A. Dahlhoff

The ATLAS level-1 calorimeter trigger architecture

The architecture of the ATLAS Level-1 Calorimeter Trigger system (L1Calo) is presented. Common approaches have been adopted for data distribution, result merging, readout, and slow control across the three different subsystems. A significant amount of common hardware is utilized, yielding substantial savings in cost, spares, and development effort. A custom, high-density backplane has been developed with data paths suitable for both the em//spl tau/ cluster processor (CP) and jet/energy-summation processor (JEP) subsystems. Common modules also provide interfaces to VME, CANbus and the LHC timing, trigger and control system (TTC). A common data merger module (CMM) uses field-programmable gate arrays (FPGAs) with multiple configurations for summing electron/photon and /spl tau//hadron cluster multiplicities, jet multiplicities, or total and missing transverse energy. The CMM performs both crate- and system-level merging. A common, FPGA-based readout driver (ROD) is used by all of the subsystems to send input, intermediate and output data to the data acquisition (DAQ) system, and region-of-interest (RoI) data to the level-2 triggers. Extensive use of FPGAs throughout the system makes the trigger flexible and upgradable, and several architectural choices have been made to reduce the number of intercrate links and make the hardware more robust.

ATLAS level-1 calorimeter trigger: subsystem tests of a Jet/Energy-sum Processor module

The ATLAS Level-1 Calorimeter Trigger consists of a Preprocessor, a Cluster Processor (CP), and a Jet/Energy-sum Processor (JEP). The CP and JEP receive digitised trigger-tower data from the Preprocessor and produce trigger multiplicities and total and missing energy for the final trigger decision. The trigger will also provide region-of-interest (RoI) information for the Level-2 trigger and intermediate results of the data acquisition (DAQ) system for monitoring and diagnostics by using readout driver modules (ROD). The Jet/Energy-sum Processor identifies and localises jets, and sums total and missing transverse energy information from the trigger data. The Jet/Energy Module (JEM) is the main module of the Jet/Energy-sum Processor. The JEM prototype is designed to be functionally identical to the final production module for ATLAS, and have the full number of channels. Three JEM prototypes have been built and successfully tested. Various test vector patterns were used to test the energy summation and the jet algorithms. Data communication between adjacent Jet/Energy Modules and all other relevant modules of the Jet/Energy-sum Processor has been tested. Recent test results using the Jet/Energy Module prototypes are presented and discussed.

Prototype cluster processor module for the ATLAS level-1 calorimeter trigger

The Level-1 Calorimeter Trigger consists of a Preprocessor, a Cluster Processor (CP), and a Jet/Energy -sum Processor (JEP). The CP and JEP receive digitised triggertower data from the Preprocessor and produce trigger multiplicity and region-of-interest (RoI) information. The CP Modules (CPM) are designed to find isolated electron/photon and hadron/tau clusters in overlapping windows of trigger towers. Each pipelined CPM processes a total of 280 trigger towers of 8-bit length at a clock speed of 40 MHz. This huge I/O rate is achieved by serialising and multiplexing the input data. Large FPGA devices have been used to retrieve data and perform the cluster-finding algorithm. A full-specification prototype module has been built and tested, and first results will be presented.

Study of jet shapes in inclusive jet production in pp collisions at {radical}(s)=7 TeV using the ATLAS detector

Jet shapes have been measured in inclusive jet production in proton-proton collisions at sqrt(s) = 7 TeV using 3 pb^{-1} of data recorded by the ATLAS experiment at the LHC. Jets are reconstructed using the anti-kt algorithm with transverse momentum 30 GeV < pT < 600 GeV and rapidity in the region |y| < 2.8. The data are corrected for detector effects and compared to several leading-order QCD matrix elements plus parton shower Monte Carlo predictions, including different sets of parameters tuned to model fragmentation processes and underlying event contributions in the final state. The measured jets become narrower with increasing jet transverse momentum and the jet shapes present a moderate jet rapidity dependence. Within QCD, the data test a variety of perturbative and non-perturbative effects. In particular, the data show sensitivity to the details of the parton shower, fragmentation, and underlying event models in the Monte Carlo generators. For an appropriate choice of the parameters used in these models, the data are well described.

Measurement of the inclusive isolated prompt photon cross section in pp collisions at {radical}(s)=7 TeV with the ATLAS detector

A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy root s = 7 TeV is presented. The measurement covers the pseudorapidity ranges vertical bar eta(gamma)vertical bar < 1: 37 and 1: 52 <= vertical bar eta(gamma)vertical bar < 1: 81 in the transverse energy range 15 <= E-T(gamma) < 100 GeV. The results are based on an integrated luminosity of 880 nb(-1), collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading-order perturbative QCD calculations.A measurement of the cross section for the inclusive production of isolated prompt photons in pp collisions at a center-of-mass energy √s=7TeV is presented. The measurement covers the pseudorapidity ranges |ηγ|<1. 37 and 1.52≤|ηγ|<1.81 in the transverse energy range 15≤ETγ<100GeV. The results are based on an integrated luminosity of 880nb-1, collected with the ATLAS detector at the Large Hadron Collider. Photon candidates are identified by combining information from the calorimeters and from the inner tracker. Residual background in the selected sample is estimated from data based on the observed distribution of the transverse isolation energy in a narrow cone around the photon candidate. The results are compared to predictions from next-to-leading-order perturbative QCD calculations.

Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at {radical}(s{sub NN})=2.76 TeV with the ATLAS Detector at the LHC

By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.