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Precision SM tests at the LHC using ATLAS and CMS
Peter R Hobson
School of Engineering & Design
Brunel University
Talk given at RAL on 13 June 2005
Contents
• ATLAS & CMS• Jets• Drell-Yan• B physics• Top physics• Electroweak (TGC)• Single photons
ATLAS
CMS
Day 1 of LHC p+p
From F Gianotti, LHC Physics, La Thuile 2005
Year 1 at the LHC
From F Gianotti, LHC Physics, La Thuile 2005
Year 1 at the LHC
From F Gianotti, LHC Physics, La Thuile 2005
Effects on physics reach
Effects on physics reach
From G Polisello, Les Houches 2005
b-tagging in ATLAS
Jet Physics
Atla
s
• Measure jet ET spectrum, rate varies over 11 orders of magnitude• Test QCD at the multi-TeV scale
ET of jet
Events
> 1 TeV
4106
> 2 TeV
3104
> 3 TeV
400
Inclusive jet rates for 300 fb-1:
From J Mnich, Physics at the LHC, Vienna 2004
Jet signatures• Test of pQCD in an energy regime never probed!Test of pQCD in an energy regime never probed!• The measurement of di-jets and their properties (ET and η1,2)
can be used to constrain p.d.f.’s• Inclusive jet cross section: αs measurement with 10%
accuracy• Multi-jet productionMulti-jet production is important for several physics studies:
– Top-pair production with hadronic final states– Higgs production in association with tt and bb – Search for R-parity violating SUSY (8 – 12 jets).
• Systematic uncertaintiess (statistical will be small):– luminosity (dominant uncertainty 5% -10% ) – jet energy scale– calorimeter response (linearity)– jet trigger efficiency– knowledge of p.d.f.’s– value of strong coupling constant, αs – uncertainties in parton shower modeling
- -
ET Jet [GeV]
From VA Mitsou, QCD Conference Montpellier 2004
LHC 1 fb-1
Drell-Yan Lepton-Pair Production
pT > 6 GeV
|| < 2.5
• Total cross section pdf parton lumi search for Z, extra dim. , ... Much higher mass reach ascompared to Tevatron
/Zq
q
e,
e+, +
Inversion of e+e qq at LEP
Z pole
From J Mnich, Physics at the LHC, Vienna 2004
Forward-backward asymmetry estimate quark direction assuming xq > xq
Measurement of sin2W effective • 2004: LEP & SLD
sin2W = 0.23150 0.00016
AFB around Z-pole• large cross section at the LHC
(Z e+e) 1.5 nb
• stat. error in 100 fb-1 incl. forward electron tagging
(per channel & expt.)
sin2W 0.00014
Systematics (probably larger)• PDF• Lepton acceptance• Radiative corrections
Drell-Yan Lepton-Pair Production
Atla
s
[%]
From J Mnich, Physics at the LHC, Vienna 2004
Drell-Yan processes• QCD effects enter DY production in initial state only
predictions less uncertain• Reconstruction of leptons (e, μ) unambiguous
identification ( opposed to jets )• Di-lepton productionDi-lepton production constrains proton structure at
Q2 ≈ mℓℓ2
• W and Z production: huge statistical samples• ~105 events containing W (pT
W > 400 GeV, L=30 fb-1)• ~104 events containing Z (pT
Z > 400 GeV, L = 30 fb-1)
• WW±± production production:– higher cross-section for W+ than for W-
– different yW -distributions: W+ forward; W- central– constrain quark and anti-quark densities in the
proton [ud(bar)W+; u(bar) d W-]
• W+jet productionW+jet production study colour coherence• Z productionZ production provides accurate reconstruction of
final state (no neutrino!) • Pair production (WW, ZZ, WZ)Pair production (WW, ZZ, WZ) study triple gauge
boson constants
pTmiss>20 GeV
|ηℓ|<2.5
NLO calculation
Represent background sources to many new phenomena searches
From VA Mitsou, QCD Conference Montpellier 2004
B Physics at ATLAS & CMS
From VM Ghete Physics at LHC Vienna, 2004
B Physics at ATLAS & CMS
From VM Ghete Physics at LHC Vienna, 2004
