Two fast trigger algorithms based on 3 innermost hits in the CMS Inner
Tracker are presented. One of the algorithms will be applied at LHC low
luminosity to select B decay channels. Performance of the algorithm is
demonstrated for the decay channel Bs->Ds+pi. The second algorithm will be used
to select tau-jets at LHC high luminosity.Comment: 10 pages, 10 figures, to be published in the Vertex 2001 Conference
  Proceedin

Andrey Starodumov

Angarano

Danek Kotliński

Lenzi

Schnetzer

English

arXiv

Crossref

High level tracker triggers for CMS

Two fast trigger algorithms based on 3 innermost hits in the CMS Inner Tracker are presented. One of the algorithms will be applied at LHC low luminosity to select B decay channels. Performance of the algorithm is demonstrated for the decay channel Bs -&gt; Ds+pi. The second algorithm will be used to select tau-jets at LHC high luminosity

Kotlinski, Danek

Starodumov, Andrei

CERN Document Server

High Level Tracker Triggers for CMSDanek Kotlinski a Andrey Starodumov b,1aPaul Scherrer Institut, CH-5232 Villigen, SwitzerlandbINFN Sezione di Pisa, Via Livornese 1291, 56010 S.Piero a Grado(PI), ItalyAbstractTwo fast trigger algorithms based on 3 innermost hits in the CMS Inner Tracker arepresented. One of the algorithms will be applied at LHC low luminosity to select Bdecay channels. Performance of the algorithm is demonstrated for the decay channelB0s ! D−s + pi+. The second algorithm will be used to select τ -jets at LHC highluminosity.Key words: Triggering, B-physics, Pixel detectors, CMSPACS: 29.40.Gx1 IntroductionThe collision rate at LHC [1] is foreseen to be 40MHz. To handle such rates apowerful DAQ system and also fast and ecient trigger algorithms are needed.The CMS Collaboration [2] has chosen a trigger scheme based on two com-ponents: a hardware Level 1 Trigger and a software Level 2 Trigger. As anycollider detector, CMS is composed of tracker, calorimeter and muon systems.At Level 1 the information from the muon and calorimeter systems is used.At Level 2, called Higher Level Trigger (HLT) in CMS, also the tracker con-tributes to the event selection.The CMS tracker system [3] is based on two technologies: 1) silicon pixel de-tectors, which are closest to the beam, provide 3-D hits used for vertexing, and2) silicon strip detectors placed at larger radii are used for pattern recognitionand track reconstruction. The amount of information coming from the pixelEmail addresses: danek.kotlinski@psi.ch (Danek Kotlin´ski),andrey.starodumov@pi.infn.it (Andrey Starodumov).1 On leave from ITEP, Moscow, RussiaPreprint submitted to Elsevier Science 12 March 2002detector is about 10% of the full tracker, hence it is reasonable to use it at thebeginning of the HLT algorithms. In this article we present track reconstruc-tion algorithms based on 3 innermost tracker hits. At low LHC luminosity(1033cm−2s−1), when only two pixel layers are foreseen, the third hit is takenfrom the lowest radius strip detector layer. At high luminosity (1034cm−2s−1)the algorithm is based purely on the pixel detector.2 Detector layoutThe results shown here were obtained with a full GEANT based simulationof the CMS detector using the CMSIM [4] and ORCA [5] software packages.The CMS inner tracker layout used in the simulation will be briefly mentionedbelow, more details can be found in Ref.[3,6{8]. The pixel detector consistsof three barrel layers located at mean radii 4.3 cm, 7.2 cm and 11.0 cm.The 52 cm long pixel barrel is supplemented by two endcap disks on eachside. With this conguration the pixel detector provides 3 hit coverage up torapidity jηj  2.1. For low luminosity only two barrel layers will be installedgiving 2 pixel hits up to jηj  2.1. The 1st silicon strip barrel layer used inour reconstruction algorithm is 89.2cm long and is located at a mean radiusof 23.5cm3 The triggering schemeThe goal of the CMS trigger system is to reduce the event rate from 40MHzdown to 100Hz that will be stored for o-line analysis. The DAQ architectureis designed in such a way that it should sustain maximum output rate of Level1 trigger of 100kHz and provide HLT acceptance of about 10−3. As mentionedin the Introduction the Level 1 trigger decision is based on the muon andcalorimeter systems, at HLT the tracker information will be added. Even ifthe full detector information is accessible at HTL, fast algorithms which selectevent topologies of interest and use partial information should be applied rst.4 Track and vertex reconstructionTwo algorithms based on the track reconstruction from the 3 innermost trackerlayers will be discussed below, both start from the two innermost pixel lay-ers. For the 3rd hit the rst strip layer is used in one algorithm, the secondalgorithm uses the third pixel layer.24.1 Track reconstruction using the pixel detectorThe pixel track nding algorithm has been explained in Ref.