EPJ Plus – Determination of a time shift in the OPERA setup using high-energy horizontal muons in the LVD and OPERA detectors

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Published on 10 August 2012

The halls of the INFN Gran Sasso Laboratory (LNGS) were built in the 1980s based on the design of A. Zichichi and oriented towards CERN for experiments on neutrino beams. In 2006, the CERN Neutrinos to Gran Sasso (CNGS) beam was ready and the search could start for tau-neutrino appearances in the muon-neutrino beam produced at CERN. The OPERA detector was designed and built for this purpose.

In 2011, OPERA reported that the time of flight (TOF) of neutrinos measured on the 730km distance between CERN and LNGS was approximately 60ns shorter than that of light. Since the synchronisation of two clocks in different locations is a very delicate operation and a technical challenge, OPERA had to be helped by external experts in metrology.

This paper presents a much simpler and completely local check, by synchronising OPERA and LVD. Both detectors at LNGS are at about 160m distance from each other along the axis of the so-called "Teramo anomaly". This structural anomaly in the Gran Sasso massif, established by LVD many years ago, lets through high-energy horizontal muons at the rate of one every few days, penetrating both experiments.

The LVD-OPERA TOF measurement shows an offset of OPERA comparable with the claimed superluminal effect during the period in which the corresponding data were collected, and no offset in the periods before and after that data taking (when OPERA had corrected equipment malfunctions), as shown in the figure.

The result of this joint analysis is the first quantitative measurement of the relative time stability between the two detectors and provides a check that is completely independent of the TOF measurements of CNGS neutrino events, pointing to the existence of a possible systematic effect in the OPERA neutrino velocity analysis.