[Eg6_analysis] No meeting tomorrow

[Nathan Baltzell]

Dear F.X. and Everyone,

I was doing something related (below), and in the process I noticed
we need significant phi correction to account for solenoid feld (10
degrees phi shift between target windows).  Maybe that has big affect
on your x/y pion vertex.

It may be possible to use upstream window in same way as downstream
window for beam offset, and then maybe combine the two to get beam angle:

http://clasweb.jlab.org/rungroups/lowq/wiki/index.php/Beam_Offset_and_Alignment

-Nathan




On Thu, 01 Nov 2012 14:22:55 -0500, Francois-Xavier Girod  
<fxgirod at jlab.org> wrote:

>
> 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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