Simulation of Anti-Nucleus – Nucleus and Nucleus-Nucleus interactions in GEANT-4
QCD Processes in the Nucleus
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Transcript of QCD Processes in the Nucleus
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QCD Processes in the Nucleus
Will BrooksJefferson Lab
QCD and Hadron Physics Town Meeting, Rutgers UniversityJanuary 12, 2007
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Outline
• Introduction to fundamental processes in QCD
• Transverse momentum broadening and quark energy loss in nuclei
• Color transparency
• Hadron formation lengths (time permitting)
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Main Physics Focus
QCD in the space-time domain:• How long can a light quark remain deconfined?
– The production time p measures this– Deconfined quarks lose energy via gluon emission– Measure p 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 f
h measures this– Measure f
h via hadron attenuation in nuclei
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“How long can a light quark remain deconfined?”
pT Broadening, Production Time, and Quark Energy Loss
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Nucleus “A”
pT
e
e’*
+
Definitions
pT broadening: DIS
DT
DIS
ATT ppp 222
Production time: lifetime of deconfined quark, e.g.,
dxdEc
zp
)1(
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pT Broadening and Quark Energy Loss
nucleus
propagating quark
quarks in nuclei
gluons
L
• Quarks lose energy by gluon emission as they propagate– In vacuum– Even more within a medium
Medium-stimulated losscalculation by BDMPS
ThisThisenergy loss is manifested by energy loss is manifested by pT2
pT2 is a signature of the production time p
E ~ L dominates in QEDE ~ L dominates in QEDE ~ LE ~ L22 dominates in QCD? dominates in QCD?
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Energy Loss in QCD
Baier, Schiff, Zakharov, Annu. Rev. Nucl. Part. Sci. 2000. 50:37-69
• 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
c
kc
2
Coherence length ~ formation time of a gluon radiated by a group of scattering centers
Three regions: if mean free path is , and medium length is L, then →
cLc
Lc
Incoherent gluon radiation
Coherent gluon radiation
‘Single-scatter’ gluon radiation
Two conditions emerge: LdxdE
EdxdE
CriticalLL
CriticalLL L
E
LCritical
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ppTT22 vs. vs. for Carbon, Iron, and Lead for Carbon, Iron, and Lead
C
Pb
Fe
ppTT
22 (G
eV (G
eV22 ))
(GeV)(GeV)
~ 100 MeV/fm (perturbative formula)
~dE
/dx
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Production length from JLab/CLAS 5 GeV data (Kopeliovich, Nemchik, Schmidt, hep-ph/0608044)
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ppTT22 vs. vs. for Carbon, Iron, and Lead for Carbon, Iron, and Lead
Strongly suggests quadratic behavior of total energy loss!Data generally appear to ‘plateau’
Only statistical errors shown
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pT Broadening - SummaryWhat we have learned• Precise, multivariable measurements of pT
2 are feasible• Quark energy loss can be estimated
– Data appear to support the novel E ~L2 ‘LPM’ behavior– ~100 MeV/fm for Pb at few GeV, perturbative formula
• Deconfined quark lifetime can be estimated, ~ 5 fm @ few GeV• Much more theoretical work needed for quantitative results
Outstanding questions• Is the physical picture accurate?
– Transition to E ~L*E1/2 behavior at higher ?– Can asymptotic behavior E→0 be observed, →∞?– Hadronic corrections under control? Consistent with DY?
• Plateauing behavior due to short p or energy loss transition?• Provide quantitative basis for jet quenching at RHIC/LHC?
JLAB12/EIC
THEORY
EICE906JLAB12
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“How fast do color singlets expand?”
Color Transparency(at low energy)
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Definitions
TA
Q2
Complete transparency
Glauber
Color transparency: • produce a hadron configuration of small size• small hadron has small cross sectionHadron 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
(A)
o
TA =
)(2
22 QMVc
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Color Transparency – Physics PictureThree 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 ExperimentsThree-quark systems• A(e,e’p) in quasi-free kinematics (JLAB)• D(e,e’p) final-state interactions (JLAB)
Precise measurements, CT not observedTwo-quark systems•
– High-energy diffractive dissociation (FNAL) CT observed– n→-p (JLAB) Onset of CT???– A(e,e’) (JLAB) Onset of CT??
• – Exclusive electroproduction at fixed coherence length
(HERMES, JLAB) Onset of CT?
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plation)(Data/Simulation)(Data/Simu T A
A. Larson, G. Miller and M. Strikman, nuc-th/0604022
D. Dutta et. al, JLab experiment E01-107
Direct Pion Electroproduction
pion nucleus total cross-sectionproton nucleus total cross-section
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Kawtar HafidiSearch for the onset of CT Chile 2006
Q2 (GeV2)
TFe
Preliminary Results from CLAS EG2 data
Preliminary results show a clear increase in transparency, in good agreement with CT model!
