Transcript Document 7319153

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
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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
b-tagging in ATLAS
From G Polisello, Les Houches 2005
Jet Physics
• Measure jet ET spectrum, rate varies over 11 orders of magnitude
• Test QCD at the multi-TeV scale
Inclusive jet rates for 300 fb-1:
ET of
jet
>1
TeV
>2
TeV
>3
TeV
Event
s
4106
From J Mnich, Physics at the LHC, Vienna 2004
3104
400
Jet signatures
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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 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).
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ET Jet [GeV]
Systematic uncertaintiess (statistical will be small):
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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
From VA Mitsou, QCD Conference
Montpellier 2004
Drell-Yan Lepton-Pair Production
q
q
/Z
pT > 6 GeV
|| < 2.5
e,
e+, +
Z pole
Inversion of e+e  qq at LEP
• Total cross section
pdf
parton lumi
search for Z, extra dim. , ...
Much higher mass reach as
compared to Tevatron
LHC 1 fb-1
From J Mnich, Physics at the LHC, Vienna 2004
 Forward-backward asymmetry
estimate quark direction
assuming xq > xq
 Measurement of sin2W effective
• 2004: LEP & SLD
sin2W = 0.23150  0.00016
AFB around Z-pole
• large cross section at the LHC
(Z  e+e)  1.5 nb
[%]
Drell-Yan Lepton-Pair Production
• stat. error in 100 fb-1
incl. forward electron tagging
(per channel & expt.)
sin2W  0.00014
 Systematics (probably larger)
• PDF
• Lepton acceptance
• Radiative corrections
From J Mnich, Physics at the LHC, Vienna
2004
Drell-Yan processes
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QCD effects enter DY production in initial state only
 predictions less uncertain
Reconstruction of leptons (e, μ) unambiguous
identification ( opposed to jets )
Di-lepton production constrains proton structure at
Q2 ≈ mℓℓ2
W and Z production: huge statistical samples
NLO
calculation
• ~105 events containing W (pTW > 400 GeV, L=30 fb-1)
• ~104 events containing Z (pTZ > 400 GeV, L = 30 fb-1)
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W± 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-]
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W+jet production  study colour coherence
Z production provides accurate reconstruction of
final state (no neutrino!)
Pair production (WW, ZZ, WZ)  study triple gauge
boson constants
pTmiss>20 GeV
|ηℓ|<2.5
 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
σ (mb)
Process
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
cc
bb
-
Events/year
(L = 10 fb-1)
7.8
~8·1013
0.5
~ 5·1012
ψ´
J/ψ
ggbb
gggg, g bb
gbgb
– 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
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:
Process
N/s
N/year
Total 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 ?
0.02
105
?
H (130)
sˆ  sx1 x2 ; x1 x2 ~ 103
~90% gg
~10% qq
Opposite
@ FNAL
– Top production cross
section
approximately
100x Tevatron
LHC is a top factory!
From S Bentvelsen, QCD Conference Moriond 2004
Golden-plated MTop channel
Br(ttbbjjl)=30%
for electron +
muon
Lepton side
• 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:
Hadron side
Typical selection efficiency: ~5-10%:
•Isolated lepton PT>20 GeV
•ETmiss>20 GeV
– enormously reduces background
(physics and combinatorial)
– clean up the reconstruction
•4 jets with ET>40 GeV
Background: <2%
•>1 b-jet (b40%, uds10-3, c10-2)
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
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|mlb -<mjjb>| < 35 GeV
Kinematic fit to the tt hypothesis,
using MW constraints

Selection efficiency 5-10%
j2
j1
W-mass
b-jet
t
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
Challenge:
determine the mass of the top
around 1 GeV accuracy in one year of LHC
From S Bentvelsen, QCD Conference Moriond 2004
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
Top mass from J/
• Use exclusive b-decays with high mass products (J/)
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Higher correlation with Mtop
Clean reconstruction (background free)
BR(ttqqb+J/)  5 10-5
 ~ 30%  103 ev./100 fb-1
(need high lumi)
MlJ/
Different systematics
(almost no sensitivity
to FSR)
M(J/+l)
M(J/+l)
Uncertainty on the bquark fragmentation
function becomes
the dominant error
Mtop
From S Bentvelsen, QCD
Conference Moriond
2004
Top During Commissioning
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Determination MTop in initial phase
– Use ‘Golden plated’ lepton+jet
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Period
Stat Mtop
(GeV)
Stat /
1 year
0.1
0.2%
1 month
0.2
0.4%
1 week
0.4
2.5%
No background
included
Selection:
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Calibrating detector in comissioning phase
Assume pessimistic scenario:
-) No b-tagging
-) No jet calibration
-) But: Good lepton identification
Isolated lepton with PT>20 GeV
Exactly 4 jets (R=0.4) with PT>40 GeV
Reconstruction:
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Select 3 jets with maximal resulting PT
– Signal can be improved by
kinematic constrained fit
From S Bentvelsen, QCD Conference
Moriond 2004
• Assuming MW1=MW2 and
MT1=MT2
Top During Commissioning
• 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)
• Signal plus background at initial
phase of LHC
L = 150 pb-1
(2/3 days low lumi)
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 toppeak should be visible at LHC
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:

R(|Vtb|)=


(with care in choosing coherent cuts for the two processes, to
avoid the reintroduction of the same errors in a subtler way)
From A Giammanco, Les Houches 2005
Single top: “how to”
General strategy (both s/t-ch.):
1 isolated lepton
2 high Et jets
at least 1 tagged b-jet
missing Et
l+MET: MT compatible with W
Ht (scalar sum of all Et’s)
M(lb) in a window around Mt
s/t-channel separation:
2(b-t-b)/1 tagged b-jets
0/1 jets in the forward calo
2/1 central jets
angular distance between the
reco top and the remaining jet
From A Giammanco, Les Houches 2005
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.
3rd jet: b
(mostly undetectable)
T-channel
CMS note 1999/048
2nd jet: recoil
1st jet: b from t
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 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 to
soft gluon emission
 Understanding irreducible background from
fragmentation in gg fusion: crucial for Hγγ
searches
From VA Mitsou, QCD Conference Montpellier 2004
LO
γγ production