QCD at LHC with ATLAS

Arthur M. Moraes

University of Sheffield, UK(on behalf of the ATLAS collaboration)

APS April 2003 Meeting – Philadelphia, PA

APS April 5, 2003QCD physics at ATLASA. M. Moraes APS April 5, 2003QCD physics at ATLASA. M. Moraes

OutlineLHC and ATLAS.

Precision tests & measurements in unexplored kinematic region.

Jet physics.

Parton luminosities and p.d.f.’s ( high-Q2 processes at LHC: parton-parton collider ).

Direct photon production ( fg(x), background to H → γγ, parton dynamics ).

Measurement of the αS at very large scales.

Background processes: multi-parton interaction, minimum-bias and the underlying event.

Conclusion.

APS April 5, 2003QCD physics at ATLASA. M. Moraes

LHC (Large Hadron Collider):

• p-p collisions at √s = 14TeV

• bunch crossing every 25 ns (40 MHz)

low-luminosity: L ≈ 2 x 1033cm-2s-1

(L ≈ 20 fb-1/year)

high-luminosity: L ≈ 1034cm-2s-1

(L ≈ 100 fb-1/year)

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Mass reach up to ~ 5 TeV

~ 1010100Inclusive jetpT > 200 GeV

~ 1070.8Inclusive tt

~ 103~10-8Inclusive jetET > 2 TeV

~ 10135 x 105Inclusive bb

Events/year (L = 10 fb-1)

σ (nb)Process

Test QCD predictions and perform precision measurements.

Production cross section and dynamics are largely controlled by QCD.

-

-

large statistics: small statistical error!

APS April 5, 2003QCD physics at ATLASA. M. Moraes

ATLAS: A Toroidal LHC AparatuS

~44m

~22m

7,000 tons• Multi-purpose detectorcoverage up to |η| = 5;design to operate at L= 1034cm-2s-1

Most of the QCD related measurements are expected to be performed during the “low-luminosity” stage.

• Inner Detector (tracker)Si pixel & strip detectors + TRT;2 T magnetic field;coverage up to |η|< 2.5.

• Calorimetryhighly granular LAr EM calorimeter( | η |< 3.2);hadron calorimeter – scintillator tile( | η |< 4.9).

• Muon Spectrometerair-core toroid system(| η | < 2.7).

Jet energy scale: precision of 1% ( W → jj; Z ( ll) + jets)

Absolute luminosity: precision ≤ 5% ( machine, optical theorem, rate of known processes)

APS April 5, 2003QCD physics at ATLASA. M. Moraes

LHC Parton Kinematics

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100100

101

102

103

104

105

106

107

108

109

fixedtarget

HERA

x1,2 = (M/14 TeV) exp(±y)

Q = M

M = 10 GeV

M = 100 GeV

M = 1 TeV

M = 10 TeV

66y = 40 224

Q2

(GeV

2 )x

Essentially all physics at LHC are connected to the interactions of quarks and gluons (small & large transferred momentum).

Accurate measurements of SM cross sections at the LHC will further constrain the pdf’s.

The kinematic acceptance of the LHC detectors allows a large range of x and Q2 to be probed ( LHC coverage: |y| < 5 ).

This requires a solid understanding of QCD.

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Jet physics• Test of pQCD in an energy regime never probed!

40> 3 TeV3 x 103> 2 TeV4 x 105> 1 TeVNeventsJet ET

0 < |η| < 1

1 < |η| < 2

2 < |η| < 3

ET Jet [GeV]

dσ/d

ET [n

b/G

eV] NLO QCD

10-11

10-9

10-7

10-5

10-3

10-1

1

0 1000 2000 3000 4000 5000

At the LHC the statistical uncertainties on the jet cross-section will be small.

• Systematic errors:jet algorithm,calorimeter response (jet energy scale),jet trigger efficiency,luminosity (dominant uncertainty 5% -10% ),the underlying event.

• The measurement of di-jets and their properties (ET and η1,2) can be used to constrain p.d.f.’s.

• Multi-jet production is important for several physics studies: a) tt production with hadronic final statesb) Higgs production in association with tt and bb c) Search for R-parity violating SUSY (8 – 12 jets).

- --

• Inclusive jet cross section: αS(MZ) measurement with 10% accuracy.( can be reduced by using the 3-jet to 2-jet production )

10 5

10 6

10 7

10 8

0 1 2 3

log(1/x)

Q2 [G

eV2 ]

L = 30 fb-1

● 0 < |η| < 1○ 1 < |η| < 2■ 2 < |η| < 3

dσ/d

ET

[nb/

GeV

]

ET Jet [GeV]

Q2

[GeV

2 ]

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Measuring parton luminosities and p.d.f.’s

Uncertainties in p-p luminosity (±5%) and p.d.f.’s(±5%) will limit measurement uncertainties to ±5% (at best). • Using only relative cross section measurements,

might lead eventually to accuracies of ±1%.

