QCD Processes in the Nucleus

1
QCD Processes in the Nucleus

• Will Brooks
• Jefferson Lab
• QCD and Hadron Physics Town Meeting, Rutgers
  University
• January 12, 2007

2
Outline

• Introduction to fundamental processes in QCD
• Transverse momentum broadening and quark energy
  loss in nuclei
• Color transparency
• Hadron formation lengths (time permitting)

3
Main Physics Focus

• QCD in the space-time domain
• How long can a light quark remain deconfined?
• The production time tp measures this
• Deconfined quarks lose energy via gluon emission
• Measure tp and dE/dx via medium-stimulated gluon
  emission
• How fast do color singlet systems expand?
• Color transparency at low energies measures this
• Access via nuclear transparency vs. Q2
• How long does it take to form the full color
  field of a hadron?
• The formation time tfh measures this
• Measure tfh via hadron attenuation in nuclei

4
How long can a light quark remain
deconfined?pT Broadening, Production Time, and
Quark Energy Loss
5
Definitions
pT broadening
p
e
g
pT
Nucleus A
e
Production time lifetime of deconfined quark,
e.g.,
6
pT Broadening and Quark Energy Loss

• Quarks lose energy by gluon emission as they
  propagate
• In vacuum
• Even more within a medium

• This energy loss is manifested by DpT2
• DpT2 is a signature of the production time tp
• DE L dominates in QED
• DE L2 dominates in QCD?

Medium-stimulated loss calculation by BDMPS
7
Energy Loss in QCD

• Partonic energy loss in QCD is well-studied
  dozens of papers over past 15 years
• Dominant mechanism is gluon radiation elastic
  scattering is minor
• Coherence effects important QCD analog of LPM
  effect

Coherence length formation time of a gluon
radiated by a group of scattering centers
Incoherent gluon radiation
Three regions if mean free path is l, and medium
length is L, then ?
Coherent gluon radiation
Single-scatter gluon radiation
DE
Two conditions emerge
L
LCritical
Baier, Schiff, Zakharov, Annu. Rev. Nucl. Part.
Sci. 2000. 5037-69
8
DpT2 vs. n for Carbon, Iron, and Lead
Pb
dE/dx
100 MeV/fm (perturbative formula)
Fe
C
DpT2 (GeV2)
n (GeV)
9
Production length from JLab/CLAS 5 GeV data
(Kopeliovich, Nemchik, Schmidt, hep-ph/0608044)
10
DpT2 vs. A1/3 for Carbon, Iron, and Lead
Strongly suggests quadratic behavior of total
energy loss!
Data generally appear to plateau
Only statistical errors shown
11
pT Broadening - Summary

• What we have learned
• Precise, multivariable measurements of DpT2 are
  feasible
• Quark energy loss can be estimated
• Data appear to support the novel DE L2 LPM
  behavior
• 100 MeV/fm for Pb at few GeV, perturbative
  formula
• Deconfined quark lifetime can be estimated, 5
  fm _at_ few GeV
• Much more theoretical work needed for
  quantitative results
• Outstanding questions
• Is the physical picture accurate?
• Transition to DE LE1/2 behavior at higher n?
• Can asymptotic behavior DE?0 be observed, n?8?
• Hadronic corrections under control? Consistent
  with DY?
• Plateauing behavior due to short tp or energy
  loss transition?
• Provide quantitative basis for jet quenching at
  RHIC/LHC?

JLAB12/EIC
EIC
E906
JLAB12
THEORY
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How fast do color singlets expand?Color
Transparency(at low energy)
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Definitions

• Color transparency
• produce a hadron configuration of small size
• small hadron has small cross section
• Hadron must remain small over nuclear dimensions
  to observe effect
• Common signature of CT increase of the nuclear
  transparency TA
• with increasing hardness of the reaction (Q).
• Coherence length lc also affects TA

TA
Complete transparency
Glauber
Q2
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Color Transparency Physics Picture

• Three aspects accessible experimentally
• Existence and onset of color transparency
• Coherence in hard pQCD quark-antiquark pair
  production
• Evolution of pre-hadron to full hadron

Kopeliovich, Nemchik, Schaefer, Tarasov, Phys.
Rev. C 65 035201 (2002)
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Color Transparency Recent Experiments

• Three-quark systems
• A(e,ep) in quasi-free kinematics (JLAB)
• D(e,ep) final-state interactions (JLAB)
• Precise measurements, CT not observed
• Two-quark systems
• p
• High-energy diffractive dissociation (FNAL) CT
  observed
• gn?p-p (JLAB) Onset of CT???
• A(e,ep) (JLAB) Onset of CT??
• r
• Exclusive electroproduction at fixed coherence
  length (HERMES, JLAB) Onset of CT?

