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Multiplicity Structure in DIS and DDIS at HERA

Multiplicity Structure in DIS and DDIS at HERA. Eddi A. De Wolf University of Antwerpen, Belgium On behalf of H1 collaboration. Outline Charged particle multiplicity distributions Kinematic dependences of <n> in DDIS Comparison DIS/DDIS Summary. Motivation.

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Multiplicity Structure in DIS and DDIS at HERA

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  1. Multiplicity Structure in DIS and DDIS at HERA Eddi A. De Wolf University of Antwerpen, Belgium On behalf of H1 collaboration • Outline • Charged particle multiplicity distributions • Kinematic dependences of <n> in DDIS • Comparison DIS/DDIS • Summary

  2. Motivation • H1 analysis on 94 DDIS data: • Dependence of <n> on MX DDIS: W,x,Q2, β, xIP ,t, MX Which ones are relevant for multiplicity? • H1 analysis on 2000 DDIS data: • Large statistics allowsmore differential study: W, Q2,  dependences at fixed MX • Compare DIS and DDIS

  3. Charged particle multiplicity of the hadronic final state • The multiplicity distribution P(n) • Independent emission of single particles: Poisson distribution • Deviations from Poisson reveal correlations and dynamics • Mean multiplicity <n> of charged particles • Particle density in Rapidity Space • Koba-Nielsen-Olesen (KNO) scaling (z) • energy scaling of the multiplicity distribution • (z) = <n> Pn vs z = n/<n>

  4. Diffraction: ep -> e’XY ~ 10% of DIS events have a rapidity gap MX: inv. mass of X

  5. Models for diffraction Combine QCD & Regge theory: resolved IP model • Proton infinite momentum frame • Colourless IP is built up of quarks/gluons • Based on QCD and Regge factorization: naïve, probably incorrect but works…! • Needs subleadingIR component

  6. Models for diffraction cont’d Colour dipole approach • In proton rest frame: * splits into q-qbar dipole • Model the dipole cross section, for example Saturation model Golec-Biernat and Wusthoff (GBW)

  7. Data selection DIS and DDIS • 2000 nominal vertex data: 46.65 pb-1 • Data corrected via Bayesian unfolding procedure: • DIS MC: DJANGOH 1.3, proton pdf CTEQ5L • DDIS MC: RAPGAP resolved pomeron DIS selection: • Good reconstruction of scattered electron • Kinematic cuts: • 0.05 < yav < 0.65 • 5 < Q2 < 100 GeV2 • 80 < W < 220 GeV DDIS selection: • Rapidity gap: • No activity in the forward detectors • max < 3.3 • Kinematic cuts: • 4 < MX < 36 GeV • xIP < 0.05

  8. Track selection • Primary vertex fitted tracks • 15 <  < 165o and pT > 150 MeV • Boost to hadronic *p CMS use tracks with * > 1 acceptance resolution * *

  9. Multiplicity results Charged particles with *>1 in *p CMS frame • Multiplicity distribution and moments • Q2 dependence of <n> in DIS/DDIS at fixed W •  dependence of <n> in DDIS at fixed MX • W dependence of <n> in DDIS at fixed MX • Comparison of DIS and DDIS • Density in Rapidity • KNO scaling

  10. Kinematics: Bjorken Plot DIS W2 ~ Q2/x ymax=ln (W/mπ) DDIS

  11. Q2 dep. of <n> in DIS/DDIS at fixed W • <n> vs Q2 • DIS/DDIS data (at fixed MX) • No stat. signif. dependence on Q2 • Weak W- dependence • Fit <n> to <n> = a + b log(Q2)

  12. Q2 dep. of dn/d(y-ymax) in DIS at fixed W • (1/N)dn/d(y-ymax) [ymax= ln(W/m)] • Weak Q2 dependence • QCD scaling violations of fragmentation function DIS: <n> predominantly function of W, not Q2

  13. Q2 dep. of dn/d(y-ymax) in DDIS at fixed W • dn/d(y-ymax) • ymax= ln(W/m) • Weak Q2 dependence • Weak W dependence • Effect of the gap at lowest W! DDIS: <n> predominantly function of MX, not Q2

  14. dependence of <n> • Intuitive picture: expect no  dependence at fixed MX (since no Q2 dependence observed)

  15. dependence of <n> low <n> triplet/anti-triplet • dipole models: • relative fraction of q and g fragmentation depends strongly on : • expect <n> depends on  octet/octet high <n>

  16. dep. of <n> at fixed MX in DDIS • MX kept fixed • No obvious  dependence DDIS: <n> predominantly function of MX, not Q2 or  separately

  17. W dependence of <n>? • Changing W changing the gap width • Gap ~ ln(1/xIP) • Investigate <n> dependence on xIP

  18. Wdep. of <n> at fixed MX in DDIS All Q2 • At fixed MX: change W means change xIP • Regge factorisation: diffractive pdf’s independent of xIP • Breaking of Regge factorisation • In resolved IP model: pomeron + reggeon

  19. Wdep. of <n> at fixed MX in DDIS • Fit <n>: <n> = a + b log W2 • Regge factorisation breaking expected eg. in multiple scattering models • Effects predicted to diminish with increasing Q2 ( shorter interaction time); not seen here Factorisation breaking not dependent on Q2 within errors

  20. Particle Density in y: DIS  DDIS • (1/N) dn/d(y-ymax) • Central region: particle density similar for DIS and high MX DDIS DDIS & DIS particle density not much different although MX<< W

  21. Comparison DIS/DDIS: KNO scaling • Negative particles • (z) = <n> Pn vs z = n/<n> • Approximate KNO scaling for DIS and DDIS • Shape of KNO distribution similar for DIS and DDIS • Means: Correlations very similar

  22. Summary Charged particle multiplicity • studied for DIS and DDIS in ep at HERA • <n> in DIS main dependence only on W, not Q2 or x separately • <n> in DDIS main dependence on MX (and a bit on W), not on Q2 and  separately • Lack of  dependence is surprising (q-qbar; q-qbar g) • DIS and DDIS: density in rapidity similar at highest MX • DIS and DDIS: approximate KNO scaling & same shape. Kinematic dependences Comparison DIS and DDIS

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