Feedback from Joerg

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e-mail from Joerg Reinhold 4/15/20
OK, here we go:

The first question that comes to my mind is a matter of definition: Is this a new experiment proposal or more a 
proposal to analyze data that is being taken anyhow?

So, I looked first at whether the approved CPP and this proposal are compatible. My conclusion is that there are 
two issues that need to get clarified in order to really run NPP at the same time as CPP: the target choice, 
Sn vs Pb, and the TOF trigger.

Details:

My understanding is that the experimental configuration is the same up to the FCAL. The CPP muon detectors, 
go all behind the FCAL. 

The CPP experiment will remove the start counter. NPP shows the same configuration.

The one big difference I do find is the target: CPP lists a 116Sn target, while NPP lists a Pb target. Did CPP 
change the target after the experiment has been approved? Interestingly, the CPP experiment presents MC 
studies with a Pb target.

The remaining question is the trigger: 

CPP 

For this experiment, the total hadronic rate for a 107γ/s of collimated flux between 5.5 and 6 GeV incident on a 5%
radiation length Sn target is less than 3.5 kHz, well below the DAQ limit of 20 kHz. The trigger condition will also 
be open and the selection of the two pion signal will be accomplished during offline reconstruction. The FCAL 
will be used for triggering on the pion pair, but in order to be efficient for

our two-pion trigger, the thresholds in the FCAL will need to be reduced below 100 MeV. Figure 23 shows the 
distribution of hadronic events surviving a single 30 MeV threshold cut in the FCAL, which eliminates the events
produced by photons with energies of less than about 3 GeV, well below the coherent peak. However, most of the 
trigger rate for low thresholds in the FCAL is due to the electromagnetic backgrounds, which contribute 10 kHz at 
a threshold of 100 MeV. In order to reduce the threshold in the FCAL even further, we will need to select coincident 
hits in the time-of-flight scintillators and veto on energy in the BCAL.

NPP (comparatively short description)

The Primakoff reaction will transfer all the energy of the beam into four photons, which are going forward. All this 
energy will be deposited in the FCAL, except for leakage down the beampipe.
                                                                                              
We expect a simple trigger with an energy threshold in the FCAL should have very high efficiency for any events 
that can be reconstructed.

I would assume that any FCAL threshold with good charges pion efficiency also would be efficient for 4 photons. However, 
the proposed coincident TOF trigger for CPP would be incompatible with NPP.


Joerg