Saturday, June 17, 2023

4058 - NUCLEAR FORCES - to understand atoms?

 

-    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.

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---------------   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.

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-    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.

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-    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.

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-    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.

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-  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.

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-    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.

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-    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”.

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-   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.

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-    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.

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-    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.

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-    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.

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-   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!

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June 17,  2023      NUCLEAR FORCES  -  to understand atoms?           4058

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