Modulation of the waterfall by a gauge field
  We present the first complete calculation of the curvature perturbation generated during the hybrid inflation waterfall, caused by the coupling of the waterfall field to a gauge field A whose kinetic function f^2 depends on the inflaton field. We impose an upper bound on the field W=f*A which ensures that it has a negligible effect before the waterfall. We confirm the claim of Soda and Yokoyama, that the perturbation \delta W generates a statistically anisotropic spectrum and bispectrum, which could easily be observable. We also discover a new phenomenon, whereby the time-dependent `varyon' field W causes the inflaton contribution to vary during the waterfall. The varyon mechanism might be implemented also with a scalar field and might not involve the waterfall. 
David H. Lyth and Mindaugas Karciauskas
astro-ph.CO hep-ph hep-th