Updating Magic Universe
A new conundrum about neutrinos
Billions of them are passing through your body right now. With no electric charge and very little mass, neutrinos are the most penetrating of the commonplace particles of the Universe – and the shyest. Detecting them calls for experiments on a monumental scale. To figure out their properties requires immense patience too, and big steps forward are few and far between. But, as announced today at the Neutrino 2010 meeting in Athens, a multinational team is 95% sure that there’s a greater contrast between the masses of different kinds of anti-neutrinos, than between different kinds of neutrinos.
The work’s being done in the MINOS experiment (Main Injector Neutrino Oscillation Search) wherein a beam of muon neutrinos travels below the ground for 735 km, from Fermilab in Illinois to detectors deep in the Soudan mine in Minnesota. On the way, some of the muon neutrinos νμ change their “flavour” to become tau neutrinos ντ or, more rarely, electron neutrinos νe. Ditto for muon anti-neutrinos, when the Main Injector is set to create those instead.
Jenny Thomas of University College London, MINOS co-spokesperson, is quoted as saying:
We do know that a difference of this size in the behaviour of neutrinos and anti-neutrinos could not be explained by current theory. While the neutrinos and anti-neutrinos do behave differently on their journey through the Earth, the Standard Model [of particle physics] predicts the effect is immeasurably small in the MINOS experiment. Clearly, more anti-neutrino running is essential to clarify whether this effect is just due to a statistical fluctuation or the first hint of new physics.
If confirmed, this will be an important update for the story in Magic Universe called “Neutrino oscillations: when ghostly particles play hide-and-seek”. It tells the tale of Raymond Davis’s failure to detect the expected number of neutrinos from solar nuclear reactions, with a pioneering experiment in Homestake Mine in the Black Hills of Dakota. It led to the joky question, “Is the Sun still burning?”.
This mystery was solved only when it was confirmed that many neutrinos had “disappeared” by changing into a different type that the detectors couldn’t see. The climax of the story, that neutrinos could indeed change, or “oscillate” to other flavours is told in Magic Universe as follows:
Joy erupted among neutrino experts meeting in Takayama, Japan, in 1998, when Takaaki Kajita of Tokyo spoke. He announced that an underground experiment nearby had found evidence that neutrinos could indeed change their flavours. Kajita was reporting on behalf of a 120-strong Japanese-US team working with a neutrino detector 1000 metres down, in the Kamioka zinc mine.
Compared with Davis’s pioneering equipment, Super-Kamiokande was vastly bigger, and speedier in operation. A tank containing 50,000 tonnes of ultra-pure water was lined with more than 11,000 large light-detectors — photo-multipliers. They watched for flashes created when the invisible neutrinos set other particles moving faster than light’s speed in water.
The 1998 results concerned, not electron-neutrinos from the Sun, but muon-neutrinos created high in the Earth’s atmosphere by the impact of energetic cosmic rays arriving from the Galaxy. The sensational result from Super-Kamiokande was that more muon-neutrinos entered the water tank from above than came into it from below. Our planet is so transparent to neutrinos that the only real difference was the distance from their point of origin.
The neutrinos coming straight down from the upper air over Japan, and then through a sliver of crust to the detector, travelled only 10-20 kilometres. Those made in the air over the South Atlantic, directly below Japan, travelled 13,000 kilometres, right through the Earth. In reality, most neutrinos came in on a slant, but it remained the case that any coming from below had much farther to travel.
The extra journey time evidently allowed some of the muon-neutrinos to change to a flavour undetectable by Super-Kamiokande. That almost certainly meant tau-neutrinos. The verdict was that neutrinos do indeed play hide-and-seek, and must have at least a little mass. Most of the original atmospheric products were muon-neutrinos. The most likely change was to a tau-neutrino — a flavour then predicted but not yet observed.
Within two years, Fermilab near Chicago had verified the existence of tau-neutrinos. They were seen to create taus, the super-heavy electrons, in collisions with atomic nuclei. Finding the tau tracks, just a millimetre long in photographic emulsion, required 3-D visualization techniques using computer-controlled video cameras, developed and implemented in Nagoya.