[G12] Photon energy reconstruction issue in MC

[Andrea Celentano]

Hi,
yes, will try to exclude gpp from the sim. chain and see if there’s still an error with the photon energy.
MK: I agree that for MC the correction is trivial since you now the generated 4vector. 

The kinfit is not used at all for MC. It is used for real data, where you need to derive the “true” photon momentum from data itself. I did so as the missing photon from gamma p -> p pi0 -> p e+ e- gamma, by doing a kinfit to the hypothesis gamma p -> p pi0 -> e+ e- (gamma) and using the missing 4-momentum from kinfit as the “real” photon momentum.

Bests,

Andrea

> On Jan 16, 2019, at 14:24, John W. Price <jprice at csudh.edu> wrote:
> 
> Hi Andrea:
> 
> Can you do a test *without* including gpp?  This might help isolate the cause of the issue.
> 
> John
> 
> -- 
> John W. Price
> Professor, CSUDH Department of Physics
> Coordinator, Science, Mathematics, and Technology Program
> 310-243-3403
> 
> On Jan 16, 2019, at 05:15, Michael Kunkel <mkunkel at jlab.org <mailto:mkunkel at jlab.org>> wrote:
> 
>> Greetings Andrea,
>> 
>> I believe you just verified what Diane Schott and I had been trying to convince other of for many years. :)
>> 
>> However, the correction for simulation is quite trivial since we already know the generated 4vector. 
>> 
>> I don’t believe the kinfit package handles this problem.
>> 
>> BR
>> MK 
>> ----------------------------------------
>> Michael C. Kunkel, USMC, PhD
>> 
>> On Jan 16, 2019, at 05:59, Andrea Celentano <andrea.celentano at ge.infn.it <mailto:andrea.celentano at ge.infn.it>> wrote:
>> 
>>> Hi MK,
>>> last meeting before holidays I presented the corrections necessary for photons, for both MC and data. For MC, the reconstructed photon energy is affected by a major shift with respect to the generated one - much larger than what happens for real data. I remember you pointed out that this may be due to a non correct parametrization of the EC sampling fraction in the MC chain (gsim + gpp + a1c). For data, this effect is not present, since the difference between the measured photon energy and the “real” one (as obtained from the kin. fit of gamma p -> p pi0 -> p e+ e- (gamma)) is much smaller.
>>> Therefore, I suspect this effect is related to an error in gsim or gpp rather than a1c, or that different reconstruction routines are used in a1c for data and MC.
>>> 
>>> Can you comment more on this?
>>> 
>>> Thanks
>>> Bests,
>>> Andrea
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