In the first phase of LHC data-taking ATLAS will measure the charged-particle
density at the initial center-of-mass energy of 10 TeV and then at 14 TeV. This
will allow us improve our knowledge of soft QCD models and pin-down
cross-sections of different classes of inelastic collisions at LHC energies. In
particular, the dominant non-diffractive interaction is a key process to
understanding QCD backgrounds when we reach higher luminosities. We highlight
two minimum-bias triggers, sensitive to particles in complementary ranges in
pseudo-rapidity, one based on signals from the Inner Detector, the other
explicitly designed to trigger on inelastic processes. Studies of their trigger
efficiencies as well as possible trigger biases are presented.Comment: 3 pages, 4 figures, poster proceedings for ICHEP 200

Kwee, Regina

English

arXiv

In the first phase of LHC data-taking ATLAS will measure the charged-particle density at the initial center-of-mass energy of 10 TeV and then at 14 TeV. This will allow us improve our knowledge of soft QCD models and pin-down cross-sections of different classes of inelastic collisions at LHC energies. In particular, the dominant non-diffractive interaction is a key process to understanding QCD backgrounds when we reach higher luminosities. We highlight two minimum-bias triggers, sensitive to particles in complementary ranges in pseudo-rapidity, one based on signals from the Inner Detector, the other explicitly designed to trigger on inelastic processes. Studies of their trigger efficiencies as well as possible trigger biases are presented

