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Early physics with Atlas at LHC
Bellisario Esposito (INFN-Frascati)

On behalf of the Atlas Collaboration

Outline
З З З З З З Atlas Experiment Physics goals Next LHC run conditions Physics processes observable with early data In-situ detector calibration with collision events Early measurement of physics processes Conclusions

14th Lomonosov Conference on Elementary Particle Physics Moscow , 19 August 2009

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Atlas Physics goals
З Search and discover of:
- the Higgs Boson for masses ~ 0.1-1 TeV - Supersymmetry - New Physics foreseen by other models beyond SM

З Precision measurements of SM processes З Ability to detect and measure unexpected effects due to unforeseen scenarios

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At which conditions the full physics goals can be achieved?
The necessary conditions are :
On the LHC side
З High collision energy З High integrated luminosity

On the Atlas side З Achievement of the detector nominal performances З Accurate measurement of the characteristics of the most frequent physics processes which constitute background for the rare processes

This will require years of LHC running and of Atlas data analysis
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2010

2012

2014

2016

2018

2020

2022

year
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What are the perspectives of the first Atlas run ?

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Next LHC run conditions
LHC will start in fall 2009 Energy will be 3.5 - 5 TeV per beam Luminosity will be ~ 1031 - 1032 The run will continue in 2010 Luminosity Integrated will be ~ 100 pb-1

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Physics processes observable with early data
The observability of a process depends on:
З the number of events produced = Lint З З the trigger efficiency (- acceptance, pT cuts)

З the background

1000 events produced / 100 pb-1 10 events produced / 100 pb-1

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With the first ~100 pb-1 of collision data at 10 TeV
Measurement of Physics processes
З З З З Particle multiplicity in minimum bias Jet cross-section W, Z cross-sections tt cross-section ............

Improvement of MC calculation ingredients
З Improve knowledge of PDF with W/Z З Tuning of MC (minimum-bias, underlying event, tt, W/Z+jets, QCD jets,...)

New discoveries
SUSY up to gluino and squark masses of ~ 0.75 TeV ? Discover a Z' up to masses of ~ 1 TeV ?

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Detector performance and in-situ calibration
Initial
ECAL uniformity e/ E-scale Jet E-scale ID alignment Muon alignment ~2.5% 2-3% 5-10% 20-200 m 40-1000 m

Ultimate
0.7% <0.1% 1% 5 m 40 m

Physics samples for calibration
Isolated electrons, Zee J/, Z ee, E/p for electrons Z + 1j, W jj in tt events Generic tracks, isolated , Z Straight , Z

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Minimum Bias
Goals of the study of the min. bias events :
Measure the properties of the inelastic pp interaction processes in a new energy regime Determine the characteristics of the background at high luminosity due to pile-up events

Detector performance required :
Unbiased trigger Tracking efficiency at low p
T

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ID tracking
З The ID consists of many layers of Pixel , Si microstrip (SCT) and TRT (gas based transition radiation detector) in a Solenoidal field of 2 Tesla

З Tracks with pT>500 MeV traverse the full inner detector
З Tracks with pT>150 MeV traverse the full Si precision tracker (Pixel and SCT)

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Low pt tracking performance
ATLAS
ATLAS

low pT tracking default tracking
low pt tracking default tracking

low pT tracking default tracking

Tracking efficiency vs pT
( -2.5 < < 2.5 )

Tracking efficiency vs
( PT > 150 MeV )
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Measurement of the and pT distributions



distribution

pT

distribution
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Present

Expectations

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Present

Expectations



distribution

pt

distribution

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QCD jet physics
Goals of the study of the high pT jet events :
Measure the properties of the very hard pp interaction processes Look for deviations from QCD predictions due to New Physics (quark substructure, resonant production, large extra dimensions,...) Determine the characteristics of the background from QCD events for the observation of other processes

