At the edge of science

[The SC]

Neutrino 'flavour' flip confirmed

An important new discovery has been made in Japan about neutrinos.

These are the ghostly particles that flood the cosmos but which are extremely hard to detect and study.

Experiments have now established that one particular type, known as the muon "flavour", can flip to the electron type during flight.

The observation is noteworthy because it allows for the possibility that neutrinos and their anti-particle versions might behave differently.

If that is the case, it could be an explanation for why there is so much more matter than antimatter in the Universe.

Theorists say the counterparts would have been created in equal amounts at the Big Bang, and should have annihilated each other unless there was some significant element of asymmetry in play.

"The fact that we have matter in the Universe means there have to be laws of physics that aren't in our Standard Model, and neutrinos are one place they might be," Prof Dave Wark, of the UK's Science and Technology Facilities Council (STFC) and Oxford University, told BBC News.

The confirmation that muon flavour neutrinos can flip, or oscillate, to the electron variety comes from T2K, an international collaboration involving some 500 scientists.

The team works on a huge experimental set-up that is split across two sites separated by almost 300km.

At one end is the Japan Proton Accelerator Research Centre (J-Parc) located on the country's east coast.
Continue reading the main story
The 'ghostly' neutrino particle
T2K

Second most abundant particle in the Universe, after photons of light
Means 'small neutral one' in Italian; was first proposed by Wolfgang Pauli in 1930
Uncharged, and created in nuclear reactions and some radioactive decay chains
Shown to have a tiny mass, but hardly interacts with other particles of matter
Comes in three flavours, or types, referred to as muon, tau and electron
These flavours are able to oscillate - flip from one type to another - during flight
Could be a Majorana particle - that is a particle that is equal to its anti-particle

It generates a beam of muon neutrinos that it fires under the ground towards the Super-Kamiokande facility on the west coast.

The Super-K, as it is sometimes called, is a tank of 50,000 tonnes of ultra-pure water surrounded by sensitive optical detectors.

These photomultiplier tubes pick up the very rare, very faint flashes of light emitted when passing neutrinos interact with the water.

In experiments in early 2011, the team saw an excess of electron neutrinos turning up at Super-K, suggesting the muon types had indeed changed flavour en route.

But just as the collaboration was about to verify its findings, the Great Tohoku Earthquake damaged key pieces of equipment and took T2K offline.

Months of repairs followed before the project was able then to gather more statistics and show the muon-electron oscillation to be a formal discovery.

Details are being reported on Friday at the European Physical Society Conference on High Energy Physics in Stockholm, Sweden.

"Up until now the oscillations have always been measured by watching the types disappear and then deducing that they had turned into another type. But in this instance, we observe muon neutrinos disappearing and we observe electron neutrinos arriving - and that's a first," said Prof Alfons Weber, another British collaborator on T2K from the STFC and Oxford.

Neutrino oscillations are governed by a matrix of three angles that can be thought of as the three axes of rotation in an aeroplane - roll, pitch and yaw.

Other research has already shown two of the matrix angles to have non-zero values. T2K's work confirms that the third angle - referred to as theta-one-three - also has to have a non-zero value.

This is critical because it allows for the oscillations of normal neutrinos and their anti-particles, anti-neutrinos, to be different - that they can have enough degrees of freedom to display an asymmetrical behaviour called charge parity (CP) violation.

CP-violation has already been observed in quarks, the elementary building blocks of the protons and neutrons that make up atoms, but it is a very small effect - too small to have driven the preference for matter over anti-matter after the Big Bang.

However, if neutrinos can also display the asymmetry - and especially if it was evident in the very massive neutrinos thought to have existed in the early Universe - this might help explain the matter-antimatter conundrum. The scientists must now go and look for it.

It is likely, though, that much more powerful neutrino laboratories than even T2K will be needed to investigate the issue.

"We have the idea for a Hyper-Kamiokande which will require an upgrade of the accelerator complex," Prof Weber told BBC News.

