  Visualization using tracer particles is a relatively new tool available for
the study of superfluid turbulence and flow, which is applied here to
oscillating objects submerged in the liquid. We report observations of a
structure seen in videos taken from outside a cryostat filled with superfluid
helium at 2 K, which is possibly a vortex loop attached to an oscillator. The
feature, which has the shape of an incomplete arch, is visualized due to the
presence of solid H2 tracer particles and is attached to a beam oscillating at
38 Hz in the liquid. It has been recorded in videos taken at 240 frames per
second (FPS), fast enough to take around 6 images per period. This makes it
possible to follow the structure, and to see that is not rigid. It moves with
respect to the oscillator, and its displacement is in phase with the velocity
of the moving beam. Analyzing the motion, we come to the conclusion that we may
be observing a superfluid vortex attached to the beam and decorated by the
hydrogen particles. An alternative model, considering a solid hydrogen
filament, has also been analyzed, but the observed phase between the movement
of the beam and the filamentary structure is better explained by the superfluid
vortex hypothesis.
