  The black hole transient MAXI J1820+070 displayed optical P-Cyg profiles and
other wind-related emission line features during the hard state of its
discovery outburst. We present near-infrared (nIR) spectroscopy covering the
different accretion states of the system during this event. Our 8-epoch data
set (VLT/X-shooter) reveals strong variability in the properties of the nIR
emission lines. This includes the presence of absorption troughs and extended
emission line wings with kinetic properties that are remarkably similar to
those inferred from the wind signatures observed in optical emission lines,
indicating that they are most likely tracing the same accretion disc wind.
Unlike the optical features, these nIR signatures are not exclusive of the hard
state, as they are also witnessed across the soft state with similar
observational properties. This supports the presence of a relatively steady
outflow during the entire outburst of the system, and represents the first
detection of an accretion disc wind in a black hole soft state at energies
other than X-rays. We discuss the visibility of the wind as a function of the
spectral band and the potential of nIR spectroscopy for wind studies, in
particular during luminous accretion phases.
