Scaling violations

Whereas the preceding paragraphs dealt mainly with the semi-soft limit of the hadron spectra, promising results from H1 and ZEUS were presented to this workshop on the scaling violations at larger values of the scaled momentum x_p = 2p/Q [14,15]. In QCD, scaling violations of the fragmentation function are expected, in full analogy with scaling violations of structure functions, due to increased gluon radiation. This leads to softer particle spectra with increasing energy. In principle, the scaling violations at large x_p allow a measurement of $\alpha_S$ and have been exploited for that purpose in e⁺e^- annihilation. Whereas different e⁺e^- experiments must be combined to cover a sufficient range in centre-of-mass energy, this can be accomplished for ep collisions in a single experiment.

H1 data provides evidence for violation of scaling in the current region of the Breit frame. The corresponding ZEUS data, shown in Fig. 2, have also been compared with the CYCLOPS NLO calculation [19] incorporating fragmentation functions taken from e⁺e^- [15]. The data at large x_p show a weak dependence on the input proton parton densities but a clear sensitivity to $\alpha_S$.With analysis of more, already existing, data it should become possible to use the complete NLO calculations to extract $\alpha_S$ in DIS.

  

Figure: $1/\sigma d\sigma/dx_p$ measurement as a function of Q². The preliminary ZEUS data are compared to data from e⁺e^- annihilation experiments and the CYCLOPS NLO calculation of the inclusive charged hadron spectra.