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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 analysis at low and 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 and . The 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 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 cross section
in the photoproduction regime
which comes as a by-product of the primary analysis. This result
is then
compared with theoretical predictions.

** Next:** Bayes' unfolding - short
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Giulio D'Agostini
2004-05-05