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Recent results on multiplicity from ZEUS

Recent results on multiplicity from ZEUS . Michele Rosin. University of Wisconsin, Madison on behalf of the ZEUS Collaboration Hadron Structure 2004 September 1, 2004. DESY Hamburg, Germany. H1. ZEUS. e’(k’). e(k).   (q). remnant. p(P). q’. HERA description & DIS kinematics.

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Recent results on multiplicity from ZEUS

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  1. Recent results on multiplicity from ZEUS Michele Rosin University of Wisconsin, Madison on behalf of the ZEUS Collaboration Hadron Structure 2004 September 1, 2004

  2. DESY Hamburg, Germany H1 ZEUS e’(k’) e(k)  (q) remnant p(P) q’ HERA description & DIS kinematics • 920 GeV p+ (820 GeV before 1998) • 27.5 GeV e- or e+ • 318 GeV cms (300 GeV) • Equivalent to a 50 TeV Fixed Target • DIS Kinematics: Inelasticity 0 ≤ y ≤ 1 Virtuality of photon Fraction of p momentum carried by struck parton

  3. Multiplicity e+e- and pp e+e- e+e-: pp vs. √q2had pp vs. √spp pp Invariant mass of pp • Agreement between e+e- and pp plotted vs. pp invariant mass

  4. e+e- & ep : Breit Frame DIS event • Breit Frame definition: • “Brick wall frame” incoming quark scatters off photon and returns along same axis. • Current region of Breit Frame is analogous to e+e-. Lab Frame Breit Frame PT Breit Frame PL

  5. Measurement vs. Q • Consistent with e+e- 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 e+e- European Physics Journal C11 (1999) 251-270

  6. Multiplicity: ep vs. e+e- (1) • e+e-: boson with virtuality √s produces 2 quarks & hadronization is between 2 colored objects q and q • 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 e+e- • If we use Q as scale to compare to e+e-, multiply hadrons by 2

  7. Multiplicity: ep vs. e+e- (2) • 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 e+e-, but it isn’t • This asymmetric configuration leads to migration of particles from the current region to the target region Breit Frame diagram

  8. q q q q g g Gluon radiation, Q, and 2*EBreit 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 2*EBreit and 2*Nch for comparing with e+e- N < N expected With migrations: No migrations:

  9. Multiplicity vs. 2*Ebreit • Measure multiplicity dependence on 2*EBreit and compare to previous ZEUS measurement vs. Q, and to e+e- • Points agree with the e+e- 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 e+e-..

  10. Visible multiplicity in Breit and hadronic center of mass (HCM) frames Visible Part • Breit Frame: 90% of hadrons in current region visible in detector, only 30% of target region hadrons are visible • HCM Frame: Particles in proton hemisphere unseen in detector. Photon region dominated by contribution from target region of Breit frame (~80% of visible hadrons) All Hadrons Current Region Breit Frame Proton remnant Visible Part All Hadrons Photon Region HCM Frame Proton Region HCM Frame

  11. Moving to the hadronic center of mass (HCM) frame • Using HCM frame is intuitive because all interactions in e+e- and pp happen in the center of mass frame. • Photon region of HCM frame has BIG contribution from target region of Breit frame • To be consistent with Breit frame measurement we measure • 2<nch> vs. 2*E of correspondent hadronic system

  12. Multiplicity vs. W in HCM

  13. Multiplicity in current region of Breit and HCM frames compared to e+e- and pp • Measurement in current region of Breit frame agree with e+e- and pp points. • Measurement in photon hemisphere of HCM frame agrees with LEP points. Request for preliminary

  14. Summary • The hadronic final state has been investigated in DIS ep scattering in terms of the mean charged multiplicity and respective invariant mass of the charged and neutral particles, Meff • Measurement in current region of the Breit frame show same dependence as e+e- if 2*Ecurrent 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 show no strong dependence on x or Q2 • Full comparison of current and target region is still underway, the main problem being that visible part of the target region is only 30% of the total, and MC studies expect that the multiplicity behavior changes when we go closer to the proton remnant.

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