TPC T0 adjustment for Run 14 FF Cosmics

 We decided to use the charge step method to determine the T0 for the dedicated FullField cosmic ray data acquired just before the start of Run 14 (see this thread). For this purpose, I used TPC hits (with flag==0) stored in event.root files produced by Yuri from days 31-34.

(note: most plots can be opened in a separate window/tab to get a higher resolution view)

After some study, I found that the charge step appeared to slide off of a fixed position over the course of a few runs, then slide back over a few more runs. This is demonstrated in this plot of the charge step for east (shifted by -3 cm) and west (shifted by +3 cm) vs. a run index (using the last 3 digits of the run number, +0 for day 31, +200 for day 32, +300 for day 33, + 400 for day 34; I have written the day numbers near their data as a guide):


The anomaly is seen at the end of day 32 and beginning of day 33 (run index near 300), and is much more apparent on the west than east. Since T0s tend not to vary gradually like this, I took a look at the drift velocities and found what appears to be the culprit:


The west drift velocity has a blip right around the start of day 33 which is likely erroneous.

Excluding runs 15032056-15033019, here is what we see for the east and west charge steps overlaid (and reversed for east to clarify):


There are a couple features worth noting:

1. There is a bump near -0.8 cm (about 0.7-0.8 cm in from the charge step), for which I currently have no theories. The bump does not correspond to any particular hit flags.
2. The east and west distributions are offset by ~0.1 cm. The are many potential reasons for this (e.g. east and west drift velocities different [or differently wrong], unaccounted differences in drift length, errors in the sector-by-sector or padrow-by-padrow T0s, etc.), so I will spend no further time here trying to figure it out.

Thus, we can only expect to get the charge step T0 calibrated to a precision of ~0.1 cm in z, or ~0.002 μs, and the resulting distribution of hits across the central membrane will still show some features from the bumps. Here then are some overlaid distributions of the z distribution with no separation of the east and west other than to separately add T0 shifts in 0.002 μs steps (under the assumption of a constrant 5.500 cm/μs drift velocity).


The initial T0 shift for the Run 14 AuAu15 physics data has been set to 0.266 μs (which was determined with less precision than is being done here and will likely be refined soon). One can see that 0.266 μs works reasonably well here, but the best choice is perhaps close to 0.274 μs. I will insert the shift with 0.274 μs into the database, but I infer that the T0 is probably the same in reality for both the cosmic and physics data: whatever T0 is determined as final for the physics data should be set in the database to apply for all Run 14 data unless something new develops. [update: entered into the DB with beginTime 2013-12-20 00:02:00]

It will be interesting to see what effect this T0 adjustment has on track projections to the HFT (my impression was that they expected only half this much shift based on the east vs. west track projections that they saw).

-Gene