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Phase-2 Muon Performance Projections and Evaluation

Phase-2 Muon Performance Projections and Evaluation. D. Abbaneo, M. Abbrescia, E. Barberis, C. Bedoya, M. Dallavalle, J. Hauser, K. Hoepfner, V. Khotilovich, S. Krutelyov, D. Nash, P. Paolucci, A. Safonov , A. Sharma, D. Trocino. Phase-2 Near Tagger ME-0 Scenario.

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Phase-2 Muon Performance Projections and Evaluation

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  1. Phase-2 Muon Performance Projections and Evaluation D. Abbaneo, M. Abbrescia, E. Barberis, C. Bedoya, M. Dallavalle, J. Hauser, K. Hoepfner, V. Khotilovich, S. Krutelyov, D. Nash, P. Paolucci, A. Safonov, A. Sharma, D. Trocino

  2. Phase-2 Near Tagger ME-0 Scenario • Near tagger ME-0 at the back of present HE • Coverage: 2.1<|h|<4.0 • Upper portion of 2.1<|h|<2.5 has trigger capabilities • Lower portion is only used in the offline m new HE

  3. Physics Scenarios • Scenario A: recover the high eta muon from H->ZZ • Look for the 4th muon in events with 3 reconstructed leptons • Increasing muon coverage to h=4.0 adds about 50% to the current CMS acceptance • The vertex in this scenario is known (tagged by 3 leptons) • Scenario B:a search for exotic two photons plus a muon events • Will also see a large increase in acceptance • More difficult as the vertex is not known (photons don’t tell you which vertex is yours) • Question: can we find the 4th muon effectively? • If the new system reconstructs a lot of “muons” (fakes) in every event, we will conclude that the answer to above is “No” • Qualification of “a lot” is analysis dependent (having a few fakes that don’t fit the kinematics of your search signature can often be okay)

  4. Scenario 1: Pixel Tracker Only • Search for H->ZZ->4mu as 3mu+forward pixel extension track in events with 3 reconstructed leptons • Scenario A requirements: we don’t want many reconstructed “4thmuons” in either signal or background • Tracks of interest: pT>5 GeV (p~25-100 GeV at this h) • Simulation at generator level (@13 TeV): charged particles/side per minbias event • Start at lower pT (3 instead of 5) to be conservative, double the number to account for photon conversions • Ntrks<0.25 “muons” (fakes) in 2.0<h<4.0 per BX (we know the vertex) • Scenario B: vertex is not known, need to multiply by NPU • Ntrks~50 “muons” (fakes) for NPU=200 per BX • Summary: one can possibly get away with a tracker-only solution for such over-constrained signatures as HZZ, but in general the fake rate is way too high

  5. Scenario 2: Muon System Only • A new muon system is used to look for the 4th muon in events with 3 reconstructed leptons • Assume no tracking, so a muon is just a hit and no momentum information • Scenario A: we don’t want many reconstructed “4thmuons” fitting reasonable analysis selections in either signal or background • Assumes a well-shielded and multi-layer new system, we use CSC LCT numbers to extrapolate to higher eta coverage (1.3 LCTs/BX at PU=25) • Number of LCTs in CSC: Nm(CSC)~ 1.3/ BX at PU=25 • Dominated by soft muons (neutron contribution is suppressed) from all sources (b-jets, decays in flight, decays in the calorimeter/punch-through) • The threshold to get through the material is over 3 GeV in p (not pT) • Simulation: the mean number of tracks with p>3 GeV in 1<|h|<2 (CSC coverage): N~1.2 per min-bias event • The mean number of tracks with p>3 GeV in 2<|h|<4 (ME-0 coverage): N~13 per min-bias event • Final estimate for the total number of “muons” in ME-0: • Nm(ME-0, PU=200) = Nm(CSC, PU=25) x 200 / 25 x 13 / 1.2 ~100 muons per BX at PU=200 • Conclusion: this will not work even for H->ZZ->4mu (equally bad for either physics scenario)