B Physics at ATLAS & CMS
From VM Ghete Physics at LHC Vienna, 2004
c & b production• Dominant production mechanism for
heavy quarks (b and t) is gg fusion• Cross-section calculation:
pQCD processes leading to QQ state non-pQCD to transform into colour-singlets tuning with Tevatron data
• Measurements of heavy quark production will provide constraints on the gluon density
• Jet-flavour identification (c-jet or b-jet): – high-pT muons (ε ≈ 85%, σ=39 MeV)– b-tagging (vertexing detectors)
• b-quark– lower-pT mesons are experimentally
accessible compared to charm-quarks– 10-4<x<0.1
• b-b(bar) correlations:– Δφμμ≈π mostly LO QCD– Δφμμ≈0 only NLO QCD
Process σ (mb) Events/year
(L = 10 fb-1)
cc 7.8 ~8·1013
bb 0.5 ~ 5·1012
--
ψ´
J/ψ
gbgb ggbb-
gggg, g bb-
From VA Mitsou, QCD Conference Montpellier 2004
Top production• Cross section determined to NLO precision
– Total NLO(tt) = 834 ± 100 pb
– Largest uncertainty from scale variation • Compare to other production processes:
– Top production cross section approximately 100x Tevatron
Opposite @ FNAL
32121 10~ ; ˆ xxxsxs
~90% gg~10% qq
Process N/s N/yearTotal collected before start LHC
W e 15 108 104 LEP / 107 FNAL
Z ee 1.5 107 107 LEP
tt 1 107 104 Tevatron
bb 106 1012-13 109 Belle/BaBar ?
H (130) 0.02 105 ?
LHC is a top factory!
From S Bentvelsen, QCD Conference Moriond 2004
Golden-plated MTop channelLepton side
Hadron side
Br(ttbbjjl)=30%for electron + muon
• Golden channel– Clean trigger from isolated lepton
• The reconstruction starts with the W mass:– different ways to pair the right jets
to form the W
– jet energies calibrated using mW
• Important to tag the b-jets: – enormously reduces background
(physics and combinatorial)
– clean up the reconstructionTypical selection efficiency: ~5-10%:
•Isolated lepton PT>20 GeV
•ETmiss>20 GeV
•4 jets with ET>40 GeV
•>1 b-jet (b40%, uds10-3,
c10-2)
Background: <2%
W/Z+jets, WW/ZZ/WZ
Lepton + jet: reconstruct top
• Hadronic side– W from jet pair with closest invariant mass to MW
• Require |MW-Mjj|<20 GeV
– Assign a b-jet to the W to reconstruct Mtop
• Kinematic fit– Using remaining l+b-jet, the leptonic part is
reconstructed • |mlb -<mjjb>| < 35 GeV
• Kinematic fit to the tt hypothesis, using MW constraints
j1
j2
b-jet
tW-mass
Selection efficiency 5-10%
From S Bentvelsen, QCD Conference Moriond 2004
Top mass systematics
– Method works:• Linear with input Mtop
• Largely independent on Top PT
– Biggest uncertainties:• Jet energy calibration
• FSR: ‘out of cone’ give large variations in mass
• B-fragmentation
– Verified with detailed detector simulation and realistic calibration
Source of uncertainty
Hadronic
Mtop
(GeV)
Fitted Mtop (GeV)
Light jet scale 0.9 0.2
b-jet scale 0.7 0.7
b-quark fragm 0.1 0.1
ISR 0.1 0.1
FSR 1.9 0.5
Comb bkg 0.4 0.1
Total 2.3 0.9
Challenge:
determine the mass of the top around 1 GeV accuracy in one year of LHC
From S Bentvelsen, QCD Conference Moriond 2004
• Use exclusive b-decays with high mass products (J/)– Higher correlation with Mtop – Clean reconstruction (background free)– BR(ttqqb+J/) 5 10-5 ~ 30% 103 ev./100 fb-1
(need high lumi)
Top mass from J/
Different systematics (almost no sensitivity to FSR)
Uncertainty on the b-quark fragmentation function becomes the dominant error
M(J/+l)Mtop
M(J/+l)
MlJ/
From S Bentvelsen, QCD Conference Moriond 2004
Top During Commissioning• Determination MTop in initial phase
– Use ‘Golden plated’ lepton+jet
• Selection:– Isolated lepton with PT>20 GeV
– Exactly 4 jets (R=0.4) with PT>40 GeV
• Reconstruction:– Select 3 jets with maximal resulting PT
– Signal can be improved by kinematic constrained fit
