- 4058 - NUCLEAR FORCES
- to understand atoms? To test the strong nuclear force,
physicists turned to the helium-4 nucleus, which has two protons and two
neutrons. When helium nuclei are excited, they grow like an inflating balloon
until one of the protons pops off.
-
--------------- 4058 - NUCLEAR FORCES - to understand atoms?
- Excited helium
nuclei will inflate like balloons. This
offers physicists a chance to study the “strong nuclear force”, which binds the
nucleus’s protons and neutrons. A new
measurement of the strong nuclear force confirms previous hints of an
uncomfortable truth: We still don’t have a solid theoretical grasp of even
these simplest nuclear systems.
-
- Surprisingly, in a
recent experiment, helium nuclei didn’t swell according to plan: They ballooned
more than expected before they burst. A measurement describing that expansion,
called the “form factor,” is twice as large as theoretical predictions.
-
- The swelling
helium nucleus is a sort of mini-laboratory for testing nuclear theory because
it’s like a microscope it can magnify deficiencies in theoretical calculations.
Physicists think certain peculiarities in that swelling make it supremely
sensitive to even the faintest components of the nuclear force, factors so
small that they’re usually ignored.
-
- How much the
nucleus swells also corresponds to the squishiness of nuclear matter, a
property that offers insights into the mysterious hearts of neutron stars. There is a significant problem in nuclear
physics. Our best understanding of
nuclear interactions, a framework known as chiral effective field theory has
fallen short.
-
- Atomic nucleons,
that is protons and neutrons, are held together by the “strong force”. But the
theory of the strong force was not developed to explain how nucleons stick
together. Instead, it was first used to explain how protons and neutrons are
made of elementary particles called quarks and gluons.
-
- For many years,
physicists didn’t understand how to use the strong force to understand the
stickiness of protons and neutrons. One problem was the bizarre nature of the
strong force, it grows stronger with increasing distance, rather than slowly
dying off like the other forces.
-
- In 1990, Steven
Weinberg found a way to connect the world of quarks and gluons to sticky
nuclei. The trick was to use an effective field theory, a theory that is only
as detailed as it needs to be to describe nature at a particular size (or
energy) scale. To describe the behavior of a nucleus, you don’t need to know
about quarks and gluons. Instead, at these scales, a new effective force
emerges, the strong nuclear force, transmitted between nucleons by the exchange
of “pions”.
-
- Weinberg’s work helped
physicists understand how the strong nuclear force emerges from the strong
force. It also made it possible for them to perform theoretical calculations
based on the usual method of approximate contributions. The theory “chiral
effective theory” is now widely considered the best theory we have,for
calculating the forces that govern the behavior of nuclei.
-
- In 2013,
researchers used this effective field theory to predict how much an excited
helium nucleus would swell. But when comparing calculation to experiments
performed in the 1970s and 1980s, they found a substantial discrepancy.
-
- Researches excited
the nuclei by shooting a beam of electrons at a tank of cold helium gas. If an
electron zipped within range of one of the helium nuclei, it donated some of
its excess energy to the protons and neutrons, causing the nucleus to inflate.
This inflated state was fleeting, the nucleus quickly lost grasp of one of its
protons, decaying into a hydrogen nucleus with two neutrons, plus a free
proton.
-
- As with other
nuclear transitions, only a specific amount of donated energy will allow the
nucleus to swell. By varying the electrons’ momentum and observing how the
helium responded, scientists could measure the expansion. They compared this
change in a nucleus’s spread with a variety of theoretical calculations. None
of the theories matched the data. But, strangely, the calculation that came
closest used an oversimplified model of the nuclear force that was not the
chiral effective field theory.
-
- Physicists have
several reasons to suspect that this simple measurement would probe the limits
of our understanding of nuclear forces.
Wow, there is still more to learn!
-
-
June 17, 2023 NUCLEAR FORCES
- to understand atoms? 4058
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