  6. A Combined System • Minimal requirements for muon extension: • What is required detector segmentation? • Should be less than multiple scattering for target range of muon pT • At these h, pT~5 GeV corresponds to p~20-100 GeV (“Higgs range”) • Can it work for offline? • Do we get so many reco tracks matching a single muon hit within the matching window that such tagging stops being useful? • Do we get too many muon hits so that an event has too high probability of having a high momentum muon reconstructed given the matching windows used? • Assume adequate shielding and multiple redundancy so that muon hits are primarily due to muons (primary or secondary) like in the current system. • Can it improve momentum measurement using larger lever arm? • What segmentation (spatial resolution) are needed to improve track fit momentum measurement? • Is the bending of the track in the magnetic field at the muon tagger z exceeding what is expected deviation due to multiple scattering at the targeted pt? Is segmentation sufficient to measure the difference?

  7. Establishing Basics: Multiple Scattering • Estimate multiple scattering: • Use FastSim muon gun with pT=20 GeV and current muon propagator • Assumes the same amount of material as in the current detector • New muon detector at z=560 cm • For pT=5 GeV, multiply RMS by ~4 • Daniele Trocino et al (NEU) D. Torcino et al. (NEU) “sim-hit” PSIM pT=20 GeV ΔM PGEN ΔMP ΔP gen track PREC r = (x2 + y2)½ reco pixel track IP 560 cm z

  8. Summary of Important Numbers • Bend is currently back of envelope (can be off by up to a factor of 2-3) • Accuracy in propagating a straight pixel track: assume 100 mm precision at Z=250 cm (halfway between IP and muon system) and perfect vertex finding (we are in offline!) • A 2-sigma matching window size (determined by multiple scattering) range DhxDf~0.05x0.04=0.002 for pT=5 GeV • Assume we are not constrained by the too crude detector segmentation here

  9. Combined System: Estimates • We are matching tracks and muon hits • The aim is to minimize fakes from accidental overlaps of muon hits and “energetic” pixel tracks (pT=t qualifies as energetic) • The matching window is about DhxDf=0.002 (pT=5, for higher pT it’s smaller) • Total area of the detector: DhxDf=2x2xp=12 • Total area fits 6000 matching windows • Scenario A (event vertex is known): • About 0.25 pixel tracks and about 100 muon hits per BX • Crudely, an average number of accidental matches with pT>5 GeV: N~ 0.25 tracks *100 muon hits / 6000 windows ~ 0.004 fake muons per BX • Scenario B (event vertex is not known): • The number has to be multiplied by NPU • For PU=200, N~0.8 fake muons/BX for pT=5 GeV • For pT>10 GeVN~0.08 fakes • Very crude estimate, additional muon-tracker compatibility checks can presumably reduce this figure further down, detailed studies are needed • However, the conclusion is clear: such system will work and will be a general purpose system

  10. Muon Momentum Measurement • The new system can potentially contribute to momentum measurement • At h=4.0, the bend is equal to the amount of multiple scattering • Seems like a showstopper • But at h=2.5 the bend is x10 larger than multiple scattering • For pT=100, it is 5 mm at h=2.4, should be easily measurable • This need to be studied • Tracker-only resolution for tracks with pT=2, 10 and 100 GeV

  11. Intermediate Observations • Neither tracker alone nor muon system alone can provide a general purpose muon tagging in the very forward region • A combined system of forward pixel extension and a new multi-layered muon detector meet the requirements • Redundancy is important, else we will be swamped by muon backgrounds! • The best place for the muon system is in front of ME-1/1: • At ME-2/1 the area of the matching window is twice larger doubling the fraction of overlaps • If we hope for any kind of momentum measurement improvement (or confirmation) from the muon system, it cannot be further away than ME-1/1 • Elsemagnetic field bend is too small due to radial field while multiple scattering is larger

  12. Trigger Considerations:Region of ETA 1.5-2.1

  13. GEM-CSC “BENDING ANGLE” View from the top of the CMS down Not to scale • L1 muon momentum resolution can be improved with a second detector: • Inner tracker tracks at L1 – best solution, but not available until LS3 • A second muon system could improve momentum resolutionif one can measure the “bending angle” • Used in the Barrel; CSCs are too thin (~11 cm) to see the bend • A new detector in YE-1/1 (least affected by scattering, largest B) • Increase “lever arm” from 10 cm to Dz=30-50 cm (physical constraints) • Need ~2 mmor better trigger resolution to effectively discriminate 5 GeVmuons from 20+ GeV ones

