  In this paper we report on the timing resolution of the first production of
50 micro-meter thick Ultra-Fast Silicon Detectors (UFSD) as obtained in a beam
test with pions of 180 GeV/c momentum. UFSD are based on the Low-Gain Avalanche
Detectors (LGAD) design, employing n-on-p silicon sensors with internal charge
multiplication due to the presence of a thin, low-resistivity diffusion layer
below the junction. The UFSD used in this test belongs to the first production
of thin (50 {\mu}m) sensors, with an pad area of 1.4 mm2. The gain was measured
to vary between 5 and 70 depending on the bias voltage. The experimental setup
included three UFSD and a fast trigger consisting of a quartz bar readout by a
SiPM. The timing resolution, determined comparing the time of arrival of the
particle in one or more UFSD and the trigger counter, for single UFSD was
measured to be 35 ps for a bias voltage of 200 V, and 26 ps for a bias voltage
of 240 V, and for the combination of 3 UFSD to be 20 ps for a bias voltage of
200 V, and 15 ps for a bias voltage of 240 V.
