Calorimetry as inference: the information content of particle showers and the limits of energy measurement

Abstract

A calorimeter does not measure the energy of a particle, it infers it from the record the absorption process leaves behind. Energy conservation writes every event as an accounting ledger, E = E_dis + E_B + E_esc + E_nu, of which only the dissipated component contributes to signal, so resolution is limited by how well the record of the dissipation constrains the missing entries. We formalize this: records form a lattice on which information is monotone; thinning a Poisson record scales Fisher information exactly by the retention; clustered records read out as sums cost the 1/(1+cv^2); and the quadrature budget a/sqrt(E) (+) b is the floor of every summed readout. The "ultimate limits" of hadronic calorimetry are identified as the first rung of a ladder, the residual spread of the invisible energy around a single correlated observable. We measure the ladder above that rung on Geant4 truth records of hadronic showers in copper, from the dissipated-energy sum to time-, position- and multiplicity-resolved records, with estimators validated on exactly solvable toy models. The achieved full-record value, model-conditional, is about 5.5%/sqrt(E) (+) 0.3% over 20-100 GeV, a factor 1.7-1.9 below the canonical one-functional construction on the same events (1.29% vs. 2.14% at 20 GeV). Under retrained high-precision neutron transport it moves from 1.29% to 1.30%, the dissipated-energy sum by 7%, and the gain from time structure becomes 0.17% in place of 0.28%. Dual-readout supplies the largest share of the information. Shower shape is next, almost entirely longitudinal, saturating at about one interaction length. Time structure adds nearly as much with or without shower shape, and stopped-muon neutrino losses set an estimated floor of 0.99% at 20 GeV. Every readout dimension has a measured information exchange rate, separating what new hardware can buy from what better algorithms can recover.