  HR4796A is surrounded by a debris disc, observed in scattered light as an
inclined ring. Past observations raised several questions. First, a strong
brightness asymmetry detected in polarized reflected light recently challenged
our understanding of scattering by the dust particles in this system. Secondly,
the morphology of the ring strongly suggests the presence of planets, although
no planets have been detected to date. We obtained high-angular resolution
coronagraphic images of the circumstellar environment around HR4796A with
VLT/SPHERE during the commissioning of the instrument in May 2014 and during
guaranteed-time observations in February 2015. The observations reveal for the
first time the entire ring of dust, including the semi-minor axis that was
previously hidden either behind the coronagraphic spot or in the speckle noise.
We determine empirically the scattering phase function of the dust in the H
band from 13.6deg to 166.6deg. It shows a prominent peak of forward scattering,
never detected before, for scattering angles below 30deg. We analyse the
reflectance spectra of the disc from the 0.95 to 1.6 microns, confirming the
red colour of the dust, and derive detection limits on the presence of
planetary mass objects. We confirm which side of the disc is inclined towards
the Earth. The analysis of the phase function suggests that the dust population
is dominated by particles much larger than the observation wavelength, of about
20 microns. Compact Mie grains of this size are incompatible with the spectral
energy distribution of the disc, however the observed rise in scattering
efficiency beyond 50deg points towards aggregates which could reconcile both
observables. We do not detect companions orbiting the star but our
high-contrast observations provide the most stringent constraints yet on the
presence of planets responsible for the morphology of the dust.