),,(),,()( 221 XgqqQxxpdfLXppN theoryppevents →××=→ − σ

quark flavour tagged γ-jet final states;use inclusive high-pT μ and b-jet identification

(lifetime tagging) for c and b;use μ to tag c-jets;5-10% uncertainty for x-range: 0.0005 – 0.2

γc, γb, sg→Wc

γ-jet studies: γ pT > 40 GeVx-range: 0.0005 – 0.2γ-jet events: γ pT ~ 10-20 GeVlow-x: ~ 0.0001±1%

precise measurements of mass and couplings;huge cross-sections (~nb);small background.x-range: 0.0003 – 0.1± 1%

γ-jet, Z-jet, W±-jet

(high-Q2 reactions involving gluons)

W± and Z leptonicdecays

(high-mass DY lepton pairs and other processes dominated by qq )-

(u,d)

• For high Q2 processes LHC should be considered as a parton-parton collider instead of a p-p collider.

qq-

g

s, c, b

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Understanding photon production:Higgs signals (H→γγ) & background;prompt-photon can be used to study the underlying

parton dynamics;gluon density in the proton, fg(x)

ATLAS: high granularity calorimeters ( |η| < 3.2 ) allow good background rejection.

Isolation cut: reduces background from fragmentation (π0)

qg→γqqq→γg-

Production mechanism:dominant (QCD Compton scattering)

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

200 400 600 800 1000

| ηγ | < 2.5

pTγ (GeV)

dσ/d

p T (

nb/G

eV)

pTγ > 40 GeV

Direct photon production

0

0.25

0.5

0.75

1

0 0.5 1 1.5 2

|η|E

ffic

ienc

y

All γ,sUnconverted γ,sConverted γ,s

Low luminosity run: the photon efficiency is more than 80% ( LArcalorimeter ).

Background: mainly related to fragmentation ( non-perturbativeQCD)

( requires good knowledge of αs)

( cone isolation)

|η|

APS April 5, 2003QCD physics at ATLASA. M. Moraes

• However, measurements of αS(MZ) will not be able to compete with precision measurements from e+e- and DIS (gluon distribution).

• Differential cross-section for inclusive jet production (NLO )ECM = 14 TeV, -1.5 < ηjet < 1.5

• A and B are calculated at NLO with input p.d.f.’s.

• Fitting this expression to the measured inclusive cross-section gives for each ET bin a value of αS(ET).

• Systematic uncertainties:p.d.f. set ( ±3%),parametrization of A and B,renormalization and factorization scale

( ±7%).

( ) ( ) ( ) ( )TRSTRST

EBEAdEd μαμασ 32~ +

• Verification of the running of αS : check of QCD at the smallest distance scales yet uncovered:

αS= 0.118 at 100 GeVαS~ 0.082 at 4 TeV

Determination of αs: scale dependence

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Multiple parton interactions (MPI)p p

Aσ^

σ^B

• AFS, UA2 and more recently (and crucially!) CDF, have measured double parton interactions.

σeff = 14.5 ± 1.7 mb

σeff has a geometrical origin;parton correlation on the transverse space;it is energy and cut-off independent.

( )eff

BAcutTD mp

σσσσ

2=

• σD decreases as pT→∞ and grows as pT→ 0.• σD increases faster with s as compared to σS.

Multiple parton collisions are enhanced at the LHC!

4-jet production: 2 → 4 v (2 → 2)2

• Source of background:WH+X→ (lν) bb+X,Zbb→ (lν) bb+X,W + jets, Wb + jets and Wbb + jets,tt → llbb,final states with many jets pT

min ~ 20 – 30 GeV.

-- -

- --

2 → 4

(2 → 2)2

b

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Minimum-bias and the underlying eventMinimum bias events

Experimental definition: depends on the experiment trigger! “Minimum bias” is usually associated to non-single diffractive events(NSD), e.g. ISR, UA5, E735, CDF.

difndifddifselastot ... σσσσσ +++=

σn.dif ~ 55 - 65mbσtot ~ 102 - 118 mb

Underlying event in charged jet evolution (CDF style analysis)

It is not only minimum bias event!

Δφ = φ − φljet

In a hard scattering process, the underlying event has a hard component (initial + final-state radiation and particles from the outgoing hard scattered partons) and a soft component (beam-beam remnants).

Best described with MPI models!

(PYTHIA) (PHOJET) (PYTHIA) (PHOJET)

APS April 5, 2003QCD physics at ATLASA. M. Moraes

Conclusions:LHC will probe QCD to unexplored kinematic limits;

Jet studies (test of pQCD, constrain p.d.f.’s, physics studies);

Luminosity uncertainties can be reduced by measurements of relative luminosities: high-Q2 and wide x-range;

Prompt-photon production will lead to improved knowledge of background levels (H→γγ), fg(x) and parton dynamics;

αs at high-energy scales (test of the running of αs);

Multiple parton scattering: source of background and/or new physics channels;

Minimum-bias and the underlying event: improved understanding of events dominated by soft processes.