PASS 0 mode

This mode is usually used on FALCON and serves as pre-processing. In this mode, the TPC drift velocity and the VDET hot channels list can be determined.

Introduction

The steering is equivalent to :

*------ Detectors to be processed ---------------------------
PROC 'SKEL', 'VDET', 'TPC '

The PROC card is not needed, however only one of the two PASS 0 actions can be performed by selecting the corresponding detector my means of a PROC data card. For the TPC, the dE/dx reconstruction is not performed and the Kalman filter is not run. For the VDET, only the clustering is performed.

This mode requires two input files and produces one output file. The input files are :

• the raw data file, described in the FILI card
• the PASS 0 reference file which is a BOS card file, described in the READ card in the following example.

The output file, is a BOS card file, described in the PAS0 card. Below, is an example of JULIA data cards used on FALCON for PASS 0 running mode.

FILI 'RAW.NATIVE | NATIVE'
PAS0 'OUTPASS0.CARDS'
READ 'REFPASS0.CARDS'
* PASC allow to change  PASS0 cuts , PASC is on the DAF
*
* first parameter : minimum number of 'good for pass0' events needed
*                   to update the drift velocity
* second          : maximum number of 'good for pass0' events used
*                   to update the drift velocity
* third           : maximum allowed elapsed time since last laser
*                   measurement to use vz_laser is there is too few events
* fourth          : maximum allowed absolute value for delta-t0 (mus)
*                   in order to update the drift velocity
*
*PASC -1 / 4 1 50  250 -1. 0.200
ENDQ

How does it works

The purpose is to determine the TPC drift velocity along the z axis. Events are reconstructed with a guess of what this drift velocity should be, the guess should not be too far away from the real value otherwise the reconstruction would fail.

Then by selecting good tracks, and by measuring for those tracks the mean z impact parameter on each half of the TPC, it is possible to determine, knowing the input drift velocity to JULIA ( the guess ) the real drift velocity value : the new one.

Good tracks are selected, according to the following criteria (those lines are taken from a standard JULIA log file):

  TPC T0 MONITOR initialized on first call        
  -----------                                     
  TPC-T0 : Default cuts used **********           
  1) min. number of fitted tracks on one side....          2
  2) min./max. lambda for a track (degrees).......      0.000 /     50.000
  3) max. allowed d0 (cm)........................      2.000
  4) max. allowed z0 (cm)........................     10.000
  5) max. invers radius..........................    0.00200
  6) number of required hits on a track..........         10

The relation between, the old and the new drift velocity is given by the formula :

where 220. is the TPC half length, stands for the TPC and for the TPC drift velocity along the z axis. being half the difference along the z axis, between the vertices found in the two TPC halves.

One should note that it is impossible with this method to distinguish between a change in the drift velocity and a change in the TPC . Usually, the value is obtained by other means and one simply states that . However, any brutal change in the TPC drift velocity as a function of time would indicate a change of value.

The output of JULIA is a card file (OUTPASS0.CARDS in the previous data card example). This card file contains,

• a short JULIA summary,
• the PASS 0 summary bank (JPAS), which will go onto the POT run header after the JULIA processing,
• the new TPC drift velocity bank (TPDV),
• the new VDET hot channel bank (VHOT).

The running of JULIA in PASS 0 mode, is driven by the PASC data base bank. A minimum number of events is required to allow for a new determination of the drift velocity etc.. If the conditions are not met, the drift velocity is taken from the previous run for which the conditions were met (usually it is the input card file REFPASS0.CARDS in the previous example). If the conditions are met, on FALCON, the OUTPASS0.CARDS is renamed in REFPASS0.CARDS and used as input for the next run.

Example of an output of PASS 0, produced on FALCON :

PON
*PASS0 --- Summary for Run  27639 -  823 Z read
*PASS0 Date  940725 Time    1221
*PASS0 Julia 274.02 Alephlib  20.10 Daf   191 from  940624
*PASS0 Min. nb of evts :    50 - TLAS max. elapsed time :   -1.00 h.
*PASS0 TLAS too OLD :   280. hours
*PASS0 T0 difference, 0.5*(TPC end-A - TPC end-B) for   377 events
*PASS0 delta-T=   0.00284 +- 0.00179 mus; rms= 0.00121 mus
*PASS0 Number of VDET HOT channels :    70 Previous run :    65
*PASS0 TPC Vz ref. from Run  27638 VHOT from Run  27638
*PASS0 Vold 5.24144 Vnew 5.24179+- 0.00022 Dv/v -0.00007
*PASS0 ----> Reference Drift Velocity updated <----
 * START OF TEXT-RECORD
 JPAS $FMT 2I,(3I,9F,4I)
 JPAS 27639/ 17 1 823 377 50 0.00284068 0.00178954 0.00120732 5.24491
 5.24144 5.24179 223.47E-6 -1. 279.666 940725 1221 999999 65 70
 TDPV $FMT 2I,(4F)
 TDPV 27639/ 4 2 -900.E-6 0.0035 5.24179 223.47E-6 900.E-6 -0.0035
 5.24179 223.47E-6
 VHOT $FMT 2I,(I)
 VHOT 27639/ 1 70 7080433 11799026 10324000 23169057 29198370 29460515
    .......
 ENDQ