In 2012, the Higgs particle was detected by the ATLAS and CMS experiments at CERN. Since then, one often hears that the Standard Model of particle physics is complete. "Not quite true!" says Alain Blondel, professor of physics at the University of Geneva. There is still the neutrino, which, as it is known today, does not fit into the Standard Model. Exciting news about the elusive particle was published recently: New observations by the T2K neutrino experiment in Japan provide first indications shedding light to a central question of modern physics: Why does the universe consist only of matter while the associated antimatter is missing?
In 1930 the Austrian theoretical physicist Wolfgang Pauli was working at ETH Zurich. In that year, he postulated the existence of a neutral elementary particle, known today as 'neutrino'. Since then almost 90 years have passed. Science has established that there are three types of neutrinos (electron-neutrino, muon-neutrino, tau-neutrino) - and that the neutrino, contrary to an earlier assumption, has mass (although a very tiny one). "Since the neutrino has mass, the Standard Model of particle physics is not complete, actually the Standard Model is based on a massless neutrino," says Alain Blondel, a leading neutrino expert, who, together with Teresa Montaruli, turn the University of Geneva into an epicentre of Swiss neutrino research.
Advances in neutrino research
Even 90 years after the discovery, the neutrino is still a big mystery. Few know better than Alain Blondel. The native French explored this mysterious particle for nearly 50 years. In his doctoral thesis in the early 1970s, the physicist trained at the Ecole Polytechnique Paris and in Berkeley (USA) dealt with the neutrino for the first time. At the time, he did his research at the Gargamelle bubble chamber. With this chamber a new interaction between neutrinos and matter, called 'neutral currents' was discovered at CERN in 1973. The hull of this bubble chamber is today a museum piece on the CERN site in Meyrin. "The bubble chamber cracked in 1978 at the time and I was forced to complete my dissertation sooner than planned," says Blondel with a smile.
In the 1980s Blondel carried out very precise measurements of neutrino properties at CERN with the ALEPH experiment at the Large Electron Positron collider (LEP) - the predecessor of the current proton-proton collider, the LHC, that occupies the same tunnel as LEP. From these results the researchers could deduce that there are no more than the three known types of neutrinos. Another neutrino research result, in which Blondel was involved, dates from 2013: At that time, neutrino researchers were excited to discover at the T2K experiment in Japan that muon neutrinos transform into electron neutrinos. This transformation is also called 'neutrino oscillation'. With the T2K experiment, started in 2010, physicists send 10,000 billion neutrinos every second on a 300 kilometer journey from the east coast (Tokai) to the west coast (Kamioka) of Japan (from Tokai to Kamioka, short: T2K). The 300-kilometer journey lasts only a millisecond; but that is exactly the right time for the neutrinos to oscillate, thus turning into other types of neutrinos, as the scientists can determine with their experiments.
Neutrinos and antineutrinos behave differently
In the beginning of August this year, the T2K collaboration, involving 500 scientists from eleven countries including Switzerland, were again reporting a fascinating result. This time the neutrino researchers have evaluated how many of the muon neutrinos transformed into electron neutrinos along their journey. They further evaluated how many of the anti-muon neutrinos turned into anti-electron neutrinos. The still provisional findings of the researchers suggest that muon neutrinos and antineutrinos do not transform with the same probability - that means: the neutrinos (matter) seem to behave differently from antineutrinos (antimatter).
If this observation will be confirmed during the next few years by recording and analyzing more data from the T2K experiment and also from a competitor experiment in the USA, this would have huge consequences: it would be an important piece of the puzzle to answer the important question: why does our universe consist essentially of matter while the same quantity of antimatter, which must also have existed during the Big Bang, seems to have disappeared? In order to confirm the matter-antimatter asymmetry in neutrino oscillations in a statistically significant way, much more data will have to be taken and analyzed at T2K in the coming years. In order to make this possible, the experiment has to be improved constantly. A 10-times more powerful detector, named 'HyperK', is now being proposed for funding by the Japanese Ministry of Education, Culture, Sports, Science and Technology; it is expected to be commissioned in Kamioka from 2025 onwards. If the collaboration succeeds in proving this matter-antimatter asymmetry in the neutrino sector, a further Nobelprize could be looming.
Swiss physicists at the forefront
Swiss particle physicists have made significant contributions to the T2K experiment since its beginnings in 2002. With five PhD theses at the CERN NA61 experiment, they have provided basic input for determining the number of neutrinos and antineutrinos produced at the source of the T2K experiment. Important contributions were made by Swiss researchers for the detector in Tokai, which is used to study the neutrinos before they begin their 300 km journey: physicists from ETH Zurich (Group of Prof. Andrea Rubbia) and the University of Bern (Group of Prof. Antonio Ereditato) have cared about the magnet with which those particles are controlled that later will decay to those neutrinos being sent across Japan. Researchers from the University of Geneva around Alain Blondel have supported the construction of the so-called 'tracking chamber' of the T2K near detector. Swiss scientists also contributed to the management of the experiment.
There is still much to be done to unravel the neutrino puzzle. In the future, scientists would like to explain the strange fact that the spin of neutrinos always turns anti-clockwise (left-handed chirality), even though the neutrino, like all particles that have a mass, should be possible rotating with either spin directions (left- and right-handed chirality). Neutrino theories, like the hypothesized see-saw mechanism, suggest that neutrinos with a clockwise spin would exist, but with a much higher mass than their anti-clockwise spinning counter parts, such that these right-handed neutrinos cannot be observed with the instruments available today, as their mass is too large. "It is a very natural hypothesis that this right-spinning neutrino should be involved to explain why our universe consists of matter only and not of an equal amount of antimatter," says Alain Blondel, adding, "If we could explain the surplus of matter, one of the main scientific questions of the day, that would be fantastic. The most recent results are a first step in that direction."
Author: Benedikt Vogel
The T2K Collaboration has published new results showing the strongest constraint yet on the parameter that governs the breaking of the symmetry between matter and antimatter in neutrino oscillations.Image: Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo