Neutrinos scoop the Nobel Prize in Physics 2015

The prize has been awarded to two physicists who discovered a strange property of neutrinos, which Imperial scientists are still unravelling today.

Neutrinos have the smallest mass of any known particle, and are created in several ways, including during radioactive decay, directly in the Sun’s core, and when cosmic rays from the Sun hit the Earth’s atmosphere. Thousands of billions of neutrinos are streaming through our bodies each second.

Today’s physics laureates, Professor Arthur B. McDonald of the Sudbury Neutrino Observatory Collaboration in Canada and Professor Takaaki Kajita of the Super-Kamiokande Collaboration in Japan, discovered that neutrinos can change their identities, a phenomenon known as oscillation.

That neutrinos can do this reveals that they in fact have mass, where they were previously thought to be massless, changing physicists’ ideas about the innermost workings of matter.

Professor Kajita made his seminal discoveries at the experiment where Imperial scientists are also working today to uncover the secrets of neutrinos. Super-Kamiokande, or Super-K, is a gigantic underground experiment in Japan that detects atmospheric neutrinos.

Super-K can also detect neutrinos created by the J-PARC laboratory at Tokai Village on the eastern coast of Japan, 295 km away. The collaboration that measures differences in neutrinos created in Tokai and detected at Super-K is known as T2K.

Changing identities

Through interactions with other matter, neutrinos are known to come in three identities, or 'flavours,' – one paired with the electron (called the electron neutrino), and two more paired with the electron's heavier cousins, the muon and tau leptons (called the muon and tau neutrinos).

The fact that neutrino masses and flavours do not exactly overlap each other means that the three different flavours of neutrinos can spontaneously change into each other as they travel, and this is neutrino oscillation.

Dr Morgan Wascko, from Imperial’s Department of Physics, who is the international co-spokesperson for T2K, explains Professor Kajita’s discovery:

“When cosmic rays interact in the Earth's atmosphere, they create many electron and muon neutrinos, which Super-K can differentiate. However, Super-K detected far less muon neutrinos coming from below compared to those coming from above, meaning those that had travelled through the Earth were ‘disappearing’.

“This phenomenon is called neutrino disappearance and its discovery by Super-K is considered the dawn of a new field of particle physics. It is now known that the muon neutrinos were in fact changing identities into tau neutrinos.”

Several scientists from Imperial’s Department of Physics are using Super-K in the T2K experiments to reveal more about the strange identity-shifting behaviour of neutrinos. In 2013, Imperial’s Dr Yoshi Uchida and Dr Morgan Wascko were part of a team that announced a new kind of neutrino oscillation.

In his Nobel prize-winning work, Professor McDonald used the Sudbury Neutrino Observatory (SNO) to detect solar neutrinos streaming from the Sun, which are all electron neutrinos. However, there were less electron neutrinos arriving than expected – again, this was because the neutrinos were actually shifting their identity, this time becoming muon and tau neutrinos.

A matter of antimatter

Only a few months ago, physicists from Imperial and elsewhere using Super-K announced they had recorded the same identity-shifting behaviour in antineutrinos – the antimatter equivalent of neutrinos.

Every type of particle that makes up the universe has an antimatter counterpart – an identical particle with the opposite charge. Physicists predict that during the Big Bang, the creation of the universe, equal amounts of matter and antimatter should have been created.

However, matter and antimatter annihilate each other, so the persistence of matter making up our universe is a mystery. Small differences in the way matter and antimatter behave could explain why one survived at the expense of the other.

Differences between the identity-shifting behaviour of neutrinos and antineutrinos could explain why the universe is made up of normal matter, and was not obliterated by antimatter shortly after the Big Bang.

Worthy winners

On hearing the news that the 2015 Nobel Prize in Physics had been awarded to Kajita and McDonald, Dr Wascko said:

“Neutrino oscillation is the first, and still the only, observed phenomenon that is not allowed within the Standard Model of particle physics. The remarkably small mass of the neutrino is a strong suggestion that there is something we don't understand going on in nature, but we don't yet know what it is. 

“Neutrino oscillation is sometimes called the navel of particle physics, because it indicates that the Standard Model has a bigger, deeper origin than we can currently see, but it doesn’t tell us exactly what it is.

“This Nobel prize is a clear recognition of the importance of neutrino oscillation to particle physics, cosmology, and astronomy.  It is also a testament to the incredible experiments that made these discoveries, and their remarkable leaders. Art McDonald and Kajita-san are very worthy Nobel prize winners indeed!”

Dr Asher Kaboth, a Post-Doctoral researcher from the Department of Physics who announced the recent antineutrino result, said:

“It’s fantastic that both Professor Kajita and Professor McDonald have won this prize for their work on Super-Kamiokande and SNO. The Super-K and SNO experiments were the ones that brought us into our current era of neutrino experiments.

“Before them, there were many experiments that showed that something strange and unexplained was going on with neutrinos, but it was these two experiments that provided the definitive evidence that neutrinos undergo flavour-changing oscillations, and ushered us into the era where we can precisely study these effects.

“There is still much to be learned about and from neutrinos—and many people hard at work around the world—but we would not be doing it without the accomplishments of SNO and Super-K and without Professor McDonald and Professor Kajita’s leadership.

“I hope that the next 20 years of neutrino physics are as remarkable as the last 20 years!”