BBC / VPD Vertex Investigation

I have made several changes to the BBC/VPD vertex calibration outlined here. On this page I will describe the changes I've made and display the results.

The first change I made was adding another QA condition to the PPV vertices that I use. When doing the calibration, I use the MuDst files so I can get the PPV vertex and compare it to the vertices I reconstruct using the BBC and VPD. In addition to being the first listed primary vertex and having at least two good tracks pointing to it, I now also require that all the PPV vertices I use have a non-negative ranking.

The second group of changes I made concerns the QA cuts I put on the BBC and VPD time differences. Valid BBC and VPD time differences come from events with ADC values above a threshold and largest TAC values within a certain window. The previous thresholds were: East summed ADC > 0 && West summed ADC > 0; 1000 < Largest TAC value < 3000. The new thresholds are listed below, see P4 and P5 here for spectra:

• East summed ADC > 30 && West summed ADC > 30
• BBC East TDC Range: 200 - 3700
• BBC West TDC Range: 200 - 3400
• VPD East TDC Range: 200 - 3450
• VPD West TDC Range: 200 - 3650

The third change I made was to impose a cut on allowed reconstructed vertices. I now throw away events with a reconstructed vertex greater than 160 cm or less than -160 cm. This restriction cuts out events that may have very low or very high time differences which would lead to bad vertex reconstructions. The calibration check pdf contains plots showing vertex distributions before and after this restricted vertex condition is applied.

The fourth change I made has to do with the calibration constants I used when I calculated the z vertex using the TAC values I access directly. I can calculate the z vertex with the formula: Zvert = (maxWestTAC - maxEastTAC) * A * c/2. Here the TAC values are the same ones discussed above, c is the speed of light, and A is a calibration constant giving ns/TAC Bin. Bill Llope provided me with the value A = 0.017 for the VPD and I had been using the same value for the BBC. Below I show plots allowing me to confirm the value for the VPD and calculate a new calibration constant for the BBC. In addition to the linear fits which give me a value for A, I also add constant offsets when running to make the mean of the vertex distribution line up with that of the PPV vertex distribution. The offsets are -8.127 for the VPD and 11.587 for the BBC.

Figure 1: This plot shows (maxWestTAC - maxEastTAC) Vs. PPV Z vertex for the VPD. The distribution is fit with the line y=3.92*x. The calibratioin constant discussed above can be obtained from the slope with the formula: A = 1/(slope * (c/2)). This gives: 1/(3.92*14.99) = 0.017.

Figure 2: This plot shows (maxWestTAC - maxEastTAC) Vs. PPV Z vertex for the BBC. The distribution is fit with the line y=4.77*x. This gives a calibration constant: 1/(4.77*14.99) = 0.014. This fit was designed to 'span the space' of most numerous vertex values in the range of most numerous time differences.

The final changes I made were setting the fit parameters I use to get the Z vertex from the online time difference. In addition to the calculation above, I can find a Z vertex by fitting a line to the online time differenc Vs. PPV distribution and then using the fit parameters to convert a given online time difference into a Z vertex.

Figure 3: This plot shows the online time difference Vs. PPV Z vertex for the VPD. The distribution is fit with the line: y = -0.216 * x + 897.5.

Figure 4: This plot shows the online time difference Vs. PPV Z vertex for the BBC. The distribution is fit with the line: y = -0.250 * x + 1016.0.

The plots I used to make and check these corrections are shown below.

The Calibration Histograms show the online time differences and vertex distributions calculated from TAC differences after various QA conditions were applied as well as several other quantities.

The Check Histograms compare the vertex distributions from the online time difference and the TAC difference to the PPV vertex distribution. Plots are shown before and after the restricted vertex condition was applied.

The MuDst Histograms show the spectra of several kinematic variables before and after the various BBC and VPD vertex corrections have been applied and compares them to the spectra one would see if the PPV vertex was used.