A fast on-line event reconstruction procedure based on a linearized least-squares fit

We have developed and implemented a procedure suited for fast on-line reconstruction of particle physics events, in particular events with twobody final states, which are detected by a set-up consisting of triggering counters, multiwire proportional chambers and a polarized target (implying a strong magnetic field). The parameter vector describing the event is obtained from the vector of observations (MWPC coordinates) by multiplication with a reconstruction matrix, which is the pseudoinverse of the design matrix, obtained by a standard least-squares technique. A goodness-of-fit parameter (chi-square) is computed at the same time and allows testing of various reaction hypotheses and discrimination against background. The design matrix, which models the experiment in a linear approximation is obtained beforehand on a large computer. On-line reconstruction is then performed on events satisfying the desired trigger condition and having successfully passed tests made on the MWPC coordinates by a fast decision logic. The reconstruction is performed by a dedicated minicomputer and consists of aligment checks in individual telescopes, substitution of missing coordinates, computation of the various parameters describing the event, application of a correction to compensate for non-linearities in the relation between observations and parameters, and tallying of good events in multidimensional histograms to be transferred periodically to the computer PDP-11 controlling the entire experiment. (orig.)