimination of muons in jets. ID improves resolution at low p T, MS at high p T. LVL1 selects active detector regions (Region of Interest, RoI), in each event. Coincidence windows are defined on the allowed geometrical roads with their center corresponding to the infinite momentum track. At p T =6 GeV threshold, barrel efficiency is 83%, rate ~11 kHz (lumi=10 33 cm -2 s -1 ), at p T =20 GeV efficienc is 79% and the rate is ~2 kHz (lumi=10 34 cm -2 s -1 ) The Muon Spectrometer (MS) is the detector dedicated to the identification of muons. It consists of RPC and TGC trigger chambers and MDT and CSC precision chambers. Full bandwidth Total latency time TrigMoore MOORE MuId SA MuId COMB MS CALO ID 3-station coincidence trigger efficiency of TGC Endcap 6 GeV 20 GeV 75 kHz Low p T rates: muons from pions and kaons Differential cross-section for production of muons from pions and kaons production of muons from pions and kaons One of the main sources of muons at low p T are in flight decays of light mesons. A track from such decays appears with a kink, and the  2 of the fit is worse than prompt muon tracks. All possible kinematic parameters and statistical techniques must be used in order to reject such tracks. Different kinds of physic events need to share available bandwidth, that is limited. Flexible trigger menus allow to avoid saturation from few processes, and guarantee the possibility to organize the analysis depending from luminosity conditions. When the rates are too high, prescale factors can be applied to low p T thresholds at LVL1, and leave the HLT in pass through mode. The HLT capabilities are studied flagging the events without rejection. Express streams, not prescaled, at fixed thresholds, are used for calibration and monitoring purposes. Trigger Menus A typical Muon Trigger Menu for 10 31 luminosity (LHC startup). (*) mu_4 is a totally open threshold used for cosmic data limited by cabling only. (*) 1/p T resolution for EF algorithms. Single muons |  |<2.5 MuidStandAlone TrigMoore MuidCombined |  |<1.05 MuId COMB efficiency wrt LVL2 p T = 20 GeV threshold p T = 6 GeV threshold Barrel efficiencies vs p T for MuId combined 6 GeV 8 GeV 20 GeV40 GeV 6 GeV 10 33 cm -2 s -1 Barrel (Hz) Endcap (Hz) beauty580720 charm290360 top0.050.07 W2.94.0  /K 16401180 TOTAL25302260 8 GeV 10 33 cm -2 s -1 Barrel (Hz) Endcap (Hz) beauty200290 charm90130 top0.040.06 W2.84.0  /K 350310 TOTAL640740 20 GeV 10 34 cm -2 s -1 Barrel (Hz) Endcap (Hz) beauty68110 charm2644 top0.210.32 W2233  /K 4645 TOTAL160230 40 GeV 10 34 cm -2 s -1 Barrel (Hz) Endcap (Hz) beauty2.74.4 charm0.941.6 top0.050.08 W4.16.9  /K 0.160.2 TOTAL7.913.1 EF trigger rates estimated with single muon sample. Muons are present in many standard model processess. Resolution and efficiencies are evaluated on single muon samples generated at different p T values. Rates are then estimated using the theoretical contribution from different sources, heavy and light mesons, top, W, etc. End Cap Barrel Efficiency from Z   sample Tag combined triggered muon ID+MS p T > 20 GeV |η| < 2.4 Probe ID track p T > TPcut |Mμμ - MZ| < 10 GeV Δφ > 0.3 Method to evaluate performance directly from data, using one  reconstructed in both ID and MS as a tag, and the other requiring only the ID as a probe. This method can be applied to extract MS trigger efficiency Tag Probe TPcut = 20 GeV trigger measurement over threshold TPcut = 1 GeV for integrated trigger efficiency Allows threshold trigger efficiency measurements with good precision wrt to MC generator.