Spin-dependent Fragmentation Functions Research Articles

QCD analysis of unpolarized and polarized Λ-baryon production in leading and next-to-leading order

We analyze experimental data for the production of Lambda baryons in e^+e^- annihilation in terms of scale dependent, QCD evolved, Lambda fragmentation functions. Apart from the vast majority of the data for which the polarization of an observed Lambda was not determined, we also consider the recent LEP measurements of the longitudinal polarization of Lambda's produced on the Z-resonance. Such data correspond to spin-dependent fragmentation functions for the Lambda. We point out that the present data are insufficient to satisfactorily fix these. We therefore suggest several different sets of fragmentation functions, all compatible with present data, and study the prospects for conceivable future semi-inclusive deep-inelastic scattering experiments to discriminate between them. We provide the complete next-to-leading order QCD framework for all the processes we consider.

Spin-Dependent Structure Function ĝ1+ĝ2 for e++e-→B+X (Revisited)

The QCD factorization expression has been revisited to the sum of two spin-dependent structure functions ĝ1 and ĝ2 for e++e-→B+X. We find that there exist some spurious quark-mass contributions in an expression derived earlier by Lu, Li and Hu. Their removal guarantees that the measurements of ĝ1 and ĝ2 indeed construct a determination of twist-three transverse spin-dependent fragmentation function, ĝT, in the case of inclusive Λ production in electron–positron annihilation.

Fragmentation of transversely polarized quarks probed in transverse momentum distributions

It is shown that the azimuthal dependence of the distribution of hadrons in a quark jet is a probe of the transverse spin of the quark initiating the jet. This results in a new spin-dependent fragmentation function that acts at the twist-2 level. One example of a process where it contributes is semi-inclusive deeply inelastic scattering with a transversely polarized nucleon target but with an unpolarized electron beam. This process is treated in detail. Another process in the cross section for singly polarized hadron-hadron scattering when two high- p ⊥ hadrons are measured in the final state and are close to back-to-back in azimuth. The new fragmentation function is sensitive to the coupling of the fragmentation process to (spontaneous) chiral symmetry breaking.

Polarized q--> Lambda fragmentation functions from e+e---> Lambda + X.

Measurement of the helicity asymmetric cross section for semi-inclusive production of $\Lambda$-hyperons in $e^+e^-$ annihilation near the $Z^0$ resonance allows a complete determination of the spin-dependent fragmentation functions for the different quark flavors into the $\Lambda$. The parity violating, self analyzing, decay of the final state $\Lambda$ makes the experimental analysis of the helicity asymmetry possible. This experiment should be practical with present day technology at the LEP collider at CERN or at SLC at SLAC.