From neven@phys.latech.edu Thu Sep  9 07:24:49 2004
Date: Tue, 07 Sep 2004 13:33:21 -0500
From: Neven Simicevic 
To: Jim Birchall 
Cc: Dave Mack , Juliette Mammei ,
     Roger Carlini , Klaus Grimm ,
     armd@jlab.org, Mark Pitt , Allena Opper ,
     Greg Smith , Norman Morgan ,
     Mike Finn , Yongguang Liang ,
     Shelley Page , Tony Forest 
Subject: Re: Qweak working group - next meeting

Hello,

a short comment:

If the position of our collimator depends on the minitorus and if the 
minitorus can considerably influence our measurement, then it is the 
minitorus a defining element of our experiment not the collimators.

Neven


Jim Birchall wrote:

> Dave,
>
> How about noise on the asymmetry measurement? How much ripple is 
> allowable on the mini-torus field so that the noise is less than say 
> 10% of counting statistics on Qw? A small change of deflection of 
> elastic electrons could modulate the event rate nicely, especially at 
> the small angle side of the acceptance where the cross-section slope 
> is large.
>
> Jim
>
> On 7 Sep 2004, at 9:35 am, Dave Mack wrote:
>
>>
>> Allena et al,
>>
>> The mini-torus is a perturbation on the trajectory. I do not think
>> it is true to say that the acceptance-defining collimator MUST go before
>> the mini-torus. When Greg and I briefly looked at this long ago, one 
>> only
>> needed a crude field map (say 5%? help, Mark) for the resulting error on
>> absolute Q^2 to be negligible. Admittedly, it is more difficult to 
>> measure
>> a weak field to 5% accuracy than a strong field (due to zero errors 
>> in the
>> Hall probes), and presumably the error derivative for misaligments of 
>> the
>> 1/r mini-torus field are pathologically large. But I do not think 
>> there is
>> a hard constraint here, only a trade-off.
>>
>> regards,
>>
>> Dave
>>
>>
>>
>>
>>