K. Hara (University of Tsukuba)

1
ATLAS commissioning and early physics -
resonance and jet production -

• K. Hara (University of Tsukuba)
• on behalf of the ATLAS Collaboration

2
QNP2009, Sep.24 Beijing
Cosmic events for ATLAS commissioning
2009 2 weeks
2008 5 months
Cosmic events (300M events) are very useful for
detector calibration. The data taking was
valuable experiences for coordinated detector
operation, including all the detector components,
trigger and DAQ system, monitoring, offline
analysis, Another cosmic run is scheduled for
final checkout for the collision
2
3
QNP2009, Sep.24 Beijing
Pickups from cosmic results
Track impact point resolution vs. track pT -
requires overall understanding of detector
alignment.
Track p difference between ID and
MUON understanding the calorimeter material
Measured calorimeter ET for muons
3
4
QNP2009, Sep.24 Beijing
Quarkonium physics with early data

• With early data (10-100 pb-1 integrated
  luminosity), quarkonium is first physics to
  measure including
• prompt to indirect J/y cross-section ratio
• prompt J/y ?mm and prompt ??mm differential
  production cross-sections
• spin alignment of J/y and ? as a function of
  quarkonium transverse momentum
• cc cross-section? J/y g cb cross-section? J/y
  J/y and others

Large predicted cross-sections and range of
transverse momenta accessible at LHC, ATLAS can
give new insight into quarkonium production and
tests of QCD ? production mechanism of quarkonium
has many features still unexplained ? large
predicted quarkonia rates J/y and ? will play a
central role for calibrations of the ATLAS
detector and software
Predicted rates _at_ 10TeV require pTgt 4 GeV for
both muons
s(pp?Qm4m4 X) _at_10TeV J/y ?(1S) ?(2S) ?(3S) DY bb
generator-level cross section (nb) 27 18.5 10.2 8.8 0.24 16.2
rate after trig/reco/bg subtraction (nb) 17 12.1 5.5 4.1 0.14 9.5
8-12GeV mass range
color-octet model adopted in PYTHIA
17k ev/ pb-1 _at_10 TeV (we expect 100pb-1 _at_7
TeV)
The rest of the ATLAS simulation is _at_14TeV
4
5
Color Octet Model can not explain everything
CDF
J/y
Kraemer Prog.Part.Nucl.Phys.47141-201,2001
NRQCD Braaten et al.,PRD61,094005(1995) Cho et
al.,PLB346(1995)129. kT factorization Baranov,
PRD66,114003(2002)
q helicity angle between m in rest frame and y
direction in lab frame
polarization parameter a 0 (un-polarized) a 1
100 transverse a -1 100 longitudinal
color octet
Angle of m
6
QNP2009, Sep.24 Beijing
NNLO Color Singlet Model
Artoisenet et al, Phys.Rev.Lett 101152001 (2008)
? Xsec (CDF) is explained by CSM alone with
NNLO Negaitve a is predicted (D0 Run2)
LHC prediction
Precise a and Xsec measurements to high PT are
interesting at LHC
6
7
Separation of prompt and indirect production
Use decay time difference between prompt and
indirect y
B decay length 1mm, typically
8
Proper time for prompt/indirect separation
Proper time 0 prompt J/? (spreadresolution)
gt0 secondary from B decay
CERN-OPEN-2008-020
CERN-OPEN-2008-020
no misalignment
e93 purity92 _at_0.2ps
CERN-OPEN-2008-020
9
Quarkonium mass distributions

• Two different trigger strategies
• dimuon trigger m6m4 (or m4m4)
• single muon m10 (2nd m in offline)

?(1S) only
CERN-OPEN-2008-020
CERN-OPEN-2008-020
single m trigger (2nd track pT gt0.5 GeV) is to
rescue small acceptance of di-muon trigger for
forward J/? charge opposite to triggered m
no other candidate track in DRlt3 of m
d0lt0.04mm(m), 0.10mm(track)
10 pb-1
larger bkg, but mass resolution not degraded
m
This method is not justified for ? (low
S/N0.25) at 10 pb-1
J/?
10
Acceptance for spin alignment measurement
restricted cos? coverage (CDF) is a major source
of systematics.
D0 Run2 ? polarization data disagree
with theoretical models and CDF Run1 data
with single m10 (track) trigger, wider cos?
range is covered more reliable spin alignment
measurement should be possible. events generated
flat in cos ? (acceptance shape depends on PT
range more flat for high PT )
CERN-OPEN-2008-020
11
Quarkonium spin alignment sensitivity at 10 pb-1
agen is properly reconstructed (Da0.02-0.06 in
10ltPTlt20 GeV for J/y, comparable to the Tevatron
1 fb-1 data)
CERN-OPEN-2008-020
10pb-1
Determination less precise for ? (single-muon
track is not reliable for S/N0.05 at 10 pb-1 )

