Elliptic flow from color-dipole orientation in pp and pA collisions

For ultrarelativistic proton-proton and proton-nucleus collisions, we perform an exploratory study of the contribution to the elliptic flow v2 coming from the orientation of the momentum of the produced particles with respect to the reaction plane. Via the color glass condensate factorization valid at high energies, this contribution is related to the orientation of a color dipole with respect to its impact parameter, which in turn probes the transverse inhomogeneity in the target. Using the McLerran-Venugopalan model (with impact-parameter dependence) as an effective description for the soft gluon distribution in the (proton or nuclear) target, we present a semianalytic calculation of the dipole-scattering amplitude, including its angular dependence. We find that the angular dependence is controlled by soft gluon exchanges and hence is genuinely nonperturbative. The effects of multiple scattering turn out to be essential (in particular, they change the sign of v2). We find that sizable values for v2, comparable to those observed in the LHC data and having a similar dependence upon the transverse momenta of the produced particles, can be easily generated via peripheral collisions. In particular, v2 develops a peak at a transverse momentum that scales with the saturation momentum in the target.

Note:

28 pages, 22 figures; v2: extended version, new discussion about the relation between elliptic flow and eccentricity, 8 new plots and some additional references added

p p: scattering
p nucleus: scattering
quantum chromodynamics
color glass condensate
hadron: production
hadron: elliptic flow
hadron: transverse momentum
impact parameter: dependence
color: dipole
scattering amplitude: dipole

References(0)

Figures(22)

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