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The purpose of this work is to report the measurement of a time-shift in theOPERA set-up in a totally independent way from Time Of Flight (TOF)measurements of CNGS neutrino events. The LVD and OPERA experiments are bothinstalled in the same laboratory: LNGS. The relative position of the twodetectors, separated by an average distance of ~ 160 m, allows the use of veryhigh energy horizontal muons to cross-calibrate the timing systems of the twodetectors, using a TOF technique which is totally independent from TOF of CNGSneutrino events. Indeed, the OPERA-LVD direction lies along the so-called"Teramo anomaly", a region in the Gran Sasso massif where LVD has established,many years ago, the existence of an anomaly in the mountain structure, whichexhibits a low m. w. e. thickness for horizontal directions. The "abundant"high-energy horizontal muons (nearly 100 per year) going through LVD and OPERAexist because of this anomaly in the mountain orography. The total live time ofthe data in coincidence correspond to 1200 days from mid 2007 until March 2012.The time coincidence study of LVD and OPERA detectors is based on 306 cosmichorizontal muon events and shows the existence of a negative time shift in theOPERA set-up of the order of deltaT(AB) = - (73 \pm 9) ns when two calendarperiods, A and B, are compared. This result shows a systematic effect in theOPERA timing system from August 2008 until December 2011. The size of theeffect is comparable with the neutrino velocity excess recently measured byOPERA. It is probably interesting not to forget that with the MRPC technologydeveloped by the ALICE Bologna group the TOF world record accuracy of 20 ps wasreached. That technology can be implemented at LNGS for a high precisiondetermination of TOF with the CNGS neutrino beams of an order of magnitudesmaller than the value of the OPERA systematic effect.

Determination of a time-shift in the OPERA set-up using high energy horizontal muons in the LVD and OPERA detectors