[9], here onlythe most important details will be given. It’s three essential steps are shownin Fig. 1. Pixel hit pairs from the rst two layers (barrel+barrel or bar-rel+endcap) are matched in r − φ and z − r to establish track candidates.The cuts are optimized for a minimum track transverse momentum (pt) of1GeV/c and permit a maximum impact parameter in r − φ of 1mm. In thez − r plane the hit pairs have to point to a region in z within 3σ(+/− 15cm)of the LHC luminous region. Valid pixel pairs are matched with a 3rd pixel hit~ 10-30xyzy(0,0) (0,0) layer 1layer 2layer 3z04 cm7 cm4 cm7 cmFig. 1. Pixel detector track finding algorithmforming a track candidate. Using these tracks a list of primary vertices (PV)is formed at z values where at least 3 tracks cross the z axis. Tracks which donot point to any PV candidate are erased. Due to the detector overlaps in ther− φ direction for some Monte Carlo tracks more than one track candidate isfound. Track pairs which share pixel hits and are closer than 10mrad from eachother are identied. A cleaning procedure then erases one of the tracks in eachpair. The track nding eciency is illustrated in Fig. 2 where the eciency(the ratio of reconstructed to Monte Carlo tracks) is shown as a function ofthe track pt and rapidity. Only reconstructed tracks which have all 3 hits cor-rectly assigned are counted and the fraction is normalized to all Monte Carlotracks which are within the full acceptance of the pixel detector. The plottedeciency is for high luminosity LHC events in the presence of minimum biaspile-up interactions (on the average 20). However, the comparison is done forthe signal tracks only, that is tracks originating from the minimum-bias eventsare ignored. In the upper part of Fig. 2 the reconstruction eciency and theghost rate is shown as a function of pt. The lower part shows that the eciency(for tracks with pt  1GeV/c) is flat within the rapidity coverage of the pixeldetector. The solid curve shows the absolute eciency and the dashed curvethe algorithmic eciency (normalized to the number of tracks with 3 pixelhits). For all event types considered in our HTL studies the absolute tracknding eciency for tracks above 1 GeV/c is about 90%. The algorithmic ef-ciency is between 93% and 95% and the fraction of ghost tracks is typicalbetween 5% and 8%.Obviously the track parameters, tted with 3 points only, are much inferior30.20.40.60.810 2.5 5 7.5 10 12.5 15 17.5 20pT(GeV)Efficiency00.20.40.60.81-3 -2 -1 0 1 2 3ηEfficiencyFig. 2. The track reconstruction efficiency at high LHC luminosity shown as afunction of track pt (upper part) and rapidity (lower part). The dashed line in theupper part shows the ghost track rate. The dashed line in the lower part shows thealgorithmic efficiency defined as the ratio of the correctly reconstructed tracks toall MC tracks with at least 3 pixel hits (see text for more details).to the full tracker resolution. The reconstructed track’s direction is good, theresolution in the φ direction being 8mrad and in the θ direction 11mrad.The pt measurement however, is poor, mainly because of the small radius ofthe last hit (11 cm). For 2GeV/c tracks this resolution is about 7%, but for10GeV/c it reaches 20% (see Ref.[9] for more details). The curvature of thetracks is sucient to distinguish the track sign with 100% eciency up topt of 30  40 GeV/c. The primary vertex candidates found during the tracknding stage are reanalyzed. Only PVs with at least 3 valid tracks are keptand the position of each vertex is estimated as the mean value of the z impactparameters of all tracks assigned to it. In addition to the main \signal" PV,on the average 6  8 more PVs per event are found at high luminosity. The\signal" PV is usually found with a high accuracy of better than 50 µm. Thisis shown in Fig. 3 where the dierence in the z position of the reconstructedvertex and the Monte Carlo vertex is plotted. The eciency of this algorithmis high, for the HLT event samples the \signal" PV is always found with aneciency of better than 95%.4.2 Reconstruction using the pixel detector and one strip layerThis algorithm is similar to the one discussed in section 4.1. Since the algo-rithm is developed to trigger on exclusive B meson decays, selection criteria4050100150200250300-0.03 -0.02 -0.01 0 0.01 0.02 0.03Zreconstructed-Zmc(cm)CountsFig. 3. The difference (in cm) between the z position of the Monte Carlo primaryvertex and the reconstructed primary vertex. Only pixel hits are used in the vertexreconstruction.listed below are optimized for such decays. Below we describe it in more detail.