QCD at LHC with ATLAS

Theodota Lagouri

Aristotle University

of Thessaloniki

(on behalf of the ATLAS collaboration)

EPS 17-23 July 2003, Aachen, Germany

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Outline

LHC and ATLAS

Jet physics

Measurement of the αS at very large scales

Photon Physics

Lepton Physics

Background processes: minimum-bias and hard diffractive

scattering events

Beauty production and QCD

Summary

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

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 ≈ 10 fb-1/year)

high-luminosity: L ≈ 1034cm-2s-1

(L ≈ 100 fb-1/year)

Mass reach up to ~ 5 TeV

Process σ (nb) Events/year (L = 10 fb-1)

Inclusive bb 5 x 105 ~ 1013

Inclusive jet

pT > 200 GeV100 ~ 109

Inclusive tt 0.8 ~ 107

Inclusive jet

ET > 2 TeV~10-8 ~ 103 Test QCD predictions and perform precision

measurements.

Production cross section and dynamics are largely controlled by QCD.

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large statistics: small statistical error!

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EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

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- LAr for endcap&forward ( | 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)

Lepton precision: 0.1% ( or better )

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

LHC Parton Kinematics

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.

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

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

Jet ET Nevents

> 1 TeV 4 x 105

> 2 TeV 3 x 103

> 3 TeV 40

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

• Systematic errors: jet algorithm (how to best define a jet), 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.

-•αS(MZ) measurement with 10% accuracy ( can be reduced by using the 3-jet to 2-jet production )•αS measurement at very large scales (O(1 TeV)).

L = 30 fb-1

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

dσ/d

ET [

nb/G

eV]

ET Jet [GeV]

Inclusive Jet Cross-Section The measurement of jet production will allow to access

small distances due to large cross-section. The statistical uncertainties will be small: A 30 fb-1

integrated luminosity amounts to 4x105 events with ET> 1 TeV, 3000 events with ET>2 TeV and ~40 with ET> 3 TeV.

The expected statistics at large jet ET values implies a desirable control of the systematic uncertainties to a precision better than 1% for ET < 1 TeV and to ~10% for ET of ~3 TeV.

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

•αS(MZ) measurement with 10% accuracy ( can be reduced by using the 3-jet to 2-jet production )•αS measurement at very large scales (O(1 TeV)).

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Jet PhysicsDi-jet Production

1. Triple differential cross-section The measurement of di-jets and their properties can be used to constrain the parton densities inside the proton and in various kinematic regions in Q2 and x.

2. Di-jet invariant massFor an integrated luminosity of 300 fb-1, the available statistics should allow a measurement of masses up to 10 TeV/c2.

Q2

[GeV

2 ]

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

• 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

EBEAdE

d 32~

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

αS= 0.118 at 100 GeV

αS~ 0.082 at 4 TeV

Determination of αs

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

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→γq

qq→γg-

Production mechanism:

dominant (QCD Compton scattering)

pTγ > 40 GeV

Photon Physics

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

Background: mainly related to fragmentation (non-perturbative QCD)

( requires good knowledge of αs)

( cone isolation)

|η|

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Photon Physics Direct photon measurements can provide important

constraints on parton distributions, especially on the gluon distribution in the proton. The advantage of photon measurements is the better energy determination in comparison to jet measurements, thus avoiding the jet definition ambiguities.

The expected ratio of the inclusive photon cross-section has a value of about 1.5x 10-3

A rejection larger than 3x103 can be achieved while retaining an efficiency of more than 80% for photons with ET>40 GeV.

Photons with ET >40 GeV (Q2 >103 GeV2) allow to reach values of Bjorken-x down to 5x10-4 (for ||<2.5). Statistics will not be a problem, as for example more than 2x104 events with a photon of ET >500 GeV are expected for 30 fb-1

Effects of soft gluon emission can be studied directly by measuring the production of pairs of photons. At LHC it will be possible to study the photon pair production up to 1 TeV invariant mass of the pair with reasonable statistics.

