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Reconstructing mH
of Higgs candidates at LEP2
Tom JunkUniversity of Illinois at Urbana-Champaign
LC RetreatUC Santa CruzJune 27-29, 2002
• A Typical LEP detector• Reconstructing Jet energies• Error parameterization• Reconstructing mH in selected candidates
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• Optimized for Z0 pole physics• Worked fine at LEP2 energies, but some lessons learned along the way.• ALEPH, DELPHI, L3 – some better features, others not as good.
A “Typical” LEP detector
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• Tracking chambers: 1.85 meter radius jet chamber 159 axial sense wires per jet cell 4 bar of gas – Ar Ethane (good dE/dx) B = 0.435 T poor z resolution (5 cm) -- improved by outer Z chambers, inner stereo+ silicon+PV constraint; endpoint constraint
down to 1.6×10-3 with silicon; better at LEP1
A “Typical” LEP detector
GeV/109.1 32
P
P
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A “Typical” LEP detector
• Calorimetry• EM calorimeter Barrel: 9440 lead-glass blocks with PMT’s outside of coil+presampler.
24.6 radiation lengths at normal incidence
Projective, but not towards the IP (reduces effects of cracks).
• Presampler• 3 cm thick, two layers of limited streamer mode tubes – axial wires + stereo cathode strips• Mounted outside the magnet coil (1.5 rad lengths thick) and before EM calorimeter• Aids in electron ID
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A “Typical” LEP detector
During installation – showing barrel EM and hadron calorimeters.Inner radius of Barrel EM: about 2.5 meters.
EM segmentation: 1 layer thick, blocks approx.2 degrees on a side. 99.97% of solid angle covered (with endcap, gamma-catcher and lum mon)
Intrinsic resolution: With material in front: worse at higher costheta (more material) and in thebarrel/endcap overlap
]GeV[%6%2.0 E]GeV[%15 E
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EM calorimetry and increasing ECM
PMT
Light Coupler
Lead-glass block
Higher-energetic EM showers at LEP2are longer. Some spilled into the plasticlight coupler, scintillating (normal light isCerenkov light from the lead glass).
nonlinearity for high energy showers
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A “Typical” LEP detector
• Hadron calorimetry • 80 cm of iron (4.8 interaction lengths) with 9 layers of streamer tubes. Pad and strip readout.• 2.2 interaction lengths of material in front -- use both EM and HCAL for measuring hadrons.
For individual hadrons
(approx. intrinsic resolution from beamtests, and also achieved with the helpof the EM calorimeter in use).
]GeV[%120 EEE
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A bit of experience with stray material
• Make sure all material is in the MC – electronics, cables, cooling, pipes, supports, misc. Nobody ever really gets this fully right (except maybe KTeV)
• e+e- energy dip near readout cards.
• 1996: OPAL had an event with large missing PT. There was an energetic photon visible in the ECAL but nowhere near enough to balance PT. It had lost energy in a steel support wheel for the jet chamber.
The good news: it was in the MC simulation. The bad news: no background events in the relevant MC had a photon hitting the wheel.
Effect characterized with LEP1 Bhabhas – very clear what was going on. -- Need lots of calibration data at the Z0!
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Jet Reconstruction
• Would like to use tracking chambers for measuring charged-particle energy and calorimetry for neutrals.
Exception: electrons – often clustered with their FSR and bremsstrahlung photons
• Can naively use tracks + unassociated clusters, but problems with overlapping showers.
EM showers: narrow. Fine segmentation should help quite a lot; improves 0
reconstruction too. Hadron showers: much broader
Can try to separate particles more with a high B field – showers at different angles then.
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OPAL’s approach: Matching Algorithm
• Similar to ALEPH, DELPHI, L3: -- in papers the result is referred to as “energy-flow objects”
• The steps:• Associate tracks and clusters• Identify electrons (also from photon conversions) and muons.
• muons: subtract MIP energy from assoc. ECAL + HCAL clusters• electrons: subtract track energy from assoc. ECAL cluster
• Apply energy compensation to remaining clusters (costheta and energy dependent) – tuneable!• Subtract track energy from HCAL clusters. (don’t go negative). If that’s not enough,• Subtract remainder from associated ECAL cluster.
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PerformanceMeasured at the Z0 Jets in HZ at LEP2 are similar to Z jets atLEP1.
Total energy resolution: 8.7 GeV out of 91.2GeV – 9.5% for the event. (~14% per jet)
W/o HCAL (re-optimized cluster compensation): 10.6% resolution, but low Evis tail.
With HCAL Without HCAL
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Jet Resolutions
• Angular resolution for a jet in =22 mrad (1.3º)
-- estimated from acoplanarity angle distribution at the Z0 in two-jet events
• Angular resolution for a jet in is 34 mrad (2°)
Angular resolutions are more importantthan energy resolutions after beam constraintsare applied (assuming no ISR).
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Hadronic Events at LEP2
A large fraction are “radiative return” eventsto the Z0. ISR photons mainly go down thebeam pipe but some are detected.
Other events: WW, full-energy qqbar,two-photon processes, ZZ
Mvis distribution for ECM=183 GeV withqqbar, WW and two-photon portions indicated.
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Jet Error Parameterization
Important to choose the right variables(so the errors can be minimally correlated)
The usual: p, cot, and
But: variations. p, 1/p or log(p) instead of cot .. Errors depend on : (example: energy)
Jet energiesare correctedvs. costhetabefore kin.fitting formoregenerality.
