OPERA reports time-of-flight anomaly

The OPERA experiment in Italy’s INFN Gran Sasso Laboratory has sent ripples round the world with its findings that neutrinos created 730 km away at CERN arrive at the detector slightly earlier than if they were travelling at the speed of light.

The result is based on the observation of more than 15,000 neutrino events measured by the experiment, which observes the beam produced by the CERN Neutrinos to Gran Sasso (CNGS) project. Using high-statistics data taken in 2009, 2010 and 2011, the collaboration has measured the velocity of the muon-neutrinos reaching the detector with much higher accuracy than previous studies conducted using accelerator neutrinos. Upgrades to the CNGS timing system and to the OPERA detector, as well as the use of high-precision geodesy to measure the neutrino baseline, allowed the collaboration to achieve comparable systematic and statistical accuracies.

To perform the study, the OPERA collaboration teamed up with experts in metrology from CERN and other institutions to make a series of high-precision measurements of the distance between the source and the detector, and of the neutrinos’ time of flight. The distance between the origin of the neutrino beam and OPERA was measured with an uncertainty of 20 cm over the 730 km travel path. The neutrinos’ flight time was determined with an accuracy of less than 10 ns by using sophisticated instruments, including advanced GPS systems and atomic clocks. The time responses of all of the elements of the CNGS beamline and of the OPERA detector have also been measured with great precision.

The results indicate that neutrinos from CERN arrive early at Gran Sasso by 60.7 ± 6.9 (stat.) ± 7.4 (sys.) ns compared with the time that would be taken assuming the speed of light in vacuum. This anomaly corresponds to a relative difference of the muon-neutrino velocity, v, with respect to the speed of light, c, (v-c)/c = (2.48 ± 0.28 (stat.) ± 0.30 (sys.) × 10^–5.

Given the potential far-reaching consequences of such a result, independent measurements are certainly needed before the effect can either be refuted or firmly established. While OPERA continues to gather more data, the MINOS collaboration in the US is planning to improve its measurement of the neutrino time of flight with the beam from Fermilab to the Soudan Underground Laboratory, about 730 km away.