Discovery of particle physics raises hopes for a theory of everything

The Standard Model of particle physics, which describes every particle we know of and how they interact, received a lot of credit when the Higgs boson was discovered in 2012. Now measurements of a rare particle physics decay particles at the Large Hadron Collider provide additional support. for the model – but also clues to the means of discovering what lies beyond.

The Standard Model is cherished by physicists because it can explain most of nature’s fundamental phenomena by referring to a handful of elementary particles.

These particles include quarks (one of the components of an atom) and electron-like particles called leptons – as well as their so-called antiparticles which are identical but have opposite charge. The model also includes particles that transport forces between them (photons, gluons, W and Z bosons) and the Higgs.

The elementary particles which, according to the standard model, constitute matter.
By HolgerFiedler nach Benutzer:Murphee via Wikimedia Commons, CC BY-SA

The picture provided by this model is remarkably complete and accurate, given its (relative) simplicity and the wide variety of very different phenomena it can explain with astonishing accuracy.

Even the sun has spots…

But the standard model is far from perfect. For starters, it doesn’t include gravity. Moreover, the elementary particles he describes so successfully represent only 4% of the matter of the universe. The rest is a mysterious substance called “dark matter” whose composition we still don’t know. This is one of the reasons why scientists doubt that the Standard Model can be the true theory of everything.”

For quite some time now, physicists have been desperately searching for any phenomena that deviate from the predictions of the Standard Model, as they might provide clues or clues to the nature of physics beyond it. Such experimental findings could help test theories that go beyond the Standard Model. These include supersymmetry – in which there are copies of all particles – and string theory, which is an attempt to reconcile quantum mechanics and general relativity.

But so far the Standard Model has been very resilient, able to successfully explain anything experimental physicists have managed to throw at it.

That could be about to change. Two collaborations of LHC scientists – one using the Compact Muon Solenoid detector and the other performing the LHC beauty experiment – ​​at the CERN particle physics laboratory near Geneva have measured the decays of so-called B mesons. B mesons are strange particles composed of a specific quark and an antiquark. They looked at two different types of particles: a “neutral” B meson and a “strange” B meson.

The huge CMS detector that successfully measured the tiny muons.
CERN

All B mesons are short-lived and spontaneously decay into a bunch of other mesons. But this study specifically focused on the decays of B mesons into pairs of so-called muons, which are heavier versions of electrons, and their antiparticles.

These decays are of particular interest because their probabilities can be calculated in the Standard Model with little ambiguity and high accuracy. Experimentally, muons are relatively easy to detect and can be measured with great precision.

Starting point for a theory of everything

Thus, according to the Standard Model, on average about four out of every billion strange B mesons decay into the muon-antimuon pair (instead of other particles). For the neutral B meson, this number is even smaller, about one in ten billion. These are indeed very small numbers and explain why past experiments have failed to detect them.

But the new experiments have been able to observe these disintegrations, and measure their probabilities. They show that while the strange B meson decays into muons at the same rate as the standard model predicts, the neutral B meson does so about four times more often than expected (although the accuracy here is somewhat lower).

This is an important new development, as various theories that go beyond the Standard Model predict larger decay probabilities. These results will help rule out some of the theoretical possibilities for physics beyond the Standard Model. Knowing this is essential to one day designing the next Theory of Everything.

The next LHC run, which is about to begin, should provide an opportunity to improve the accuracy of these measurements and place even tighter constraints on theories that include physics beyond the model. standard – or maybe bring a find that won’t fit one of the existing ones?

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