Improved modelling of nuclear structure in francium aids searches for
new physics

Date:
August 5, 2020
Source:
University of Queensland
Summary:
Thanks to new research, we now know with much greater certainty
the nuclear magnetic moments of francium atoms.



FULL STORY ========================================================================== Thanks to researchers from The University of Queensland, we now know with
much greater certainty the nuclear magnetic moments of francium atoms.


==========================================================================
Dr Ben Roberts, a postdoctoral research fellow in UQ's School of
Mathematics and Physics, said that the nuclear magnetic moment is
a fundamental property of atoms, and knowing its value precisely is
important when testing fundamental physics theories.

"But because francium is radioactive, the standard techniques for
determining nuclear magnetic moments can't easily be applied," Dr
Roberts said.

"Using new methods, we were able to calculate moments with uncertainties
four times smaller than the previous best values.

"Take francium-211, for example: its nuclear magnetic moment was
previously determined to be in the range 3.92 to 4.08 (in the natural
unit for expressing these moments).

"Our calculations now show it's between 3.90 and 3.94." This may not
seem like a huge difference, but Dr Jacinda Ginges, an ARC Future Fellow
at UQ and Associate Investigator at the ARC Centre of Excellence for
Engineered Quantum Systems (EQUS), said that when you're talking about
atomic physics, small differences can have a huge effect, so narrowing
the range of possible values is a big deal.



==========================================================================
"Our current understanding of the fundamental particles that make up the Universe and their interactions relies on the standard model of particle physics, but we also know this model is incomplete, there are some things
it can't explain," Dr Ginges said.

"We need precise values for nuclear magnetic moments to be able to test
the validity of our atomic models, which in turn are really important
for testing the standard model of particle physics.

"By combining precision experiments in atoms with high-precision atomic
theory, we get a powerful way to search for new physics." The improvement
in precision was the result of very precise calculations of the hyperfine structure of francium -- the tiny differences in atomic energy levels
caused by its nuclear magnetic moment -- and more accurate models of
nuclear effects.

"Previous determinations assumed that the nucleus of a francium atom
was like a ball with uniform magnetisation, but in our calculation we
assumed a more realistic model that allowed the magnetisation to vary
within the nucleus," Dr Roberts said.

"The effect of non-uniform magnetisation (known as the Bohr-Weisskopf
effect) is especially large in francium, so by accurately taking this into account we were able to determine its nuclear magnetic moments much more precisely." "Our results can now be used to benchmark atomic theory,
which will help interpret experiments currently underway at Canada's
national nuclear and particle physics facility, TRIUMF," Dr Ginges said.

"They also show how important it is to accurately model nuclear effects,
and will have implications for past and future precision experiments
with heavy atoms."

========================================================================== Story Source: Materials provided by University_of_Queensland. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. B. M. Roberts and J. S. M. Ginges. Nuclear
Magnetic
Moments of Francium-207-213 from Precision Hyperfine
Comparisons. Phys.

Rev. Lett., 4 August 2020 DOI: 10.1103/PhysRevLett.125.063002 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200805091845.htm

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