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2015 Nobel prize for Physics

The 2015 Nobel prize for Physics has been awarded jointly to Takaaki Kajita
and Arthur McDonald for the discovery of neutrino oscillations. This is the
mechanism responsible for the observation that neutrinos of any particular
type (there are 3 known types: electron, muon and tau) can change their
identity if given enough time to do so.
Earlier observations of electron neutrinos from the Sun had shown that
there was a deficit in the number detected at Earth compared to the
theoretical expectation (and this work was honoured by the 2002 Nobel Prize

in Physics for the pioneering work of Ray Davis Jr and Masatoshi Koshiba). It
was the results obtained from the experiments led by Kajita and McDonald that
provided the evidence that oscillations were the explanation.

Kajita-san's work was based on observations of muon neutrinos
produced from the impact of cosmic rays with the Earth's upper
atmosphere. Using the giant Super-Kamiokande detector in
Japan, his collaboration were able to show that there were less muon neutrinos
arriving at the detector having first traversed through the Earth, than those
arriving from directly overhead. The count rate was expected to be the same
and led to the discovery that something new was happening to deplete those taking the longer route through the Earth.

The clinching evidence that the source of the observed deficits was due to neutrino
oscillations came from McDonald's solar neutrino experiment- the Sudbury Neutrino Observatory
(SNO) in Canada. In two separate types of interaction SNO used 1000 tonnes
of heavy water to simultaneously measure both the rate of solar electron neutrinos
and the total rate of all three neutrino types combined. In this way, they were able to show
that although the rate of electron neutrinos was in deficit, consistent with
that seen in previous experiments, the rate of all neutrino types combined was
as expected. This was firm evidence that something had happened to the electron
neutrinos on their route from the Sun and this was consistent with an
explanation in terms of an oscillation from electron type into the other two

Since the work of Kajita and McDonald, neutrino oscillations have been confirmed
by experiments able to not only measure deficits of a particular neutrino type,
but to directly measure the type of neutrino that emerges after the
oscillation process has happened. At the forefront of this effort is the T2K
project in Japan to which the Warwick EPP Group has made major
contributions. Using the same Super-Kamiokande detector that Kajita's
collaboration developed, T2K was the first to observe directly the transition
of a muon neutrino into an electron neutrino. Our group are now working on two
successor projects to T2K (named, Hyper-K and DUNE) which will have the capability to determine whether neutrinos and anti-neutrinos are different - a result which will have far-reaching consequences for our understanding of the early Universe.
For more background behind this years Nobel prize see:

Gary Barker for the Experimental Neutrino Group