Beating the speed of light?

There has been a lot of debate in recent times over the speed of neutrinos and whether they travel faster than light. The neutrino was first detected in 1956, yet, there are many unknowns factors surrounding it, writes C Sivaram 

In recent times, there has been a lot of excitement following the September-23 announcement by physicists involved in the OPERA collaboration that they had measured the speed of high-energy muon neutrinos (product at CERN) to be greater than the speed of light. Although the excess velocity (as compared to light) is only a few parts in a hundred thousand, the result contradicts accepted wisdom on relativistic kinematics, where no particle can travel faster than light.

In 2007, there was the MINOS neutrino experiment (which detected neutrinos from Fermilab in a Minnesota mine) which was also consistent with a superluminal speed, but the accuracy was not sufficient for a definite claim. In the OPERA experiment, neutrinos produced at CERN are detected at the Gran Sasso underground facility about 730 kilometers away. Using the Global Positioning Data (GPS), this distance is measured very accurately to about 20 centrimers and also used to synchronise the clocks in both the laboratories.

Light would take about 2.5 milliseconds to cover the distance, whereas the OPERA experiment sees the neutrinos arrive about 60 nanoseconds sooner. The neutrinos are produced by ramming protons from CERN into a graphite target. The collisions lead to copious generation of mesons which decay into muons and their associated muon neutrinos. OPERA claims to precisely time the neutrino burst to a few nanoseconds.

So it is these high-energy muon neutrinos with an average energy of 17 giga electron volts that are claimed to be superluminal. In February 1987, electron neutrinos with energies of several million electron volts were detected both in Kamiokande in Japan and IMB detector in the US from a supernova explosion in the Large Megallanic Cloud (LMC), a satellite galaxy of the Milky Way, about two lakh light years away. These neutrinos were produced when a massive star (several solar masses) collapsed in the LMC.

While its core collapses into neutron star or black hole, there is a prodigious release of 10 to the power 58 electron neutrinos in a burst of about ten seconds (a hundred trillion neutrinos would have passed through each of our bodies!). While the core collapsed, the outer parts of the star were ejected in the gigantic stellar explosion, called a supernova. These electron neutrinos reached a few hours earlier than the optical radiation, which puts an express speed of one part in a billion over that of light.

Postulated in 1930
The neutrino was first postulated by Pauli in 1930 to explain some puzzling features of nuclear beta decay where electrons are produced. If the heavier nucleus decayed into a lighter one along with only the electron, then the electron should have a specific, well-defined energy.

However, in all such nuclear decays, a continuous distribution of different electron energies was observed. Pauli realised that this required a third unobserved particle, the so-called neutrino, to be present to carry away the missing energy and momentum.

There was no need to involve drastic explanation like postulating even a violation of energy conservation in weak nuclear decays as suggested some physicists. However, the neutrino would be very weakly interacting with ordinary matter; it can traverse even light years of lead before colliding with another particle.  It would also have no charge or mass.

The neutrino was detected 25 years later in 1956 by Reines and Covan, when it was realised that large nuclear fission reactors must produce huge fluxes of such particles.

It was also realised that neutrinos would be copiously produced in nuclear reactions in the sun’s core and indeed, each of our bodies receive hundred trillion of these solar neutrinos every second! As they are so weakly interacting, a huge detector with several kilotons of water (like in Kamiokande) or a million litres of dry cleaning fluid (like in the Davis experiment) are required to detect even a few neutrino events. And they all have to be deep underground!

Davis did first report detection of the solar neutrinos, but the flux was only a third of the expected value, if only nuclear reactions generate the solar radiation. All these are electron-type neutrinos produced in nuclear beta decay.

In 1962, the muon neutrino was identified (in muon decays) by Lederman and others. In 1975, it became clear that there is yet another neutrino, the so called tau-neutrino associated with the tau-lepton decays (discovered by Martin Perl and his group).

It was also realised that the neutrinos could have a very small mass (much lighter than the electron), which could enable conversion of one neutrino type to another (a purely quantum effect where different eigen states can mix). This could explain the deficit of solar neutrinos.

Indeed the Sudbury detector in Canada, which used kilotons of heavy water as detector, and which could therefore detect all three types of neutrinos, found that there was no deficit of solar neutrinos.

This implied that our understanding of the solar interior and how thermonuclear reactions generate the sun’s energy is essentially correct, but that the neutrinos, while traversing from the solar core to the earth, undergo conversions from one type to another and that is why they were not detected in the detectors which were able to detect only electron-type neutrinos, used earlier. This also suggested that the neutrino has a small mass.

More recently, in connection with the OPERA experiments, it was suggested by Cohen and Glashow that superluminal motion would imply production of electron-positron pairs and consequent loss of energy by this process, so that the neutrinos would not have the high energies observed as it would have been damped. No such evidence has been observed.

Although neutrinos of three different types (and their antiparticles) are all around us (there is even a cosmic background of a thousand neutrinos every cubic centimeter) we still know so little about them (even whether they are faster than light)!