Post on 18-Dec-2015
b-tagging for the high-pT physics program of ATLAS
Laurent Vacavant
FZU/Academy of Sciences & FJFI/Czech Technical University – Prague – Dec 19 2008
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 2
Introduction
• b-jet tagging: identification of jets stemming from the hadronization of a b-quark, in high-pT processes
B-physics• A key ingredient for:
– top quark physics– Higgs searches and properties– SUSY and BSM models
• This talk:– a few physics cases (top, Higgs, SUSY)– b-tagging basics– tagging algorithms– performance– how to measure the performance in data
All material to be available in: Expected Performance of the ATLAS Experiment,
Detector, Trigger and Physics, CERN-OPEN-2008-20
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 3
Top-quark physics
• At least 1 btag: Ru ~200, b = 63%
Muon+jets channel
(ttbar) 23.6%
Top physics @ LHC: • production (BSM?)• precise top mass• properties (charge, spin, etc)
Signal extraction:•σ = 830 (350) pb @ 14 (10) TeV •favorable situation: S/B 10x higher than Tevatron
• b-tagging still useful to kill light jet background
Lepton+jets, 100 pb-1:S/B 2x (4x) better requiring 1 (2) b-jets
1 jet pT> 30 GeV
3 jets pT> 40 GeV +
PT(lep) > 20 GeV
Missing ET > 20 GeV
No b-tagging
modest efficiency OK
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 4
Higgs sector
SM Higgsbb (searches & properties): • natural interest for low mass region (Γff ∼ mf
2)• revived interest for WH(bb) (w/ high-pT jets) (Butterworth et al., PRL 100,242001,2008)• direct production not visible associated prod. ttH(bb) channel, WH channel
• H properties: Yukawa coupling with ttH
SUSY Higgses:• coupling to b enhanced by sinα/cosβ for 2HDM
models like MSSM @ high tanβ• production of neutral H in association with b quarks:
bbH, H
≳70% needed !
soft b-jets
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 5
Low mass Higgs: ttH
• ttH(bb) @ LHC (14 TeV), mH=120 GeV:• σ = 0.519 pb (LO), K-factor=1.2• complex final state: for l+jets:1 lepton, 6 jets (4 b-jets), Et
miss
• b-tagging crucial to reject huge tt+light jets background
• excellent efficiency required (4 b-jets)• systematics critical performance under control: b 20% change in selection eff., Ru 3-10% change
cut ttH ttbb(EW) ttbb(QCD) tt+jets
1 Lepton (fb) 56.9 141 1356 63710
+ 6 jets (fb) 36.2 76.7 665 26214
+ 4 b-jets loose (fb) 16.2 23.4 198 2589
+ 4 b-jets tight (fb) 3.76 4.2 29.6 50.7
500
Significance of lepton+jets channel, 30 fb-1: 2.2but requires precise background extraction
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 6
Exotics
• Plethora of models• Often heavy particles decay primarily to heavy quarks
t/b/c
LR Twin Higgs model
RS Extra-dim: KK-gluon
σ ~15 pb for m=1 TeV (@ 10 TeV)Large branching to top: > 90%
Common pattern for exotics: most often high-mass particles giving rise to (very) high-pT b-jets ( O(TeV) ) challenging experimentally
b-tagging basics
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 8
b-tagging basics
BBa0<0
a0>0
x
y
Secondary Vertex
Primary vertex
Jet axis
Soft lepton
Soft-lepton tagging:• Low pT electron from B (D)• Low pT muon from B (D)
Hard fragmentation of b quarks xB ~ 70%High mass mB ~ 5 GeVLifetime of B hadrons:
c ~ 470 m (mixture B+/B0/Bs) , ~ 390 m (b) for E(B) ~ 50 GeV, flight length ~ 5 mm, d0 ~ 500 m
Spatial tagging:• Signed impact parameter of tracks (or significance)• Secondary vertex
(limited by Br: around 20% each)
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 9
b-tagging ingredients
• Jets (direction):•to assign tracks:•to sign impact parameters
•Tracks:• impact parameter resolution
(35m for a central 5 GeV track)• tracking in (dense) jets• specific quality requirements
• N(hit B-layer) > 0 • shared hits, etc
• Primary vertex:• mostly needed along Z (σ~50m)• @ high-luminosity
• Lepton ID (soft leptons)
σ(d0) ~ 10 ⊕ 150/pT√sinθ m
4.022 R
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 10
Pattern-recognition inside dense jets
Example of very different treatment (shared hits, errors) :
• Specific to tracking in jets• NewTracking can be tuned differently (e.g. r12, r14)• Important for strategy with real data ! 20% diff. in b-tagging
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 11
