The ATLAS tau trigger consists of three level trigger systems: the first one (L1) is hardware based and uses FPGAs, while the second (L2) and third levels (EF -Event Filter-) are software based and use commodity computers (2 x Intel Harpertown quad-core 2.5 GHz), running scientific linux 5. In this contribution, we discuss both the physics characteristics of tau leptons and the technical solutions to quick data access and fast algorithms. We show that L1 selects narrow jets in the calorimeter with an overall rejection against QCD jets of 300, whilst L2 and EF (referred together as High Level Trigger -HLT-) use all the detectors with full granularity and apply a typical rejection of 15 within the stringent timing requirements of the LHC. In the HLT there are two complementary approaches: specialized, fast algorithms are used at L2, while more refined and sophisticated algorithms, imported from the offline, are utilized in the EF

Tsuno, S

The ATLAS tau trigger has three levels: the ﬁrst one (L1) is hardware based and uses FPGAs, while the second (L2) and third levels (EF -Event Filter-) are software based and use commodity computers (2 x Intel Harpertown quad-core 2.5 GHz), running scientiﬁc linux 4. In this contribution we discuss both the physics characteristics of tau leptons and the technical solutions to quick data access and fast algorithms. We show that L1 selects narrow jets in the calorimeter with an overall rejection against QCD jets of 300, whilst L2 and EF (referred together as High Level Trigger -HLT-) use all the detectors with full granularity and apply a typical rejection of 15 within the stringent timing requirements of the LHC. In the HLT there are two complementary approaches: specialized, fast algorithms are used at L2, while more reﬁned and sophisticated algorithms, imported from the oﬄine, are utilized in the EF

