Transcript Nuclear effects in DIS at HERMES

Nuclear effects in DIS
at 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
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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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)
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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… the Q2 dependence
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Medium modification of
Parton Fragmentation
Semi-Inclusive DIS of
leptons on nucleons
DF
1 d 2 N h ( x, z )

Ne
dxdz
FF
 e q ( x)D ( z)
 e q ( x)
q
2
q
h
f
f
q
2
q
(LO)
f
Fragmentation functions from SIDIS multiplicities
Semi-Inclusive DIS of
leptons on nuclei
Nuclear Attenuation:
Reduction of multiplicity of the
fast forward hadrons due to:
1)
2)
Hard partonic interactions
Soft hadron interactions.
Formation time:
Erika Garutti
Antwerpen, 16-19 Sep. 2002
t f = 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

mT = transverse mass
A = collision area of two nuclei
dN/dy = rapidity density
mT dN
t 0 A dy
plasma formation time

Estimate t0 ~ tf (?)
Impact of tpf in dilepton spectrum
(P.Filip,J.Pisut Phys. Lett. B 411 (1997) 187)
 
 
 Predictions for p p  e e in Pb-Pb (LHC, @ 160 GeV) depend on tpf
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Multiplicity ratio for A/D
Multiplicity ratio for A/D
Hadron Attenuation
Data Selection
Kr: p, K, (p) from RICH
• 2.5 (4) < Ph < 15 GeV
• x > 0.06 , y < 0.85
• Q2 > 1 GeV2 , W2 > 4 GeV2
Erika Garutti
N: h, p from Cherenkov
• Ph < 1.4, 4 < Pp < 13.5 GeV
Antwerpen, 16-19 Sep. 2002
14N
z > 0.8
Unexplored
region
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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)

tf = ch n(1-z)
 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
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Direct Production on
14N
Phenomenological inter-nuclear
reinteraction model:
DIRECT PRODUCTION
(1-time scale model)
(Bialas & Chmaj, PLB 133(1983) 241)
Multiplicity ratio for 14N/D
(HERMES Coll., EPJ C20 (2001) 479)
• tf = ch n(1-z)
• s*= 0
• sh fixed for diff. h-type
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Hadron Separation vs z
1-time scale model fit:
•att(z)|p ~ att (z)|K
leading hadrons (z > 0.5)

t f  n (1  z )
cp consistent with
14N: >1
•att|
for low z:
p 1.37+/-0.18
84Kr:
1.49+/-0.10
14N
fm/Gev
fm/GeV
rescattering
cp consistent with gluon
brems. model
•att|p at high z:
Th: 1.35
fm/GeV
information on Dpq(z)
 cp > cp/K
(Guo & Wang , PRL 85 (2000) 3592)
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Hadron Separation vs n
Observations:
•att|p/p ~ att|K/K
 tfp/p ~ tfK/K
•att|p > att|p :
s(pd) @ 3s(pd) @ E = 1 GeV (?)
•att|p < att|p/K
 tfp > tfp/K (?)
Average kinematics
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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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 < 1
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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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)
h
D f ( z ) A  D hf ( z ) D  D hf ( z )
d h ( z, xB , Q 2 )
D ( z ) 
x A2
h
f
xA 
c
m p r0 A1 / 3
 Dhf(z) modification increases for small n
 Dhf(z) scales with A2/3 due to LPM interference
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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Parton Energy Loss
(X.N. Wang, hep-ph/0111404, X.N. Wang, hep-ph/0202105)
Dhf (z) Modification equivalent
to Dhf (z) Rescaling by parton
Energy loss (E/E):
h
f
D ( z) 
1
z


D

1  E / E  1  E / E 
Compared to hadron yield
in Au-Au collisions:
average over expanding system (QGP)
 dE/dx ~ 0.25 GeV/fm
expansion corr.  hot,dens system
Calculated from DIS data:
dE/dx|stat~ dE/dx|exp (RA/2t0) = 4.5 GeV/fm
cold, static system
RA ~ 6fm , t0 ~ 0.2 fm
 dE/dx ~ 0.3 GeV/fm
Erika Garutti
 Gluon density in initial stage of
Au-Au QGP is ~ 15 times that of
cold nuclear matter
Antwerpen, 16-19 Sep. 2002
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Cronin Effect
Enhancement at high pt in AA
collisions relative to pp collisions.
Agreement with multiple parton
scattering model
(E.Wang and X.N.Wang nucl-th/0104031)
 Transition at pt ~ 1-2 GeV/c
Erika Garutti
In DIS on nuclei:
Multiplicity ratio for A/D
In Heavy Ion Collisions:
• 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:
att|p < att|p/K
Interpretation: tfp > tfp/K (?)
 Nuclear modification of Dhf(z)
 Hadronization times
 Parton energy loss and Cronin effect
unambiguous info
on quark transport
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
Erika Garutti
Antwerpen, 16-19 Sep. 2002
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