[Halld-cal] Fwd: Re: Curvature

[Elton Smith]

Hi Tegan,

Thanks for the explanation. It is a nice summary of what Andrei did and 
how this can help us in collecting hits into a clusters.

Cheers, Elton.

Elton Smith
Jefferson Lab MS 12H3
12000 Jefferson Ave STE 4
Newport News, VA 23606
(757) 269-7625
(757) 269-6331 fax

On 8/6/15 11:08 AM, beattite wrote:
> Re-sending.  Some people missed it.  I'll shove it on DocDB at some 
> point as well, I suppose.
>
>
> -------- Original Message --------
> Subject: Re: Curvature
> Date: 13.07.2015 14:51
> From: beattite <beattite at uregina.ca>
> To: <halld-cal at jlab.org>
>
> Accidentally sent the e-mail before attaching things.  Here you are.
>
>
> On 13.07.2015 14:48, beattite wrote:
>> Hello.
>>
>> Last week, Mark asked for some sort of plot showing the effects of
>> the shower curvature for why we might want to be doing a correction to
>> the IU shower code, since it's been a while since Andrei's original
>> work and we all want to be on the same page.
>>
>> The attached picture (15degrees.jpg) is point r vs. point z for a 
>> simulated 5 GeV
>> photon gun at 15 degrees.  The picture shows all reconstructed points
>> in the run.  It gives the point z distribution for an incident photon
>> at 15 degrees (I chose such a large energy, 5 GeV, so that there are
>> enough points in layer 4 to talk about).
>>
>> If we expect the centroid of the point z distribution to lie along
>> the trajectory of the photon, we would expect the centroid in the four
>> layers to be r/tan(15):
>>   Layer 1 - 242 cm
>>   Layer 2 - 256 cm
>>   Layer 3 - 274 cm
>>   Layer 4 - 302 cm
>>
>> What we actually see are centroids in the four layers of:
>>   Layer 1 - ~253 cm
>>   Layer 2 - ~257 cm
>>   Layer 3 - ~260 cm
>>   Layer 4 - ~265 cm
>>
>> This is similar to what Andrei's plots from a lot time ago show, as
>> in, the centroid in layer one is shifted in the positive z-direction
>> from the photon trajectory while the centroids in layers three and
>> four are shifted in the negative z-direction from the photon
>> trajectory.  The centroid of layer two lines up nicely with the
>> trajectory.
>>
>> The IU code uses the Moliere radius around the trajectory to include
>> points.  At low angles like 15 degrees, the spread of the cone in z is
>> something like +\- 75 cm.  That is to say, the code is looking for
>> points in the shower between z = 230 and z = 370 or so in layer four.
>> While this does actually cover the shower including the curvature (in
>> this case), it also has a huge acceptance in the positive z-direction
>> that is unneeded and which possibly catches points which belong to a
>> different shower.  So, the idea is to take the initial energy estimate
>> from the IU shower along with the angle estimate of the IU shower
>> (which is based off points in layers 1, 2, and 3, and so should be
>> close to the angle of the actual incident particle), then use the
>> curvature tables provided by Andrei which simulated in more detail the
>> energy deposition of an incident photon in the BCAL at different
>> angles and energies.  The tables give energy deposition centroids in
>> each layer which we can then use to fill the showers with points
>> rather than the Moliere radius that the IU code used.
>>
>> The effect is less pronounced at, say, 50 degrees (50degrees.jpg).
>> Here, the centroid in layer four should be around 68, but it lies
>> more around 65, for instance.
>
>
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