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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 in DDIS Comparison DIS/DDIS Summary. Motivation. - PowerPoint PPT Presentation
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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
Outlinei. Charged particle multiplicity distributions
i. Kinematic dependences of <n> in DDIS
ii. Comparison DIS/DDISii. Summary
Low-x 2005 Sinaia E.A.De Wolf 2
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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