Kwee, R

CERN Document Server

ATL-DAQ-PROC-2008-00701December200834th International Conference on High Energy Physics, Philadelphia, 2008Minimum Bias Triggers at ATLAS, LHCRegina Kwee (on behalf of the ATLAS Collaboration)CERN, Switzerland/Humbolt-Universita¨t zu Berlin, GermanyIn the first phase of LHC data-taking ATLAS will measure the charged-particle density at the initial center-of-massenergy of 10 TeV and then at 14 TeV. This will allow us improve our knowledge of soft QCD models and pin-downcross-sections of different classes of inelastic collisions at LHC energies. In particular, the dominant non-diffractiveinteraction is a key process to understanding QCD backgrounds when we reach higher luminosities. We highlight twominimum-bias triggers, sensitive to particles in complementary ranges in pseudo-rapidity, one based on signals fromthe Inner Detector, the other explicitly designed to trigger on inelastic processes. Studies of their trigger efficienciesas well as possible trigger biases are presented.1. INTRODUCTION AND MOTIVATION FOR MINIMUM BIAS TRIGGERSThe cross-section predictions at LHC-energies vary around 50 % due to theoretical uncertainties. The totalcross-section is estimated to σtot ≈ 100 mb depending strongly on Monte Carlo models and different theoreticalestimates [1]. In particular, at LHC-energies the particle multiplicities are unknown. Contributions to σtot comefrom elastic and inelastic processes, where the inelastic part dominates with σinel ≈ 80 mb. The leading inelasticprocesses are non-diffractive interactions with σnd ≈ 55 mb, producing mainly low-pT particles in the central detectorregion. They are generally associated with minimum bias events, but technically the exact definition of a minimumbias event is detector dependent and made by the trigger system. Further inelastic contributions arise from singleand double diffractive dissociations, with σsd ≈ 14 mb and σdd ≈ 10 mb respectively, producing particles mostlyin the forward direction. Merely for completeness, so called central-diffractive dissociations are mentioned wherea proton leaves a high diffractive mass also in the central detector part. This kind of interaction is not relevantfor understanding the dominant QCD background events as their cross-section with σcd ≈ 1 mb is expected tobe negligibly small. These inelastic soft interactions were often approximated in a phenomenological frame, theirmeasurement will advance the theory for different soft QCD models. To improve our knowledge of the physics ofthese dominant processes, ATLAS [2] will measure them at low luminosities, where events still contain single pp-interactions. This is essential for understanding the pile-up of collisions they form at high luminosities.ATLAS will use two Minimum Bias Triggers complementary in pseudo-rapidity to make these measurements. One isbased on signals of the Inner Detector (ID), the other one is a dedicated experimental setup consisting of MinimumBias Trigger Scintillators (MBTS). In these studies we investigated the initial scenario for data-taking. With a bunchpitch of 75 ns and a luminosity of L = 1031cm−2s−1, roughly 90 % of the events are empty: the beam bunchespenetrate each other without interacting, but leaving only electronic noise. They represent the main backgroundto be rejected. After the commissioning phase, the minimum bias triggers also need to be efficient in suppressingbeam-gas and halo events.2. MINIMUM BIAS TRIGGER SCINTILLATORSThe MBTS consist of 16 counters per side, of which 8 form an inner ring and another 8 an outer ring coveringin total a pseudo-rapidity range of 2.1 < |η| < 3.8 in the forward direction of each beam. They are mounted at|z| = 3.6 m between the ID and the Liquid-Argon-Calorimeter. The signals are read out through the electronicsof the hadronic calorimeter [2] which broaden the fast scintillator signals by roughly a factor of 50. A secondtrigger path has been installed to take advantage of the good time resolution of the scintillators which will allowto distinguish pp-collisions from beam-induced background events. The MBTS, as a relatively simple and robust134th International Conference on High Energy Physics, Philadelphia, 2008detector system and sensitive to a minimum of detector activity, play a key role for detector commissioning [3].After 3 to 4 months of running at luminosities of L ≈ 1033cm−2s−1, major radiation damage is expected.   η-6 -4 -2 0 2 4 6Trigger Bias   0.70.750.80.850.90.9511.051.1emptysdiffddiffndiffbeamgasFigure 1: Trigger bias for a requirement ofany 2 hits in total.   η-6 -4 -2 0 2 4 6Trigger Bias   0.70.750.80.850.90.9511.051.1emptysdiffddiffndiffbeamgasFigure 2: Trigger bias for a requirement ofa 1-hit coincidence.Performance of Minimum Bias Trigger ScintillatorsTo efficiently suppress empty events several trigger configurations have been studied for the MBTS requiring eithera hit coincidence or total hit multiplicity. The efficiency was defined as the ratio of reconstructed events passing thetrigger to the total number of reconstructed events and was computed for several simulated event samples. Due totheir expected significance, non-diffractive events served as signal and empty events as background. Less significantbut still not negligible, single and double diffractive samples were also classified as signal and beam-gas events wereconsidered as background. With a trigger threshold set to 40 mV, noise events can be highly suppressed whilethe non-diffractive events are still very efficiently retained with an expected signal amplitude of around 100 mV. Adifference of these