[Olympus] momentum spectrum

Tue May 13 11:14:01 EDT 2014

Hello all,

To follow up, I was able to isolate the problem in the MWPC digitization =

which caused the strange MC momentum distributions in Kirill's =

presentation yesterday.  The gist of the problem was that MC hits were =

smeared as single 2D hits, rather than as 1D hits in each plane (which =

is what happens in the real detector).  This caused the hit error to be =

overestimated for certain combinations of all three wires, which led to =

the excessive broadening of the MC momentum distribution that we saw.  =

This effect was hidden prior to the geometry change by the geometry =

errors to a certain extent, and does not show up when the GEMs are =

included since their resolution dominates and "over-smeared" MWPC hits =

have less effect on the tracking.

I have attached a plot showing the results for a single run (7909), with =

the quickest improvement to the MWPC digitization that I could think =

of.  Rather than taking a single 2D hit in the MWPC middle plane as the =

old MWPC digitization did, I take the three individual plane hits, smear =

them individually, and reconstruct a 2D hit in the same fashion as is =

done in real data. As you can see, this improves the agreement between =

real data and MC significantly (although of course it can still be =

improved, especially since this was the quickest/easiest improvement I =

could think of).

This issue further highlights the need to move to the use of 1D MWPC =

hits only rather than 2D hits, as I have alluded to in previous =

meetings.  I will make this my top priority so that analyses using the =

MWPCs can continue with minimal delay.

Brian

On 05/13/2014 09:38 AM, Brian S. Henderson wrote:
> Hello all,
>
> To clarify, I would expect the MC for MWPCs only to look more like what
> is shown on the second slide of the document Stan sent around (and the
> addition of external bremsstrahlung would help to fill in the low
> momentum tail, as has been seen in the past).  On the first slide of
> what Stan sent around, you can see some results I ran yesterday for
> MWPC-only tracking.  As you can see, the real data momentum distribution
> looks more like we would expect than MC.  While the broadness is due to
> the low resolution of the MWPCs alone,  there is a shift in the
> peak/shape/broadening that indicates a problem in the MC for the MWPCs,
> most likely in the digitization stage.  While the MC I ran yesterday
> agrees with the distributions show by Kirril yesterday, I think the
> appropriate conclusion to draw is that the MWPCs are still not well
> modeled in the MC.
>
> Since the agreement between data and MC used to be better (as I alluded
> to yesterday and as shown by the plot included from Yuri on slide 2 of
> what Stan sent), my main guess is that I introduced some sort of bug
> into the MWPC digitization when I changed things to account for the
> recent geometry changes.  I am currently re-examining the MWPC
> digitization and testing for possible problems.
>
> Brian
>
> On 05/13/2014 09:26 AM, Belostotski, Stanislav wrote:
>> Dear Brian and colleagues,
>> Thanks a lot for your plot.  In view of our Monday discussion, the
>> result is as expected.
>> The broad Kirill's distribution is due to reconstruction done without
>> GEMs with MWPC only. The MWPC spatial resolution is about 0.5mm while
>> the total deflection of the lepton in the magnetic field  is 2. to
>> 2.5mm, i.e. momentum resolution (without GEMs) must be very poor.
>> This has also been demonstrated by Yuri in his MC studies.
>> I included Brian's plot in a the attached file where various variants
>> (with or without GEMs reconstruction) are presented.
>> A good cross check is made.
>> With best regards, StanB
>>
>> On 13.05.2014 14:43, Brian S. Henderson wrote:
>>> Hello again,
>>>
>>> I have attached a set of plots that looks at the various reconstruction
>>> parameters for MWPC only tracking for two combined electron runs (7909
>>> and 7910) and two combined positron runs (7950 and 7951) with the
>>> corresponding MC data sets.  The only thing like a cut applied was to
>>> accept only the single best fit track per event (with very loose cuts on
>>> the max chi^2 and number of iterations).
>>>
>>> As you can see, the momentum distributions from the MC look similar to
>>> what you showed yesterday.  It looks like something has definitely
>>> changed in the MC since I last used MWPC only tracking, so I apologize
>>> for questioning your momentum distributions yesterday.  Oddly, the data
>>> looks more like what I would expect than the MC (at least for the first
>>> 10 plots which are left arm), especially the missing energy distribution
>>> which peaks at around 250 MeV for the MC.  This makes me think that
>>> there could be an issue in the way the MWPC digitization was changed to
>>> correspond with the GDML changes made to fix the coordinate system
>>> issues. It also could be that the estimate of the MWPC resolution
>>> currently used in the digitization is not a good model.  I will take a
>>> look at this right away this morning to see if I can figure anything
>>> out.
>>>
>>> Brian
>>>
>>> On 05/12/2014 12:59 PM, Suvorov, Kirill wrote:
>>>> Hi Brian,
>>>>
>>>> Could you please send pictures of reconstructed momentum distribution
>>>> for radiative generator (without GEM digitization) and tell what cuts
>>>> did you use?
>>>>
>>>> best regards, Kirill
>>>>
>>>> p.s. We trying to understand why my pictures of momentum distribution
>>>> are worse.
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