Experimental Observation of Quantized Vortex Reconnection and Turbulence in Superfluid Helium

We present experimental studies of the first direct visualization of reconnecting quantum vortices and the decay of superfluid turbulence in $^{4}$He. Micron-sized solid hydrogen particles are used for particle tracking. The cores of the superfluid vortices trap the hydrogen particles, thereby allowing direct visualization of the dynamics of the line-like defects. We generate superfluid turbulence by driving a thermal counterflow. After pulsing the counterflow, the system relaxes through a cascade of reconnection events. We examine the dynamics of pairs of particles trapped on reconnecting vortices and observe that these particles separate as power laws in time with a scaling exponent distributed about the predicted value of $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern- .15em\lower.25ex\hbox{$\scriptstyle 2$} $. We show that reconnection leads to power-law tails in the velocity probability distribution function, which is in stark contrast to the Gaussian tails that are ubiquitous in classical turbulence and thermal motion.