On 23 March 2021, Geneva-based CERN, the world’s largest particle physics laboratory, announced evidence of strange particle behaviour.
The idea that matter is made up of tiny indivisible particles is ancient. The word atom is derived from the ancient Greek word atomos, which means uncuttable.
However, as particle physics has progressed, the uncuttable atom has been disassembled into many smaller particles in a quest to find nature’s ultimate granularity and possible insights into the origins of the universe.
For a long time it was believed the smallest components of atoms were electrons, neutrons and protons. Physicists have since discovered that neutrons and protons, which are two types of hadrons, are clusters of smaller particles known as quarks.
Currently, there are 30 known elementary particles and a possible extra one known as the graviton, which has never been observed.
As physicists dig deeper into the sub atomic world they formulate and refine theories that predict how matter should behave. When known particle behaviour does not fit a theory it suggests that some undiscovered elementary particles might exist.
The Standard Model of particle physics is a theory that has largely stood the test of time, despite its inability to account for such phenomena as gravity and the accelerating expansion of the universe. However, this week, CERN announced the results of an experiment that may have revealed a new weakness in it.
Engineers and physicists at CERN have been sending particles containing beauty quarks (AKA bottom or b quarks) racing around the LHC (Large Hadron Collider) particle accelerator, CERN’s largest, to crash into each other. The collisions were observed at the LHCb detector – the b refers to the beauty quark. These collisions sometimes result in temperatures a billion times hotter than the sun and can be done at a rate of around 40 million collisions per second for hours at a time.
Very rarely, at a rate of roughly one in two million, the bottom quarks in these particle collisions decay into an electron or a muon, another elementary particle similar to the electron but roughly 200 times heavier.
Part of the Standard Model, known as lepton flavour universality, predicts that electrons and muons, two elementary particles belonging to a group of elemental particles known as leptons, should emerge from these decay events at the same rate.
However, data from the recent CERN experiment strongly suggest that electrons and muons do not emerge at the same rate. If so, reality would be at odds with lepton flavour universality and the Standard Model.
So what might be going on?
One possibility is that some new particle, a hypothetical leptoquark, exists. Another is that there is some unknown interaction between currently known elementary particles.
There is a small possibility that a decay difference doesn’t exist. But it is unlikely. The large quantities of data1 produced by the experiment yielded results at the 3.1 sigma level, which means there is only a 1 in 1,000 probability that it happened by chance. Results exceeding 3 sigma are typically considered “evidence” that something occurred. Results above 5 sigma are deemed “observations” ie something happened.
The results of other experiments seem to point in the same direction as this recent one. It may be that CERN’s engineers and physicists are on the cusp of an exciting new discovery that changes our understanding of the physical universe.
The recent experiment is part of the LHC beauty experiment set up to explore what happened after the Big Bang. The video below explains the broader experiment.
Further experiments will be run next year after the LHCb detector has been upgraded.
1 CERN produces around 90 petabytes of data a year – 1 petabyte is 1 million gigabytes