(1) Any pair of hits from two separate pixel layers are connected with astraight line in the transverse plane. We utilize the fact that the tracks wewant to reconstruct are relatively sti (pT  0.9GeV/c) and the lever armis small ( 7cm). Then the impact parameter (IP, the distance from theline to the primary interaction point in the transverse plane) is calculated.If it is less than 2mm, the two hits create a track candidate.(2) Extrapolating the straight line that connects two selected hits to the third(strip) layer in the r− z plane, the expected position (zexp) of a third hitis calculated. If there is a reconstructed hit with its coordinate zrec closeto zexp, z = jzexp−zrecj  1.5mm, it is taken as the third hit of the trackcandidate. The momentum and the charge of the track is determined.In Fig. 4 we show the IP and dz distributions for decay (1) to illustrate theabove cuts. If the transverse momentum of the reconstructed track is greaterthen 0.9GeV/c it is accepted for further analysis.5 B-physics triggersThe Level 1 B physics trigger in CMS is mainly based on the muon system(the single muon trigger rate is assumed to be 10kHz). At the Level 2 in-formation from the muon and calorimetric systems will be reanalyzed with51010 210 30 0.2 0.4 0.6 0.8 1IP (cm)Arbitrary units1010 210 30 0.5 1 1.5 2∆z (cm)Arbitrary unitsFig. 4. Impact parameter (left) and ∆z (right) distributions for final state tracks ofdecay (1).00.511.522.533.543 4 5 6 7 8 9 10 11 1210 GeV/c1 GeV/cNumber of hits∆PT/PT (%)Fig. 5. Transverse momentum resolution as a function of number of hits for trackswith transverse momentum of 1 and 10GeV/c.ne granularity and more sophisticated algorithms. However, soft products ofB decays usually do not manifest themselves in the calorimeters and moreprecise calculation of the trigger muon momentum can suppress the rate byonly a factor of 2. Thus, one needs to apply further selection algorithms whichare specic to B decays, e.g. reconstruction of B decay products (tracks) andcalculation of invariant masses of intermediate resonances if any.The idea of using only 3 innermost hits at HLT to select the exclusive B decaymodes is based on the observation that the momentum resolution providedby these 3 hits is only a few times worse than that obtained with the fulltracker. Fig. 5 shows the transverse momentum resolution in percentage versusthe number of continues hits used for track reconstruction. Two distributionsrepresent results for tracks of 1 and 10GeV/c. One can see that for tracks60510152025303540450.98 1 1.02 1.04 1.06 1.08M(KK) (GeV/c2)Arbitrary units05101520253035401.8 1.85 1.9 1.95 2 2.05 2.1 2.15 2.2M(φpi) (GeV/c2)Arbitrary unitsFig. 6. Left - an invariant mass of two opposite charged tracks (assuming they areKaons) and right - an invariant mass of reconstructed φ and adjacent pi for theevents of decay mode (1).with pT ’ 1GeV/c the transverse momentum resolution obtained with 3 hitsis about 1.5%, 2 times worse than with the full tracker.The decay mode (and its charge conjugate)B0s ! D−s + pi+ ! φ + pi− + pi+ ! K+ + K− + pi− + pi+ (1)has been chosen as a benchmark. Only the Ds decay chain is triggered at thisstage which allows us to select also the following decay modes of the B meson:B0s ! D−s + X (2)where X = a+1 , K+, D+s and µ+ + X.Applying the reconstruction algorithm discussed in Section 4.2 we obtain themomentum and the charge for each reconstructed track. To trigger the decaymodes (1,2) we reconstruct all tracks with pt  0.9GeV/c and then nd all φand Ds candidates by trying dierent track combinations. An event is selectedif two conditions are fullled. Firstly, that there are two oppositely chargedtracks close to each other (R  0.3), and their invariant mass (assumingthey are kaons) is compatible with the φ mass, jMKK − Mφj  15MeV/c2.Secondly, there is a third track in the cone R = 1.0 around the reconstructedφ candidate and the invariant mass of this φ candidate and the third track(assuming it is a pion) agrees with the Ds mass, jMφpi −MDs j  70MeV/c2.Mass cuts have been dened based on expected mass resolutions shown inFig. 6 for the decay mode (1): σ(Mφ) ’ 5MeV/c2 and σ(MDs) ’ 25MeV/c2.The proposed algorithm reduces the single muon trigger rate by factor of 30while providing a signal eciency of about 75%. The fraction of ghost tracks is7relatively small (about 10%). Further reduction of trigger rate will be based onmore strict cuts using the full CMS tracker capability to reconstruct exclusiveB decays.6 Tau triggerThe \pixel" τ trigger proposed here is an example of a tracker HLT appli-cation which uses only the pixel data. As signal events we used SUSY Higgsbosons decaying into two τ leptons with two τ hadronic jet(s) in the nalstate. The main background for such events are QCD 2-jet events in the ptrange 50-230 GeV/c. More details about the algorithm and the event samplesFig. 