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

new HE

m

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)

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 “4th muons” 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

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 “4th muons” 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)

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?

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

r = (x2 + y

2) ½

z560 cm

ΔM

ΔMPΔP

PGEN

PSIM

PREC

gen track

“sim-hit”

IPreco pixel

track

pT=20 GeV

Daniele Trocino et al (NEU)

D. Torcino et al. (NEU)

Summary of Important NumbersMean bend in magnetic field (cm)

RMS due to multiple scattering

Propagated pixel reco uncertainty at muon detector surfacecm Dh

pT=20 GeV h~2.5 2.5 cm 2.7 mm 0.003 ?

pT=20 GeV h~4.0 0.1 cm 1 mm 0.005 ?

pT=5 GeV h~2.5 10 cm 10 mm 0.012 ?

pT=5 GeV h~4.0 0.4 cm 4 mm 0.02 ?

Very high pT track 0 0 0 0.2 mm• 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

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 GeV N~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

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

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 • Else magnetic field bend is too small due to radial field while multiple

scattering is larger

TRIGGER CONSIDERATIONS:REGION OF ETA 1.5-2.1

13

GEM-CSC “BENDING ANGLE”• 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 resolution if 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 mm or better trigger resolution to effectively discriminate 5 GeV muons from 20+ GeV ones

YE-1/1 pT=5 GeV pT=20 GeVDxDz=30 cm 12±3mm 3±1mm

Not to scaleView from the top of the CMS down

14

Bending angle at L1 Granularity

• 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

Df(CSC-GEM)~ 4 mradDx ~8 mm

pT=20 GeVpT=5 GeV

“Close” chamber pairs “Far” chamber pairs

V. Khotilovich et al. (TAMU)

GE-1/1 trigger rate

V. Khotilovich et al. (TAMU)

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 x5Muon gun pT>5 GeV

Efficiency includes track finding only. No muon system

inefficiencies incorporated.

Reco’ed stubs pT>2Stubs from true particles w/ pT>2

Tracker

Muon System

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

YE-

2/1 Y

E-3/

1 YE-

4/1

YE-

0GE-1/1

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

Bending Angle Power• 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

S. Krutelyov et al. (TAMU)

Trigger Rate Reduction PowerSignal: Muons w/ pT=30 GeV

Ideal trigger rate reduction

Assumed realistic resolution

Realistic rate reduction

Limitation

ME-0 only 95-98% x3-4 ~0.5-1 mm x2.8-3.2

GE-2/1 only 95-98% x1.9-2 1-2mm (?) x1.4-1.5 Resolution of ME-2/1

Combined x5-6 (?) x3-4 (?)Demand presence of combined ME0-ME1/1 stub

98+% (?) N/A x1.2 Start using new additional redundancy

Demand combined ME0-ME11 stubs and stubs in YE-2, 3, 4

98+% (?) N/A x1.5 Assume full redundancy (GEMs and GRPCs)

Combined + 4/4 stubs

95% (?) N/A x5-6 ME-0

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)

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