[Eg6_analysis] No meeting tomorrow

[Nathan Baltzell]

Dear all,

I realized what I said about phi correction for solenoid was wrong.
It is just phi-acceptance moving with z-vertex, as it should in the field.

But the first section at that wiki link is still useful (beam position
 from both target windows).

-Nathan



On Thu, 01 Nov 2012 15:36:24 -0500, Nathan Baltzell <baltzell at anl.gov>  
wrote:

> 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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