by Nicole Casal Moore
May 08, 2013
A graphical representation of the pear-shaped
nucleus of an exotic atom. The shape of the
nucleus could give clues to why the universe
contains more matter than antimatter.
Image credit: Liam Gaffney and Peter Butler,
University of LiverpoolA graphical
of the pear-shaped nucleus of an exotic atom.
The shape of the nucleus could give clues to why
the universe contains more matter than
antimatter. Image credit: Liam Gaffney and Peter
Butler, University of LiverpoolANN ARBOR - An
international team of physicists has found the
first direct evidence of pear shaped nuclei in
The findings could advance the search for a new fundamental force in
nature that could explain why the Big Bang created more matter than
antimatter - a pivotal imbalance in the history of everything.
"If equal amounts of matter and
antimatter were created at the Big Bang, everything would have
annihilated, and there would be no galaxies, stars, planets or
Tim Chupp, a University of
Michigan professor of physics and biomedical engineering and
co-author of a paper on the work published in the May 9 issue of
Antimatter particles have the same mass
but opposite charge from their matter counterparts.
Antimatter is rare in the known
universe, flitting briefly in and out of existence in,
When they find each other, matter and antimatter particles mutually
destruct or annihilate.
What caused the matter/antimatter imbalance is one of physics' great
mysteries. It's not predicted by
the Standard Model - the
overarching theory that describes the laws of nature and the nature
The Standard Model describes four fundamental forces or interactions
that govern how matter behaves:
Gravity attracts massive bodies
to one another
The electromagnetic interaction
gives rise to forces on electrically charged bodies.
The strong and weak forces
operate in the cores of atoms, binding together neutrons and
protons or causing those particles to decay.
Physicists have been searching for signs
of a new force or interaction that might explain the
The evidence of its existence would be
revealed by measuring how the axis of nuclei of the radioactive
elements radon and radium line up with the spin. The researchers
confirmed that the cores of these atoms are shaped like pears,
rather than the more typical spherical orange or elliptical
The pear shape makes the effects of the
new interaction much stronger and easier to detect.
"The pear shape is special," Chupp
said. "It means the neutrons and protons, which compose the
nucleus, are in slightly different places along an internal
The pear-shaped nuclei are lopsided
because positive protons are pushed away from the center of the
nucleus by nuclear forces, which are fundamentally different from
spherically symmetric forces like gravity.
"The new interaction, whose effects
we are studying does two things," Chupp said.
"It produces the matter/antimatter
asymmetry in the early universe and it aligns the direction of
the spin and the charge axis in these pear-shaped nuclei."
To determine the shape of the nuclei,
the researchers produced beams of exotic - short-lived - radium and
radon atoms at CERN's Isotope Separator facility
The atom beams were accelerated and
smashed into targets of nickel, cadmium and tin, but due to the
repulsive force between the positively charged nuclei, nuclear
reactions were not possible.
Instead, the nuclei were excited to
higher energy levels, producing gamma rays that flew out in a
specific pattern that revealed the pear shape of the nucleus.
"In the very biggest picture, we're
trying to understand everything we've observed directly and also
indirectly, and how it is that we happen to be here," Chupp
The research was led by University of
Liverpool Physics Professor Peter Butler.
"Our findings contradict some
nuclear theories and will help refine others," he said.
The measurements also will help direct
the searches for atomic EDMs (electric dipole moments) currently
being carried out in North America and Europe, where new techniques
are being developed to exploit the special properties of radon and
"Our expectation is that the data
from our nuclear physics experiments can be combined with the
results from atomic trapping experiments measuring EDMs to make
the most stringent tests of the Standard Model, the best theory
we have for understanding the nature of the building blocks of
the universe," Butler said.
The paper is titled "Studies of nuclear
pear-shapes using accelerated radioactive beams."
Answers to FAQ’s
I have received a number of questions about this above work that will be
Since I am pursuing the EDM experiments,
my answers will emphasize this aspect of the impact of the research.
How is the finding of a
pear-shaped nucleus likely to affect nuclear physics
research? Will it have any influence on our understanding of
things like nuclear fusion?
This work does not address fusion in the sense that it is a
possible future energy source. As for nuclear physics
research, we will continue to use this technique to study
radon and radium atoms at ISOLDE as well as making use of
other techniques at other labs.
Perhaps the most significant
effect on nuclear-physics research is that this provides
increased confidence on the prospects of EDM experiments and
What dictated your choice of
radium and radon?
These are two systems that are both accessible to EDM
experiment for different reasons. Radon is a noble gas like
helium or neon and thus has special features that allow a
Radium has many attractive
features and it can be trapped by lasers for an EDM
measurement. From the nuclear-physics perspective, these are
nuclei that were predicted to have strong pear-shaped
Why hasn't the shape of
nuclei been studied in such detail before?
Studies of the shapes of nuclei have been going on for a
long time in particular for spherical and oval (rugby-ball
or watermelon) shapes. The pear-shaped nuclei are not
stable, that is they undergo radioactive decay to more
symmetric shapes, so special technology is required to
produce and accelerate these.
This was developed at CERN by
the REX-ISOLDE program. The ability to make the measurements
of the pattern of gamma rays from the excited nuclei is a
feature of the MINIBALL detector developed by a large team.
The analysis of the data in
terms of the nuclear shapes is also quite specialized with
the greatest contributions coming from the University of
What role does the EDM play
in a nucleus being pear-shaped? How is its study likely to
lead to the finding of new physics phenomena?
The nuclear pear shape is a consequence of nuclear forces,
while the EDM would arise due to much weaker, undiscovered
forces. BUT the pear shape will make the effects of these
undiscovered forces much stronger and easier to detect.
Could you explain the role of
the strong nuclear force and the weak force in the octupole
deformation of nuclei?
It is the strong-nuclear force that determines how the
neutrons and protons move within the nucleus. This nuclear
force on neutrons and protons has the important effect that
it is not completely central and thus pushes protons and
neutrons into unusual places (gravity is a central force
that depends on the distance between two objects centers).
The weak force has very, well,
weak effect on the shape or structure of nuclei.