CERN Document Server

ATL-DAQ-SLIDE-2009-34023October2009The ATLAS tau triggerSoshi Tsuno (KEK) on behalf of ATLAS collabora9onOct.27.2009 ?IEEE, FloridaChallenge of tau triggerIden%fying the hadronic tau lepton at trigger.   Collimated tracks (1‐3‐prong) in core region.    Isola9on from other tracks.    non zero impact parameter (cτ~87µm).   Collimated deposi9on in EM.       (50% of energy by π0 in 1‐prong τ)     Use shower shape.    Both EM and HAD component.    Isola9on cone.    Reconstruc9on of π0 sub‐cluster.Tracking :Calorimeters :Rela%vely high QCD jet rejec%on can be achieved.Our challenge : combined performance of tracking, EM                              and Hadronic calorimeters.Oct.27.2009 ?IEEE, FloridaPhysics mo9va9onHiggs100010TeV14TeV200HzNeed to suppress QCD background with order ~106 .LHC is “QCD jets” collider.The tau trigger will play an important rule  to suppress the rate by exploring the combined  menu.   double hadronic taus ( τhτh )    lepton + tau ( l+τh )          VBF Higgs ‐>  ττ ‐> hh / lh    tau + Missing ET  (τh + MET )          MSSM Charged H+    high pT tau          Z’ / W’ searches. To suppress the trigger rate, Oct.27.2009 ?IEEE, FloridaLevel 1 tau trigger   Hardware : ASICS / FPGA    latency < ~2.5 µs, operates on 40MHz,    L1 acceptance : 75kHz,    7200 coarse projec9ve trigger towers                                           (ΔφxΔη=0.1x0.1)    10bits ADC (1 ADC count = 250MeV) Cluster processor :Basic spec :  L1 Calorimeter   RoI Core : 2x2 tower EM and Had ET maximum,    Tau cluster is formed as 1x2 EM + 2x2 Had towers.    EM/Had isola9on :  total 16 towers around RoI Core. Maximum 8 trigger thresholds, EM isola9on     6GeV~300 VME modulesL1 TAU :Oct.27.2009 ?IEEE, Florida€ ≤High Level Trigger (HLT)SoPware triggers :    PC farm : 2300 PCs, SLC4‐>5, Intel 2.5GHz (4‐core)    Implemen9ng on RoI/object,    L2 operates 75kHz, processing 9me ~40ms,    EF operates 3kHz, processing 9me ~4s,    Accept 200Hz (go in tape, 1.6M bytes/evts).    Full reconstruc9on at EF. Tau algorithm performance : tau20iLevel 2 Event FilterRoI unpacking ~3.8 ms ‐‐‐Calorimeter ~8.0 ms ~13.4 msTracking ~15.0 ms ~269.5 msCombined ~2.0 ms ~80.7 msTotal 9me ~19.7 ms ~67.5 msNote : QCD jets sample was used. No prescale. “Total” is ms/event, otherwise ms/RoI.Oct.27.2009 ?IEEE, FloridaTau iden9ﬁca9on at triggerEMRadius Isola%on frac%onLevel 2 / Event Filtertau QCD jetstau QCD jets# of trackstau QCD jetsTrack isola%onEMRadius : transverse EM                       shower proﬁle, Isola%on frac%on: frac9on of              energy in iso.ring region, n tracks     : # of tracks, track isola%on: frac9on of                     tracks in core region, EM frac%on: EM energy frac9on, isola%on frac%on, leading track pT , EM ET …… etc. At EF, same reconstruc9on algorithm is used with oﬄine tau reconstruc9on.Oct.27.2009 ?IEEE, FloridaAt HLT (L2 and EF), the tau  iden9ﬁca9on criteria is  applied :(normalized to 1)tau QCD jetsTau trigger performancetau16_looseAt each level, the trigger eﬃciency reaches fairly ﬂat.The “tau16” means the tau with pT>16GeV. tau16_loose : L1/L2/EF = 1.9k / 408 / 265 Hz  tau50_loose :                      26 / 1.97 / 0.89 Hz Acceptable as single menuThe trigger rate at L=1031cm‐2s‐1 : Used in the combined menuOct.27.2009 ?IEEE, FloridaTau trigger menuStart up menu ( L=1031cm‐2s‐1 , 10TeV)   Validate trigger. Stay simple as much as possible.    L1 : low pT threshold, looser isola9on,    HLT : running with pass‐through mode,    Combined menu will be 9ghten.    tau+MET will be studied for W‐>τhν xsec.At L=1033cm‐2s‐1 , 14TeV   Complex signature and  high pT threshold will be  necessary to achieve the  physics goal.L=1031cm‐2s‐1 , 10TeVTrigger rate and acceptance at L=1031cm‐2s‐1 ,14TeV, loose cutOct.27.2009 ?IEEE, FloridaTrigger eﬃciency measurement from dataTag & probe method :probetage, µtau trigger ﬁre?τhZ‐>ττ events   Require tag lepton which passes single lepton trigger,    Ask if tau trigger ﬁres with respect to oﬄine tau,    Direct measurement from data,    Suﬀer from the sta9s9cs, and limited pT range.Using QCD jets or electron :   The tau is between “electron” and “jet”,    Gain high sta9s9cs, extend high pT region,    Assume the transfer of jet/electron to tau                   by adjus9ng iden9ﬁca9on criteria.Using \bar events :   The top event can be another control sample.    Using three jets trigger, ask if one of W ﬁre      the tau trigger.Oct.27.2009 ?IEEE, FloridaTau trigger data quality monitoringGREY : reference GREEN : Good RED : Bad Data is monitored w.r.t. the reference.This example depicts        EM ET is ﬁne, but problem in total ET.Problem was in hadronic ET.  It was conﬁgura9on problem.Oct.27.2009 ??IEEE, FloridaTau trigger performance on cosmic dataThe tau trigger performance is checked with respect to the oﬄine reconstruc9on. The plots show the pT diﬀerence as a func9on of EF tau (lex) and the energy correla9on  between trigger and oﬄine reconstruc9ons (right).  No eta dependence was found. The diﬀerence is within the expected diﬀerence  between trigger and oﬄine reconstruc9on. Oct.27.2009 ??IEEE, FloridaHun9ng the MIP clusterThe tau signature gives an unique opportunity to hunt the MIP cluster in tau object. muonEMHADphotonThe tau uses tracks and  EM and hadronic energy. Hun9ng the cluster  associated in the muon  track. Source of the three peaks :   The MIP cluster itself ﬁres the tau trigger (~10GeV).    MIP energy in Tile calorimeter (~3‐4 GeV)    MIP energy in LAr. (typically 200MeV), but cut oﬀ by the cell energy threshold (~1GeV).Events are categorized as EM and HAD seeded events.Oct.27.2009 ??IEEE, Florida(The muon track is extrapolated on to calorimeter surface, then ﬁnd a MIP cluster.)SummaryIn the tau triggering,  Performance is well‐evaluated by the MC simula9on :   High rejec9on of QCD background,    Combined menu (2tau, l+tau, tau+MET) explores sensi9vi9es for new physics.Level 1         : narrow jet and isola9on with coarse projec9ve trigger tower, Level 2         : fast tracking, fast clustering in RoI, fast iden9ﬁca9on, Event Filter : sophis9cated iden9ﬁca9on, same reconstruc9on with oﬄine.Tested, demonstrated on the cosmic muon data :   Monitoring the data quality,    Validated and understood with respect to the oﬄine reconstruc9on. Our challenge is to understand the combined performance of tracking,  EM and hadronic calorimeters. We are eagerly wai%ng for DATA.Oct.27.2009 ??IEEE, Florida

