Problems with the perturbative QCD interpretation of HERMES data on semi-inclusive lepto-production of pions

TL;DR

This paper critically examines the limitations of standard perturbative QCD analysis in interpreting HERMES semi-inclusive pion production data, highlighting issues with isospin symmetry assumptions and the potential breakdown of factorization at small z.

Contribution

It demonstrates that applying conventional perturbative QCD with isospin symmetry fails to accurately describe the data, suggesting the need for revised theoretical approaches.

Findings

Standard perturbative QCD poorly fits deuteron data at small z.

Allowing isospin violation improves fit but is physically questionable.

Factorization assumptions may not hold in the analyzed kinematic region.

Abstract

A theoretical analysis has been performed of the HERMES semi-inclusive deep inelastic data on the difference between $\pi^{+}$ and $\pi^{-}$ multiplicities. It turns out that the application of a standard perturbative QCD analysis, using, as usual, isospin symmetry for the fragmentation functions, leads to a poor description of the deuteron data at small z values. If one allows a breaking of isospin invariance, a good fit to both proton and deuteron data can be achieved for all measured z, but the level of isospin violation, especially at small values of z, is simply not credible. We suspect that the problem is a consequence of using the factorized QCD treatment in a kinematic region where it is unjustified.