Charged Higgs – Uppsala 2006 C.H. Shepherd-Themistocleous- RAL 1
C. H. Shepherd-Themistocleous Rutherford Appleton Laboratory, UK
Identification of tau particles in the CMS detector
Rutherford Appleton Laboratory
cH±arged 2006, Uppsala University, Sweden, 13-16 September 2006
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Outline
Properties of particles
CMS detector
identification techniques in hadronic decays at CMS
• Isolation• Decay length• Impact parameter• Invariant mass
N.B. HLT performance later this afternoon
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Characteristics of Tau decays
Lifetime c 87 m (0.29 ps) , m1.78 GeV/c2
Decays: 65% hadronic , 35% leptonic• Hadronic
– 1 prong 50 % : n
– 3 prong 15 % : 3n
tau jets at LHC:• Very collimated
• Low multiplicity
– One, three prongs
• Hadronic, EM energy deposition
– Charged pions
– Photons from 0
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tau tagging
Properties used for tagging at CMS– Narrow jets
• ECAL isolation
• Tracker isolation
– Significant lifetime
• Impact parameter
• Decay length
– Invariant Mass
Backgrounds – QCD jets
– Electrons
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Compact Muon Solenoid
Tracker
4T solenoid
Muon chamber
s
HCAL
Iron yoke
Total weight: 12,500 tOverall diameter: 15 mOverall length: 21.6 mMagnetic field: 4 T
Si microstripsPixels
BarrelDrift tubes (DT)Resistive platechambers (RPC)
EndcapsCathode Strip Chambers (CSC)Resistive platechambers (RPC)
Plastic scintilator/brass sandwich
Scintillating PbWO4
crystals
ECAL
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The CMS Tracker
5.4 m
Endcap Strips
Outer Barrel Strips
2.4
m
Inner Barrel Strips
PixelsThe World’s largest
Silicon Tracker = 250 m2
!
10 layers of Silicon Strip Sensors surrounding 2-3 layers of Silicon
Pixel Sensors.
15000 silicon modulescontaining 76000000 pixels +
strips !
TEC
TOB
TID
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ECAL
High resolution electromagneticcalorimetry is central to the CMS design
m / m = 0.5 [E1/ E1 E2
/ E2 / tan( / 2 )]
Where: E / E = a / E b c/ E
Aim: Barrel End cap Stochastic term: a = 2.7% 5.7% (p.e. stat, shower fluct, photo-detector, lateral leakage)Constant term: b = 0.55% 0.55% (non-uniformities, inter-calibration, longitudinal leakage)Noise: Low L c = 155 MeV 770 MeV High L 210 MeV 915 MeV(dq relies on interaction vertex measurement)
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Events used
tau sample – taus only i.e. no pile up, no underlying event
– pT jet > 30 GeV, uniform in |
QCD sample
– di-jets events in Pythia ET 30-150 GeV, Rsep > 1.5
– True energy is that found when using cone size 0.5.
Matching: R(Calorimeter jet axis – MC jet axis) < 0.2
Efficiency for QCD events to pass preselection and matching ~12%
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ECAL isolation
candidates Pisol < cut value
Efficiency of rejection of QCD jets increases with ET. — Low pT tracks (<2 GeV/c) bent out of cone
Achieve 80% efficiency with bkg rejection of factor of 5 for QCD jets with pT > 80 GeV/c(wrt presel. and matching)
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Tracker Isolation
axis of calo jet
Jets reconstructed with iterative cone algorithm
Look for tracks inside jet-track matching cone Rm (0.1) with pt > 6 GeV
Form signal cone around track with highest pT.
Tracks inside Rs with z d0 within z (2mm) of leading track deemed to be from tau
Tracks reconstructed within Ri.
Require pT > pTi (1 GeV) and within
z (2mm) of leading track.
Tracks 8 Si hits with at least 2 pixel hits.
Isolation requires no non-tau tracks within Ri.
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Tracker Isolation Performance
Single Tau (30<ET<150 GeV)
QCD jets50<ET<170 GeV
Ri Ri
Bins 130-150, 80-110, 50-70, 30-50 GeV
ET
inc
Single tau simulated events QCD events generated in bins of pT.
