Is There A Problem With the Standard Model of Particle Physics?
This is just a short talk on the Science Breakthroughs of 2021 for OLLI at UC Irvine.
The Muon Anomalous Magnetic Moment Experimental Discrepancy
The muon is essentially a heavier version of the electron, of negative charge and of spin 1/2. It is 207 times heavier than the electron.
The charge of elementary particles are quantized to be the same as the electron, or oppositely positive, like the positron or proton, or a multiple of them.
But the anomalous magnetic moment of an elementary particle is due to the currents and particles running around inside of it.
The physicist Paul Dirac formulated the first application of quantum mechanics and relativity to spin 1/2 particles or Fermions. He discovered that they all started out with a magnetic moment g = 2.
Then Julian Schwinger calculated that the correction to that for a virtual photon coupling to the electron or muon while it is interacting magnetically, added an extra alpha/2*pi electromagnetic coupling where alpha is the dimensionless form of the electromagnetic coupling e*e
Alpha = e^2 / hbar*c, where hbar is Plank’s constant over 2*pi, and c is the speed of light. Alpha = 1/137.037…
Thus, g = 2 (1 + alpha/2*pi). Lower case “a” is the anomalous magnetic moment defined in a = (g – 2) / 2 = alpha / 2*pi + …
It is the … which has to be calculated from the experiments and theory of the virtual particles which form a cloud around the muon, and compared to experiment.
When that is done with the latest, most accurate experiment, there is a very tiny, but enormously significant discrepancy of 4.2 sigma in a Bell curve, or only 1 in 47,000 chance of its just being a statistical fluctuation.
This is the slide of contributions from a talk by Dr. Aida El-Khadra of the theory group of Fermilab near Chicago, where the experiment was done. She led the theory group which calculated the virtual hydronic contributions.
The same day as this experimental result came out, so did a new pure calculational QCD result, which comes close to the experiment. This new result did not agree with the data that was used in the experimentally seeded results of the hadronic contribution, and is therefore not as substantive.
There are many theories of new particles, which are too massive to have yet been produced, which may explain this. It really is good evidence to pursue all of the experimental consequences of these theories. One of them is Supersymmetry, where each standard model particle has a Supersymmetric image, but with spin 1/2 becoming spin 0 or 1, and spin 0 or 1 becoming spin 1/2.
Recently, there has been a reanalysis of an earlier experiment determining the weak W boson mass, which disagrees with other experiments. There are also discrepancies in B meson and K meson physics. Particle physicists are also trying to find out what the dark or uncharged matter is that brings galaxies together gravitationally. And finally, the nature of dark energy.
