EIC electron energies are not the same as ePIC MC samples

[simonge]

Is your feature request related to a problem? Please describe. The electron beam energy being used by the EIC for simulations are different from the nominal values used for all ePIC MC samples in the order of a few percent.

5 GeV = 5.0674572870932 GeV 10 GeV = 9.7813741161107 GeV 18 GeV = 17.846262619763 GeV

While small, these differences are not insignificant. Over the drift vacuum passing the Low-Q2 taggers this results in a few mm steering offset.

The electron beam backgrounds produced by @nat93 are based on the machine simulation so use the more precise energy values. The energy-flux differences in this region are exponential so any mismatch between MC samples will significantly influence the rates and fraction of background electrons seen by the taggers

Recently @nat93 has showed the synchrotron sample shouldn't be effected much. The Touschek, Bremstrahlung and Coulomb samples however will need some consideration.

The same may be applicable to the ion beams.

Describe the solution you'd like I don't have a preferred solution at the moment.

Describe alternatives you've considered Some short and longer term solutions.

• Run background MC samples with the magnet strengths scaled appropriately and mix them afterwards
• Scale the energy of the background MC sample, either as a pre-processing stage before merging, or option when running a simulation.
• Recreate all of the MC samples with the corrected EIC energies and advise any future samples to use them too.
• Recreate the background files with the nominal 5/10/18 GeV energies.

[ruse-traveler]

I noticed similar behavior when looking at single neutron samples with @Abiolade1 a couple months ago. My initial assumption was that it was due to something like kinetic energy being reported in one place and total energy in another, but none of the differences lined up with what I would expect if that were the case...

Also tagging @vanekjan , @nathalysns , and @Magicyop since this is relevant for the BHCal calibration studies...