netproton

A net proton is a quantity used in particle and nuclear physics that represents the difference between the number of protons and the number of antiprotons observed in a defined region of phase space or in a collision event. Formally, it is N_p minus N_pbar, and it often serves as a practical proxy for the net baryon number in experiments where neutrons and other baryons are more difficult to detect.

Net-proton measurements are common in relativistic heavy-ion experiments. Protons and antiprotons are identified with tracking detectors combined with particle identification methods such as energy loss (dE/dx), time-of-flight, and Cherenkov detectors, within a specified acceptance in rapidity and transverse momentum. The resulting net-proton counts are analyzed to study how baryon number is distributed after collisions and how it depends on collision energy and centrality.

A major application of net-proton studies is the investigation of event-by-event fluctuations in heavy-ion collisions. The distribution of net-proton numbers is characterized by its moments—mean, variance, skewness, and kurtosis—to gain insight into the properties of strongly interacting matter and to search for signatures of the quantum chromodynamics (QCD) critical point and the phase transition between hadronic matter and quark-gluon plasma.

Interpretation requires careful corrections for detector efficiency, acceptance, and isospin effects, since net protons are related to but not identical with the total net baryon number. See also baryon number, proton, antiproton, heavy-ion collision, and QCD phase diagram.