L1 Track Trigger for SuperCMS

1. L1 Track Trigger for SuperCMS D. Newbold, J. Brooke, R. Frazier CMS Collaboration • CMS Upgrade for SLHC • L1 Trigger strategy at SLHC • Track trigger architecture • Work in progress! • Next steps Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

2. The CMS Detector • General-purpose detector • Outer muon tracking / 4T solenoid • High-performance calorimetry • Robust all-silicon tracker • Not currently used in L1 trigger decision Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

3. Super LHC • Assume “SLHC” to be 14TeV, 1035/cm2/s, 20MHz • Base BX rate could still be 40Mhz in ‘high-low’ scheme • Estimate ~350 inelastic events per crossing • ~20000 charged tracks within CMS acceptance • The “Strait Plot” • 2015 timescale also driven by detector lifetime Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

4. “Super CMS” • The physics programme • Many scenarios where increased luminosity essential / desirable • New physics <1TeV ? => extend search;complement ILC • No new physics <1TeV ? => SLHC is the only game in town • Why upgrade the detector? • Inner tracking elements will be at end-of-life • Trigger and DAQ can take advantage of new tech development • In the case of trigger, new algorithms are also required • What can be done? • Muon system should cope at SLHC • Calorimeters probably out of scope for upgrade (cost, time) • Inner tracking will be replaced • Trigger and DAQ - upgrade where necessary / feasible Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

5. L1 Trigger Performance • CMS L1 trigger strategy at LHC • Use of coarsified muon / calorimeter information only • Identification of prompt leptons / photons above threshold • Trigger on leptons in combination with jets, energy sums • Use inclusive signatures + (potentially) topological criteria at L1 • How to adapt for SLHC? • “Natural scale” for lepton thresholds set by W / Z mass • Cannot simply raise thresholds for most physics channels • Control of rate via higher thresholds may not be robust • Increased use of ‘exclusive’ triggers is desirable • Maintain acceptance for precise lower mass-scale physics • Track triggering • Use information from central tracker into L1 decision • Reinforce muon / calo trigger rejection power • The same job currently done in first stage of software HLT • Add new information (precise pt, charge, z-vertex) to L1 candidates Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

6. L1 Trigger Performance • Muon L1 rate versus threshold (1034, CMS DAQ TDR) • Limited rejection power at high pt without tracker information Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

7. Track Trigger Requirements • What is needed / possible? • For muons: confirmation from tracker of isolated high-pt muon candidates • + refinement of pt measurement with extra points • For calo: increased rejection of fake e/ objects • + refinement of isolation and  ID • Rejection of uncorrelated (different primary vertex) combinations • Do not require a stand-alone track trigger • Constraints • Operate within few s fixed L1 latency • So cannot perform ‘selective readout’ of tracker, or iterative algos • Do not add substantially to tracker material / power budget • Reasonable bandwidth for readout • Reasonable processing density off-detector • Robust w.r.t background, inefficiency, alignment, etc • Interface to any non-upgraded elements of current L1 system • Muon / calo trigger upgrade can provide enhanced space / pt resolution Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

8. Technical Challenges • On-detector • Raw information from tracker is huge in size • Communication bandwidth likely to drive on-detector power budget • Tracker is a highly integrated electromechanical system • Trigger functionality must be integral part of new design from the start • Off-detector: experience from existing L1 trigger • Communications density / data concentration is the key problem • In particular: dealing with overlaps / edges can be very hard • Processing density not a constraint; deep pipelining possible • Heavy on-detector data reduction is required • Communication between tracker layers is probably impractical • Require multiple stand-alone measurements of candidate tracks • Implementation of a track trigger will be challenging! • Focus on reduction of the key parameter: readout bandwidth Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

