General Trigger Philosophy The definition of ROI’s is what allows, by transferring a moderate amount of information, to concentrate on improvements in defining the trigger. All Level 1 trigger are based on either MUON’s or calorimeter information.

Trigger Procesors There are 4 different LV-1 trigger processors to prepare the trigger data. They interact via the TTC to send a trigger-accept that allows to start the data readout

LV-1 MUON Trigger The MUON trigger philosophy is based on opening a cone (which defines a given p(t) threshold) around a point in a pivot plane (that contains non-overlapping geometry). The barrel includes a a 3-out-of-4 trigger logic for low p(t), combined with a 1-out-of-2 confirmation logic for high p(t) The end-cap requires a 3-out-of-4 logic combined with a 2-out-of-3 logic in the inner layer. The low p(t) is obtained by a non-linear combination in the inner layer, while linear for the high p(t) The end-cap has a more robust logic, due to the higher background conditions, combined with the fact that the stations are located in a non-magnetic region.

Problems with the integrated magnetic field The cone includes both the charges, they do not need to be symmetric and can be programmed. Due to the interference between the magnetic fields of the barrel and end-cap toroids, there are points where the integrated magnetic field is small. This can be solved either by masking these points or by taking the data and corrected at the LV-2, using the tracking chambers.

MUON P(T) distributions are exponential Since the p(t) distributions are exponential, a very small acceptance for low p(t) MUON’s leads to a high trigger rate. P(t) of 6 GeV/c enhances the production of b quarks, while p(t) of 20GeV/c enhances the hard processes.

Calorimetric LV-1 trigger Need to optimize the number of cells to ensure low trigger rate while keeping low complexity. All conditions are programmable for the EM and Hadron calorimeter cells to allow the definition of isolated clusters for e, gamma, tau and jets.

Integration time plays an important role Since the L-Ar signals extend over many beam crossings, analog signals are sent. To be able to determine the right bunch-crossing, the signals are differentiated; however this leads to undershoot, which is very important for energy sums. The signals are therefore compensated for each tower, in order to allow for corrected energy sums. Digitized and corrected sums are then transferred to the cluster finding processors to obtain the various trigger quantities.

The isolation criteria is crucial to determined the trigger rates Having the flexibility of setting the isolation criteria, allows small corrections to be made to keep up with a reasonable trigger rate. Isolation provides only factors of 10, while p(t) distributions are exponential.

Isolation is important for TAU definition The exact definition of the towers allows the TAU isolation to be determined and its contribution to the trigger rates to be reduced.

Large rapidity coverage is crucial for missing E(T) Forward going particles carry a non-negligible E(T), for this reason including the Forward Calorimeter in the sum is crucial to allow a proper missing E(t) cut

Typical trigger menu at the LV-1 Final trigger menus depend on the running conditions and the available luminosity. Various high rate triggers are pre-scaled to allow for control samples. Calibration triggers are important to keep up with a proper calibration and ensure that at the LV-2 trigger, the best calibration values are used. The programmability of the system allows optimization of the cuts in view of the physics needs.

LV-2 trigger The general Philosophy of the LV-2 trigger in ATLAS is to improve the definitions of the LV1 and to add some particular qualities. The use of ROI is what allows transfer of a small amount of information, related to a particular PHI-ETA bin, to evaluate the quality of the trigger further. Various elements are then applied: –For MUON’s partial tracking is done through the MUON spectrometer (and also can use ID matching) to allow a sharpening of the threshold. –For e, photon, tau, the use of of clustering shapes allows the definitions to be sharpened. –For energy sums, weighting factors using the longitudinal shower information are introduced to improve energy resolution. –For e and tau, ID detector information is included, for p-E (electrons) and for tau multiplicity (1-3). –For jets, a fast determination of the existence of secondary vertices is performed, that allows to tag B-jets.

Progress on algorithm performance Progress in the muon slice: –Performance studies of combining the muon and ID information at LVL2 in the barrel region –Hypothesis algorithms for LVL2 muon algorithm in the barrel are ready –Extension of the LVL2 track reconstruction in the end-cap regions ongoing: Field inhomogeneity in the end-cap breaks all the attempts to build a simple track model as in the barrel. Ongoing study of how to measure the muon p T and parametrize its track path. –First evaluation of Event Filter End-Cap rates vs p T thresholds at Barrel

Progress on algorithm performance (2/4) Egamma trigger progress: –Study of cut-dependent trigger performance with simulated Z  e + e - and W  e events. –Study of the photon trigger with simulated H  –Study of different track-fitting algorithms including bremsstrahlung recovery for EF e25i T2Calo efficiency variation for Z  ee M (H  )

Present status of trigger algorithms

Typical improvements using LV-2 Starting with a rate of 25KHz in LV-1, one can reduce it by factor of 100

More realistic trigger menus for low luminosity

Detailed trigger menu for 10**33

Example of Exclusive Physics triggers The ATLAS trigger system contains enough flexibility to be able to optimize the cuts for any of the channels. Still a lot of work is needed to perform the optimization, taking into account unforeseen backgrounds and status of the calibrations.

Conclusions Most of the hardware construction of the ATLAS detector is nearing completion. The installation of the detector components is barely consistent with being completed at the end of August 07, to register first collisions. The trigger system of ATLAS is flexible enough to be able to adapt to the various LHC background and luminosity conditions to optimize the Physics outcome. A large effort is still needed to perform this trigger optimization, based on the Physics expectations.