configurations is visible for forward directed events. A total hit multiplicity requirement wouldpass ∼ 30 % more of the diffractive events, but also 15 % of the beam-gas events. The trigger bias was computedusing the same event samples, but taking into account only truth quantities with a trigger threshold fixed to 40mV for all counters. To estimate the bias, the ratio of stable, primary charged truth particles passing the triggerrequirements and the same particles from all events was calculated. Comparing the two configurations from Fig. 1,the total hit requirement seems much more favorable. The bias is flat for all event types over a large η-region up tothe coverage of the MBTS of |η| < 3.8. Requiring a hit-coincidence as shown in Fig. 2 seems to bias both kinds ofdiffractive events strongly.3. INNER DETECTOR BASED MINIMUM BIAS TRIGGERBased on signals from the silicon sub-detectors of the ID, this minimum bias trigger covers a pseudo-rapidity rangeof |η| < 2.5. The innermost tracking devide in ATLAS is built of pixel sensors with about 80 million channels. Itis surrounded by a silicon strip-detector, the Semi-Conductor-Tracker (SCT), with a read-out limited to roughly 6million channels. The pixel hits provide already 3-dimensional position information forming SpacePoints. In contrast,the SCT forms its SpacePoints by requiring a coincidence of two hits in strips that overlap in polar and azimuthalcoordinates η and φ. The SCT provides thereby an intrinsic noise suppression. However, to keep flexibility, both pixeland SCT signals are exploited for the trigger. This trigger is configured to start off a random trigger at Level-1 (L1)to introduce no bias in the event selection. At the second trigger level (L2) detector activity is measured by countingthe SpacePoints in the entire silicon-detector, which will be used to reject noise events. To discard beam-gas events,silicon tracks are reconstructed at the third trigger level, the Event Filter (EF), and tracks close to the interactionpoint are counted.234th International Conference on High Energy Physics, Philadelphia, 2008SCT Space Point Threshold   0 20 40 60 80 100 120 140 160 180 200Trigger Efficiency   00.20.40.60.81emptysdiffddiffndiffbeamgasATLASFigure 3: Trigger efficiencies for a 3 SCTSpacePoints requirement. It drops quickly forempty events.   η-6 -4 -2 0 2 4 6Trigger Bias   0.70.750.80.850.90.9511.051.1emptysdiffddiffndiffbeamgasFigure 4: Trigger bias for the requirementof 3 SCT and 12 pixel SpacePoints at L2 and2 tracks with a |z0| < 200 mm at EF.Performance of the Inner Detector based Minimum Bias TriggerThe trigger performance was studied with the same event samples as for the MBTS. The trigger efficiency forrequiring a minimum number of SCT SpacePoints at L2 is shown in Fig. 3. From the simulations we can expectthat noise events can be strongly suppressed while keeping almost 100 % of the non-diffractive events with acut of e.g. 3 SCT SpacePoints. A similar plot has been made for the pixel SpacePoint requirement and showedcomparable efficiencies for a cut at 12 pixel SpacePoints. The trigger efficiency was also computed for bothSpacePoints requirements at L2 and an additional track requirement at the EF. The event is only accepted at theEF if a certain number of tracks could be reconstructed with a longitudinal impact parameter |z0| < 200 mm.Setting the trigger threshold to 2 tracks would reject another 10 % of the total beam-gas events, achieving a totalrejection of more than 60 %. In order to retain most of the diffractive events, this trigger condition should notbe largely increased. However, for the non-diffractive sample even requiring 10 tracks leaves the trigger highlyefficient with almost 100 %. The trigger bias was also calculated from the truth η-distributions having in thenumerator the truth tracks which contain at least 12 pixel and 3 SCT SpacePoints and the total number of tracksin the denominator. Adding the track requirement of 2 tracks did not to influence the bias at all. The biasfor both L2 and EF requirements is shown in Fig. 4. We see that the non-diffractive events are selected in anunbiased way, while a small bias can be observed in the diffractive samples towards the end of the ID acceptance(η ∼ 2.5). Once, higher luminosities of L≈ 1033cm−2s−1 are reached, a pure random trigger becomes more important.AcknowledgementsThe work presented was a collaboration of many people, especially those joining the Minimum Bias Trigger and Min-imum Bias Working Group. The credits go also to W.H. Bell (University of Glasgow), D.Berge (CERN), C.Buttar(University of Glasgow), B.Demirkoz (CERN), E.Feng (University of Chicago), J.Husten (Michigan State University),N.Ellis (CERN), B.T.Martin (Michigan State University), A.Messina (CERN), S.Kama (DESY), M.Leyton (LBNL)and P.Steinberg (BNL). Particulary, I would like to thank William Bell, Ricardo J. Goncalo (Royal Holloway Uni-versity of London), David Berge and Andrea Messina for their valuable input to the poster of this proceedings andthis paper itself. Also, I would like to thank the organisers of ICHEP08 for the opportunity to present this work.References[1] J.P. Guillaud and A. Sobol, “Simulation of diffractive and non-diffractive processes at the LHC energy with PYTHIA and PHOJETMC event generators ”, LAPP-EXP 2004-06, July 2004[2] ATLAS Collaboration, “The ATLAS Experiment at the CERN Large Hadron Collider”, 2008 JINST 3 S08003, 14.Aug. 2008[3] A. Messina et al., “Commissioning of the ATLAS Level-1 Central Trigger system”, talk at TWEPP-08, 19.Sep. 20083

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