1. ATLAS and the Trigger System The ATLAS (A Toroidal LHC ApparatuS) Experiment [1] is one of the four major experiments operating at the Large Hadron Collider (LHC). The LHC aims to create proton-proton collisions with a centre of mass energy of 14 TeV, at the design luminosity of 10 34 cm -2 s -1. With these parameters, the total proton-proton interaction rate is around 1 GHz. However, due to resource limitations, the event storage rate is limited to ~200 Hz, for events averaging 1.5 MB in size. The ATLAS trigger system, divided into three steps, is responsible for reducing this rate whilst keeping any event that may contain useful physics. Level-1 (L1) is a hardware based system, reducing the rate to around 75 kHz using relatively coarse detector information. In an event, if there is a high energy physics object which is worth studying and it successfully passes the L1 trigger selection, a Region of Interest (RoI) with the location of the object in η-φ is sent to the Higher Level Trigger (HLT). The HLT consists of the Level-2 trigger (L2) and the Event Filter (EF). Both of these use software based selection algorithms run on computer farms. By accessing the full granularity of the detector, L2 reduces the rate to 3.5 kHz so the EF can then reduce it to 200 Hz. Selection and classification of events takes a few seconds with all events passing the HLT written to permanent storage. Offline Monitoring with Electrons and Photons Studies have been done that look at trigger efficiencies to improve hardware performance using the calorimeter deposits (clusters) that come from electrons and photons reconstructed offline in events triggered independently of L1 electromagnetic trigger items (e.g. with Minimum Bias triggers) in order to determine the trigger performance. Trigger efficiencies are calculated for all of the electromagnetic thresholds. They determine how well the L1Calo hardware performs in producing RoIs for electron and photon candidates with different transverse energies. The aim is to get the efficiency turn-on curve for each threshold to reach 100% as quickly as possible and then ensure that the efficiency stays at 100% as the energy increases. Electrons and photons with ET > 3 GeV are capable of producing a RoI. Objects with lower energies than this are seen to trigger creating a ‘bump’ in the efficiencies. It was assumed this was an artefact of having reduced granularity trigger towers and that these objects were matching to RoIs from significantly higher energy candidates nearby. It has now been shown that candidates below 3 GeV can produce a RoI due to issues with calibration and noise in L1Calo. Other studies have involved comparing the energy and position resolution of L1Calo RoIs to see how closely they can match to electrons and photons using the reduced granularity information available as well as the effect of energy calibration on the ability to trigger. Cluster Processor Algorithm The Cluster Processor algorithm is based on a window of 4x4 trigger towers in both the electromagnetic and hadronic calorimeters. The algorithm identifies 2x2 clusters of EM towers that are local transverse energy (ET) maximums for all possible 4x4 windows and for which at least one of the four 1x2 or 2x1 component sums exceeds an energy threshold. A candidate object must satisfy the following criteria: The EM cluster (e/γ) or HAD cluster (τ/h) must have an ET greater than the electromagnetic or hadronic threshold under consideration. The ET in the EM and HAD isolation rings must be less than their respective thresholds. For EM clusters, the total ET in the hadronic inner core must be less than a threshold. 16 sets of thresholds can be implemented in the hardware (8 exclusively for EM thresholds). The multiplicities of the candidate object for each set of thresholds are transmitted to the CTP. If a RoI is produced, it will indicate which thresholds are passed. Missing Electromagnetic Trigger Events If sufficient energy is seen in the L1Calo PPr, a RoI is produced. If a RoI is not produced for a candidate, then the electromagnetic trigger tower (TT) near the object will see little or no energy and consequently the PPr will not see enough energy. The energy can be lost in several places prior to the PPr so finding the underlying causes for this is important. Candidates are studied by looking at the different parts of L1Calo leading up to the RoI production process as well as using the Atlantis Event Display to understand the entire event. Some of the reasons for an electron or photon not to produce a RoI include: The TT has been switched off for being too noisy or due to faulty electronics. Electromagnetic shower spread across many TTs so not enough ET seen in one. BCID indicates the bunch is out of time so the TT will not measure any ET. The transition region for the electromagnetic calorimeters (1.37 < |η| < 1.52) is currently a problem as the trigger towers across it are not all timed correctly yet. An independent problem comes from failed optical transmitters so analogue calorimeter information cannot be received. Using all of the information, an η-φ efficiency map is then made to view the ability of the TTs to produce RoIs and identify regions with poor performance. By analysing the most recent data, it is possible to keep track of problems so that L1Calo experts can be informed of them. Monitoring the ATLAS Level-1 Calorimeter Trigger with electrons and photons Hardeep Bansil - University of Birmingham Calorimeter DepositsL1 Trigger Towers ρZ Projection of ATLAS Inner Detector & Calorimeters Overview of Level-1 Calorimeter Trigger The ATLAS Detector ATLAS Trigger System Overview Cluster Processor Sliding Window Algorithm 6 GeV 4 2 0 ETET 4 2 0 ETET η-φ Efficiency Map for Electrons with E T > 6 GeV ATLAS Work in Progress Level-1 Energy Resolution for all Triggered Photons ATLAS Work in Progress Level-1 Efficiency Curves for Electrons and Photons passing any EM Thresholds ATLAS Work in Progress ATLAS Work in Progress References [1] The ATLAS Collaboration (2008), Expected Performance of the ATLAS Experiment Detector, Trigger and Physics, CERN-OPEN-2008-020. [2] ATLAS Level-1 Trigger Group (1998), ATLAS First-Level Trigger Technical Design Report, ATLAS TDR-12, CERN/LHCC/98-14. Level-1 Calorimeter Trigger The first stage of the Level-1 Calorimeter Trigger [2] (L1Calo) is the Preprocessor system (PPr). The PPr receives ~7200 analogue trigger towers, each with a reduced granularity of 0.1x0.1 (ΔηxΔφ). Here the analogue signals are digitised, then Bunch Crossing Identification (BCID) and noise suppression are performed. The PPr output is received in parallel by the Cluster Processor (CP) and Jet/Energy Processor (JEP) systems. The CP system identifies the electron/photon and tau/hadron candidates. The JEP system is for jet identification as well as determining missing and total energy in an event. The CP and JEP systems merge their information to a detector level result in the Common Merger Module (CMM) for each bunch-crossing. The results are sent to the Central Trigger Processor (CTP) where the Level-1 decision is made.