Detector and analysis performance required :
Use of a jet algorithm appropriate for comparison with theoretical calculations (colinear and infrared safe)
Absolute calibration of the jet energy scale
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Expected Jet inclusive ET distribution

s 14TeV

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s 1.8TeV

10 pb-1 @ 14 TeV -> O(100) jet pT > 1TeV D0 e CDF pTMax = 700 GeV

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Calibration of the Jet Energy Scale
З The jet energy has to be corrected for detector effects(non compensation, noise, cracks....) and for physics effects (clustering, fragmentation, ISR and FSR, UE....) З The procedure is rather complex З In-situ calibration with physics processes (dijet, /Z + jet, multijet, W->jet jet) is used to estimate systematic uncertainty and resolution and to perform the final tuning of the jet energy scale
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-jet pT balance

With 100 pb-1 statistical uncertainty on JES ~ 1-2% for 100-200 19


W and Z physics
Goals of the study of the W and Z events :
Measure their production cross-sections known theoretically with uncertainty ~ 1% Measure pT distribution to probe QCD initial parton radiation Measure rapidity distribution to probe parton density functions (PDF)

Detector performance :
Use well known properties of the events to perform in-situ detector calibration (absolute energy and momentum scale, resolution, trigger and reconstruction efficiency)
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Measurement of W and Z cross-sections
( Lint 50 pb-1 )

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In-situ calibration of the reconstruction efficiency from events Z
Tag and probe method
tag : fully identified in the detector (ID and MS track) probe : ID track forming the Z mass w ith the tag

Efficiency vs and pT : in-situ calibration compared with MC truth

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tt

physics

Goals of the study of the t t events :
Measure tt cross-section Study top properties and decay

Detector performance :
In-situ detector calibration (b-tagging efficiency, light jet energy scale) using b-jet and W->jj from tt events

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tt signal
Single
3 jets pT> 40 GeV 1 jets pT> 20 GeV

Lepton

channel

s = 14 TeV 100 pb-1

1 lepton pT> 20 GeV

Е channel No b-tagging

ET

miss

> 20 GeV

tt -> Wb Wb -> b qqb

three jet mass
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s = 14 TeV s = 14 TeV

s = 14 T= 14 TeV s eV

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tt signal
Di-lepton channel : tt -> Wb Wb -> b b
2 leptons with pT>20 GeV ETmiss> 25 GeV (30 for ee/ЕЕ) Veto Z-mass window (85-95 GeV)

(pT>20 GeV)

Signal shows up with low background in the sample with Njet 2 Systematic uncertainties smaller than for the single lepton channel
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Early discoveries of New Physics ?

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10 TeV vs 14 TeV

At 10 TeV, more difficult to create high mass objects
Below about 300 GeV, this suppression is <50% (process dependent ) Above ~ 1 TeV the effect is more marked J.Stirling

Some simulation results reported have been obtained at 14 TeV They are to be scaled to 10 TeV taking into account the ratio of parton luminosities
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Z' l l
14 TeV, Z' ee

Sequential SM

З Signal is (narrow) mass peak above small and smooth SM background
З Discovery for m ~ 1 TeV possible with 100 pb-1 at 10 TeV
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The observation of Z'->ll signal does not require ultimate detector performance

Effect of misalignement of the chambers on the M



signal from Z'-> events
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SUSY
e.g.

~ q

~ g


Spectacular final states : many jets, leptons, missing transverse energy
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Simulation at s = 10 TeV ~~ for m(q,g) ~ 410 GeV

200 pb-1



Jets + ET

miss

Jets + E

T

miss

+ lepton
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Discovery reach

200 pb

-1

~ g ~ 0.75 TeV



~ q ~ 0.75 TeV

systema tics included

Discovery up to m ~ 750 GeV with 200 pb

-1

at s =10 TeV

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Detector and analysis performance required:
Understanding the fake missing transverse energy coming from instrumental effects (noise, cracks, beam gas scattering, machine background,...)

Understanding the physics background from SM processes

ETmiss can be checked with known processes Data-driven methods to estimate the background can be used

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Conclusions
З The study of a variety of SM processes in a new energy regime and the search for some of the new particles foreseen by the models beyond the SM are the physics prospects of the first LHC run. З The analysis of the data collected will also provide the verification and the tuning of the Atlas detector calibration, necessary to improve the performances and reduce the systematics. З With a well understood and calibrated detector unexpected effects possibly leading to surprising discoveries can be looked upon.

Atlas eagerly waits for LHC collisions !
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