New Cross Term in the Two-Particle Hanbury-Brown–Twiss Correlation Function in Ultrarelativistic Heavy-Ion Collisions

Using two specific models and a model-independent formalism, we show that in addition to the usual quadratic “side,” “out,” and “longitudinal” terms, a previously neglected “out-longitudinal” cross term arises naturally in the exponent of the two-particle correlator. Since its effects can be easily observed, such a term should be included in any experimental fits to correlation data. We also suggest a method of organizing correlation data using rapidity rather than longitudinal momentum differences, since in the former every relevant quantity is longitudinally boost invariant.