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
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