Output

As stated earlier, the fitter does not explicitly find a start vertex for the event. However, it will reconstruct an event to a point at a certain radius from the center of the detector, (900 cm by default). This point is given in the FTMV bank. The track direction vector is given in the FTMT bank, as per usual. The exit position of the muon is given in the FTM bank at word KFTM_EXIT_X.

Other values of interest given in the FTM bank are:

• the impact parameter P for the fit track (PFIT),
• the total length of track inside the PSUP (LENGTH),
• the number of tubes passing the charge cut (NTUBES_GOODCHARGE),
• the exit wound position and track direction of the best grid trial (GUESS_*)
• the total charge deposited in the detector per cm of track length (DEDX)
• the $\chi^2$/tube value using all tubes in the event, not just the high-charge ones (ALLTUBES_CHISQ)
• the number of tubes whose fit time was within 1 sigma of the observed time (NTUBES_AGREE). (This is a valuable measure of the goodness of fit),
• the $\chi^2$/tube for all tubes with $\vert t_{fit}-t\vert < t_{window}$ (TWINDOW_CHISQ),
• the mean and RMS values for time and charge of all the tubes (QMEAN, QRMS, TMEAN, TRMS)
• the mean and RMS values of lambda (see below) (LMEAN, LRMS)

The value of 'lambda' for tube i is the fractional distance along the track that the muon was at when it gave light that fired tube i. If a tube has lambda = 0, this indicates that it was it by light emitted by the muon as the muon entered the PSUP. If lambda = 1, this indicates the muon was leaving the PSUP as it gave off light that hit this PMT.

The distribution in lambda is of interest for two reasons. First, if the lambda values are $>>1$ or $<<0$, the fit is incorrect. Second, if the lambda values are all, say $>0.5$, this would indicate that the muon is partially contained, created in the detector halfway along the fitted track. This tool is one needing study, but is potentially useful in analysis.