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