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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 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 • 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
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>
Diffraction: ep -> e’XY ~ 10% of DIS events have a rapidity gap MX: inv. mass of X
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
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)
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
Track selection • Primary vertex fitted tracks • 15 < < 165o and pT > 150 MeV • Boost to hadronic *p CMS use tracks with * > 1 acceptance resolution * *
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
Kinematics: Bjorken Plot DIS W2 ~ Q2/x ymax=ln (W/mπ) DDIS
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)
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
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
dependence of <n> • Intuitive picture: expect no dependence at fixed MX (since no Q2 dependence observed)
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>
dep. of <n> at fixed MX in DDIS • MX kept fixed • No obvious dependence DDIS: <n> predominantly function of MX, not Q2 or separately
W dependence of <n>? • Changing W changing the gap width • Gap ~ ln(1/xIP) • Investigate <n> dependence on xIP
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
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
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
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
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