Precision SM tests at the LHC using ATLAS and CMS

1
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
2
Contents

• ATLAS CMS
• Jets
• Drell-Yan
• B physics
• Top physics
• Electroweak (TGC)
• Single photons

3
ATLAS
4
CMS
5
Day 1 of LHC pp
From F Gianotti, LHC Physics, La Thuile 2005
6
Year 1 at the LHC
From F Gianotti, LHC Physics, La Thuile 2005
7
Year 1 at the LHC
From F Gianotti, LHC Physics, La Thuile 2005
8
Effects on physics reach
9
Effects on physics reach
b-tagging in ATLAS
From G Polisello, Les Houches 2005
10
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 Events
gt 1 TeV 4?106
gt 2 TeV 3?104
gt 3 TeV 400
From J Mnich, Physics at the LHC, Vienna 2004
11
Jet signatures

• Test of pQCD in an energy regime never probed!
• The measurement of di-jets and their properties
  (ET and ?1,2) can be used to constrain p.d.f.s
• Inclusive jet cross section as 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).
• 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, as
• uncertainties in parton shower modeling

-
-
ET Jet GeV
From VA Mitsou, QCD Conference Montpellier 2004
12
Drell-Yan Lepton-Pair Production
Z pole

• Total cross section
• pdf
• parton lumi
• search for Z?, extra dim. , ...
• Much higher mass reach as
• compared to Tevatron

From J Mnich, Physics at the LHC, Vienna 2004
13
Drell-Yan Lepton-Pair Production

From J Mnich, Physics at the LHC, Vienna 2004
14
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 production constrains proton structure
  at Q2 mll2
• W and Z production huge statistical samples
• 105 events containing W (pTW gt 400 GeV, L30
  fb-1)
• 104 events containing Z (pTZ gt 400 GeV, L 30
  fb-1)
• 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-
• Wjet production ? study colour coherence
• Z production provides accurate reconstruction of
  final state (no neutrino!)
• Pair production (WW, ZZ, WZ) ? study triple gauge
  boson constants

• Represent background sources to many new
  phenomena searches

From VA Mitsou, QCD Conference Montpellier 2004
15
B Physics at ATLAS CMS
From VM Ghete Physics at LHC Vienna, 2004
16
B Physics at ATLAS CMS
From VM Ghete Physics at LHC Vienna, 2004
17
B Physics at ATLAS CMS
From VM Ghete Physics at LHC Vienna, 2004
18
c b production
Process s (mb) Events/year (L 10 fb-1)
cc 7.8 81013
bb 0.5 51012
-
-

• 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 (e 85, s39 MeV)
• b-tagging (vertexing detectors)
• b-quark
• lower-pT mesons are experimentally
  accessible compared to charm-quarks
• 10-4ltxlt0.1
• b-b(bar) correlations
• ?fµµp ? mostly LO QCD
• ?fµµ0 ? only NLO QCD

?
J/?
gb?gb
From VA Mitsou, QCD Conference Montpellier 2004
19
Top production

• Cross section determined to NLO precision
• Total ?NLO(tt) 834 100 pb
• Largest uncertainty from scale variation
• Compare to other production processes

90 gg10 qq
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 ?
H (130) 0.02 105 ?

• Top production cross section approximately 100x
  Tevatron

LHC is a top factory!
From S Bentvelsen, QCD Conference Moriond 2004
20
Golden-plated MTop channel

• Br(tt?bbjjl?)30for 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 reconstruction

• Typical selection efficiency 5-10
• Isolated lepton PTgt20 GeV
• ETmissgt20 GeV
• 4 jets with ETgt40 GeV
• gt1 b-jet (?b?40, ?uds?10-3, ?c?10-2)

Background lt2 W/Zjets, WW/ZZ/WZ
21
Lepton jet reconstruct top

• Hadronic side
• W from jet pair with closest invariant mass to MW
• Require MW-Mjjlt20 GeV
• Assign a b-jet to the W to reconstruct Mtop
• Kinematic fit
• Using remaining lb-jet, the leptonic part is
  reconstructed
• ml?b -ltmjjbgt lt 35 GeV
• Kinematic fit to the tt hypothesis, using MW
  constraints

• Selection efficiency 5-10

W-mass
From S Bentvelsen, QCD Conference Moriond 2004
22
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
23
Top mass from J/?

• Use exclusive b-decays with high mass products
  (J/?)
• 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) Uncertainty on the b-quark fragmentation
function becomes the dominant error
M(J/?l)
M(J/?l)
From S Bentvelsen, QCD Conference Moriond 2004
Mtop
24
Top During Commissioning

• Determination MTop in initial phase
• Use Golden plated leptonjet
• Selection
• Isolated lepton with PTgt20 GeV
• Exactly 4 jets (?R0.4) with PTgt40 GeV
• Reconstruction
• Select 3 jets with maximal resulting PT

Calibrating detector in comissioning phase Assume
pessimistic scenario -) No b-tagging -) No jet
calibration -) But Good lepton identification
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

• Signal can be improved by kinematic constrained
  fit
• Assuming MW1MW2 and MT1MT2

From S Bentvelsen, QCD Conference Moriond 2004
25
Top During Commissioning

• Most important background for top W4 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 W4 extra light jets Jet PTgt10, ?lt2.5,
?Rgt0.4 No lepton cuts Effective ? 2400 pb

• Signal plus background at initial phase of LHC

L 150 pb-1 (2/3 days low lumi)
With extreme simple selection and reconstruction
the top-peak should be visible at LHC
measure top mass (to 5-7 GeV) ? give feedback on
detector performance
From S Bentvelsen, QCD Conference Moriond 2004
26
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)
From A Giammanco, Les Houches 2005
27
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
• lMET MT compatible with W
• Ht (scalar sum of all Ets)
• M(l?b) in a window around Mt

For MET and Ht, single top lies in the middle
between non-top and ttbar bkgs. S-channel
S/Blt0.2, main bkgs ttbar-gt2l (1 lost), Wbb,
t-channel. T-channel is much easier to select,
due to higher cross section and unique topology.

• 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

CMS note 1999/048
From A Giammanco, Les Houches 2005
28
TGC
From M Dobbs, Hadron Collider Physics 2004
29
TGC
From M Dobbs, Hadron Collider Physics 2004
30
QGC
From M Dobbs, Hadron Collider Physics 2004
31
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

32
Status of CMS W? Analysis

• Signal
• BAUR NLO MC
• Used in CMSJET studies
• Backgrounds
• Wjet main background
• Radiative W decay
• Quark-Gluon fusion

• Cuts
• isolated high pt photon, lepton and missing
  energy.
• pT(?)gt 100 GeV
• pT(l)gt 25 GeV
• pT(?)gt 50 GeV
• MT(?,l,?) gt 90 GeV
• ?R(?,l) gt 0.7
• pT 2nd Jet lt 25 GeV
• ? lt 2.5

Peter Hobson, Kate Mackay
33
Status of CMS W? Analysis
Peter Hobson, Kate Mackay
34
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 as )
• Background jets with a leading p0

-

• Isolation cut low hadronic activity in a cone
  around the photon
• ATLAS high granularity calorimeters
• ( ? lt 3.2 ) allow good ?/jet separation
• Di-photon production m?? and ?f?? sensitive to
• soft gluon emission
• Understanding irreducible background from
  fragmentation in gg fusion crucial for H???
  searches

LO ?? production
From VA Mitsou, QCD Conference Montpellier 2004