Automatic detection of small angle deflection in beam tracks on bubble chamber photographs

For an experiment where one wished to detect scatters (small angle deflections) in the range 3-30 mrad occurring in beam tracks on bubble chamber pictures, programs have been developed which will search for these scatters with far less bias than that obtainable when they are searched for by eye. The input data for these programs are a series of coordinates obtained during a raster scan of the picture using a mechanical digitizer (HPD). Typically 40000-60000 coordinates are generated for each picture. The method used relies heavily on the good collimation and small momentum spread of the incoming beam. Tracks which do not belong to the beam or which are confused on the film with other beam tracks are rejected at an earlier stage. The remaining tracks are followed across the film and the average points, which are computed at regular intervals, are subsequently used in the algorithms which test for the presence of a scatter. All candidate events found by the program are subsequently checked by eye using the presence of a short recoil track as a means of confirming the interpretation of the event. Both the methods used and the performance obtained on 650000 tracks are described. (3 refs).