16
Direct Pion Electroproduction
pion nucleus total cross-section
proton nucleus total cross-section
A. Larson, G. Miller and M. Strikman,
nuc-th/0604022
D. Dutta et. al, JLab experiment E01-107
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Preliminary Results from CLAS EG2 data
?0 Electroproduction at Fixed Coherence Length
TFe
Preliminary results show a clear increase in
transparency, in good agreement with CT model!
Q2 (GeV2)
JLab experiment E02-110
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12 GeV
E12-06-106
E12-06-107
r0 electroproduction
r0 Transparency
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Color Transparency - Summary

• What we have learned
• CT in 3q systems not apparent for Q2lt10 GeV2
• CT in 2q systems exists at high energies
• Onset of CT in 2q systems at few-GeV2?
• Several strong hints
• One/two clearly positive signals, more theory
  needed
• Outstanding questions
• How fast does color singlet expand?
• Studies of medium thickness, higher energy/Q2
• Better understanding of dynamics

JLAB12
THEORY
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How long does it take to form the full color
field of a hadron?Hadron Attenuation
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Definitions

• Hadronic multiplicity ratio

Airapetian, et al. (HERMES) PRL 96, 162301 (2006)
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Hadron Attenuation Physics Picture

• Hadrons lost from incident flux through
• Quark energy loss
• Interaction of prehadron or hadron with medium

zh0.5, larger n, less attenuation
zh?1, smaller n, more attenuation
Accardi, Grünewald, Muccifora, Pirner, Nuclear
Physics A 761 (2005) 6791
23
HERMES Data Kr for p, K, p
24
Hermes Data Dependence on pT and A
25
Examples of multi-variable slices of preliminary
CLAS 5 GeV data for Rp
Q2 dependence
n dependence
Four out of 50 similar plots for p! K0, p0, p-,
more, underway
pT2 dependence
zh dependence
26
Cronin Effect Dependence on zh
Theoretical prediction ?
Probes reaction mechanism
Carbon
Iron
Lead
CLAS preliminary data z0.5 and 0.7
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Accessible Hadrons (12 GeV)
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12 GeV Anticipated Data
12 GeV Anticipated Data
Bins in yellow accessible at 5 GeV
p
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Hadron Attenuation - Summary

• What we have learned
• Hadronic multiplicity ratios depend strongly on
  hadron species, are universally suppressed at
  high z
• Main ingredients prehadron cross sections, gluon
  radiation, formation lengths possible exotic
  effects
• Verified EMC observation Cronin-like phenomenon
  in lepto-nuclear scattering new dependence on A,
  Q2, x, z observed
• Outstanding questions
• Energy loss or hadron formation?
• How do hadrons form? Optimal method to extract
  formation lengths?

JLAB12
THEORY
JLAB12
THEORY
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Conclusions

• Fundamental space-time processes in QCD finally
  becoming experimentally accessible
• Parton propagation, color transparency, hadron
  formation
• Plenty of exciting opportunities for the future!

31
Backup Slides
32
Kinematics for pT Broadening

• Choose kinematics favoring propagating quark
  in-medium
• z (Eh/n) gt 0.5 enhance probability of struck
  quark
• z ltlt 1.0 and n/mh gtgt 1 maximize production
  length to ensure ctp gtgt nuclear size
• z, x such that nucleon factorization holds, to
  suppress target fragmentation influence
• x gt 0.1 to avoid quark pair production

33
Hadron-Nucleus Absorption Cross Sections
Hadronnucleus absorption cross section
Fit to
Hadron momentum 60, 200, 280 GeV/c
? lt 1 interpreted as due to the strongly
interacting nature of the probe
Experimentally ? 0.72 0.78, for p, K, ?
A. S. Carroll et al. Phys. Lett 80B 319 (1979)
34
FNAL E665 experiment
E? 470 GeV
Adams et al. PRL74 (1995) 1525
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How long can a light quark remain deconfined?
Physical picture, DIS

• Ubiquitous sketch of hadronization process
  string model

• Alternative gluon bremsstrahlung

Microscopic mechanism not known from experiment
Kopeliovich, Nemchik, Predazzi, Hayashigaki,
Nuclear Physics A 740 (2004) 211245
36
How long can a quark remain deconfined?
Characteristic time scales

• Two distinct dynamical stages, each with
  characteristic time scale

Formation time tfh
Time required to form color field of
hadron Signaled by interactions with known hadron
cross sections No gluon emission (Hadron
attenuation)
Production time tp
Time during which quark is deconfined Signaled by
medium-stimulated energy loss via gluon
emission (pT broading)
These time scales are essentially unknown
experimentally
S. J. Brodsky, SLAC-PUB04551, March 1988
37
Quasi-free A(e,ep) No evidence for CT
Transparency
Conventional nuclear physics calculation by
Pandharipande et al. gives adequate description
38
A(e,ep) Results -- A Dependence
Fit to s s0 A?
a
a constant 0.75
for Q2 gt 2 (GeV/c)2
Close to proton-nucleus total cross section data!
39
Color Transparency in D

• Experimental ratios s(lt0.3)/s(0.1),
  s(0.25)/s(0.1) and s(0.5)/s(0.1)
• Black points 6 GeV CLAS data already taken
• Magenta points 11 GeV projections with (solid)
  and without (open) CT
• Dotted red PWIA
• Dashed blue Laget PWIAFSIIC

40
A(?-,di-jet) Fermilab E791 Data
Coherent ?- diffractive dissociation with 500
GeV/c pions on Pt and C.
a
Fit to s s A
0
? gtgt 0.76, p-nucleus total cross-section
Aitala et al., PRL 86 4773 (2001)
Brodsky, Mueller, Phys. Lett. B206 685 (1988)
Frankfurt, Miller, Strikman, Nucl. Phys. A555,
752 (1993)
41
Pion Electroproduction
70 degrees
90 degrees
42
?0 Electroproduction at Fixed Coherence Length
HERMES Nitrogen data TAP0 P2Q2 P2 (0.097 ?
0.048stat ? 0.008syst) GeV-2
Airepetian et al. (HERMES Coll.) Phys. Rev. Lett.
90 (2003) 052501
43
A Dependence of Transparency
Usually s (A) s0 Aa
T Aa-1
a
from fit of T(A) Aa-1 at fixed Q2