Hanbury Brown-Twiss (HBT) interferometry relative to the triangular flow plane in heavy-ion collisions

The PHENIX Collaboration reported third-order harmonic oscillations of the source radius parameters when measuring the Hanbury Brown-Twiss correlation function for charged hadrons relative to the triangular flow angle. They interpreted their result geometrically as a measurement of the final triangular deformation at freeze-out. Such a purely geometric interpretation is not tenable: we show that a triangular source deformation can not lead to third-order oscillations unless the source also features radial flow.-- Using a simple Gaussian source featuring both geometric deformations and anisotropic flow, we analyze possible origins of the observed third-order oscillations. It is known that second-order oscillations of the HBT radii relative to the elliptic flow plane can arise from an elliptic spatial deformation of the source at freeze-out, both with and without radial flow, as well as from elliptic flow; in general the first (geometric) effect is strongest. We show that this is different for third-order oscillations relative to the triangular flow plane. We present the dependence of the third-order oscillation amplitudes of all HBT radii on the model parameters. In particular we explore the origin of the observed much larger third-order oscillation amplitude for Rout than Rside.