Efficiency wrt preselected and matched events
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Impact parameter tag1 prong 3 prong
QCD tail due to fake tracks. Hits onreconstructed track from various true tracks.Majority at large Extrapolation distances larger
Little discriminationin 3-prong events
IP for highest pT track
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d0 performance
• Efficiency of transverse d0 significance cut. d0 < 300 m
• Mean error ~ 15 m (1-) 16.7 m (3-) : QCD 17.9 (1-) 22.2(3-) m
Tracker isolation required Significance cut
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Electron Rejection
Electrons can fake 1-prong taus.
Events selected requiring ECAL and Tracker isolation
Background suppressed using HCAL information
Minimum requirement on energy of most energetic HCAL tower in the jet.
HCAL cut electron jet
ET 40-60 GeV
jet
ET 100-140 GeV
> 1 GeV 0.08 0.936 0.977
> 2 GeV 0.03 0.854 0.942
Performance of HCAL cut for leading track pT > 10 GeV
All distributions normalised to 1.
Tail due to gaps in ECAL
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Decay length tag
Very collimated jets lead to shared hits in pixel layers.Decay length < 35 mm required.
Events required to pass tracker isolation and have 3 tracks in the signal cone.
Probability ~ 63% for 3-prong decays
b and c quark jets not a major problem(~12% c 3% b)
-jetsQCD jets
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SV - decay length
Secondary vertex resolution in jet events
Resolution transverse to jet axis Resolution parallel to jet axis
• Analysis used the Kalman vertex fitter (KVF)
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Decay length performance
Performance as a function ofsigned transverse decay lengthsignificance
Error in decay length dominated by secondary vertex. - Primary vertex:
QCD events use pixel vertex finder events smear z by 60m
Rejection factor of 5 possible for a signal efficiency of 70-80%(efficiency calculated wrt MC preselection and matching and tracker isolation)
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Mass tag
Mass reconstruction uses track momenta and energy of ECAL clusters.
Clusters matched to tracks are removed to avoid double counting. — Clusters only used if track - cluster R > 0.08 — Use clusters within cone of 0.4
Due to 1-prong decays
ET 30-50 GeV ET 130-150 GeV
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Mass tag performance
— Tracker isolation required
— Mass cut < 2.5 GeV/c2
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tag efficiency determination
Method: Use in single muon triggers
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efficiency II
Principal backgrounds: t t , W + jet, QCD
Error on tag tag efficiency using 30fb-1 of data.
This method allows verification of MC at Z energies. The efficiency is a function of jet energy. – Greater collimation
• lower probability of signal tracks outside signal cone• greater probability of tracks sharing hits
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ID performance
From R. Kinnunen’s talk on Wednesday
For this channel: Rm = 0.1, Rs = 0.07, Ri = 0.4 with veto on tracks with pT > 1 GeV in the isolation coneECAL isolation for jet: ET
cell (0.13<R<0.4) < 5.6 GeV, R defined around the jet direction Electron contamination suppressed with a cut on maximal HCAL cell (ET > 2 GeV) inside the jet cone pleading track / E > 0.8, to exploit the opposite helicity correlations in in the H± -> and W± -> decays, leading to harder pions from H± ->
Efficiency: signal 11-15%, tt->WWbb -> bbl 5%, W+3jets 1%, tt->WWbb -> bbl’’l 2%
Signal process defined as gg -> tt -> W±H±bb->lbb, l = e or mH+ = 140 GeV
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ID performance II
From R. Kinunnen’s talk on Wednesday
Associated production gg -> tbH± , H± -> Level 1 jet trigger ET > 93 GeV
-selection efficiencies including pre-selection, trigger and off-line mmH± (GeV) (GeV) 200 400 tt Wt W+3j QCD
Total efficiency 3.8% 9.0% 5.0x10-4 6.7x10-4 1.9x10-3 9.2x109.2x10-5-5
Offline Offline identification:- jet reconstruction in the direction of the triggeredjet, ET > 100 GeV- leading track within R < 0.1 around the jet direction- small signal cone around the leading track, r=0.04- one or three tracks in the signal cone- isolation of the signal cone in 0.04<R<0.4- addional quality cuts for the leading track: transverse impact parameter < 0.3 mm and at least 10 hits in the tracker (signal efficiencies ~95%)- ET of maximal HCAL cell in the jet > 2 GeV to remove electron contamination- pleading track/E jet > 0.8, exploits the helicity correlations
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Summary
Reconstruction utilizes characteristics of tau jets
Principal methods are:– Isolation in ECAL & tracker– Decay length– Impact parameter – Invariant mass
A method for determining efficiency from data has
been studied.