"And in America there's something called the LBNE, which again would have bigger detectors, more sensitive detectors and more intense beams, as well as a longer baseline to allow the neutrinos to travel further."

• Second most abundant particle in the Universe, after photons of light
• Means 'small neutral one' in Italian; was first proposed by Wolfgang Pauli in 1930
• Uncharged, and created in nuclear reactions and some radioactive decay chains
• Shown to have a tiny mass, but hardly interacts with other particles of matter
• Comes in three flavours, or types, referred to as muon, tau and electron
• These flavours are able to oscillate - flip from one type to another - during flight
• Could be a Majorana particle - that is a particle that is equal to its anti-particle

BBC News - Neutrino 'flavour' flip confirmed

 

[The SC]

Neutrinos from the cosmos hint at new era in astronomy

An experiment buried beneath the ice of the south pole has for the first time seen high-energy neutrino particles originating outside our Solar System.

They are produced in our atmosphere and in the cosmos's most violent processes, but the IceCube experiment has seen the first energetic "cosmic neutrinos".

It detected 28 of the exceptionally fast-moving neutrinos - but it remains unclear exactly where they came from.

The pioneering finds could herald an entirely new branch of astronomy.

The results were presented on Wednesday at the IceCube Particle Astrophysics Symposium in Wisconsin, US.
Neutrino events The experiment has captured a variety of neutrino events

Researchers have gathered there to discuss the findings of the world's largest neutrino detector, occupying a cubic kilometre. It is made up of 86 strings sunk into the Antarctic ice, each with 60 sensitive light detectors strung along it like "fairy lights".

As neutrinos pass, they very rarely bump into the nuclei of atoms in the ice, producing a brief flash that the detectors can catch. With more than 5,000 detectors catching flashes at different times, the direction of the neutrinos' arrival can be determined.

IceCube is just one of a number of neutrino detection experiments around the globe. Low-energy neutrinos from the dying throes of a star were spotted by Japanese researchers in 1987 - the first-ever neutrinos from beyond our cosmic neighbourhood, leading to the 2002 Nobel Prize in physics.

They can also be produced in the Sun and our own atmosphere here on Earth - IceCube picks up about 100,000 of those a year.

However, previous attempts to associate higher-energy neutrinos with more far-flung cosmic processes, such as those described in April 2012, had turned up nothing.

But in April this year, the IceCube collaboration reported seeing two neutrinos - nicknamed Bert and Ernie - of energies greater than a "petaelectronvolt".

That is 150 times higher than the energy to which particles within the Large Hadron Collider can currently be accelerated.

Now the team reports 26 more events, each higher than 50 teraelectronvolts (a twentieth of a petaelectronvolt), which they expect will also be of cosmic origin.

But Francis Halzen, principal investigator on the IceCube experiment, said that "of course, there's much more to do".

"It's after you find them that the work starts; these events are very difficult to analyse," Prof Halzen told BBC News.
Particle pictures

For centuries, stargazers have relied only on light of a wide range of wavelengths - many far beyond those we can see - to get pictures of the cosmos.

But these first cosmic neutrino detections open the possibility for doing astronomy instead using particles - developing pictures of the Universe's most active corners by analysing the directions and energies of the neutrinos they produce.

Prof Halzen recalled discussions with Frederick Reines, who shared the 1995 Nobel Prize in physics for first discovering the neutrino in the mid-1950s.

"He would tell me that as soon as he discovered that the neutrino was real, everybody had the idea that you had a particle that you could do astronomy with. In 1960, several people wrote rather detailed papers on how to do it."

Only later did it become clear that a detector as monumentally large as IceCube would be required to launch such a new era in astronomy - an era that for the first time seems to be taking shape.

"It is incredibly exciting to work with the final IceCube configuration," Prof Halzen said.

"It not only shows that we built the right detector, it promptly delivered results. What it means for astronomy is in our future, hopefully our very near future. The tools are in place and the first harvest of events is in."


BBC News - Neutrinos from the cosmos hint at new era in astronomy