New data throws more support behind the theory that neutrinos are the
reason the universe is dominated by matter.
The current laws
of physics do not explain why matter persists over antimatter — why the
universe is made of ‘stuff’. Scientists believe equal amounts of matter and
antimatter were created at the beginning of the universe, but this would mean
they should have wiped each other out, annihilating the universe as it began.
Instead,
physicists suggest there must be differences in the way matter and antimatter
behave that explain why matter persisted and now dominates the universe. Each
particle of matter has an antimatter equivalent, and neutrinos are no
different, with an antimatter equivalent called antineutrinos.
They should be
exact opposites in their properties and behaviour, which is what makes them
annihilate each other on contact.
Now, an
international team of researchers that make up the T2K Collaboration, including
Imperial College London scientists, have found the strongest evidence yet that
neutrinos and antineutrinos behave differently, and therefore may not wipe each
other out.
Dr Patrick
Dunne, from the Department of Physics at Imperial, said: “This result brings us
closer than ever before to answering the fundamental question of why the matter
in our universe exists. If confirmed — at the moment we’re over 95 per cent
sure — it will have profound implications for physics and should point the way
to a better understanding of how our universe evolved.”
Previously,
scientists have found some differences in behaviour between matter and
antimatter versions of subatomic particles called quarks, but the differences
observed so far do not seem to be large enough to account for the dominance of
matter in the universe.
However, T2K’s
new result indicates that the differences in the behaviour of neutrinos and
antineutrinos appear to be quite large. Neutrinos are fundamental particles but
do not interact with normal matter very strongly, such that around 50 trillion
neutrinos from the Sun pass through your body every second.
Neutrinos and
antineutrinos can come in three ‘flavours’, known as muon, electron and tau. As
they travel, they can ‘oscillate’ — changing into a different flavour. The fact
that muon neutrinos oscillate into electron neutrinos was first discovered by
the T2K experiment in 2013.
To get the new
result, the team fired beams of muon neutrinos and antineutrinos from the
J-PARC facility at Tokai, Japan, and detected how many electron neutrinos and
antineutrinos arrived at the Super-Kamiokande detector 295km away.
They looked for
differences in how the neutrinos or antineutrinos changed flavour, finding
neutrinos appear to be much more likely to change than antineutrinos.
The available
data also strongly discount the possibility that neutrinos and antineutrinos
are as just likely as each other to change flavour. Dr Dunne said: “What our
result shows is that we’re more than 95 per cent sure that matter neutrinos and
antineutrinos behave differently. This is big news in itself; however we do
already know of other particles that have matter-antimatter differences that
are too small to explain our matter-dominated universe.
“Therefore,
measuring the size of the difference is what matters for determining whether
neutrinos can answer this fundamental question. Our result today finds that
unlike for other particles, the result in neutrinos is compatible with many of
the theories explaining the origin of the universe’s matter dominance.”
While the result
is the strongest evidence yet that neutrinos and antineutrinos behave
differently, the T2K Collaboration is working to reduce any uncertainties and
gather more data by upgrading the detectors and beamlines, including the new
Hyper-Kamiokande detector to replace the Super-Kamiokande. A new experiment,
called DUNE, is also under construction in the US. Imperial is involved in
both.
Imperial researchers
have been involved in the T2K Collaboration since 2004, starting with
conceptual designs on whiteboards and research and development on novel
particle detector components that were key to building this experiment, which
was finally completed and turned on in 2010.
For the latest
result, the team contributed to the statistical analysis of the results and
ensuring the signal they observe is real, as well as including the effects of
how neutrinos interact with matter, which is one of the largest uncertainties
that go into the analysis.
Professor Yoshi Uchida said: “When we started, we knew that seeing signs
of differences between neutrinos and antineutrinos in this way was something
that could take decades, if they could ever be seen at all, so it is almost
like a dream to have our result be celebrated on the cover of Nature this
week.”
Journal article: https://www.nature.com/articles/s41586-020-2177-0
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