Study of vertex reconstruction in transverse X-Y plane, pp 500 data from 2009, high PT events from W-stream

Large variance in the initial determination of beam line constrain for pp 500 data has been observed. The concern was that reconstruction accuracy for high PT TPC electron tracks from W decay may be not sufficient.

At first simpleminded  idea of increasing minimal PT of used tracks and imposing high track multiplicity did not improve accuracy of vertex determination in the transverse plane.

Next we look at individual events passing the following selection criteria, passing through most likely primary tracks candidates from the pool of global tracks:

• use only global tracks matched to BTOW,ETOW, or central membrane
• require nFit/nPoss>51%, and Rxy @ DCA to X=Y=0 below 3 cm
• require PPV finds valid vertex along Z direction, require delZ <3cm
• require global pT in range [0.8, 8.0] GeV/c
• require Sti extrapolation error in transverse plane is below 3 mm
• require at least 5 tracks passed criteria listed above

  
Fig 1. Typical spectra for some of the cut parameters for W-stream pp 500 events

Tracks passing selection are approximated by straight lines in the vicinity of DCA to X=Y=0 and shown in Fig 2. Z-axis range  is always 6 cm, centered at the max likelihood of PPV.
The following encoding was added to plots:
*head of arrows indicates direction of the momentum vector 
*size of the arrow is proportional to track PT, max PT for given set of tracks (event) is in the title of the left histograms
* thickens of the line is proportional to the weight of  track in vertex (or beam line) determination, I used formula:
       width= 3.* (0.15*0.15)/sig/sig; , where sig=sigYlocal from Sti .
(The last 2 conditions sometimes interfere, since the thicker line increases also the arrow size, but still plots should help us to gain intuition).

Fig 2, Projections of global tracks at most likely vertex location. One event per row, two projections: Y vs. X and Y vs. Z.
Stray tracks are most likely form pileup or from decays matched to fired EMC towers.
The width of arrows is proportional to likelihood the vertex is below it (~1/track error^2)

Attachments A,B show more real data events.

Attachments C,D show M-C Pythia QCD events with partonic pT>10 & 20 GeV, respectively. C has fixed vertex offset, D has varied vertex offset.

Conclusion:

*Very often one sees 2 jets what impedes determination of transverse vertex position on event by event basis, in particular if vertex finder is not returning non-diagonal covariance matrix element covXY (see last event in fig 2.)

* we will pursue alternative method of beam line determination by fitting its equation directly to preselected tracks from multiple events. We try to skip event by event vertex determination.