Charged Particle Multiplicity in DIS

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Charged Particle Multiplicity in DIS

• ZEUS Collaboration Meeting

M. Rosin, D. Kçira, A. Savin, and L.
Shcheglova University of Wisconsin March 4, 2004
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Outline

• Universal dependence of mean charged
  multiplicity, ltnchgt, on effective energy going
  into particle production, Whad, for ee-, pp, and
  ep.
• Introduction of Meff and motivation for its use
  as an energy scale
• Data selection simulation
• Resolutions and systematics
• Measurements of ltnchgt vs. effective mass
• Comparison to second analysis
• Summary and plan

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Early experimental evidence for universality

• In the current region of the Breit
  frame, Q for ep reactions corresponds to
  Whad for ee- reactions
• Mean charged multiplicity, ltnchgt , vs. Q
  shows logarithmic dependence for both ee-
  and ep on the effective energy going into
  hadronization
• Universal dependence of ltnchgt observed in
  ee- and ep reactions in Breit frame

ee-
ep
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Motivation for the use of Meff as energy scale
Similarity of particle production at ee- and ep
colliders
Whad

• Similarity of Whad dependence on ltnchgt has been
  observed
• A common Whad dependence on ltnchgt implies the
  production of secondary particles is similar in
  the different interactions
• Study the dependence of ltnchgt of the observed
  part of the produced HFS on its total invariant
  mass, Meff

Whad
Meff
Whad HFS measured in full phase space Meff HFS
measured in the detector where the tracking
efficiency is maximized
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1996-97 Data sample

• Event Selection
• Scattered positron found with E gt 12 GeV
• A reconstructed vertex with Zvtx lt 50 cm
• scattered positron position cut x gt 15 cm or
  y gt 15cm (in RCAL) Box cut
• 40 GeV lt E-pz lt 60 GeV
• Track Selection
• Tracks associated with primary vertex
• ? lt 1.75
• pT gt 150 MeV
• Physics and Kinematic Requirement
• Q2 da gt 25 GeV2
• y el lt 0.95
• y JB gt 0.04
• 70 GeV lt W lt 225 GeV ( W2 (q p)2 )

8,700,692 events before cuts 208,550 events
after all cuts
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Event simulation

• Ariadne 97 6v2.4
• Matrix elements at LO pQCD O(?s)
• Parton showers CDM
• Hadronization String Model
• Proton PDFs CTEQ-4D

(Simulates both 96 and 97 data no changes in
detector)
747,003 events before cuts 50,884 events after
all cuts
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Eta dependent study using generated events

• The dependence of ltnchgt on the Meff of the
  produced system for ep generated events is
  the same in different regions of phase
  space
• Can use the observed part of the
  produced hadronic final state, with good
  tracking(h lt 1.75) for studying this
  dependence
• Impact of restricted h measurement on Meff

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Resolutions of kinematic variables
s 6.6
s 9

• Resolutions well behaved
• Use standard deviations for excursions
  in systematic studies

s 15.5 cm
s 22
s 6.8
s 10
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Correlated Uncorrelated Systematics
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1995 ZEUS measurement in lab frame
ZEUS 95 PRELIMINARY RESULT (never published)

• Compare ltnchgt vs. Meff dependence in ee-,
  pp, and ep (ZEUS). universality?
• ltnchgt proportional to log Meff
• ltnchgt 15 above corresponding ee-
• Suggestion difference due to ep color
  dynamics at the pre- hadronization stage.

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ltnchgt vs. Meff 96-97 vs 95

• This analysis compared to 1995 study
• Data corrected to hadron level
• Full error bars statistical systematic
  uncertainties added in quadrature
• Inner error bars statistical uncertainties
• Reasonable agreement with 1995 ZEUS
  preliminary result

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Comparison to 2nd analysis
2nd Analysis by Dorian Kçira
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Trigger studies by L. Shcheglova
Lydia has investigated the possibility to go to
lower Q2. Because of changing prescales for DIS01
and changing radius for DIS03, must use a
weighting scheme Created a mixed sample of DIS01
DIS03 to get agreement with MC The weighting
scheme is described in detail here http//amzeus.
desy.de/sumstine/trigger_study/weighting_foils.ps
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Results of reweighting
Good agreement between data and MC down to Q2
15 or 10 GeV2 Currently Q2 gt 25, but lowering Q2
cut can increase the kinematic lever arm
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Summary

• The dependence of ltnchgt on the Meff of the
  produced system for ep generated events is the
  same for restriced eta reigions
• Systematic errors are small, dominated by CAL
  energy scale
• Trigger study shows possibility of going to lower
  Q2
• Agreement between 1st and 2nd analyses less than
  1
• ltnchgt vs. Meff agrees with 1995 ZEUS preliminary
  results
• Plan
• Generate more ARIADNE to increase statistics
• Generate LEPTO monte carlo for systematic studies