Tracks with shared hits
0.951
Tagging algorithms
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 13
Impact parameter-based b-tagging
Simplest algorithm: • counting tracks with large d0 or d0/σ
Impact parameter: • signed w.r.t. jet direction• use significance (d0/σ)
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 14
JetProb tagging algorithmJetProb algorithm: (à la ALEPH)
• compatibility of tracks with primary vertex•resolution function extracted from data
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 15
Likelihood ratio IP taggers
trN
i i
ijet Su
SbW
1 )(
)(ln
IP likelihood ratios: • smoothed & normalized significances for the two hypotheses: b(S), u(S) requires PDFs (from MC or data)• for each track: ratio b(S)/u(S)
In 3D: • use 2D PDF for (d0,z0) correlations
R impact parameter significanceZ
im
pac
t p
aram
eter
sig
nif
ican
ce
Z vs R
trN
i ZiRi
ZiRijet SSu
SSbW
1 ),(
),(ln
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 16
Inclusive vertex:1. Find all two-track vertices (tracks away from primary vertex: L3D/σ > 2)
2. Remove vertices compatibles w/ γ conv., Ks, Λ, material inter.
3. Fit all remaining tracks into single inclusive vertex4. Remove iteratively worst tracks5. Use distance to PV or properties of the fitted vertex for discrimination:
• number of two-track vertices• mass of the vertex• energy fraction
(NB: Lxy not used, correlated with IP)
Secondary vertex-based b-tagging
N
Ks Λ
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 17
Secondary vertex-based b-tagging
SV
X m
ass
M
F ratio
b-jets
light jets
),,(
),,(log
),(
),(log
1 00
00
NFMu
NFMb
zdu
zdb
WWW
trackN
i ii
ii
vertextracksjet
Basic algorithm:• SV0 tagger: returns L3D/σ
MF
LR approach: templates (PDFs) for b and light hypotheses:• SV1 tagger: 1D for N’, 2D for (M’, F’)• SV2 tagger: 3D for (N’, M’, F’)• Fold in also probability to find vertex
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 18
Main idea: try to constrain all tracks from the B/D hadron decays to intersect the same flight axis (instead of the common vertex approach)
JetFitter: a new dedicated Kalman filter fitting:
Treatment of incomplete/1-prong topologies:• on average 1.9 (1.7) OK tracks from D (B) decays
Based on likelihood:• categories for topology • vertex mass• energy fraction• significances of {disti}
~20% improvementin light jet rej.
promising for b/c separation
Topological reconstruction of B/D decay
charm
jet
reje
ctio
n
light
jet
reje
ctio
n
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 19
Tagging with Soft Muons
• Br(bX)+Br(bcX) = 11% + 10%• Muon reconstruction: high-efficiency down to 4 GeV/c pT (90% above 5 GeV),
low fakes (5p1000)
• 2 steps: association muon-jet (cone ∆R < 0.5), likelihood ratios (1D, 2D) for b/light hypothesis using pT, pTrel.
Rejection of 300 for 10% b-tagging efficiency (incl. Br) Fake muons vs pile-up/cavern bckgd: 15% impact on rej. (@ 2.1033)
Their Performance
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 21
Performance of b-tagging algorithms
IP3D+SV1, w>4, tt
Factorization (≠ channels): dependency on jet pT, and env. (∆R)
Degraded performance:•low pT: MS, secondaries•high η: lever-arm (z0),secondaries•high pT: next slide
•Track counting: R~30 @ 60%•Soft muon: R~300 @ 10% (i.e. 80% w/ BR)
•Soft electron: R~100 @ 8%•HLT: R~20 @ 60%•Charm rejection: 5 to 7 @ 60%, up to 20 with JetFitter
5% dead pixels
tt
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 22
Tagging high and very high-pT jets
Example: Z’(2 TeV) bb, uuafter retuning of IP3D+SV1:(x3 worse otherwise)
Challenges:• fixed ∆R for track/jet assoc dilution for jets of high pT
• gains (x2,x3) possible for non-isolated jets• pattern-recognition issues• ‘late’ B-decays
Require dedicated treatment for clustering, pattern-recognition
Fraction of jets (WH400):
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 23
Impact of residual misalignments
Very important studies:
MC geometry includes misplacements as expected in detector (incl. surveys)10-100 m for modules, mm for elements, some systematic deformations (global shifts,
clocking effects,…) next slide
1. all displacements known in reco2. 1 + additional pixel random misalignments introduced in reco only3. actual ATLAS alignment procedure !
