Using the new TPC alignment, Matt Posik & I observed a notable loss of west-side (positive z & η) primary tracks for use in other calibrations. Here are the raw η distributions of primary (red) and global (blue) tracks using the old alignment (first plot) and new alignment (second plot) from the first 8 events of st_physics_13132003_raw_5010005:
[note: open the plots in a new tab/window to see higher resoltuion]


After some investigation, I found that the global T0 for the TPC was not valid for this data using the new alignment. Here are plots showing the z distribution of TPC hits using the old alignment (black) and new alignment (red). The plots are:

• Top left: full z distribution
• Top right: zoom-in near z=0 (central membrane)
• Bottom left: zoom-in near z=-210 cm (east-side prompt hits)
• Bottom right: zoom-in near z=+210 cm (west-side prompt hits)

The shift in z due to erroneous TPC T0 is very apparent. For the new alignment, the database table for Calibrations/tpc/tpcElectronicsB had a tZero value of -0.119 μs. In essence, the primary vertex positions appear to be generally defined by the east-side tracks, so the east-side tracks still form primaries well, but the west-side tracks do not (an interesting finding on its own)!

I first tried to correct this by matching the west-side prompt hits (fitting the peaks with a Gaussian), and obtained this result by adding 0.426 μs to the tZero:

The west-side prompt hits look very good, but the east-side hits still show a difference from the old alignment, and there seems to be a gap in hits near the central membrane, indicating that the T0 has not been shifted enough.

Matching the east-side prompt hits gave a tZero shift of 0.496 μs:

As expected, this offsets the west-side prompt hits. Further, it now appears that there is an excess of hits across the central membrane. This means the T0 has been shifted too much.

So, for a preliminary calibration of the T0, I used a value half-way between the two: -0.119+0.5*(0.426+0.496) = 0.342 μs, leaving neither prompt hit peak matching their positions from the old alignment, but making the distribution of hits across the central membrane considerably smoother:


The prompt hit peaks are fit as follows: gaussian to the entire range of the below histograms, then restricting the range to the peak position ±2σ. The old and new prompt hit peak positions are tabulated and plotted here:

Positions & Widths [cm]	East Inner	West Inner	East Outer	West Outer
Old Alignment	μ = -209.14 σ = 0.19	μ = 209.14 σ = 0.18	μ = -209.36 σ = 0.26	μ = 209.35 σ = 0.26
New Alignment	μ = -209.36 σ = 0.17	μ = 208.96 σ = 0.16	μ = -209.54 σ = 0.25	μ = 209.16 σ = 0.27

(errors on all values in the table are below 0.01 cm)


We see prompt hit peaks that were at pretty much the same |z| with the old alignment, but the new alignment separates them by ~0.4 cm. Yuri notes that his new alignment shifts the east side overall by 0.0156 cm, but the observed shift is significantly larger than this.

...Anyhow, the η distributions with this calibration now look like this (much better than before):

-Gene