What is a Neutrino?

A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half-integer spin. The neutrino (meaning "little neutral one" in Italian) is denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have mass but the upper bounds established for their mass are tiny even by the standards of subatomic particles.

Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale. Therefore a typical neutrino passes through normal matter unimpeded.

Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type is associated with an antiparticle, called an "antineutrino", which also has neutral electric charge and half-integer spin. Whether or not the neutrino and its corresponding antineutrino are identical particles has not yet been resolved, even though the antineutrino has an opposite chirality to the neutrino.

Most neutrinos passing through the Earth emanate from the Sun. About 6.5×1010 solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.

Physicists at the CERN laboratory have put the final nail in the coffin for the idea that neutrinos can travel faster than the speed of light. They also confirmed that the groundbreaking results from 2011 can be blamed on faulty equipment.

[partner id="wireduk"] Back in September 2011, a team of particle physicists detected neutrinos moving faster than the speed of light as they traveled from CERN to the Gran Sasso lab. They smashed the universal speed limit by 60 nanoseconds — a result that was constant, even after 15,000 repetitions of the process.

The results seem to run counter to a century’s worth of physics and would overturn Einsten’s special theory of relativity if true. As such, CERN called for more experiments to double-check the findings.

“When an experiment finds an apparently unbelievable result, it’s normal procedure to invite broader scrutiny,” CERN research director Sergio Bertolucci said at the time. “We need to be sure that there are no other, more mundane, explanations.”

At the International Conference on Neutrino Physics and Astrophysics in Kyoto on June 8, CERN research director Sergio Bertolucci presented results on the travel time of neutrinos from CERN to the INFN Gran Sasso Laboratory, on behalf of four experiments — Borexino, Icarus, LVD and Opera.

All four experiments measured a neutrino time of flight that was below the speed of light, confirming that neutrinos respect Einstein’s cosmic speed limit. The previous anomaly was “attributed to a faulty element of the experiment’s fibreoptic timing system.”

“Although this result isn’t as exciting as some would have liked,” said Bertolucci, “it is what we all expected deep down.”

“The story captured the public imagination, and has given people the opportunity to see the scientific method in action — an unexpected result was put up for scrutiny, thoroughly investigated and resolved in part thanks to collaboration between normally competing experiments.”

In March 2012, Antonio Ereditato — spokesperson of the Opera experiment — resigned from his post. That was following revelations that the results could have been affected by an unaccounted-for error, and a follow-up test from Icarus that ran counter to Opera’s findings.