Nuclear%20effects%20in%20DIS%20at%20HERMES

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Nuclear effects in DISat HERMES
Erika Garutti (NIKHEF) On behalf of the HERMES
collaboration

• Inclusive DIS on nuclei (14N and 84Kr)
• Semi-inclusive DIS on nuclei
• Nuclear Attenuation ? Hadron Formation Times
• Link to heavy-ion physics
• Conclusion and Outlook

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Medium modification of Parton Distributions
Late 60s Inclusive high energy DIS of leptons
on nucleons
Parton Distributions
80s Inclusive lepton DIS on nuclei
Medium effects (shadowing,EMC effect)
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the Q2 dependence
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Medium modification ofParton Fragmentation
DF FF


Semi-Inclusive DIS of leptons on nucleons
(LO)
Fragmentation functions from SIDIS multiplicities
Semi-Inclusive DIS of leptons on nuclei

• Nuclear Attenuation
• Reduction of multiplicity of the
• fast forward hadrons due to
• Hard partonic interactions
• Soft hadron interactions.
• Formation time

tf lf / c
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Motivation

• Why is formation time of interest?
• Dependence on hadron type
• Comparison to model predictions
• Link to Heavy Ion physics
• Formation time in Heavy Ion physics
• Initial energy density in Quark Gluon Plasma
• plasma formation time ? Estimate t0
  tf (?)
• Impact of tpf in dilepton spectrum
• (P.Filip,J.Pisut Phys. Lett. B 411 (1997)
  187)
• ? Predictions for in
  Pb-Pb (LHC, _at_ 160 GeV) depend on tpf

mT transverse mass A collision area of two
nuclei dN/dy rapidity density

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Hadron Attenuation
14N
Multiplicity ratio for A/D
Multiplicity ratio for A/D
z gt 0.8 Unexplored region

• Data Selection
• Kr p, K, (p) from RICH
• 2.5 (4) lt Ph lt 15 GeV
• x gt 0.06 , y lt 0.85
• Q2 gt 1 GeV2 , W2 gt 4 GeV2

• N h, p from Cherenkov
• Ph lt 1.4, 4 lt Pp lt 13.5 GeV

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Gluon Bremsstrahlung Model

• (B.Kopeliovich et al., hep-ph/9511214)
• pQCD based model for high-z mesons
• Highly virtual quark looses energy via gluon
• emission till final meson is formed
• induced radiation
• nuclear suppression
• (dominant)
• ?
• ? cp 1.35 fm/GeV c
• Suppression of high-z mesons
• high-z quarks emit only few gluons
• correspond to small tf
• long time for hadron-Nucleus interaction

tf ch n(1-z)
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Direct Production on 14N
(HERMES Coll., EPJ C20 (2001) 479)

• Phenomenological inter-nuclear
• reinteraction model
• DIRECT PRODUCTION
• (1-time scale model)
• (Bialas Chmaj, PLB 133(1983) 241)
• tf ch n(1-z)
• s 0
• sh fixed for diff. h-type

Multiplicity ratio for 14N/D
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Hadron Separation vs z

• 1-time scale model fit
• leading hadrons (z gt 0.5)
• ?cp consistent with 14N
• 14N 1.37/-0.18 fm/Gev
• 84Kr 1.49/-0.10 fm/GeV
• ?cp consistent with gluon brems. model
• Th 1.35 fm/GeV
• cp gt cp/K

• att(z)p att (z)K
• ?
• attp gt1 for low z
• rescattering
• attp at high z
• information on Dpq(z)
• (Guo Wang , PRL 85 (2000) 3592)

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Hadron Separation vs n

• Observations
• attp/p- attK/K-
• ? tfp/p- tfK/K-
• attp gt attp
• s(pd) _at_ 3s(pd) _at_ E 1 GeV (?)
• attp lt attp/K
• ? tfp gt tfp/K (?)
• Average kinematics

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Hadron Separation vs n
Enhancement at high n due to rescattering of
produced hadron on target protons Leading
protons have Ratt lt 1
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Fragmentation Function Modification Model
(Guo Wang , PRL 85 (2000) 3592) Modification
of Dhf(z) in medium due to multiple parton
scattering (in QCD)
? Dhf(z) modification increases for small n
? DDhf(z) scales with A2/3 due to LPM
interference
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Parton Energy Loss
(X.N. Wang, hep-ph/0111404, X.N. Wang,
hep-ph/0202105)
Compared to hadron yield in Au-Au collisions
average over expanding system (QGP)
Dhf (z) Modification equivalent to Dhf (z)
Rescaling by parton Energy loss (DE/E)
? dE/dx 0.25 GeV/fm

• expansion corr. ? hot,dens system
• dE/dxstat dE/dxexp (RA/2t0) 4.5 GeV/fm
• RA 6fm , t0 0.2 fm
• Gluon density in initial stage of Au-Au QGP is
  15 times that of cold nuclear matter

Calculated from DIS data cold, static system
? dE/dx 0.3 GeV/fm
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Cronin Effect
In Heavy Ion Collisions Enhancement at high pt
in AA collisions relative to pp collisions.
In DIS on nuclei
Multiplicity ratio for A/D
Agreement with multiple parton scattering model
(E.Wang and X.N.Wang nucl-th/0104031) ?
Transition at pt 1-2 GeV/c

• Similar pt2-enhancement
• Clean information on quark
• transport in cold nuclear matter

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Conclusions Outlook

• Strong nuclear attenuation at high z and low n
• Similar attenuation for p and K
• Observed attp lt attp/K
• Interpretation tfp gt tfp/K (?)
• Nuclear modification of Dhf(z)
  unambiguous info
• Hadronization times
  on quark transport
• Parton energy loss and Cronin effect in
  cold matter
• more to come
• additional HERMES data on 4He and 20Ne
• pt broadening for p, K, p
• comparison to theory for separate hadron type