Polarization in hadronicΛhyperon production and chiral-odd twist-3 distribution

Abstract

Polarization of the Λ hyperon produced with a large transverse momentum in the unpolarized nucleon-nucleon collision is analyzed in the framework of QCD factorization. We focus on the mechanism in which the soft-gluon component of the chiral-odd spin-independent twist-3 quark distribution E F (x, x) becomes a source of the polarized quark fragmenting into the polarized Λ. Our simple model estimate for this contribution indicates that it gives rise to a significant Λ polarization at large x F . This is in parallel with the observation that the soft gluon pole mechanism gives rise to a large single transverse spin asymmetry in the pion production at x F → 1.

Azimuthal and single spin asymmetries in hard scattering processes

“…The first contribution has been studied in Ref. [197], where by adopting a simple ansatz on φ (3) a/p (x 1 , x 2 ), namely a proportionality relation with the chiral-odd transversity distribution, the main features of the data are reproduced, although some discrepancies still remain.…”

Hyperon polarization in unpolarized scattering processes

“…The above expansion allows us to integrate over three of the four components of each of the loop momenta k i , and the hadronic tensor W µν will depend on the chiral-odd spinindependent twist-three quark-gluon-antiquark correlation function [3,22],…”

“…Similar to the SSA in inclusive hadron production in p ↑ p → πX, the Hyperon polarization in unpolarized pp scattering pp → Λ ↑ X receives the contributions from (naive)-time-reversal-odd effects in the distribution and fragmentation parts [21,22]. In order to understand the experimental observations of pp → Λ ↑ X, one has to take into account both contributions.…”

“…In particular, theoretical developments have been made to understand the Λ polarization in unpolarized hadronic reactions [19,20,21,22]. Similar to the SSA in inclusive hadron production in p ↑ p → πX, the Hyperon polarization in unpolarized pp scattering pp → Λ ↑ X receives the contributions from (naive)-time-reversal-odd effects in the distribution and fragmentation parts [21,22].…”

“…To describe these effects, one can calculate the Λ polarization in the twist-three approach [22] or use the TMD mechanism [23]. For example, the contribution from the T-odd effects in the distribution part is associated with the so-called Boer-Mulders TMD quark distribution h ⊥ 1 [23] multiplied by the TMD transversity fragmentation function H 1T when the transverse momentum of the produced Λ is much smaller than the hard scale P Λ⊥ ≪ Q where Q is the virtuality of the virtual photon in DIS process.…”