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Backup slides
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Choice of jet cone size
Cone size of 0.4 chosen. Contains 98% of jet energy and good energy resolution
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Energy scale corrections
Tau jets need softer corrections to their
energy, wrt QCD jets.– For the same transverse energy,
pions in Tau jets have harder transverse momentum than pions in QCD jets
– In Tau jets there is a larger amount of electromagnetic energy (due to the presence of p0)
– Corrections parametrized as function of ET and
The jets corrections optimised for true hadronic taus significantly underestimate energy scale for QCD jets
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Performance
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trigger
Level-1 (~µs) 40 MHz High-Level ( ms-sec) 100 kHzEvent Size ~ 106 Bytes
Level-1 (~µs) 40 MHz High-Level ( ms-sec) 100 kHzEvent Size ~ 106 Bytes
40 MHzClock drivenCustom processors
100 kHzEvent drivenPC networkTotally software
100 HzTo mass storage
two trigger levelstwo trigger levels
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L1 trigger
QCD ET samples in the range 50-170 GeV were used for the HLT studies. They represent more than the 90% of the total L1 Rate
A factor ~103 of QCD background rejection is required at HLT
— Reduce rate from ~kHz -> ~Hz
Active towers patternsallowed for tau jetscandidates
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HLT
Two trigger algorithms “Calo+Pxl trigger ” and “Trk trigger”
– “Calo+Pxl trigger”: only pixel hits and calorimeter isolation used• Fast – limited track reconstruction• Good performance for isolation• preferred for decays with two taus in the final state (like A/H-
>tautau)
– “Trk trigger”: (some) hits of the microstrip inner tracker used, no calorimeter isolation
• slower than “Calo+Pxl”• much better resolution for track momenta • useful in channels like charged Higgs boson decay ( plus missing energy selection)
– tight cut on the pT of the leading track
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ECAL isolation
Efficiency evaluated for bbH bb+ samplew.r.t. L1 trigger.
QCD di-jet events in range pThat:50-170
GeV used to evaluate background suppression.
Rejection factor 3 is given by Pisol < 5.0 GeV
Used with Pixel isolation to form trigger
Isolation is applied to the most energetic) HLT calorimeter jet.
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Calo + Pixel HTL
Track isolation alg. similar to offline. Tracks constructed from 3 pixel hits only.
Isolation cone varied from 0.2 to 0.6 (step 0.05). Signal cone 0.07, matching cone 0.1, leading track pT > 3 GeV/c.
Single tag Double tag
Efficiency calculated w.r.t L1 trigger
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Charged Higgs Trigger
Channel considered
gg->tbH+, gb->tH , H+ -> (tau hadronic decay)
L1 output rate: ~ 3kHz
HLT selection
ETmiss>65 GeV: output rate ~30 Hz
After applying Tracker isolation + momentum cut (PT
LT>20GeV):— output rate: ~ 1Hz
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performance in example channel
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HCAL
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H0/A0 decaysProvides best reach large tan : + , + had, had + had had+had final state:
Backgrounds: QCD ( muli-jet fake ) ; Z/* tt ; W+jet, W . Requires hadronic trigger Large associated production allows good rejection with b tag. “jet” (1-, 3- prong) tagging, lifetime
Potential SUSY background. - decays. negligible
b tagging
QCD ~ 106 Mass resolutionrejection ~ 15%
~
ExploitbbH0/A0
production
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Provides clear signature for BSM physics.
Production: – mH
± < mt: tt , t H± b– mH
± > mt: gb t H± ,gg tbH ±, qq’ H±
Backgrounds: tt ; Wtb, W
Signal : Look for lepton from top + had
Spin correlations from H harder than from W . Require 80% jet energy carried by +
Plot transverse mass. (missing >1 )– Signal endpoint ~ mH
– Background endpoint mw
H± decays
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add properties of sub detectors
resolutions