OPERA detector at Gran Sasso

The Gran Sasso National Laboratory (LNGS)

LNGS is situated deep underground (1400 meters) to shield experiments from cosmic rays, which can interfere with sensitive measurements. The rock overburden acts as a natural filter, reducing background noise. The laboratory hosts several experiments, including Borexino (studying solar neutrinos), ICARUS (searching for proton decay), and GERDA (searching for neutrinoless double beta decay). The unique environment makes it ideal for studying rare events and fundamental physics.

The OPERA Experiment: Purpose and Methodology

The OPERA experiment, conducted between 2006 and 2012, was designed to study oscillations of muon neutrinos from CERN to the Gran Sasso Laboratory, a distance of approximately 730 kilometers. Neutrino oscillation is a quantum mechanical phenomenon where a neutrino changes its flavor (electron, muon, or tau) as it propagates. The experiment aimed to confirm this oscillation and precisely measure the parameters governing it.

• Neutrino Source: CERN’s Super Proton Synchrotron (SPS) produced a beam of muon neutrinos.
• Detection System: OPERA used a complex detector consisting of a target, a magnetic horn to focus the neutrinos, and a massive emulsion-spectrometer tracker.
• Emulsion-Spectrometer: This was the core of the detector, comprising layers of lead and nuclear emulsions. Emulsions record the tracks of charged particles created when neutrinos interact.
• Electronic Detectors: Electronic detectors were used to precisely measure the arrival time of neutrinos.

The Faster-Than-Light Anomaly (2011)

In September 2011, the OPERA collaboration announced that their measurements indicated that muon neutrinos were traveling slightly faster than the speed of light (approximately 20 parts per million faster). This result, published in arXiv, sent shockwaves through the physics community, as it directly contradicted Einstein’s theory of special relativity, a cornerstone of modern physics. The initial measurements were based on the time-of-flight calculation, determined by the distance between CERN and Gran Sasso and the measured arrival time of the neutrinos.

Scrutiny and Error Identification

The initial announcement was met with skepticism and intense scrutiny from physicists worldwide. Numerous independent experiments were launched to verify the OPERA results. Several potential sources of error were identified:

• GPS Synchronization: The timing system relied on GPS for synchronization between CERN and Gran Sasso. It was discovered that the GPS receiver at Gran Sasso had a faulty fiber optic connection, leading to timing errors.
• Clock Synchronization: Issues were found with the clock synchronization system, which was not accurately accounting for the stretching of the fiber optic cables used for timing.
• Measurement of Distance: A slight inaccuracy in the measured distance between the neutrino source and detector was also identified.

Corrected Results and Implications

After careful re-analysis and correction of the systematic errors, the OPERA collaboration retracted its initial claim in February 2012. The corrected measurements showed that the neutrinos traveled at a speed consistent with the speed of light, within the experimental uncertainties. The incident highlighted the importance of rigorous error analysis and independent verification in scientific research. While the faster-than-light claim was disproven, the OPERA experiment successfully confirmed neutrino oscillations and provided valuable data for understanding neutrino properties.

Further Research and Legacy

The OPERA experiment concluded in 2012, having achieved its primary goals. The data collected continues to be analyzed, contributing to our understanding of neutrino physics. Subsequent experiments, such as T2K in Japan and NOvA in the United States, have further refined our knowledge of neutrino oscillations and are exploring other neutrino properties, like the CP violation in the lepton sector.