[G12] g12: EC corrections and resolution

[Andrea Celentano]

Second picture is attached



On 11/27/18 6:17 PM, Andrea Celentano wrote:
> Dear all,
> I tried to derive photons corrections for MC and data, following the 
> procedure described in this note: 
> https://misportal.jlab.org/ul/Physics/Hall-B/clas/viewFile.cfm/2011-004.pdf?documentId=632
> Basically, this is based on the exclusive reaction gamma p -> p pi+ 
> pi- gamma, *where all the particles are measured*. The measured gamma 
> 3-momentum is compared to the 3-momentum obtained from a 1C kin. fit 
> to the  gamma p -> p pi+ pi- (gamma) hypothesis.
> For Kin. fit, I am using the package provided by MK. Simulations have 
> been performed with MK DB and runIndex = 10.
>
> After deriving MC corrections (and waiting for data to be skimmed), I 
> also looked at photon resolution and covariance matrix for MC. For MC, 
> cov. matrix can be determined comparing the "true" gamma 3-momentum 
> with the measured one. The "true" 3-momentum can be the generated one, 
> or the one obtained from the kin. fit. For data, the same procedure 
> can be adopted, and the "true" gamma 3-momentum will be the missing 
> one obtained from the kin. fit. However, when using as "true" 
> 3-momentum the one obtained from the kin. fit, the resolution (i.e. 
> the width of the "kin.fit-measured" distribution) is given by the 
> (quadratic) sum of the error on the measured quantity, and the error 
> on the missing photon 3-momentum from kin. fit. If the latter is 
> "small", it can be neglected.
>
> I attach a plot (MC1.png) showing the MC distribution of:
>
> * Generated photon energy - kin. fit photon energy (black)
> * Measured photon energy - generated photon energy (red)
> * Kin. fit photon energy - measured photon energy (green)
>
> The width of the black curve is smaller than the one of the red curve, 
> showing the the kin. fit photon energy error is much smaller than the 
> measured photon energy error. This guarantees that the resolution 
> studies based on comparing the kin. fit photon energy to the measured 
> one are not biased.
>
> I also attach a plot (MC2.png) showing the MC distribution of:
>
> * Generated photon theta - kin. fit photon theta (black)
> * Measured photon theta - generated photon theta (red)
> * Kin. fit photon theta- measured photon theta (green)
>
> In this case, the width of the "generated - kin. fit" distribution 
> (black) is LARGER than the width of the "generated - measured" 
> distribution (red), as if the experimental error on the measured 
> photon theta angle is SMALLER than the error on the theta angle of the 
> missing photon from kin. fit. If this is correct, the corresponding 
> resolution can't be extracted using the kin. fit information as the 
> "true" one: not a problem for MC, but for data.
> Or, maybe, the EC simulation (gsim+gpp) is not smearing enough the EC 
> response for angles - however, I checked that EC resolution determined 
> from generated 3-momentum is compatible with what described, for 
> example, in 
> https://misportal.jlab.org/ul/Physics/Hall-B/clas/viewFile.cfm/2009-011.pdf?documentId=533
>
> Do you think it may be possible that the error on the measured photon 
> angle in EC is smaller than the missing-photon angle (after applying 
> the kin. fit?). Note, finally, that the same conclusion applies even 
> when not using the kin. fit, but just taking the missing photon 
> 3-momentum from final state p, pi+, pi-.
>
> Thanks!
> Bests,
> Andrea
>
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