EXPERIMENTAL neutrino physics reached a major milestone on February 24 when a neutrino fired through the earth from a “neutrino factory” at the Japan Proton Accelerator Research Complex (J-PARC) at Tokai village, 100 kilometres north of Tokyo, was detected by the giant Super-Kamiokande (SK) underground detector located 295 km away, near the western coast of Japan. This marks the beginning of a new generation of experiments in the study of these ghostly, elusive particles by the international collaboration called T2K – short for Tokai to Kamioka – led by KEK, the Japanese High Energy Accelerator Research Organisation at Tsukuba.
This new neutrino (nu) beam facility at J-PARC has been in the works for nearly 10 years. It was first proposed in 2001 and was approved in 2003, when its construction began. The beams in a nu-factory are produced in the following way. High-energy protons are directed onto a carbon target where their collisions produce charged particles called pions. The pions travel through a helium-filled volume where they decay to produce a beam of neutrinos. The J-PARC beamline is designed to deliver the world’s most powerful neutrino beams to study the mysterious phenomenon known as neutrino oscillations. The SK detector, located about half a kilometre underground, is a 40 m × 40 m cylindrical Cherenkov detector comprising a 50,000-tonne tank of ultra-pure water surrounded by 10,000 photo multiplier tubes, each sensitive enough to detect a single photon.
Neutrinos are extremely weak-interacting chargeless particles that come in three types, or flavours as physicists like to call them: the electron neutrino (nu-e), the muon neutrino (nu-mu) and the tau neutrino (nu-tau). Each type is associated with its more familiar charged cousin: electron, muon and tau respectively – and together they are called leptons. Neutrino oscillation is a quantum mechanical phenomenon in which neutrinos can switch their identities, that is, a neutrino of one type can transmute to another.
For such oscillations to occur, at least one of the neutrino types should have mass, however small, because quantum theory tells us that the probability of oscillation between neutrinos is proportional to the difference in their squared masses. But ever since the neutrino was theoretically predicted by Wolfgang Pauli in 1930 – it was only detected in 1956 by Clyde Cowan and Frederick Reines – it was always thought to have strictly zero mass.
Beginning in the 1960s, several experiments found that the number of nu-e’s arriving on the earth from the sun, where they are produced in the nuclear reactions that power it, were between one-third and half the number predicted. This was a great paradox because the extreme weak interaction of neutrinos with matter meant that physicists could predict their fluxes on the earth very accurately. But if nu-e’s could oscillate into the other types, the deficit could be explained as the detection techniques used could detect only nu-e’s.
The Standard Model (SM) of particle physics, which was otherwise extremely successful, too was built on the premise that neutrinos were massless. But if the neutrinos did have mass, they were too small to be measured unambiguously and unequivocally and there was no other direct evidence of oscillation either.
The clear evidence for oscillation came in 1998 when the SK detector measured the ratio of nu-mu’s to nu-e’s produced by high-energy cosmic ray interactions in the upper atmosphere. The ratio varied as a function of the length of the flight path from the points of their generation, which matched the theoretical calculation of neutrino oscillation with finite masses. This showed that atmospheric neutrinos changed flavours as they travelled to the detector. Subsequently, this was established conclusively in other higher precision experiments such as KamLAND (Kamioka Liquid Scintillator Anti-Neutrino Detector).
The T2K experiment is designed to make measurements of known neutrino oscillations with unprecedented precision using the artificial neutrino beams from the nu-factory so as to get a more complete understanding of the parameters governing neutrino oscillations. The chief goals of the T2K experiment are to look for the as-yet-unseen oscillation of nu-mu’s into nu-e’s in J-PARC’s super beam by the time they reach SK across the 295-km-long baseline and to measure a crucial parameter called theta-13, besides others.
T2K is not, however, the first long baseline neutrino oscillation experiment. This was, in fact, pioneered at KEK itself between 1999 and 2004 by the K2K (KEK to Kamioka) experiment in which 1.3 giga electronvolt (GeV) neutrinos were produced by the 12 GeV proton synchrotron accelerator and detected at SK.
This and the other previous neutrino experiments have only observed the disappearance of muon neutrinos in a beam, leading to a deficit in the numbers detected at the distant detector. Specific oscillation from nu-mu to nu-e has not, however, been observed. The reason for this is that the parameter theta-13, which controls the probability of this oscillation, is very small. T2K is a more powerful and more sophisticated version of the same experiment, with a more intense neutrino beam produced by the newly built 30 GeV synchrotron at J-PARC.
Before the neutrinos leave the J-PARC facility, their properties are determined by a sophisticated “near” detector, located 280 m from the origin. This makes a detailed measurement of the neutrino beam’s energy, direction and type before it travels to the “far” SK detector. The first detection of the neutrino beam by the near detector was done in January. Having achieved its detection at the far detector, the T2K experiment is now poised to have physics runs until summer by which time the experiment expects to have made the most sensitive measurements of oscillations between all the three nu types. These should provide critical tests for theoretical predictions.
Over the next few years, upgrades in the experiment will allow comparison between oscillations of neutrinos and anti-neutrinos, the antimatter counterparts of the three nu-types. This, theorists believe, will enable an understanding of one of the great mysteries of fundamental physics – why is there more matter than anti-matter in the universe?
“The hunt has just begun,” said Koichiro Nishikawa, the director of the Institute of Particle and Nuclear Studies at KEK and the founder of T2K. T2K is a collaboration of 508 physicists from 62 institutes in 12 countries (Japan, South Korea, Canada, the United States, the United Kingdom, France, Spain, Italy, Switzerland, Germany, Poland and Russia).
These achievements also underline the importance of the proposed India-based Neutrino Observatory (INO). Mooted way back in 2000-02, the proposal was to locate the underground laboratory at Singara in the Nilgiris within the complex of the Tamil Nadu Electricity Board’s PUSHEP hydroelectric project. Unfortunately, in November 2009, the Ministry of Environment and Forests rejected the proposal on the grounds that the site fell in the buffer zone of the Mudumalai-Bandipur Tiger Reserve, a new tiger reserve that was notified in 2008. Now scientists are evaluating alternative sites.
The significance of a neutrino observatory in India arises from its importance as the “far” detector in future long baseline experiments for beams from nu-factories in Japan and CERN (European Organisation for Nuclear Research).
An INO site in southern India happens to be around the “magic baseline” of 7,200 km – about 6,560 km from J-PARC and 7,150 km from CERN – that will enable a precise measurement of one of the outstanding questions in particle physics, namely, charge-parity violation in the leptonic sector of fundamental particles.
R. Ramachandran
