Hello David and Tong,

 

Today, Helen mentioned you are trying to use offline VPD information to mimic online VPD algorithm to pin down the weird BBC activity as a function of Vz observed in VPD-5 trigger. Here, I would like to share some materials related to this emulation for both online and offline data.

 

The major differences between online and offline VPD vertex are:

1. In run15 pAu collisions, online VPD vertex is calculated by timing difference between the earliest hits of both sides, while offline VPD vertex is calculated by timing difference between mean timing of both sides

2. 16 of 19 tubes (each side) are used for online VPD vertex while 19 tubes are used for offline VPD vertex. 

3. The readout electronics between online and offline are different. The timing resolution of offline is much better. Online electronics uses common stop which means the largest TAC corresponds to the earliest hit. Offline electronics uses common start which means the smallest TDC corresponds to the earliest hit. (TAC or TDC corresponds to timing information)

4. The slewing tables between online and offline are different. Online slewing table only has 8 ADC bins while offline slewing table has 128 TOT bins for each VPD tube. (ADC or TOT corresponds to amplitude information)

 

Online VPD variable (read from MuDst) and algorithm,

In principle, you can directly read the fastest east/west VPD TAC in DSM board by

Fastest east: mMuDst->event()->triggerData()->vpdEarliestTDC(east, 0) // 0 means current bunch crossing

Fastest west: mMuDst->event()->triggerData()->vpdEarliestTDC(west, 0)

 

If you plan to calculate the online vertex using QT information (raw online VPD information), The example of grabbing related QT variables can be found in,

Read QT variables: /star/u/syang/run15/mtdTrig/offline/StRoot/PicoMaker/PicoMaker.cxx  (bbq VPD is for online VPD trigger)

Apply slewing correction and calculate online vertex: /star/u/syang/run15/mtdTrig/offline/checkOnlineVpdRes/pAu200/analysis.cxx

 

Online VPD algorithm documents (earliest timing):

QT algorithm: https://www.star.bnl.gov/public/trg/TSL/Software/qt_v6_d_doc.pdf

DSM algorithm: https://www.star.bnl.gov/protected/lfsupc/syang/BBC_pAu_2015_060515.pdf

 

Offline VPD information:

How to read raw individual VPD TOT and timing information, apply T0 offset, and implement slewing correction can be found in the STAR official StVpdCalibMaker. You can manually select the earliest corrected offline TDC (smallest TDC but with seasonable TOT value) to mimic the online algorithm

 

I hope this will help. Please let me know if I do not make myself clear or you have any question.

 

Best Wishes,

Shuai