9. Trigger Concept: On-Detector • Use ‘stacked layer’ concept: • Presented before in inner pixels context • See talks of J. Jones et al • Use two space-points from sensors separated by some mm • Correlate 2D hits for track stub position & slope in /  • Window cut in  excludes low-pt minbias tracks • Also cuts down correlation logic • Modularity to match calo trigger towers(0.0875 in / ) • One trigger / readout ASIC per TT performs hit correlation • Output 4 candidate stubs (<nch> < 2 at r=0.6m for pt > 3GeV/c) • If >4 candidates, count and flag as possible jet activity • Detailed information is not required, since isolation cut already failed Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

10. Trigger Concept: On-Detector • Two specialised trigger planes in ‘outer tracker’ • Low occup. / long lever arm at large r allows coarse-resn sensors • Reduced cost and power consumption compared to a true pixel layer • Outer (r=1.2m) plane allows correlation with muon / calo objects • Inner (r=0.6m) plane gives b/g rejection, allows  / z measurement Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

11. On-Detector System Parameters • Design for track acceptance for pt > 4GeV/c • Largely above minbias spectrum, still good acceptance for jet /  tracks • Track curvature lies within ±1TT in ; limits data-sharing requirements • Sensor doublet must cover all physical high-pt track trajectories • Hit resolution requirements • Resolution in  dictated by practical limit of layer spacing • Spacing upper limit from accidental coincidence reduction - to be tuned • At 10mm spacing, resolution > 0.5mm • Resolution in z dictated by slope-matching + vertex z-resolution • At 10mm spacing, resolution > 2mm is adequate (c.f. <dzvtx> ~ 70mm) • Use full-precision 2D layers, also functioning as stereo layers? • Readout requirements • (4b+4b) position, (4b+4b) slope (with offset) will allow stub matching • Implies ~1200 x 10Gb/s readout fibres • Can reduce further (factor of 2?) using on-detector data compression • Zero-suppression; pt sorting; variable-sized payload (asynchronous links) Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

12. Trigger Concept: Off-Detector • Divide trigger processing regionally • 36 regional subsystems each process a half-detector, 20 degrees in  • Each subsystem process muon, calo and track information • Data-sharing (~25% of data) covers track propagation between regions • Simply a duplication of input data - passive optical splitting? • Track finding • Hold track segment info until muon / calo objects are available • Seed track building from candidates; define restricted ROI at inner layer • Cuts down enormously on correlation logic • Outer stub eta directions used to identify + request inner layer stubs • A match in eta / phi slope (using beam constraint) flags physical track • Output to global trigger • Fixed number of muon / calo / jet candidates with pt / Et, charge, quality • Track-based jet tag for isolation purposes • Also possibility of track-count  ID, etc etc. Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

13. Processing Topology • Generic FPGA-based board for all subsystem functions? Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

14. Technical Issues • Efficiency • There is no tracking redundancy in this scheme • What is the achievable hit efficiency for trigger sensors? • Are the geometric overlaps acceptable? Coping with dead channels? • Occupancy • Simple simulations take no account of  conversions, noise, etc • In particular: simulate / measure calo backsplash at the outer radius • Alignment • Assume stacked layers will have adequate mechanical alignment • Is this true for the alignment between the two layers? • Endcaps • Work so far concentrates on the barrel portion of CMS • Endcaps can follow the same scheme in principle • But data-sharing topology will be more complex • Resolution requirements will be different - possibly less demanding • The ‘overlap region’ is yet to considered Dave.Newbold@cern.ch TWEPP 2007, 4/9/07

15. Summary / Next Steps • SuperCMS at the SuperLHC • An programme of upgrade R&D has begun • L1 triggering a key area; current approach cannot work at SLHC • Track trigger concept • First look at ‘system architecture’ seems challenging, but promising • No obvious show stoppers at the architecture level • Correlator ASIC design & tracker integration are the key questions • Next steps • A programme of more realistic simulation is now required • Performance simulation on realistic full events is a quid pro quo • Details of on-detector implementation to be evaluated by experts • Proof-of-concept algorithm / processor design to be generated • Hope to work soon with expert collaborators to push this forward Dave.Newbold@cern.ch TWEPP 2007, 4/9/07