Multiplicity Structure in

DIS and DDIS at HERA

Eddi A. De Wolf University of Antwerpen, Belgium

On behalf of H1 collaboration

Outlinei. Charged particle multiplicity distributions

i. Kinematic dependences of <n> in DDIS

ii. Comparison DIS/DDISii. Summary

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Motivation

• H1 analysis on 94 DDIS data:

– Dependence of <n> on MX

DDIS: W,x,Q2, β, xIP ,t, MX

Which onesare relevant for multiplicity?

• H1 analysis on 2000 DDIS data:– Large statistics allows more differential study:

W, Q2, dependences at fixed MX

– Compare DIS and DDIS

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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>

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Diffraction: ep -> e’XY

~ 10% of DIS events have a rapidity gap

MX: inv. mass of X

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Models for diffraction

– Proton infinite momentum frame– Colourless IP is built up of quarks/gluons– Based on QCD and Regge factorization: naïve, probably

incorrect but works…!

– Needs subleading IR component

Combine QCD & Regge theory: resolved IP model

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Models for diffraction cont’d

– In proton rest frame: * splits into q-qbar dipole

– Model the dipole cross section, for example

Saturation model Golec-Biernat and Wusthoff (GBW)

Colour dipole approach

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Data selection DIS and DDIS

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

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

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Track selection

• Primary vertex fitted tracks

• 15 < < 165o and pT > 150 MeV

• Boost to hadronic *p CMS

use tracks with * > 1

* *

acceptance resolution

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Multiplicity results

• 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

Charged particles with *>1 in *p CMS frame

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DDIS

Kinematics: Bjorken Plot

DIS

W2 ~ Q2/x

ymax=ln (W/mπ)

2

2 2X

Q

Q M

IP

1gap ln

x

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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)

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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

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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

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• Intuitive picture: expect no dependence at fixed MX (since no Q2 dependence observed)

dependence of <n>

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dependence of <n>

• dipole models:– relative fraction of q and g

fragmentation depends strongly on

– expect <n> depends on

low <n>

high <n>

D2F

triplet/anti-triplet

octet/octet

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dep. of <n> at fixed MX in DDIS

− MX kept fixed

− No obvious dependence

DDIS: <n> predominantly function of MX, not Q2 or

separately

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W dependence of <n>?

• Changing W changing the gap width

• Gap ~ ln(1/xIP)

• Investigate <n> dependence on xIP

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Wdep. of <n> at fixed MX in DDIS

− 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

All Q2

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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

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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

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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

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− 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.

Summary

Kinematic dependences

Comparison DIS and DDIS

Charged particle multiplicity