Precision SM tests at the LHC using ATLAS and CMS Peter R Hobson School of Engineering & Design...

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