Both 1 and 2 require a specific procedure to increase the hit errors fully recovers tracking efficiency and fake levels
Impact on b-tagging:
after alignment, relative loss in rejection between 11-18%
encouraging, even though probably not all systematic deformations were accounted for
Alignment procedure suggests residual misalignments of ~ 3 m in rφ (pixels)
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 24
Impact of misalignments: CSC challenge
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 25
Improving performance: Track Categories
Idea: • use dedicated p.d.f. for variousclasses of tracks.• already used for Shared 10%(20%) improvement for b=60%(50%)
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 26
Optimizing the tracks-jet association
Distance track-jet vs pT:
Tracks from B/DOther tracks
All jets in ttbar events:
• choose f(pT) such that 95% of decay products of B/D are included for all pT
• For non-isolated jets: x3 (x2) the rejection power for (b)= 50% (60%)• Sample-dependent
Measuring b-tagging performance in data
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 28
• Key-ingredient: soft muons
• Dedicated -jet trigger:– staged jet ET thresholds
– 1 Hz 100k in 30h (1 pb-1@1031 cm-2s-1)
• Method 1: pTrel templates
– b & c templates from MC
– fit in pT, bins
• Method 2: non-linear system (à la D0)
– 2 samples – 2 different b-jet/light jet fractions
– 2 non-correlated taggers system can be solved analytically
• Both methods work well for pTjet<80 GeV
• Systematic uncertainties dominate for > 50 pb-1
Current precision on b-tag efficiency: 6%
Muon
jet
SVT tag jet
Muon jet
Calibrating b-tagging efficiency with di-jet events
(pTjet<30 GeV)
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 29
System8 on di-jet events
Measurements
system solvable analytically for , r and sample composition !
k: correlation between soft muon & track-based taggers,: sample dependency
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 30
Calibrating b-tagging efficiency with top events
W jets: ET>40 GeV, 20b-veto(weight < 3)
Hadronic side b-jet: ET>40 GeVb-tag (weight>3)
Leptonic side b-jet: ET>20 GeVNo tag requirement
Lepton: ET>20 GeV
ETmiss>20 GeV
Tag counting method:• count 1,2,3 b-tags likelihood• best accuracy on b: for 100 pb-1
±2.7%(stat)±3.4%(syst)• integrated efficiency only
Selecting unbiased b-jets (leptonic side)
Several selection methods: •topological•likelihood•kinematic fit signal purity 60-80%
b-tagging efficiency determination:
Topo. Like. Kine.
Stat. (200 pb-1) 6.4% 4.4% 5.5%
Syst. error 3.4% 14.2% 6.2%
can also be used to extract b’s p.d.fs
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 31
Tag counting in top events
• selection ttbar l+jets: 4j > 20 GeV• counting events with 1,2,3 tags• HF contents:
• 2 b-jets• possibly 1 c-jet from W• possibly cc/bb from gluon splitting
• first order:
• actual: (complex!) likelihood with rough estimate of Ru
4% total error with 100 pb-1
slightly better for di-lepton
Laurent Vacavant FZU/ASCR & FJFI/CVUT – Prague – Dec 19 2008 32
Conclusion
• Expected b-tagging performance of ATLAS studied in details• For simple taggers, light jet rejection of 30 (for b=60%)• Secondary vertex taggers can achieve a rejection of 150• Most advanced taggers can reach a rejection of 50 for b=70%,
very important for multi b-jet signatures (more is needed: NN, BDT)
• Many critical aspects studied: misalignment, material, state of detector, Monte Carlo modelling, etc – More to do: misalignment, pile-up
• Methods to measure the b-tagging efficiency in data have been established, allowing a ~5% accuracy with 100 pb-1 in top events
• Work is on-going for the measurement of mistag rates
• New developments, e.g. b/c separation, can broaden the physics reach of ATLAS
• Commissioning of the detector progressing well, our work will now focus on the commissioning of the b-tagging to transform our expectations into reality !