Tracking at Level 2 for the ATLAS High Level Trigger Mark Sutton University College London 26 th September 2006

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger2 Physics rates at the LHC LHC pp collider, 14 TeV centre of mass energy. l Bunch crossing every 25ns – 40 MHz rate l Data storage capability ~200 Hz Reduction of ~ : 1 needed! Low and high luminosity regimes, 2 x cm -2 s -1 and cm -2 s -1 Between ~5 and ~25 (soft) pp interactions per bunch crossing Interesting high p T interactions complicated by “pile-up” l ATLAS will use a Three Level Trigger… l Pipelined, hardware LVL1 l High Level Trigger farms - LVL2 and Event Filter.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger3 LVL1 LVL2 EF - Latency: 2.2  s - Hardware based (FPGA, ASIC) - Calorimeter and Muon detectors only, with coarse granularity - Latency: ~10 ms, input rate < 100 kHz - Software (specialised algorithms) - All sub-detectors, full granularity - Match different sub-detector information - Work in Regions of Interest - Latency: few seconds, input rate ~ 1 kHz - Offline-type algorithms - Full calibration and alignment information - Access to full event possible ATLAS Trigger-DAQ overview

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger4 The ATLAS Detector Calorimeter Muon Detector Inner Detector

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger5 The ATLAS Inner Detector Transition Radiation Tracker Pixel Detector SemiConductor Tracker

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger6 Tracking in the ATLAS LVL2 Trigger High-p T electron/muon identification - Match Inner Detector tracks to information from outer detector (calorimeter, muon detector) B Physics (at low luminosity) - Exclusive reconstruction of golden decays (e.g. B   ) Inclusive b-jet tagging (e.g. in MSSM H  hh  bbbb) l LVL2 is the earliest stage where … l Data from tracking detectors is available, l It is possible to combine information from different sub-detectors l Precision tracking at ATLAS predominantly from the Inner Detector: l 3 layer Pixel Detector (3 layers in the end caps) l 4 Layer Semi-Conductor Tracker, SCT (9 layers in the end caps) l Transition Radiation Tracker (TRT) l Two approaches for the Silicon tracking using space points l Lookup table based tracking - SiTrack, l Complete (all layer) silicon tracking - IdScan.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger7 LVL2 processing in Regions of Interest (RoI’s) l Most LVL1 accepted events are still uninteresting for physics studies l Decision can be made by further processing only those sections of the detector that LVL1 found interesting l Minimise data transfer to LVL2 processors l Minimize processing time at LVL2 l Average RoI data size ~2% of total event l On average, ~1.6 RoI’s per LVL1 accepted event

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger8 H  

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger9 Dealing with “Pileup” events Exploit differences between interesting (high-p T ) and uninteresting (low-p T ) interactions Each has a vertex at a different z position along the beamline. The interesting pp collision should have more high-p T tracks, at least inside the region that generated the LVL1 RoI. l Ideally, we would want to Find the z position of the interesting pp interaction before any track reconstruction Select only groups of space points consistent with that z l Only then get into combinatorial tracking.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger10 LVL2 Tracking IdScan Algorithm overview l IdScan (Inner Detector Scan) Algorithm in four stages l ZFinder to find event vertex - histogramming algorithm l Pattern recognition: HitFilter for hits compatible with this z - histogramming GroupCleaner, find hit combinations consistent with single tracks. l Track fitting with hits from previous stages - Kalman Filter Fitter, extrapolate to the TRT l Vertexing ZFinder Space Points Pattern recognition track candidate Tracks Track fitting track candidate z-coordinate

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger11 ZFinder space point selection l Designed to be fast, without the need for detailed tracking High p T tracks are (almost) linear in  –z. Use ( , z) from pairs of space points from a track for simple linear extrapolation to determine track z 0 Search for hits consistent with high p T tracks Hits from high p T tracks will lie in a restricted region of  Bin hits in thin slices of , around degrees l Treat each slice (almost) independently l Take all pairs of hits and histogram their extrapolated intersection with beam line. l Fast - reduces hit combinations from lower momentum tracks

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger12 Single electron RoI (0.2 x 0.2)    O(7) good electron hits from O(200) hits

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger13 ZFinder – Jet RoI Jet RoI from WH event (120GeV)

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger14 ZFinder performance - single leptons Resolution for single lepton events with no pile-up around 200  m. Varies with both  and  p T  l Efficiencies around 95-97%. p T (GeV)  z (mm) for muons 25 GeV electrons

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger15 ZFinder efficiency l Efficiency for finding vertex within 1 mm of true position for muons with pileup. Approaches 100% for central pseudo rapidity, even at low p T. Falls off for large |  | due to degradation of resolution.  efficiency Zero suppressed scale 6 GeV muons 26 GeV muons

