Jet Improvement in and the Status of ATLAS

1
Jet Improvement in and the Status of ATLAS

• Zachary Marshall1, Emlyn Hughes1, Ariel
  Schwartzman2
• 1Columbia University, 2SLAC
• Erice School for Subnuclear Physics
• 29 August - 7 September 2008

2
Introduction One view of a Jet
Calorimeter deposits, both hadronic and
electromagnetic, in clusters - calibrated to
average response
CALORIMETRY
?/-,?0, K,n,p,?
Charged particles created from fragmentation,
decay, and hadronization
TIME
TRACKS
g
q,g

• pp collision produces partons (quarks gluons)
• Jet defined by a cone in ?,?

p
p
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Our Technique

• Jet reconstruction in ATLAS traditionally relies
  on calorimeter information only
• Calibration is done in an average way and
  response is normalized on average
• We can improve jet energy resolution by adding
  track information
• Not energy flow! No measurements are replaced,
  no new jet energy scale is required.
• We can distinguish jet topologies individually
• We begin from jets with normalized average
  response

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Jet Topology
ATLAS MC Priliminary

• Define
• For jets made of ?/- /? 0, this is related to
  hadronic energy fraction
• Expect something between 1/2 and 2/3

ATLAS MC Priliminary
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Response,

• Response depends on ftrack
• Resolution is artificially broadened due to
  dependence
• By correcting the response, we improve the jet
  energy resolution
• Right Jets with 40-60 GeV of pT
• Most improvement for pTlt100 GeV

ATLAS MC Preliminary
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Results

• Resolution is improved by 10 at low pT

ATLAS MC Preliminary
ATLAS MC Preliminary
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Additional Variables
ATLAS MC Preliminary

• We can continue the technique with other
  track-based variables
• One can evaluating additional variables in terms
  of response difference at /- 1 ?
• Left Number of tracks in a jet (highly
  correlated to pT)
• After corrections for ftrack, we still see some
  potential for jet response improvement

ATLAS MC Preliminary
2?
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Jet Improvement Summary

• We have developed a new technique to improve jet
  energy resolution in ATLAS using track
  information
• Correct individual jets by understanding their
  topology
• gt10 jet energy resolution improvement
• Can remove biases in missing energy, for example
• Different from energy flow techniques
• Adds information to the calorimeter-based
  reconstruction to improve response jet-by-jet
• Techniques can be extended to additional
  variables
• Some power to distinguish, for example, quark and
  gluon jets

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ATLAS
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LHC All sectors cold!
Beam already to P7, P3
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What is On Schedule?

• Everything harmonizes with me, which is
  harmonious to you, O Universe. For me nothing is
  too early or too late if it is in due time for
  you.
• -- Marcus Aurelius, 180AD

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ATLAS
Configured all parts of the detector (last bit -
pixel system - configured last Thursday
night) Have seen curved cosmic tracks Expecting
first SINGLE beam in 10 days or less Expecting
first 900 GeV CoM collisions this month (?)
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Turn on Information

• They will be posting information here
• http//lhc-first-beam.web.cern.ch/lhc-first-beam/W
  elcome.html
• Once the actual day arrives (Sept 10?), there
  will be a webcast here
• http//webcast.cern.ch/live.py
• Begins at 830am CERN time