A new cross term in the two-particle Hanbury-Brown-Twiss correlation function

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.