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Higgsstrahlung Signatures
bbqqHZee 00 bbHZee -00
bbeeHZee -00
bbHZee -00 -00 qqHZee bbHZee 00
b
b
b
b
q
q
b
b
b (q)
)q( b
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Main Backgrounds
• Z0Z0 -- mostly irreducible because the Z0
decays to b quarks and is produced with another Z0. But mrec comes out near mZ (usually), and the cross-section is small.
• Some W+W- with fake b-tags -- mostly important for 4-jet channel with mispairing. Other channels need two leptons (or no leptons).
• qqbar -- + gluon radiation: 4jet background + missing ISR with some PT, or mismeasured jets: EMIS channel.
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Kinematic Fits in Hadronic Events
• Depends on what kind of analysis you’re doing!
• Taking advantage of the clean e+e- environment:
“4C” fits – constrain E = 2Ebeam
px=0, py=0, pz=0
and use the jet errors to readjust the jet 4-vectors. Jets can have fixed masses or be constrained to be massless.
• In 4-jet events, m12+m34 measured ~4x better than m12-m34 due to total energy constraint (configuration-dependent!)
• Then again, you may not want the constraints. -- Initial State Radiation
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Kinematic Fits and ISR
ISR photon
e+e-
Very common atLEP2. Can befound with kin.fit + beamconstraint if thephoton is notdetected.
ISR
e+e-
ISR
Double-ISR.
Radiative Return to the Z0
Less common afterrequiring photonsof significant energy.
Can fake Emissignatures (eventpasses pz cuts).
Can bias mrec distibsupwards.
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Kinematic Fits in 4-jet Events
• “5C” fits: Beam energy and momentum plus another constraint.
• Fits in W+W- events: constrain mW1=mW2.
• 4-Jet e+e-H0Z0 candidates: • Choose a pairing (of six possible) of assignments of jets to H0 and Z0
-- use fit 2 and other variables in a likelihood• Use a 4C fit to find reconstructed dijet masses for the Z and H.• Define mH,rec=(mdijet1+mdijet2-mZ) -- reduces combinations from six to three. Swapping Z and H dijets results in the same mH,rec
• Resolution: approx. 3 GeV
• Jet angles are more constraining than energies.
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More Kinematic Fits used for mH
• HZbb: Constrain missing mass to mZ
and do a 5C (-3C) fit to get the visible bb mass with improved resolution.
• HZbbee: Use the recoil mass to the electron pair. Use ECAL for electron energy measurement. Do not constrain mee=mZ (although we could have)
• HZbb: Use a 4C kinematic fit to improve the muon momentum resolution (tracking not very precise at these energies). Then find the recoil mass.
• HZbb, qq. Either constrain mqq or m
to mZ in the fit. Large errors on the tau momenta.
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Resolutions of Reconstructed Higgs Mass
FromA. Quadt
Depends on channel and HZ mms
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Reconstructed mH Distributions
N.b.: Pileup of mrec near the kinematic edgefor background events (esp. qqbar events)
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Why the Pileup at the Kinematic Edge?
• Four-Jet events: gluons are preferentially radiated close to the quark jets.
1 2
3 4
Pairing jets 1+4: Z. 2+3: H. This looks likeH and Z production at rest, with mH close to themaximum allowed.
Need other information -- 3 or 4 b-tags aremore convincing in events like these.
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Why the Pileup at the Kinematic Edge?
Missing-Energy Candidates• High cross-section of qqbar at low acollinearity. Some of them will be missing lots of energy
• Mismeasurement of jets• Energy of neutrinos from semileptonic b and c decay
Nearly collinear jets means the Z0 looks atrest, and so mH is reconstructed at the kin. edge.
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Rec. mH Is not Everything!
• Cut on b-tags, other variables, plot mrec
• Or: cut on mrec, plot b-tags.• Better still, do a 2D analysis!
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How significant are the candidates?
Local s/b depending on mrec and other variableslike b-tags, tau tags, acollinearity, etc.
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A Poorly-Measured Candidate
• One electron heads for the barrel-endcap overalpregion• Lots of material here• Neither signal nor background hyp. predicts many events like this one. Low s/b.• Track is also poorly measured.• Important to start with models trying to explain events, not the other way around!
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Complicated: e+e-h0A0
• Six pairings possible in 4-jet events, all equally good.• Two reconstructed masses• The strategy: Do a 4C fit (E, P) and get two mrec’s plus errors for each combination.• For each test mh, mA pair, re-constrain to a “6C” fit using the recon. masses and errors.• Use the smallest of six 2’s to pick the pairing.• Can compare with WW (ZZ) 6C 2
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Summary• Matching tracks and clusters is necessary.
• Fine segmentation helps, but unclear how much it can pay off with HCAL -- benefit of HCAL is not much on OPAL to begin with.
• Beam constraints are very powerful.• Jet angles are most constraining -- energy measurements less so. Of course a better energy msmnt may switch the roles.• ISR can bias measurements when using a beam constraint. ISR is a bigger effect with increasing ECM. Be sure to cover polar angles close to the beam. Sitting on a resonance helps!• Jet assignment makes a big difference.
• Take lots of calibration data. Test beams don’t tell you everything.
• Analyzers will be clever.