• Assuming MW1=MW2
and MT1=MT2
PeriodStat Mtop (GeV)
Stat /
1 year 0.1 0.2%
1 month 0.2 0.4%
1 week 0.4 2.5%No background
included
Calibrating detector in comissioning phase
Assume pessimistic scenario:
-) No b-tagging
-) No jet calibration
-) But: Good lepton identification
From S Bentvelsen, QCD Conference Moriond 2004
Top During Commissioning
• Signal plus background at initial phase of LHC
• Most important background for top: W+4 jets– Leptonic decay of W, with 4 extra ‘light’ jets
Alpgen, Monte Carlo has ‘hard’ matrix element for 4 extra jets(not available in Pythia/Herwig)
ALPGEN:
W+4 extra light jets
Jet: PT>10, ||<2.5, R>0.4
No lepton cuts
Effective : ~2400 pb
With extreme simple selection and reconstruction the top-peak should be visible at LHC
L = 150 pb-1
(2/3 days low lumi)
measure top mass (to 5-7 GeV) give feedback on detector performance
From S Bentvelsen, QCD Conference Moriond 2004
Direct |Vtb| extraction: single top / single W
Moreover, in principle, many theoretical errors would disappear by normalising s-channel events over single W events:
(with care in choosing coherent cuts for the two processes, to avoid the reintroduction of the same errors in a subtler way)
R(|Vtb|)=
From A Giammanco, Les Houches 2005
Single top: “how to”
General strategy (both s/t-ch.):1 isolated lepton2 high Et jetsat least 1 tagged b-jetmissing Et
l+MET: MT compatible with WHt (scalar sum of all Et’s)M(lb) in a window around Mt
s/t-channel separation:2(b-t-b)/1 tagged b-jets0/1 jets in the forward calo2/1 central jetsangular distance between the reco top and the remaining jet
1st jet: b from t2nd jet: recoil
3rd jet: b(mostly undetectable)
T-channel
For MET and Ht, single top lies in the middle between non-top and ttbar bkgs. S-channel: S/B<0.2, main bkgs: ttbar->2l (1 lost), Wbb, t-channel.T-channel is much easier to select, due to higher cross section and unique topology.
CMS note 1999/048
From A Giammanco, Les Houches 2005
TGC
From M Dobbs, Hadron Collider Physics 2004
TGC
From M Dobbs, Hadron Collider Physics 2004
QGC
From M Dobbs, Hadron Collider Physics 2004
TGC CMS studies• W (Kate Mackay, Peter Hobson, Karlsruhe Group)
– CMSJET studies with BAUR generator (Phys Rev D41 1476 (1990))
– Full background study– CMS Notes: 2000/017, 2001/052, 2001/056, CMS Thesis
1999/019
• Z (Kate Mackay, Peter Hobson, Davy Machin, Karlsruhe Group)– CMSJET studies with BAUR Z generator– Full background study– CMS notes: 2000/017, 2002/028, CMS Thesis 2005
• WZ– No CMS specific study
• W (Richard Croft)– CMSJET study with W2GRAD generator
Status of CMS W Analysis• Signal
– BAUR NLO MC – Used in CMSJET studies
• Backgrounds– W+jet – main background– Radiative W decay– Quark-Gluon fusion
Cuts: isolated high pt photon, lepton and missing energy.
• pT()> 100 GeV• pT(l)> 25 GeV• pT()> 50 GeV• MT(,l,) > 90 GeV• R(,l) > 0.7• pT 2nd Jet < 25 GeV• || < 2.5
Peter Hobson, Kate Mackay
Status of CMS W Analysis
Peter Hobson, Kate Mackay
Direct photon• Two main contributions:
– qg→q QCD Compton scattering (dominating)
– qq→g annihilation process• Information on gluon density in the
proton ( requires good knowledge of αs )
• Background: jets with a leading π0
Isolation cutIsolation cut: low hadronic activity in a cone around the photon
ATLAS: high granularity calorimeters( |η| < 3.2 ) allow good γ/jet separation
Di-photon production: mγγ and Δφγγ sensitive tosoft gluon emission
Understanding irreducible background from fragmentation in gg fusion: crucial for Hγγ searches
-
From VA Mitsou, QCD Conference Montpellier 2004
LOγγ production