[Eg6_analysis] No meeting tomorrow

[Francois-Xavier Girod]

Dear all,
(
I looked at the eg6 transverse vertex position as a function of the longitudinal position. For this purpose I selected (in CLAS) events with an electron, a positive and a negative pion in the 6 GeV sample (I further keep only events with exactly one of each). Then I used a linear extrapolation from the position and direction of each track to find 3 closest approach points, one for each pair. The vertex for the 3 particles is then defined by the "gravity" center (or barycenter) of those 3 points. Tracks are weighted with a vertex resolution ~ theta/sqrt[p] but that does not change the result much.

In the attached plot
- top left is the electron z (longitudinal vertex position in cm) (n.b. this is for events with a pi+pi- pair)
- bottom left is the corresponding vz of the triplet, the resolution of which seems degraded 
- top center is the vx of the triplet for electron -80 < e_vz < -75 cm
- top right is the vx of the triplet for electron -52.5 < e_vz < -47.5 cm
- bottom center is the vy of the triplet for electron -80 < e_vz < -75 cm
- top right is the vy of the triplet for electron -52.5 < e_vz < -47.5 cm

The transverse coordinates were fitted with a gaussian to estimate the beam position. I do not see much change in the transverse position over the target length, less than 500 microns. I find a beam position 3 mm away from the ideal value, which could change the azimuth of a 100 MeV track by 1 degree. 

Maybe I should try to find the vertex using another reaction, and/or using a helix instead of a straight extrapolation. Comments/suggestions would be welcome.

Best regards,
FX


----- Original Message -----
From: "Stepan Stepanyan" <stepanya at jlab.org>
To: "eg6 analysis" <eg6_analysis at jlab.org>
Sent: Wednesday, October 31, 2012 7:10:51 PM
Subject: [Eg6_analysis] No meeting tomorrow

Hello everyone,

I am on vacation and will not be able to call the meeting.
I propose to cancel tomorrow's meeting and meet next week.

Sorry for late email.

Regards, Stepan
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