Scientists at the University of Sussex have measured a property of the
neutron — a fundamental particle in the universe — more precisely than ever
before. Their research is part of an investigation into why there is matter
left over in the universe, that is, why all the antimatter created in the Big
Bang didn’t just cancel out the matter.
The team — which
included the Science and Technology Facilities Council’s (STFC) Rutherford
Appleton Laboratory in the UK, the Paul Scherrer Institute (PSI) in Switzerland,
and a number of other institutions — was looking into whether or not the
neutron acts like an “electric compass.” Neutrons are believed to be slightly
asymmetrical in shape, being slightly positive at one end and slightly negative
at the other — a bit like the electrical equivalent of a bar magnet. This is
the so-called “electric dipole moment” (EDM), and is what the team was looking
for.
This is an
important piece of the puzzle in the mystery of why matter remains in the
Universe, because scientific theories about why there is matter left over also
predict that neutrons have the “electric compass” property, to a greater or
lesser extent. Measuring it then it helps scientists to get closer to the truth
about why matter remains.
The team of physicists found that the neutron has a significantly
smaller EDM than predicted by various theories about why matter remains in the
universe; this makes these theories less likely to be correct, so they have to
be altered, or new theories found. In fact it’s been said in the literature
that over the years, these EDM measurements, considered as a set, have probably
disproved more theories than any other experiment in the history of physics.
The results are reported today, Friday 28 February 2020, in the journal Physical
Review Letters.
Professor Philip
Harris, Head of the School of Mathematical and Physical Sciences and leader of
the EDM group at the University of Sussex, said:
“After more than
two decades of work by researchers at the University of Sussex and elsewhere, a
final result has emerged from an experiment designed to address one of the most
profound problems in cosmology for the last fifty years: namely, the question
of why the Universe contains so much more matter than antimatter, and, indeed,
why it now contains any matter at all. Why didn’t the antimatter cancel out all
the matter? Why is there any matter left?
“The answer
relates to a structural asymmetry that should appear in fundamental particles
like neutrons. This is what we’ve been looking for. We’ve found that the
“electric dipole moment” is smaller than previously believed. This helps us to
rule out theories about why there is matter left over — because the theories
governing the two things are linked.
“We have set a
new international standard for the sensitivity of this experiment. What we’re
searching for in the neutron — the asymmetry which shows that it is positive at
one end and negative at the other — is incredibly tiny. Our experiment was able
to measure this in such detail that if the asymmetry could be scaled up to the
size of a football, then a football scaled up by the same amount would fill the
visible Universe.”
The experiment
is an upgraded version of apparatus originally designed by researchers at the
University of Sussex and the Rutherford Appleton Laboratory (RAL), and which
has held the world sensitivity record continuously from 1999 until now.
Dr Maurits van
der Grinten, from the neutron EDM group at the Rutherford Appleton Laboratory
(RAL), said:
“The experiment
combines various state of the art technologies that all need to perform
simultaneously. We’re pleased that the equipment, technology and expertise
developed by scientists from RAL has contributed to the work to push the limit
on this important parameter”
Dr Clark
Griffith, Lecturer in Physics from the School of Mathematical and Physical
Sciences at the University of Sussex, said:
“This experiment
brings together techniques from atomic and low energy nuclear physics,
including laser-based optical magnetometry and quantum-spin manipulation. By
using these multi-disciplinary tools to measure the properties of the neutron
extremely precisely, we are able to probe questions relevant to high-energy
particle physics and the fundamental nature of the symmetries underlying the
universe. “
50,000 measurements
Any electric
dipole moment that a neutron may have is tiny, and so is extremely difficult to
measure. Previous measurements by other researchers have borne this out. In
particular, the team had to go to great lengths to keep the local magnetic
field very constant during their latest measurement. For example, every truck
that drove by on the road next to the institute disturbed the magnetic field on
a scale that would have been significant for the experiment, so this effect had
to be compensated for during the measurement.
Also, the number
of neutrons observed needed to be large enough to provide a chance to measure
the electric dipole moment. The measurements ran over a period of two years.
So-called ultracold neutrons, that is, neutrons with a comparatively slow
speed, were measured. Every 300 seconds, a bunch of more than 10,000 neutrons
was directed to the experiment and examined in detail. The researchers measured
a total of 50,000 such bunches.
A new international standard is set
The researchers’
latest results supported and enhanced those of their predecessors: a new
international standard has been set. The size of the EDM is still too small to
measure with the instruments that have been used up until now, so some theories
that attempted to explain the excess of matter have become less likely. The
mystery therefore remains, for the time being.
The next, more
precise, measurement is already being constructed at PSI. The PSI collaboration
expects to start their next series of measurements by 2021.
Search for “new physics”
The new result
was determined by a group of researchers at 18 institutes and universities in
Europe and the USA on the basis of data collected at PSI’s ultracold neutron
source. The researchers collected measurement data there over a period of two
years, evaluated it very carefully in two separate teams, and were then able to
obtain a more accurate result than ever before.
The research
project is part of the search for “new physics” that would go beyond the
so-called Standard Model of Physics, which sets out the properties of all known
particles. This is also a major goal of experiments at larger facilities such
as the Large Hadron Collider (LHC) at CERN.
The techniques
originally developed for the first EDM measurement in the 1950s led to
world-changing developments such as atomic clocks and MRI scanners, and to this
day it retains its huge and ongoing impact in the field of particle physics.
Journal article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.081803
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