Anisotropic transverse flow and the Hanbury-Brown-Twiss correlation function

Utilizing the Lorentz invariance of the correlation function we study the effects of anisotropic transverse flow on the Hanbury-Brown-Twiss (HBT) correlation function. In particular we show that directed flow would evidence itself by nonzero ''side-long'' and ''side-out'' terms in the correlation function. We also show that the study of ratios of the correlation functions evaluated on different event subsamples and/or different Lorentz systems provides information on the source dimensions and velocity which could be less biased by corrections of different kinds. We present a fitting technique appropriate for HBT analysis which does not require binning and is especially useful for multidimensional fitting.