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Charged Particle Multiplicity in DIS

Charged Particle Multiplicity in DIS. ZEUS Collaboration Meeting. Michele Rosin University of Wisconsin Oct. 15, 2003. Outline. Universality of mean charged multiplicity, <n ch >, and effective energy going into particle production, W had

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Charged Particle Multiplicity in DIS

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  1. Charged Particle Multiplicity in DIS ZEUS Collaboration Meeting Michele Rosin University of Wisconsin Oct. 15, 2003

  2. Outline • Universality of mean charged multiplicity, <nch>, and effective energy going into particle production, Whad • Introduction of Meff and motivation for its useas an energy scale • Data selection & simulation • CTD acceptance and CAL energy studies • Measurements of <nch> vs. effective mass • Summary and plan

  3. Early experimental evidence for universality • In the current region of the Breit frame, Q for ep reactions corresponds to Whad for e+e- reactions • Mean charged multiplicity, <nch> , vs. Q shows logarithmic dependence for both e+e- and ep on the effective energy going into hadronization • Universal dependence of <nch> observed in e+e- and ep reactions in Breit frame e+e- ep

  4. Motivation for the use of Meff as energy scale Similarity of particle production at e+e- and ep colliders Whad • Similarity of Whad dependence on <nch> has been observed • A common Whad dependence on <nch> implies the production of secondary particles is similar in the different interactions • Study the dependence of <nch> of the observed part of the produced HFS on it’s 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

  5. 1996 Data sample • Event Selection • Scattered positron found with E > 12 GeV • A reconstructed vertex with |Zvtx| < 50 cm • scattered positron position cut: |x| > 15 cm or |y| > 15cm (in RCAL) “Box cut” • 40 GeV < E-pz < 60 GeV • Track Selection • Tracks associated with primary vertex • || < 1.75 • pT > 150 MeV • Physics and Kinematic Requirement • Q2 da > 25 GeV2 • y el < 0.95 • y JB > 0.04 8,700,692 events before cuts 208,550 events after all cuts

  6. Event simulation • Ariadne ’97 6v2.4 • Matrix elements at LO pQCD O(s) • Parton showers: CDM • Hadronization: String Model • Proton PDF’s: CTEQ-4D 747,003 events before cuts 50,884 events after all cuts

  7. Track acceptance study • Understanding the CTD acceptance is key to this measurement • Orange assignment of Sinistra electrons to tracks problematic CTD h e- track Problem with electron matching tracks/hadrons CAL h e- (cells) h

  8. Ariadne 1996 Data Track acceptance study • Applied stand alone electron track matching routine (Nils Krumnack) • Matches electron to closest track in eta-phi space entries tracks/hadrons h tracks

  9. Ariadne 1996 Data Tracks in Eta bins • Comparison of 1996 data to Ariadne in eta bins Tracks RCAL (-1.75- -0.875) Tracks BCAL (-0.875-0) Tracks BCAL (0-0.875) Tracks FCAL (0.875-1.75)

  10. Calorimeter energy scale Correction factors Eta distribution of the energy in the cells Data / MC Det h = 1.2 RCAL 0.972 RBCAL 1.019 FBCAL 0.981 FCAL 0.890 Need to correct energy of cells h Ratio after Correction h = 0.15 Data/MC det h = - 0.9 RCAL RBCAL FBCAL FCAL h h

  11. Effect of correction on Meff Improved agreement as a result of energy correction FCAL only: 1.2 < h < 1.75 Full eta range: -1.75 < h < 1.75 entries entries Meff Meff

  12. Eta dependent study using generated events • The dependence of <nch> on the Meff of the produced system for DIS generated events is the same in different regions of phase space • Can use part of the string for studying this dependence • Impact of restricted h measurement on Meff

  13. 1995 ZEUS measurement in lab frame • Investigation of degree of universality in particle production • <nch> proportional to log Meff • <nch> 15% above corresponding e+e- • Understand color dynamics at the pre- hadronization stage at HERA

  14. This analysis (statistical errors only) <nch> vs. Meff • This analysis compared to 1995 study • Corrected to hadron level • Changed definition of final state hadrons • (before) stable hadron: ZDIS particles • (now) stable hadron: lifetime > 0.3ns • Statistical errors only for this analysis • Reasonable agreement with 1995 ZEUS preliminary result

  15. Summary • Event kinematics and tracking well understood and simulated • CTD acceptance understood • Electron removed from tracks sample • CAL studied, preliminary correction applied to the cells • <nch> vs. Meff in better agreement with 1995 results since last presentation (ZEUS meeting: June 2003) • Plan • Increase statistics of data to include 1997 data • Evaluate systematic uncertainties • Generate more monte carlo • Second Analysis

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