Effects of HBT correlations on flow measurements

The methods currently used to measure collective flow in nucleus-nucleus co llisions assume that the only azimuthal correlations between particles are those arising from their correlation with the reaction plane. However, quan tum HBT correlations also produce shea range azimuthal correlations between identical particles. This creates apparent azimuthal anisotropies of a few percent when pions are used to estimate the direction of the reaction plan e. These should not be misinterpreted as originating from collective flow. In particular, we show that the peculiar behaviour of the directed and elli ptic flow of pions observed by NA49 at low p(T) can be entirely understood in terms of HBT correlations. Such correlations also produce apparent highe r Fourier harmonics (of order n greater than or equal to 3) of the azimutha l distribution, with magnitudes of the order of 1%, which should be looked for in the data. (C) 2000 Published by Elsevier Science B.V. All rights res erved.