The ATLAS tau trigger

ATL-DAQ-PROC-2009-04025November2009The ATLAS tau triggerS. Tsuno KEK, on behalf of ATLAS collaborationAbstract—The ATLAS tau trigger consists of three triggerlevels: the first one (L1) is hardware based and uses FPGAs, whilethe second (L2) and third levels (EF -Event Filter-) are softwarebased and use commodity computers. In this contribution, wediscuss both the physics characteristics of tau leptons and thetechnical solutions to quick data access and fast algorithms. Weshow that L1 selects narrow jets in the calorimeter with an overallrejection against QCD jets of 300, whilst L2 and EF (referredtogether as High Level Trigger -HLT-) use all the detectors withfull granularity and apply a typical rejection of 15 within thestringent timing requirements of the LHC. In the HLT thereare two complementary approaches: specialized, fast algorithmsare used at L2, while more refined and sophisticated algorithms,imported from the offline, are utilized in the EF.I. INTRODUCTIONTHE ATLAS trigger system consists of three level triggersystems: the first one (L1) is hardware based and usesFPGAs, while the second (L2) and third levels (EF -EventFilter-) are software based and use commodity computers 1.The L1 is designed to accept events up to 75 kHz, while theL2 and EF acceptances are 3 kHz and 200 Hz, respectively.Our challenge of the tau triggering is to suppress the QCDjets events on the order of 106. This can be archived byusing an unique feature of the hadronically decaying tauleptons measured in the detector. The hadronic tau produces acollimated energy deposition in the calorimeters in a narrowerregion than QCD jets, relatively larger energy deposition inthe electromagnetic (e.m.) calorimeter than the hadronic oneand high pT tracks in the narrow region assocated with theposition found in the calorimeters. We use the shower shapeand track information to identify the hadronic tau. The tautrigger does not have as strong a suppression of jets as theelectron and muon triggers. The thresholds for single tausare therefore higher than the single electron and muon triggerthresholds. In addition, the selection criteria can be tightened.An alternative approach to controlling the rate that is also usedis the combination of tau trigger items with electron, muon,or missing ET triggers. The combined menu is expected tobe used in the high rate environment of LHC operation atL = 1033−34 cm−2s−1, where the main focus of tau triggerswill be on the search for the Higgs boson or new physicsbeyond the Standard Model. In the early data taking period,the tau trigger will be also well tested and validated by thestandard candle processes of the W → τν, Z → ττ and t t¯events.1Currently, 2 x Intel Harpertown quad-core 2.5 GHz running on scientificlinux 5 is used.II. DESCRIPTION AND PERFORMANCE OF THE ATLAS TAUTRIGGERThe L1 tau trigger selection is fully documented in [1]. Itis a hardware trigger based on e.m. and hadronic calorimeterinformation, with 0.1× 0.1 granularity in η × φ.The selection at the HLT [2] makes use of the tracking andthe full granularity calorimeter information. Whilst fast andspecialized algorithms are used in the L2, more sophisticatedand precise determinations can be performed in the EF. In (GeV)TTrue visible E0 10 20 30 40 50 60 70Efficiency00.20.40.60.81tau12_loosetau16_loosetau16i_loosetau20i_loosetau29i_looseATLAS PreliminaryFig. 1. Overall trigger efficiency (L1 + L2 + EF) for different tau triggers.the ATLAS trigger menu, different single tau triggers are im-plemented, corresponding to different threshold requirementsin the basic tau trigger variables: tau12, tau16, tau16i,tau20i, tau29i, where the first symbol of the signaturerepresents the particle type, the following number is the ETthreshold and the “i” indicates that an isolation requirement isapplied. The last argument ”loose” is the adjustable configu-ration parameter which depends on the physics analysis andluminosity requirements. The other arguments are ”medium”and ”tight”, which apply tighter selection against QCD jetsbackground. Fig. 1 presents the overall trigger efficiency(L1 + L2 + EF) for different tau triggers. The efficiencyis normalized with respect to those tau leptons that decayhadronically with visible tau momentum (pαvis = pατ − pαν ) inthe sensitive region of the detector (|η| ≤ 2.5) and greater thanthe nominal ET threshold requirement for a given signature.Furthermore, the efficiency is optimized to select those tauleptons which are likely to be selected by the tau identificationalgorithms of the offline reconstruction software [2]. The finalrates are within the ATLAS bandwidth assigned to the tautrigger. The typical QCD rejections apply by the L1 and HLTtau triggers are 300 