UKIRT microlensing surveys as a pathfinder for $WFIRST$: The detection of five highly extinguished low-$|b|$ events

Optical microlensing surveys are restricted from detecting events near the Galactic plane and center, where the event rate is thought to be the highest, due to the high optical extinction of these fields. In the near-infrared (NIR), however, the lower extinction leads to a corresponding increase in event detections and is a primary driver for the wavelength coverage of the $WFIRST$ microlensing survey. During the 2015 and 2016 bulge observing seasons we conducted NIR microlensing surveys with UKIRT in conjunction with and in support of the $Spitzer$ and $Kepler$ microlensing campaigns. Here we report on five highly extinguished ($A_H=0.81-1.97$), low-Galactic latitude ($-0.98\le b\le -0.36$) microlensing events discovered from our 2016 survey. Four of them were monitored with an hourly cadence by optical surveys but were not reported as discoveries, likely due to the high extinction. Our UKIRT surveys and suggested future NIR surveys enable the first measurement of the microlensing event rate in the NIR. This wavelength regime overlaps with the bandpass of the filter in which the $WFIRST$ microlensing survey will conduct its highest-cadence observations, making this event rate derivation critically important for optimizing its yield.