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QCD Processes in the Nucleus

QCD Processes in the Nucleus. Will Brooks Jefferson Lab QCD and Hadron Physics Town Meeting, Rutgers University January 12, 2007. Outline. Introduction to fundamental processes in QCD Transverse momentum broadening and quark energy loss in nuclei Color transparency

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QCD Processes in the Nucleus

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  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 gluons propagating quark quarks in nuclei L nucleus • Quarks lose energy by gluon emission as they propagate • In vacuum • Even more within a medium • Thisenergy 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. 50:37-69

  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 @ few GeV • Much more theoretical work needed for quantitative results Outstanding questions • Is the physical picture accurate? • Transition to DE ~L*E1/2 behavior at higher n? • Can asymptotic behavior DE→0 be observed, n→∞? • 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

  12. “How fast do color singlets expand?”Color Transparency(at low energy)

  13. Definitions s(A) TA= Aso • 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 • withincreasing hardnessof the reaction (Q). • Coherence length lc also affects TA TA Complete transparency Glauber Q2

  14. 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)

  15. Color Transparency – Recent Experiments Three-quark systems • A(e,e’p) in quasi-free kinematics (JLAB) • D(e,e’p) 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,e’p) (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

  17. 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

  18. E12-06-106 12 GeV E12-06-107 r0 electroproduction r0 Transparency

  19. Color Transparency - Summary What 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 dynamics JLAB12 THEORY

  20. “How long does it take to form the full color field of a hadron?”Hadron Attenuation

  21. Definitions Hadron formation time tf tp Hadronic multiplicity ratio Airapetian, et al. (HERMES) PRL 96, 162301 (2006)

  22. Hadron Attenuation – Physics Picture • Hadrons lost from incident flux through • Quark energy loss • Interaction of prehadron or hadron with medium zh~0.5, larger n, less attenuation zh→1, smaller n, more attenuation Accardi, Grünewald, Muccifora, Pirner, Nuclear Physics A 761 (2005) 67–91

  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 z=0.5 and 0.7

  27. Accessible Hadrons (12 GeV)

  28. 12 GeV Anticipated Data 12 GeV Anticipated Data Bins in yellow accessible at 5 GeV p+

  29. 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? THEORY JLAB12 THEORY JLAB12

  30. 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) > 0.5 – enhance probability of struck quark • z << 1.0 and n/mh >> 1 – maximize production length to ensure ctp >> nuclear size • z, x such that nucleon factorization holds, to suppress target fragmentation influence • x > 0.1 to avoid quark pair production

  33. Hadron-Nucleus Absorption Cross Sections K p _ p a p Hadron–nucleus absorption cross section Fit to Hadron momentum 60, 200, 280 GeV/c  < 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

  35. How long can a light quark remain deconfined? Physical picture, DIS RG GY • 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) 211–245

  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,e’p) : No evidence for CT Transparency Conventional nuclear physics calculation by Pandharipande et al. gives adequate description

  38. A(e,e’p) Results -- A Dependence Fit to s = s0 A a for Q2 > 2 (GeV/c)2 a = constant = 0.75 Close to proton-nucleus total cross section data!

  39. Color Transparency in D • Experimental ratios: s(<0.3)/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 PWIA+FSI+IC

  40. A(-,di-jet) Fermilab E791 Data Coherent - diffractive dissociation with 500 GeV/c pions on Pt and C. a Fit tos = s A 0  >> 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 : TA=P0 + P2Q2 P2 = (0.097  0.048stat 0.008syst) GeV-2 Airepetian et al. (HERMES Coll.) Phys. Rev. Lett. 90 (2003) 052501

  43. pion nucleus total cross-section proton nucleus total cross-section ‘A’ Dependence of Transparency Usually s (A) = s0 Aa T = Aa-1 a from fit of T(A) = Aa-1 at fixed Q2

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