[Rivet] Deciding on a dR cut for Photos

[Emily Nurse]

Hi all,

The CDF and Dzero analyses that measure the Z pT distribution attempt  
to correct for QED effects by using Photos to correct the l+l- pT  
distribution back to the true Z pT. We therefore have to cluster back  
any photons that we think Photos would have simulated and add them to  
the lepton 4-momenta in the relevant Rivet routines. In order to do  
this we have to define a reasonable cone size to use, where any  
photons within that cone around the lepton are clustered back in. This  
is true for both electrons and muons.

In order to address the issue of where to put the dR cut I simulated   
W->enu and W->munu events using Photos and made a plot of dR between  
the lepton and any photons. The first attached plot shows the dR  
distribution for photons in W->enu and the second is for photons in W- 
 >munu.

The table below shows, for different dR cuts, the fraction of photons  
with dR < cut and the fraction of events with dR < cut (this third  
column includes events with NO photons in the numerator and  
denominator). I think the third column is the number we care about.

ELECTRONS
***************
dR cut     % of photons     % of events
1.0            94.3                  98.3
0.5            89.0                  96.8
0.2            80.7                  94.4

MUONS
*********
dR cut     % of photons     % of events
1.0            87.1                   98.0
0.5            76.5                   96.3
0.2            60.1                   93.7


Andy rightly pointed out that to do this properly we need to also get  
an idea of the number/energy/distribution of photons from other  
sources (initial state radiation, within jets etc) that should not get  
clustered back in. This I am not set up to do that easily and it will  
be a more detailed study. Perhaps we can see how much the result  
changes as we vary dR, if it changes a lot we need to consider it  
further?

In the mean time I would propose 0.5 as a default, but this is clearly  
not optimised.

Cheers,
Emily.


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