and 10 or more, depending on p T rangeand tightness, respectively.Table I gives the trigger menu proposed for initial datataking, including expected prescale factors and correspond-ing trigger rates. The un-prescaled single tau menu will beavailable above threshold ET ≥50 GeV, while the combinedmenu can achieve the necessary rate suppression with lowerthresholds.TABLE ITAU TRIGGER MENU FOR L = 1031 cm−2s−1 AT 10 TEV. TAU TRIGGERS(TAUXX) APPEAR IN COMBINATION WITH ELECTRON TRIGGERS (EXX),MUON TRIGGERS (MXX) AND MISSINGET TRIGGERS (XEXX).Trigger Prescale Rate (Hz)tau50 1 0.89±0.44tau16i_EFxe30 1 5.8±0.702tau20i 1 1.1±0.5tau12_e10 1 0.89±0.45tau16i_mu6 1 0.11±0.03III. TRIGGER TIMING STUDIESThe L1 latency, which has been determined on the finalhardware, is typically 2.5µs, dominated by cable delays andincluding all stages of the signal processing. The actual L1tau algorithm that identifies the L1 tau RoI takes < 40ns.Prior to LHC data taking, the HLT system on 2008 (com-posed of Intel(R) XEON(TM) CPU 2.20GHz machines) hasto be assessed in simulated conditions, resembling real datataking. Simulated L1 trigger signals are produced using QCDbackground events with hard parton 35 < pT < 70GeV/c.One exercise is the test of individual triggers to gauge theimpact of increasing threshold levels on the total executiontime of the trigger, presented in Table II. Although the aver-age execution time of each algorithm remains roughly equalbetween triggers, the average total time per event decreasesas a result of the lower number of RoIs per event for highenergy triggers. The results shown in Table II indicate that thetime performance of the tau trigger should be well within theconstraint for total execution time at L2 and EF.TABLE IIMEAN ALGORITHM EXECUTION TIME FOR EACH OF THE TAU TRIGGERS.ALL TIMES ARE GIVEN IN MS AND PER ROI, EXCEPT THE L2 AND EFTOTAL TIMES ARE GIVEN PER EVENT, IN MS. THE MEASUREMENTS AREPERFORMED ON A SIMULATED SAMPLE OF LIMITED STATISTICS, 950 QCDBACKGROUND EVENTS WITH HARD PARTON 35 < pT < 70 GEV/C.Threshold SignatureAlgorithm tau10i tau20i tau35iL2 Calo 8.1 8.0 8.1L2 Tracking 15.4 15.0 14.7L2 Combined 1.9 2.0 2.2L2 TotalTime 41.6 19.7 7.9EF Calo 12.3 13.4 14.0EF Tracking 289.7 269.5 247.8EF Combined 77.0 80.7 78.9EF TotalTime 149.1 67.5 24.6IV. COMMISSIONING OF THE TAU TRIGGERThe tau trigger is tested with the ATLAS commissioning runin cosmic ray events. Although there is no real tau signaturein the cosmic muon events, it is extremely worth to test thetrigger chain and all detector systems, since the tau triggeruses the track and e.m. and Hadronic calorimeter information.As shown in Fig. 2, the triggered events are compared with (GeV)TTrigger tau p0 10 20 30 40 50 60 70 80 90 100 (GeV)TOffline tau p0102030405060708090100ATLAS Preliminary2009 Cosmic-Ray DataTrigger Run OfflineFig. 2. Comparison of the EF and offline taus with respect to thereconstructed pT spectrum, where the EF and offline taus are matched within∆R ≤ 0.2.the offline reconstructed events. Since the arrival time of thecosmic ray is in principle a random process, the measuredpulse in the calorimeters synchronized with the beam clock isnot precise in the trigger reconstruction, while in the offlinereconstruction, a more sophisticated procedure is used todetermine the best optimal timing. This means that the offlineenergy reconstruction has better performance than the triggerreconstruction. Regardless of the different energy measure-ment scheme between trigger and offline reconstructions, wecan see a reasonable agreement between the EF and offlinetaus. This is an expected feature in that we observed smalldifference in each channel in the calorimeters between triggerand offline reconstructions.V. CONCLUSIONWe present an overview performance of the ATLAS tautrigger at L = 1031 cm−2s−1 for 10 TeV collisions. Thetypical tau trigger efficiency is expected to be around 90%for a wide range of physics channels, resulting in a total tautrigger rate of about 15 Hz including triggers for monitoringand calibration. The trigger menu is well tested by the cosmicdata and demonstrated its performance with respect to theoffline reconstruction. The main focus of tau triggers will beon the selection of the standard candle processes W → τν,Z → ττ and tt¯ in the early phase of the experiment, but itwill also turn to the search for the Higgs boson or new physicsbeyond the Standard Model in long term of the experiment.REFERENCES[1] J.Garvey et al., Nucl. Inst. and Meth. A 512, no. 3 (2003), 506-516.[2] ATLAS Collaboration, Expected Performance of the ATLAS Experiment -Detector, Trigger and Physics, CERN-OPEN-2008-020,arXiv:0901.0512.

The ATLAS tau trigger

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server