7. A sketch showing the basic principle of the pixel τ -jet identification algorithm.used in the simulation can be found in Ref.[10].The HLT pixel algorithm is based on track isolation criteria and is schemati-cally shown in Fig 7. The τ -jet direction is dened by the calorimeter trigger.All track candidates in the matching cone Rm around the jet direction andabove the pmt cut are considered in the search for signal tracks, that is trackswhich originate from the hadronic τ decay. The track with the highest pt isdeclared the \leading" track (tr1 in Fig. 7). Any other track which is in thenarrow cone Rs around tr1 is also assumed to come from the τ decay. A largerarea Ri is now searched for tracks above the pit cut. If no tracks are found inthe Ri cone, except the ones which are already in the Rs cone, the isolationcriteria is fullled and the jet is labeled as a τ -jet. The narrow signal coneRs around the \leading" track tr1 is needed in order to trigger on 3 prongτ decays in addition to 1 prong. Typical values of the cuts used above are :Rs=0.05, Rm=0.10, Ri=0.35, pst = 3GeV/c and pit = 1GeV/c.The algorithm works very well at low luminosity. At high luminosity, however,8it’s eciency becomes small (about 50%) due to the large number of tracksoriginating from the pile-up interactions. The eciency is improved by usingthe PVs information. The vertex of the leading track is assumed to be the\signal" PV (this assumption is correct for 99% of events). Once the \signal"PV is dened only those tracks which were assigned to it are considered inthe isolation criteria. This approach increases the eciency of our algorithmat high luminosity to the same value as at low luminosity. The variation of0.50.550.60.650.70.750.80.850.90.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4ε(QCD bkg.)ε(H(200,500 GeV)→ττ, τ→1,3h+X)Fig. 8. The variation of the QCD background and the Higgs signal efficiency withthe isolation cone size (0.20, 0.30, 0.35, 0.40, 0.50) for Higgs mass 200GeV/c2(squares) and 500GeV/c2 (circles) and for two values of the signal cone size 0.05(full marks) and 0.10 (open marks). Other parameters are Rm=0.10, pmt = 3GeV/cand pit = 1GeV/c.the QCD 2-jet background and the Higgs signal eciency with the change ofthe isolation cone size is shown in Fig. 8 for Higgs mass of 200GeV/c2 (fullsquares) and 500GeV/c2 (full circles) and for two signal cone sizes Rs=0.05and 0.10. The points from left to right correspond to the following values ofRi = 0.50, 0.40, 0.35, 0.30 and 0.20. One can see that a rejection factor of 5(for the 0.10 signal cone) can be reached with an eciency ’ 75%. A higherbackground rejection of 10 (for the 0.05 signal cone) can be achieved but at aprice of the eciency being dependent on the Higgs mass.7 ConclusionsFast HLT algorithms for B physics decays and τ -jets have been developedbased on the CMS pixel detector and the innermost strip layer. The key fea-ture of these algorithms is track reconstruction based on 3 hits only. Trackparameters are calculated based on a simple helix approach. It is demon-strated that one can obtain high track reconstruction eciency ( 90%), veryaccurate primary vertex position in the z dimension ( 50µm) and a good9momentum resolution (1%4%) for tracks with pt = 1  10GeV/c. High re-duction of background rates (about 30 for B physics channels and 5 for Higgs)with a good signal eciency of 75% are achieved.AcknowledgementsThe τ trigger algorithm has been developed together with A. Nikitenko. Wewould like to thank the organizers of the Vertex2001 conference for the pleas-ant and fruitful meeting in Brunnen.References[1] The LHC Conceptual Design, CERN/AC/95-05(LHC).[2] The CMS collaboration, CMS Technical Proposal, CERN/LHCC 94-38 1994.[3] The CMS collaboration, CMS Tracker Technical Design Report, CERN/LHCC98-6 1998.[4] CMS Simulation Package CMSIM, http://cmsdoc.cern.ch/cmsim/cmsim.html[5] Object Oriented Reconstruction for CMS Analysis, CMS Internal Note1999/001 and http://cmsdoc.cern.ch/orca/[6] M. Lenzi, Vertex2000, Nucl. Inst. and Meth. A 473(2001)31.[7] M.Angarano, The silicon strip tracker for CMS, to be published in Vertex2001Conference Proceeding.[8] S.Schnetzer, The CMS pixel detector, to be published in Vertex2001 ConferenceProceeding.[9] D. Kotlinski, CMS Internal Note 2000/22.[10] D. Kotlinski, R. Kinnunen and A. Nikitenko, CMS Note 2001/017.10

High Level Tracker Triggers for CMS

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.345.9852

High Level Tracker Triggers for CMS

Abstract

Similar works

Full text

Available Versions

Crossref

CERN Document Server