Simplified QCD fit method for BSM analysis of HERA data

The high-precision HERA data can be used as an input to a QCD analysis within the DGLAP formalism to obtain the detailed description of the proton structure in terms of the parton distribution functions (PDFs). However, when searching for Beyond Standard Model (BSM) contributions in the data one should take into account the possibility that the PDF set may already have been biased by partially or totally absorbing previously unrecognised new physics contributions. The ZEUS Collaboration has proposed a new approach to the BSM analysis of the inclusive $ep$ data based on the simultaneous QCD fits of parton distribution functions together with contributions of new physics processes. Unfortunately, limit setting procedure in the frequentist approach is very time consuming in this method, as full QCD analysis has to be repeated for numerous data replicas. We describe a simplified approach, based on the Taylor expansion of the cross section predictions in terms of PDF parameters, which allowed us to reduce the calculation time for the BSM limits by almost two orders of magnitude.