Re: back of the envelope

["Mark M. Ito" <marki@jlab.org>]

Eugene,

All of the tracks start with zero separation and at this level of 
approximation, the separation is 1/2 cm per % of deviation of B field 
per 60 cm of track length or roughly 80 microns per % of B field 
deviation per cm of track length (again for this particular 
configuration). So if we take a B field deviation of 10^-4 uniformly and 
a track length of 50 cm, we get a position error of 40 microns. This is 
small when added in quadrature with a 150 to 200 micron position 
resolution and will be "smaller" still when multiple scattering is 
considered. Again a uniform deviation is not what we are worried about, 
but this is just to set the scale (as you originally and  aptly 
suggested). As you know, the problem is that the non-uniform distortions 
in field are very difficult to calibrate away.

  -- Mark

P. S. Went back to cc'ing the list again.

Eugene Chudakov wrote:
> The separation grows with the radius.
> At the last 5 cm step the separation increased somehow.
> What is this increase? I must be small (0.5mm?), but it is
> hard to say from the plots.
>
> Thanks,
> Eugene
>
>
> On Fri, 27 Feb 2009, Mark M. Ito wrote:
>
>> Not sure I understand the question. The separation of 1 cm is mention 
>> is just obtained by eye looking at the plot.
>>
>> Eugene Chudakov wrote:
>>> Hi Mark,
>>>
>>> as far as I see these are X-Y plots (not r-phi).
>>> I guess, the deviation between two tracking steps matters. The full 
>>> deviation of the trajectory will be absorbed in the momentum
>>> measurement. The bottom plot shows the range in X (close to R for 
>>> this area) of 5cm, which is about the distance between CDC
>>> super-layers and is about 1 step in tracking. What is the increment 
>>> between
>>> the curves' separations at this step?
>>>
>>> Thanks,
>>> Eugene
>>>
>>>
>>> On Fri, 27 Feb 2009, Mark M. Ito wrote:
>>>
>>>> Folks,
>>>>
>>>> At last Monday?s tracking meeting Eugene suggested that someone 
>>>> make an estimate of the position error caused by a mistake in the 
>>>> knowledge of the magnetic field. Here is an example: 
>>>> http://tinyurl.com/btp8jx
>>>>
>>>> -- Mark
>>>>
>>
>>