Introduction

One of the main problems in the physics analysis is that of isolate the reaction of interest from all other processes which produce the same effects on the detectors, and which are therefore indistinguishable on an event-by-event basis. The quality of the results depends them on the understanding of the background and any uncertainty on it will be reflected on the quantities of interest. In the case of the $F_2$ analysis at low $x$ and $Q^2$ the main source of background is due to photoproduction in which a fake scattered electron is reconstracted by the analysis algoritm. The usual way of handling background is to substract from the observed number of events a certain fraction which correspond to the its expected number. This is done in each cell of the measured $x$ and $Q^2$. The $F_2$ analysis of the '95 Shifted Vertex Data described in [1] and [2] follows a different approach, in which the observed events are assigned, with suited probabilities, to the signal and to the background. The result is a global inference on both processes, performed, technically, by the so called unfolding. The probabilities with which the events are assigned to each physical cause are evaluated by probability inversion, i.e. starting from the probability of observing those data given a certain cause [3]. The results on $F_2$ obtained by this method have been shown in [1] and [2]. In this note I concentrate the attention on the measurement of the total hadronic $ep$ cross section in the photoproduction regime which comes as a by-product of the primary analysis. This result is then compared with theoretical predictions.