  14. Bending angle at L1 Granularity Df(CSC-GEM)~ 4 mrad Dx ~8 mm • GEM-CSC bending angle measurement using full GEANT simulation: • Muons with PT=5 and PT=20 GeV • GEM L1 Trigger pads (4 strip OR) • Good discrimination and a powerful new handle on pT resolution • An “OR” of two GEM chambers within a super-chamber is ~100% efficient “Close” chamber pairs “Far” chamber pairs V. Khotilovich et al. (TAMU) pT=20 GeV pT=5 GeV

  15. GE-1/1 trigger rate V. Khotilovich et al. (TAMU)

  16. Trigger Considerations:Region of ETA 2.1-2.5

  17. Trigger Considerations • Muon Level-1 Trigger will rely on tracking trigger and Muon matching • CMS is in a real danger to lose triggering capabilities in 2.1<|h|< 2.4 • Efficiency losses in L1 Track Trigger are due to tighter selections • Can loosen, but fakes will shoot way up • This is the exact same region where muon trigger rates shoot up: • Weakness in momentum measurement causes trigger rate to shoot up by x5 Reco’ed stubs pT>2 Stubs from true particles w/ pT>2 Tracker Muon gun pT>5 GeV Efficiency includes track finding only. No muon system inefficiencies incorporated. Muon System

  18. Saving Muon Trigger Beyond h=2.1 • Need to replicate the GE-1/1 success in h<2.1 onto this more forward region • A possible solution: • Create a category of “loose” L1 tracking trigger track candidates for forward muon triggers • Less hits means less accurate momentum measurement and allowing fake rate to shoot up • Match track to an improved muon system • Let muon requirements suppress the rate increase associated with loosening tracking selections • Optimize combined selections to keep efficiency high and trigger rate low • Assume that between the two systems we will have enough information to make a sensible decision and keep the L1 output rate at the acceptable level

  19. Scenario • New systems: a near tagger ME-0 and redundant systems GE-2/1, RE-3/1,4/1 • GE-1/1 is assumed to be in, but it is not relevant for this discussion (beyond h=2.1) • Purpose: attempt to re-utilize the bending angle idea and the additional redundancy to reduce trigger rate • Evaluate gains of a combined system and separate impact of each of the two parts • Evaluate required parameters of each system GE-1/1 YE-4/1 YE-3/1 YE-2/1 YE-0

  20. Bending Angle Power S. Krutelyov et al. (TAMU) • Assume detectors with perfect spatial resolution • But faithfully simulate multiple scattering • Measure bending angle in YE-1/1, 2/1, 3/1, 4/1 for pT=10 GeV • Assume muon trigger threshold of ~20-25 GeV: the rate is dominated by mismeasured softer muons • Important to have a handle on momentum measurement for softer muons • YE-3/1 and 4/1 have almost no power: • Tracks bent back by radial field, any residual bend is smeared by multiple scattering Width is driven by multiple scattering and by dependence of bend on eta within a chamber

  21. Trigger Rate Reduction Power

  22. Trigger Summary • No significant muon trigger rate reduction possible without ME-0, the new “near tagger”: • The only other alternative is to replace ME-1/1 with a “thick” high precision detector • ME-0 alone will provide about x3-4 in rate reduction through the use of bending angle and additional redundancy • Addition of GE-2/1 and two stations of GRPC each provide an additional reduction factor of ~1.5 • GE-2/1 can have lower segmentation compared to GE-1/1, thus lower electronics cost • GRPCs can also have lower segmentation (if we only need a confirmation)

  23. Grand Summary • This is very preliminary (!) : • By far the most effective solution for extending CMS muon coverage is the near tagger • Implies forward pixel extension • Moving the tagger further out causes a fast increase in the rate of coincidences (many fakes per event) and drastically reduces ability to do any kind of momentum measurements above 10 GeV

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