qqbar production in pa collisions
DESCRIPTION
QQbar production in pA collisions. Javier L. Albacete. QM06, Shanghai, November 2006. OUTLINE. Goal: Determination of ‘ cold nuclear’/saturation effects in quark production: Quark production in high energy nuclear collisions it is a two scale problem : Saturation scale: - PowerPoint PPT PresentationTRANSCRIPT
QQbar production in pA collisions
Javier L. Albacete
QM06, Shanghai, November 2006
OUTLINE
Goal: Determination of ‘cold nuclear’/saturation effects in quark production:
Quark production in high energy nuclear collisions it is a two scale problem:
Saturation scale: At high energies small-x gluons in the nucleus wave function saturate (maximum occupation number achieved): .
At currently available energies a non-trivial interplay between the two scales is expected for heavy quarks (charm, bottom?, extended scaling window):
Light quark production (u, d, s):
Heavy quark production:
Saturation effects in diquark production should be taken into account in moreexclusive (complex) processes ,e.g. J/production, charm energy loss puzzle.
kT factorization? NONpart scaling. Fujii et al.
Collinear factorizationNcoll scaling
Fujii et al hep-ph/0603099Kharzeev, Tuchin NPA 735:248-266,2004.
The calculation is done in the light-cone gauge of the proton, A+ =0 using light-cone perturbation theory (time-ordered diagrams).
Nuclear effects become manifest in the form of multiple rescatterings.
Eikonal approximation: Recoil of energetic quarks and gluons in their interaction with the nucleus is neglected. Coordinate Space.
Quasi-classical approximation: no more than 2 exchanged gluons per nucleon: ressummation parameter: s
2A1/3 (McLerran-Venugopalan model).
Quantum Evolution: The emission of extra soft gluons is enhanced by are resummed to all orders at leading logarithmic accuracy and in the large-Nc
limit (mean field approximation)
General Features
+ -
Kovchegov, Tuchin, Phys.Rev.D74:054014,2006.
nucleus
proton
Quasi- classical approximation
The relevant diagrams at high energies are:
qq emission wave function: Leading Nc, lowest order pQCD calculation. Fixed coupling
Eikonal propagation: Combination of Glauber-Mueller propagators
Quantum evolution QM06
At higher rapidities/energies quantum effects become important:
They have been included in the large-Nc limit of QCD (dipole degrees of freedom)
Harder gluons (y’>y) : Linear BFKL evolution
Softer gluons (y’<y) : Non-linear BK evolution
z1
z0
w1
w0
THIS WORK
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Multiplicity
Mass dependence: Infrared divergent due to the collinear singularity:
Non trivial Npart dependence:
Light quarks:
For heavy quarks:
…
Reduced at more forward rapidities
Stronger cme/Y dependence for heavier quarks
Non-interacting diquark
pt spectrum
Heavy quark mass: much harder spectrum
Finite at pt=0Assymptotic behavior ~ 1/pt
4
Single quark (antiquark) spectrum (central rapidity):
Nuclear modification factor
Strange: Cronin enhancementCharm: No enhancement. Suppresion?Bottom: No enhancement
All cases: RpA at central rapidity decreases uniformly at more forward rapidities (quantum evolution)
Nuclear modification factor
Flatness of RpA at central rapidity is compatible with experimental data:
M. Liu, Hard Probes
Indication that spectrum shape is determined by leading-twist effects
Nuclear effects affect the normalization (atomic size dependence of multiplicity?).
STAR data
SUMMARY
The formalism for pair quark production in the CGC-saturation framework has been presented. Quassiclasical approximation + quantum evolution effects (linear and non-linear dynamics non-trivially entangled). No factorization.
Multiplicity:- Divergent for zero mass (collinear singularity)- Non-trivial atomic size dependence of the total multiplicity- Stronger cme dependence for heavier quarks- Weaker Npart dependence at more forward rapidities
Single quark specturm:- Light quarks: Cronin effect at central rapidity washed out by quantum evolution at more forward rapidities. - Heavy quarks: Flat RpA : no modification of pt spectra shape. Quantum evolution makes RpA decrease.
Next:- Hadronization/fragmentation effects- Improvement in the implementation of quantum evolution: Non-perturbative input + better determination of saturation scale- Large-x corrections