  The OPERA neutrino experiment at the underground Gran Sasso Laboratory has
measured the velocity of neutrinos from the CERN CNGS beam over a baseline of
about 730 km. The measurement is based on data taken by OPERA in the years
2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the
OPERA detector, as well as a high precision geodesy campaign for the
measurement of the neutrino baseline, allowed reaching comparable systematic
and statistical accuracies. An arrival time of CNGS muon neutrinos with respect
to the one computed assuming the speed of light in vacuum of (6.5 +/-
7.4(stat.)((+8.3)(-8.0)sys.))ns was measured corresponding to a relative
difference of the muon neutrino velocity with respect to the speed of light
(v-c)/c =(2.7 +/-3.1(stat.)((+3.4)(-3.3)(sys.))x10^(-6). The above result,
obtained by comparing the time distributions of neutrino interactions and of
protons hitting the CNGS target in 10.5 microseconds long extractions, was
confirmed by a test performed at the end of 2011 using a short bunch beam
allowing to measure the neutrino time of flight at the single interaction
level.
