Abstract
The transverse polarization of forward Λ hyperons produced in high-energy p–A collisions is expected to display an extremum at a transverse momentum around the saturation scale. This was first observed within the context of the McLerran–Venugopalan model which has an x-independent saturation scale. The extremum arises due to the kt-odd nature of the polarization-dependent fragmentation function, which probes approximately the derivative of the dipole scattering amplitude. The amplitude changes most strongly around the saturation scale, resulting in a peak in the polarization. We find that the observation also extends to the more realistic case in which the saturation scale Qs is x-dependent. Since a range of x and therefore Qs values is probed at a given transverse momentum and rapidity, this result is a priori not expected. Moreover, the measurement of Λ polarization over a range of xF values actually provides a direct probe of the x-dependence of the saturation scale. This novel feature is demonstrated for typical LHC kinematics and for several phenomenological models of the dipole scattering amplitude. We show that although the measurement will be challenging, it may be feasible at LHC. The situation at RHIC is not favorable, because the peak will likely be at too low transverse momentum of the Λ to be a trustworthy measure of the saturation scale.
Highlights
It is well-known that Λ hyperons produced in collisions of unpolarized hadrons are to a large degree polarized perpendicularly to the production plane
In Ref. [1] it was shown that the available Λ polarization data for pt > 1 GeV can be described within a factorized approach by employing a polarization and transverse momentum dependent fragmentation function
The transverse polarization of Λ particles displays a peak at the saturation scale when described using the MV model for the dipole scattering amplitude
Summary
It is well-known that Λ hyperons produced in collisions of unpolarized hadrons are to a large degree polarized perpendicularly to the production plane. [1] it was shown that the available Λ polarization data for pt > 1 GeV can be described within a factorized approach by employing a polarization and transverse momentum dependent fragmentation function. This kt-odd nature implies that it is essentially accompanied by the first derivative of the partonic cross section w.r.t. kt, unlike the unpolarized Λ fragmentation function D, which is kt-even This turns the observed Λ polarization into a useful tool, which in this paper will be applied to the study of the x-dependence of the saturation scale. This discussion repeats the essentials from Ref.
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