At 7 TeV, sensitivity is not much degraded
for J/y need more luminosity (at least 100 pb-1)
for ?
produced from published ATLAS MC results
12
QCD physics at ATLAS

• QCD Physics include, e.g.
• PDF measurements (proton structure)
• Jet studies (reconstruction, rates, cross
  sections)
• Fragmentation studies
• Diffractive physics
• ?s measurements

Tevatron ETmax0.7TeV
Primary interest is to look for deviations in
high ET jet events from QCD due to new physics
O(100) jet ET gt 1TeV for 10 pb-1 _at_ 14 TeV
13
Jet cross section
Steeply falling pT spectrum control of
systematics necessary

• Scale uncertainty
• variation of ?F and ?R within pTmax/2lt?lt2pTmax
• 10 uncertainty at 1TeV

• PDF uncertainty
• uncertainty evaluation using CTEQ6, 6.1
• largest uncertainty high x gluons
• at pT ? 1 TeV around 15 uncertainty

• Jet energy scale uncertainty (largest in
  exp.)
• 1 uncertainty ?10 error on ?
• 5 uncertainty ? 30 error on ?
• 10 uncertainty ? 70 error on ?
• control to 1-2 (c.f. PDF uncertainty) is our
  target

14
Determination of jet-energy scale (JES)
Jet energy calibration is a complex task,
including calorimeter cluster reconstruction
(each tower needs to be equalized beforehand)
cluster to jet assignment jet calibration from
calorimeter to particle scale jet calibration
from particle to parton scale
Many effects from detector (non compensation,
noise, cracks.) and from physics (clustering,
fragmentation, ISR and FSR, UE.) are to be
understood
Use in-situ calibration with physics processes
(in divided ET ranges)
CERN-OPEN-2008-020
1. Zjets events (10ltETlt100-200 GeV) 1 stat.
uncertainty on JES with 300 pb-1 syst.
ISR/FSRUE 5-10 at low ET 1-2
at ET200 GeV
15
Determination of jet-energy scale (JES) contd
3. Jet balance (ETgt500 GeV) to low energy jets
with calibrated JES 2 statistical _at_1 fb-1 7
syst. from low energy jet JES
2. gjets events (100-200ltETlt500 GeV) 1-2
stat.uncertainty on JES with100 pb-1 syst.
ISR/FSRUE 1-2
g
CERN-OPEN-2008-020
CERN-OPEN-2008-020
CERN-OPEN-2008-020
CERN-OPEN-2008-020
improvement expected using data, e.g.
understanding MinBias/UE (R. Kwee talk on
Tuesday) di-jet decorrelation
16
Azimuthal di-jet decorrelation
Di-jet production result in ?f(di-jet)
f(jet1) f(jet2) p in the absence of
radiative effects Di-jet events with smaller
angle are sensitive to radiative effects,
multi-parton interactions, soft-QCD processes
A. Moraes et al., ATL-PHYS-PUB-2006-013
D0 data prefer between low ISR and increased
ISR
D0 data are from PRL 94, 221801 (2005)
17

• Summary
• Resonances are first objects to study for
  detector performance evaluation and calibration
• With 10 pb-1, J/y cross-section will be measured
  precisely (around 1 accuracy excluding e.g.
  luminosity uncertainty) with prompt and indirect
  processes well separated.
• Quarkonium spin alignment measurements will have
  the capability to distinguish quarkonium
  production models
• with reduced systematics ATLAS will provide
  competitive measurement to Tevatron with 10 pb-1
  (J/y)- and gt100 pb-1 (?)
• ATLAS will investigate high ET jets to look for
  deviations from QCD. Jet energy scale calibration
  is a crucial experimental uncertainty and various
  methods are under study to cover wide jet energy
  range.
• Di-jet azimuthal angle decorrelation will
  examine the PDFs and modeling of soft components.
  

ATLAS IS READY FOR TAKING DATA
18
Quarkonia for detector calibration
Resonance peaks are clean and useful for detector
calibration
e.g., Look at mass shifts in mmm vs. pT check
tracker momentum scale energy loss corrections
in calorimeter vs. ? and ? check correct
implementation of material effects, magnetic
field uniformity and stability vs. 1/pT(?)
1/pT(?-) check detector misalignment (?)
CERN-OPEN-2008-020
Quarkonia decays will also be used for online
monitoring (e.g. trigger efficiencies, detector
calibration)
19
CERN-OPEN-2008-020
20
QNP2009, Sep.24 Beijing
CERN-OPEN-2008-020
20
21
QNP2009, Sep.24 Beijing
CERN-OPEN-2008-020
J/y
CDF Phys.Rev.Lett.852886-2891,2000
21