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger16 The Hit Filter All space points on a track originating from a given z 0 have the same  when calculated with respect to z 0. l Steps: Put all hits in a 2D histogram in ( ,  ) - (currently use 0.005, 2.4 degrees) l Accept hits in a bin if it contains hits in at least 4 (out of 7) layers l Reject all other hits (at high luminosity, ~95% of hits are rejected!) l Limits number of combinations. l Latency scales approximately linearly with number of hits.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger17  - z view x-y view  histogram  z   Pattern recognition in Pile-up events l If correct vertex is found, Hit Filter track finding efficiency approaches 100%.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger18 Group Cleaner l A group from the Hit Filter may contain hits from more than one tracks, and/or some random hits. In GroupCleaner, we exploit the (p T,   ) information to select final track candidates Similar to Hit Filter: make a 2d-histogram in 1/p T and   Select triplets of Space Points, calculate (1/p T,   ), fill the 2d-histogram l Track candidates consist of bins with Space Points in at least 4 (out of 7) layers l If two track candidates share a significant number of Space Points, keep only the longest candidate (“clone” removal) Resulting parameters, d 0 =0, z V, 1/p T,   used as starting parameters for the Kalman fitter.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger19 LVL2 Track Fit l Two Distributed Kalman Filter track fits are available to take account of energy loss and multiple scattering. l Perigee parameter estimation: l Track state is a set of perigee parameters at the perigee, surface to surface extrapolation is not needed. l Uniform B field. l Helical track approximation. l Full track fit: l Non-uniform B-field using full ATLAS field map. l Creates the set of surfaces for the extrapolation l Extrapolates track using a parabolic approximation Gives better p T and  0 estimates for tracks with large .

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger20 Track resolution from Pixel and SCT l 20 GeV muons. l CPU time per track, Xeon 2.4 GHz l Perigee fit – 0.06 ms l Full fit ms LVL2 track fitOffline  Pull   p T / p T  0 (mrad) d 0 (  m) z 0 (mm)  

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger21 Track extrapolation to the TRT l Tracks found in Pixel and SCT are used as seed tracks for extrapolation l Track is extrapolated to the next TRT layer. l Hits within a road of extrapolated position are used. The track parameters are updated including the hits using the Probabilistic Data Association Filter (PDAF) - NIM A566 (2006), pp Pixel SCT TRT

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger22 Timing performance l Single p T = 40 GeV electron RoI at high Luminosity l Mean number of space points ~ 200 l Mean execution time ~ 1ms 1 l Efficiency ~95% l B physics (low Luminosity), full Silicon Tracker reconstruction l Mean execution time O(10 ms) l Latency scales with number of tracks. 1 CPU speed of 1GHz Latency (ms) Number of tracks

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger23 Performance - Monte Carlo data  mesons from D s decay. l K mesons efficiency the same. l O(80%) at 1 GeV. l Use for resonance reconstruction …

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger24 LVL2 Vertex Algorithm l Fast iterative algorithm based on the Kalman filter. l Geometrical vertex fit (without mass constraints) – equivalent to Billoir’s “full” vertex fit. Linear transformation of track parameters reduces track covariance to a block-diagonal form (2 x x 3) allows reduction size of measurement vector in the Kalman filter and reduction in matrix size for inversions during iterations and significant speed-up in calculations. Produces estimated vertex position (x,y,z) and estimated track parameters at vertex. l Full covariance matrix for the vertexed tracks. l Invariant mass calculation, evaluation of invariant mass variance using the full covariance matrix.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger25 Invariant mass resolution LVL2 vertex fitter (0.06 ms) M J /  /- 2.4  J /  /- 2.4 Offline vertex fitter (0.16 ms) M J /  /- 2.3  J /  /- 2.2 Without TRT tracking  J /  is 63 MeV

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger26 Resonance reconstruction - D s →(  )  Fully reconstructed D s mesons from B s decay. Search for pairs of opposite sign tracks with |m(KK) –m(φ)| < 3 σ Add additional tracks to form D s and apply mass cuts |m(KKπ) –m(D s )| < 3 σ l Combinatorial background seems acceptable. M(KK) (GeV) True KK(π) combinations

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger27 Surface Cosmic Run – Barrel SCT + TRT SCT TRT

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger28 Cosmic trigger reconstruction Surface cosmic run with no B field – no p T measurement. l Cosmics - large impact parameter with respect to nominal beamline. l Tracking optimised for tracks from the beamline. l Implement a naïve shift of the space points … l Test performance of LVL2 tracking without retuning, l Efficiency of tracking and shift.

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger29 Tracking performance in surface cosmic data l Imperfectly aligned detector. l No magnetic field - large multiple scattering at low p T. l Shift fails for large hit occupancies. l Better shift algorithm will improve efficiency. efficiency

Vertex th September, Relais San Clemente M.Sutton – Tracking at Level 2 for the ATLAS High Level Trigger30 Summary and outlook l Tracking in the ATLAS Trigger is essential to achieve the physics goals of the LHC, yet must function in a very demanding, busy environment. l Pattern recognition, track fitting and vertex finding seem to work well. l Latency performance seems acceptable l Performance in high luminosity, high occupancy data seems acceptable. l Level 2 tracking algorithms successfully operational with test beam and cosmic data l Online tracking performance with real data very encouraging. l Work is always ongoing to improve the Level 2 Tracking. l ATLAS will see its first collisions in 2007 and with full energy in 2008… l Detector and Trigger well on target for readiness within this challenging schedule.