  We present simultaneous optical and near-infrared (NIR) polarimetric results
for the black hole binary V404 Cygni spanning the duration of its 7-day long
optically-brightest phase of its 2015 June outburst. The simultaneous R and
Ks-band light curves showed almost the same temporal variation except for the
isolated (~30 min duration) orphan Ks-band flare observed at MJD 57193.54. We
did not find any significant temporal variation of polarization degree (PD) and
position angle (PA) in both R and Ks bands throughout our observations,
including the duration of the orphan NIR flare. We show that the observed PD
and PA are predominantly interstellar in origin by comparing the V404 Cyg
polarimetric results with those of the surrounding sources within the 7'x7'
field-of-view. The low intrinsic PD (less than a few percent) implies that the
optical and NIR emissions are dominated by either disk or optically-thick
synchrotron emission, or both. We also present the broadband spectra of V404
Cyg during the orphan NIR flare and a relatively faint and steady state by
including quasi-simultaneous Swift/XRT and INTEGRAL fluxes. By adopting a
single-zone synchrotron plus inverse-Compton model as widely used in modeling
of blazars, we constrained the parameters of a putative jet. Because the jet
synchrotron component cannot exceed the Swift/XRT disk/corona flux, the cutoff
Lorentz factor in the electron energy distribution is constrained to be <10^2,
suggesting particle acceleration is less efficient in this microquasar jet
outburst compared to AGN jets. We also suggest that the loading of the baryon
component inside the jet is inevitable based on energetic arguments.
