bring back the "correlated nucleon tail" #421
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The correlated tail from previous work disappeared with the most recent modifications to the LFG model here. Forcing the computed removal energy to be nonnegative makes sense for momenta drawn from the bulk of the LFG distribution; negative removal energy ordinarily means the nucleon is not bound (i.e. outside the nucleus). However, for momenta drawn from the correlated tail, they will *always* be negative relative to the calculation from the LFG model: their associated KE is much larger because their *internal* (pairwise) potential energy offsets it. Obviously the LFG computation can't be aware of that. Here we take cases where the LFG calculation winds up negative and simply reassign them a (more) sensible removal energy. In principle we would like something specific to SRC correlated pairs. In the absence of that, we fall back on the mean binding energy per nucleon, which is a (bad) proxy for the one-nucleon separation energy.
… its strength at 20% Otherwise identical to AR23_20i
… the LocalFGM nuclear model to turn use of the new Maxwell-Boltzmann distribution on/off even when not forcing positive removal energies. This allows the default configuration in config/LocalFGM.xml to retain its previous behavior. Adjust the new N24_20i configuration to turn this option on. Include the new parameter in the AR23_20i configuration with the default value of false (no change in behavior expected). Remove default arguments to LocalFGM::MaxwellBoltzmannRemovalE() since this member function is currently only called in a single place with explicit arguments.
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The correlated tail from previous work disappeared with the most recent modifications to the LFG model here. Forcing the computed removal energy to be nonnegative makes sense for momenta drawn from the bulk of the LFG distribution; negative removal energy ordinarily means the nucleon is not bound (i.e. outside the nucleus). However, for momenta drawn from the correlated tail, they will always be negative relative to the calculation from the LFG model: their associated KE is much larger because their internal (pairwise) potential energy offsets it. Obviously the LFG computation can't be aware of that.
Here we take cases where the LFG calculation winds up negative and simply reassign them a (more) sensible removal energy. The removal energy is sampled from a Maxwell-Boltzmann distribution, as is done for the "correlated spectral function" in spectral function fits. Here we use rough parameters taken from estimates by Artur Ankowski given in private communication to J. Wolcott (Ankowski paper forthcoming). The values thrown by the MB distribution are capped such that the off-shell struck nucleon in the initial state has nonnegative mass, which otherwise causes NaNs when trying to boost into its rest frame.
Because the default behavior of the "AR23" CMC would otherwise be changing under the feet of folks who want to use it, this PR also includes some changes to the AR23 configuration so that it should preserve the previous behavior. Here we also include a "N24" (NOvA 2024) CMC that turns it on.