Ckov momentum scan from 28-29 March data
Part of the data collection of the 28th and 29th of March was a momentum scan using pion beams. The important runs are from 6268 to 6278 taken on Saturday evening and Sunday morning. A motivation for the momentum scan was to evaluate the behaviour of the CKOV as a function of momentum. As part of the physics shift (and at the suggestion of Paul Soler) I made a quick analysis of these data for this purpose. The results of this analysis may be used to refine the global particle identification.
The analysis (in the attached script "ckov_momentum_scan.py") selects muons, pions, and electrons from the beam using the time of flight between TOF0 and TOF1. The selections are tabulated here;
|Beam momentum (@tgt)||run #||muon TOF range||pion TOF range||electron TOF range|
|300 MeV||6270 - 6271||28-29 ns||30-32 ns||25-27 ns|
|350 MeV||6274||27-28.3 ns||28.9-30 ns||25-27 ns|
|375 MeV||6275||27-28 ns||28.5-29.5 ns||25-26.7 ns|
|400 MeV||6276||26.7-27.8 ns||28-29 ns||26-26.7 ns|
|425 MeV||6278||26.5-27.5 ns||27.8-28.8 ns||26-26.5 ns|
These selections were motivated by the inspection of the TOF distributions attached ("ckov_tof_distributions.pdf"). The particle momentum was calculated from the time of flight using p = m / sqrt( (t/t_0)^2 - 1) where t_0 = D/c assuming D is the TOF0 to TOF1 distance from the exiting step IV drawings, or D=7862 mm (which should be updated when new surveys appear). For this distance t_0 is 26.02 ns which is consistent with the electron time of flight. A 1 ns (30 cm) difference in the time of flight can make a change of 36 MeV/c in this calculation so a better measure of the TOF positions will improve this measurement.
For each particle type at each CKOV, a 2 dimensional histogram is compiled for the number of CKOV photoelectrons against the momentum. The mean number of photo-electrons for the pions and muons are shown in the attached summary file ("ckov_scan_pvPE.eps"). The mean number of PEs from electrons was also calculated as 25 but not shown in the summary plot because the electron momentum can not be calculated from the time of flight. The figure of merit shows a clear threshold for the observation of muons in CKOVa at 260 MeV/c and a not so well defined threshold at 250 MeV/c for pions in CKOVb. This can be cleared up somewhat if run 6268 can be reconstructed (it was not in the grid reconstruction for an as yet unknown reason) as that beam line setting should fill in the region between 250 MeV/c and 280 MeV/c in the pion distribution. This is illustrated in the source 2d histograms also attached.