QQbar production in pA collisions

1. QQbar production in pA collisions Javier L. Albacete QM06, Shanghai, November 2006

2. 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): . Fujii et al hep-ph/0603099 Kharzeev, Tuchin NPA 735:248-266,2004. kT factorization? NO Npart scaling. Fujii et al. Light quark production (u, d, s): Heavy quark production: Collinear factorization Ncoll scaling At currently available energies a non-trivial interplay between the two scales is expected for heavy quarks (charm, bottom?, extended scaling window): Saturation effects in diquark production should be taken into account in more exclusive (complex) processes ,e.g. J/Y production, charm energy loss puzzle.

3. General Features Kovchegov, Tuchin, Phys.Rev.D74:054014,2006. • 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: as2A1/3 (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) - +

4. Quasi- classical approximation proton nucleus 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

5. z0 w0 z1 w1 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 THIS WORK

6. … Multiplicity Mass dependence: Infrared divergent due to the collinear singularity: Non-interacting diquark Non trivial Npart dependence: Light quarks: For heavy quarks: Reduced at more forward rapidities Stronger cme/Y dependence for heavier quarks

7. pt spectrum Single quark (antiquark) spectrum (central rapidity): Heavy quark mass: much harder spectrum Finite at pt=0 Assymptotic behavior ~ 1/pt4

8. Nuclear modification factor Strange: Cronin enhancement Charm: No enhancement. Suppresion? Bottom: No enhancement All cases: RpA at central rapidity decreases uniformly at more forward rapidities (quantum evolution)

9. 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

10. 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