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Lepton PhysicsDrell-Yan production

The measurement of Drell-Yan lepton pair production and the production of W and Z bosons (with a leptonic decay to electrons or muons) will allow one to constrain the quark and anti-quark densities of the proton at a scale given by the invariant mass of the lepton pair over a wide range in Bjorken x.

The jet rejection factors of the order of 105 can be achieved, having an efficiency of ~70% for electrons from W(Z) decays.

W/Z production In contract to the case of proton anti-proton

collisions, at the LHC there is no rapidity asymmetry for the produced W’s. However, the shape of the rapidity distributions is different for W+ and W-, where the total cross-section is larger for W+ than for W-. It is a direct consequence of the parton distribution functions responsible for W+ and W- production.

Z-production: 3x10-4 < x< 0.1 at Q2 = 8x103 GeV2

During 1 year at low luminosity about 108 (107) events with W-> e (Z-> e+e-) will be produced.

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Properties of Minimum-Bias Events and Hard Diffractive Scattering

An extension of the ATLAS detector in the forward region (beyond pseudo-rapidities of ||=5) is presently under study, in order to increase the acceptance for charged particles from inelastic interactions (possibly including a momentum measurement) and to provide tagging and measurement of leading protons from elastic and diffractive interactions.

A detailed understanding of the structure of minimum bias events is needed in order to reduce dependencies on models used to determine their effects at the design luminosity.

Each minimum bias event is expected to contribute from charged particles only about 0.5-1 GeV per unit rapidity and unit azimuth to the transverse energy.

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Beauty production and QCD

B production measurements offer an important probe of pQCD

Full NLO calculation available in QCD Tevatron single b production results show

factor ~2 cross-section discrepancy between NLO QCD and data

Tevatron has measured b-b correlations which are more sensitive to production mechanisms, however the precision was limited by statistics

ATLAS at LHC will further contribute tests of QCD and investigate cause of theory-data discrepancy at hadron interactions

b-b correlations We investigate angular

correlations in b-b production

Using J/()- angle measurements, bJ/X, b X

The ATLAS detector performance is very good for these channels

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EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

ATLAS b-b correlations in BdJ/K0s , B0

s J/, bX

Efficiency does not drop at small <1. No selection cuts requiring model dependent corrections

Including backgrounds shown before/after application of K/ rejection

B0s more difficult than Bd because of

correlated background from K

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EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Summary The LHC will provide a big sample of events with high pT signatures for QCD studies. It will extend the kinematic range to larger values of Q2 , the

hard scale of the partonic process, reaching scales of the order of TeV2 . The fraction of the proton momentum attributed to a parton will allow access

to 10-5, while keeping the scale above 100 GeV2 , in contrast to HERA measurements, where these small momentum fractions are only reached for hard

scales close to or even below 1 GeV2.

Candidate signatures to provide constraints on the quark and anti-quark distributions are the production of W and Z bosons via the Drell-Yan process as

well as lepton pair production in general. On the other hand, the production of direct photons, of jets and of top quarks can be used to get information

on the gluon distribution.

The study of diffractive processes, which allow to access the hard and soft regime of QCD at the same time, presents a significant experimental

challenge in the LHC environment and given the limited angular acceptance of the detectors.

EPS 17-23 July 2003QCD at LHC with ATLASTh. Lagouri

Summary The high beauty production rate at LHC will allow one to extend b-production measurements up to transverse momenta of several

hundred GeV. Correlations between b and b quarks and events with more than one heavy-quark pair, that were difficult to access

in previous experiments due to limited statistics, will be investigated in detail.

ATLAS can exploit the large statistics of b quark decays at the LHC to make precise b production measurements. Good prospects

for measurements of b-b correlations in muon channels. Two exclusive channels studied so far: BdJ/K0s , B0

s J/ looking for

additional muon.

A precise knowledge and understanding of QCD processes is important for the studies of the Higgs boson(s) and searches for new

physics beyond the SM, where QCD represents a large part of the background.

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