0 Electroproduction at Fixed Coherence Length
JLab experiment E02-110
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12 GeV
0 electroproductionE12-06-107
0 Tra
nspa
renc
y
E12-06-106
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Color Transparency - SummaryWhat we have learned
– CT in 3q systems not apparent for Q2<10 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 dynamicsJLAB12
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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DefinitionsHadronic multiplicity ratio
Airapetian, et al. (HERMES) PRL 96, 162301 (2006)
Hadron formation
time f
p
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Hadron Attenuation – Physics Picture• Hadrons lost from incident flux through
– Quark energy loss– Interaction of prehadron or hadron
with medium
zh~0.5, larger less attenuation
zh→1, smaller more attenuation
Accardi, Grünewald, Muccifora, Pirner, Nuclear Physics A 761 (2005) 67–91
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HERMES Data – Kr for p, K,
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Hermes Data – Dependence on pT and A
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Examples of multi-variable slices of preliminary CLAS 5 GeV data for R+
zh dependence
Q2 dependence
pT2 dependence
dependenceFour out of ~50 similar plots for +!
K0, 0, -, more, underway
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Cronin Effect Dependence on zh
Carbon
Iron
Lead
Theoretical prediction →
CLAS preliminary dataz=0.5 and 0.7
Probes reaction mechanism
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Acce
ssib
le H
adro
ns (1
2 Ge
V)
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12 G
eV A
ntici
pate
d Da
ta12
GeV
Ant
icipa
ted
Data
Bins in yellow accessible at 5 GeV+
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Hadron Attenuation - SummaryWhat 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? JLAB12THEORY
JLAB12THEORY
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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!
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Backup Slides
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Kinematics for pT BroadeningChoose kinematics favoring propagating quark in-medium:
• z (=Eh/) > 0.5 – enhance probability of struck quark
• z << 1.0 and mh >> 1 – maximize production length to ensure cp >> nuclear size
• z, x such that nucleon factorization holds, to suppress target fragmentation influence
• x > 0.1 to avoid quark pair production
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Hadron-Nucleus Absorption Cross Sections
Hadron–nucleusabsorption cross section
Fit to
Hadron momentum60, 200, 280 GeV/c
< 1 interpreted as due to the strongly interacting nature of the probe
A. S. Carroll et al. Phys. Lett 80B 319 (1979)
p p
_
Experimentally = 0.72 – 0.78, for p, K,
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FNAL E665 experiment
Adams et al. PRL74 (1995) 1525
E = 470 GeV
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How long can a light quark remain deconfined?
Physical picture, DIS• Ubiquitous sketch of hadronization process: string model
RG
GY
Microscopic mechanism not known from experimentKopeliovich, Nemchik, Predazzi, Hayashigaki, Nuclear Physics A 740 (2004) 211–245
• Alternative: gluon bremsstrahlung
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How long can a quark remain deconfined?
Characteristic time scales• Two distinct dynamical stages, each with characteristic time scale:
Formation time tfh
Production time tp
Time during which quark is deconfinedSignaled by medium-stimulated energy loss via gluon emission:
(pT broading)
Time required to form color field of hadron
Signaled by interactions withknown hadron cross sections
No gluon emission(Hadron attenuation)
These time scales are essentially unknown experimentallyS. J. Brodsky, SLAC-PUB04551, March 1988
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Quasi-free A(e,e’p) : No evidence for CT
Tran
spar
ency
Conventional nuclear physics calculation by Pandharipande et al. gives adequate description
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A(e,e’p) Results -- A Dependence
Fit to
= constant = 0.75for Q2 > 2 (GeV/c)2
Close to proton-nucleus total cross section data!
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Color Transparency in D• Experimental ratios: (<0.3)/(0.1),
(0.25)/(0.1) and (0.5)/(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 PWIA+FSI+IC
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A(-,di-jet) Fermilab E791 Data
Coherent - diffractive dissociationwith 500 GeV/c pions on Pt and C.
Fit to A0
>> 0.76, -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)
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Pion Electroproduction
70 degrees 90 degrees
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0 Electroproduction at Fixed Coherence Length HERMES Nitrogen data : TA=P0 + P2Q2
P2 = (0.097 0.048stat 0.008syst) GeV-2
Airepetian et al. (HERMES Coll.) Phys. Rev. Lett. 90 (2003) 052501
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‘A’ Dependence of Transparency
plation)(Data/Simulation)(Data/Simu T A
from fit of T(A) = A at fixed Q2
pion nucleus total cross-section
proton nucleus total cross-section
Usually A
T = A