Measurement of Dijet Production at Low Q at Hera

Jet cross sections in electron-proton collisions are successfully described by nextto-leading order (NLO) QCD calculations in most of the HERA kinematic range. However, regions of phase space have previously been observed for which NLO predictions do not reproduce the data satisfactorily [2] and leading order (LO) Monte Carlo simulations with different approaches to modelling higher order QCD effects are often more successful. In this analysis we focus on the dijet cross section in a region where we expect large deviations between measurement and the QCD predictions. This kinematic region is characterised by low photon virtualities, Q, forward jet pseudorapidities, η∗, and high inelasticities, y, (which corresponds to the region of low x γ , the fraction of the photon four-momentum carried by the parton involved in the hard scattering). The analysis is described in much more detail in a recent H1 publication [1]. Here we just recall its most important results, and in addition present one particular aspect of the analysis, namely an unphysical dependence of the NLO QCD predictions calculated using the program JETVIP [3] on a technical parameter yc. Figure 2, showing the problematic yc dependence, is the only result which is not included in [1].