HBT shape analysis with q-cumulants

Taking up and extending earlier suggestions, we show how two- and threedimensional shapes of second-order HBT correlations can be described in a multivariate Edgeworth expansion around gaussian ellipsoids, with expansion coefficients, identified as the cumulants of pair momentum difference q, acting as shape parameters. Off-diagonal terms dominate both the character and magnitude of shapes. Cumulants can be measured directly and so the shape analysis has no need for fitting.Comment: 8 pages, 6 figures for a total of 29 subfigs, revtex4. Typos corrected, three missing terms added, minor text change