Perturbative fragmentation

TL;DR

This paper refines the perturbative fragmentation model for quarks to pions by improving normalization, including all terms in the (1-z) expansion, and replacing the wave function with a light-cone model, leading to better agreement at very high z.

Contribution

It introduces a more accurate normalization and a comprehensive (1-z) expansion in the Berger model, and employs a light-cone wave function for pions, enhancing the model's validity across all z values.

Findings

The model does not confirm the (1-z)^2 behavior for z>0.5.

Agreement with data is only good at z>0.95.

Higher twist effects are significant at large z and pT.

Abstract

The Berger model of perturbative fragmentation of quarks to pions is improved by providing an absolute normalization and keeping all terms in a (1-z) expansion, which makes the calculation valid at all values of fractional pion momentum z. We also replace the nonrelativistic wave function of a loosely bound pion by the more realistic procedure of projecting to the light-cone pion wave function, which in turn is taken from well known models. The full calculation does not confirm the (1-z)^2 behavior of the fragmentation function (FF) predicted in for $z>0.5$, and only works at very large z>0.95, where it is in reasonable agreement with phenomenological FFs. Otherwise, we observe quite a different z-dependence which grossly underestimates data at smaller z. The disagreement is reduced after the addition of pions from decays of light vector mesons, but still remains considerable. The process dependent higher twist terms are also calculated exactly and found to be important at large z and/or pT.