Analysis Update: Mean Charged Multiplicity in DIS

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Analysis Update Mean Charged Multiplicity in
DIS
M. Rosin, D. Kçira, and A. Savin University of
Wisconsin L. Shcheglova Moscow State University,
Institute of Nuclear Physics ZEUS Collaboration
Meeting Padova, Italy October 20th,
2004 http//www-zeus.desy.de/sumstine/multip
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Comparing ep and ee- using current region of
Breit Frame

• ee- boson with virtuality vs produces (in L.O.)
  2 quarks hadronization is between 2 colored
  objects q and q to produce final state
• ep boson with virtuality Q produces 1 quark the
  2nd quark comes from the interaction between the
  photon and the proton
• current region of Breit frame for ep similar to
  one hemisphere of ee-
• If we use Q as scale to compare to ee-,
  multiply hadrons by 2

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Previous ZEUS Measurement ltnchgt vs. Q compared
to ee-

• multiplicity vs. center of mass energy (vs) for
  different ee- experiments data agree with one
  another
• Attempt to compare multiplicity in current region
  of Breit frame vs. Q, for ep, to multiplicity vs.
  vs for ee-.
• Multiplicity for ee- has been scaled down by
  factor of 2
• Consistent with ee- data for high Q2
• Disagreement at low Q2 may be attributed to gluon
  radiation
• Idea of current analysis Understand current and
  target multiplicity and compare to ee-

European Physics Journal C11 (1999) 251-270
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Closer look at current region

• ep Split into Current and Target Region one
  string two segments.
• In ep we have a color field between 2 colored
  objects the struck quark and the proton remnant
• When we use Q2 as a scale we are assuming the
  configuration is as symmetric as it is in ee-,
  but it isnt
• This asymmetric configuration leads to migration
  of particles from the current region to the
  target region

Breit Frame diagram
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Gluon radiation, Q, and 2EBreit
Soft Contribution
Hard Contribution
QCD Compton

• In hard and soft processes gluon radiation occurs
• These gluons can migrate to target region
• Total energy in the current region of Breit frame
  and multiplicity are decreased due to these
  migrations (Q2 is not)
• Effect is more pronounced for low Q2 more low
  energy gluons
• Must use 2EBreit and 2Nch for comparing with
  ee-

N lt N expected
With migrations
No migrations
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Multiplicity vs. 2Ebreit

• Measure multiplicity dependence on 2EBreit and
  compare to previous ZEUS measurement vs. Q, and
  to ee-
• Points agree with the ee- points
• This approximation of invariant mass partially
  takes into account the real distribution of the
  particles.
• Current region understood, would like to use some
  energy scale to compare target region for ep to
  ee-
• Start by looking at comparison of ee- and pp
  where agreement in the multiplicity dependence
  has been observed

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Breit frame vs Meff

• Similar dependence in multiplicity for visible
  parts of current and target regions of the Breit
  frame as a function of their respective effective
  mass
• Target multiplicity is higher than current
  multiplicity and can reach higher values of Meff
• combined current and target region (black points)
  show same behavior as target region
• Multiplicity and Meff are Lorentz invariant so
  can move to the lab frame

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Previous results in lab frame ltnchgt vs. Meff
compared to ee- and pp

• 1995 preliminary results
• ltnchgt plotted vs. Meff for ep, vs. qhad for pp
  and vs. vs for ee-.
• ltnchgt for ep is 15 higher than that for ee-
  and pp.
• Current analysis consistent with previous results
• Checked x and Q2 dependence no Q2 dependence,
  weak x dependence not enough to explain
  difference
• Investigate this difference look at visible
  charged multiplicity vs. Meff for current and
  target regions of the Breit frame.

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Check Compare to ee- ppusing ep dijet events
Boson-Gluon Fusion
QCD Compton

• Try to mimic final state of ee- by looking at
  dijet events
• Require events with only 2 jets, one in the
  current region of the Breit Frame and one in the
  Target region

pp collision
ee- annihilation
?remnant
?remnant
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ltnchgt vs. Meff for events with 2 jets

• First look at dijet events
• Multiplicity decreasing with increasing cut on
  jet ET
• For events with 2 jets of Etgt10 Gev, shows
  reasonable agreement with pp and ee-
• Second analysis on these points underway
• next Look at hadronic center of mass frame

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Summary

• Measurement in current region of the Breit frame
  show same dependence as ee- if 2Ecurrent is
  used as the scale
• Similar dependence observed in multiplicity for
  visible part of the current and target regions of
  the Breit frame as a function of their respective
  effective mass
• Lab frame measurements 15 above ee- and pp
  show no strong dependence on x or Q2
• Study using dijet events to better mimic pp
  collision is underway and shows decreased
  multiplicity with increasing dijet ET
• Plans
• working in ?P canter of mass frame
• paper before end of year