It’s an exciting time for particle physics. In February 2021, physicists at CERN announced a rare Higgs boson decay known as the Dalitz decay. A couple of weeks ago, the discovery of four new tetraquarks was announced. And now, physicists working on the LHCb (Large Hadron Collider beauty) experiment have announced that they have made an observation that requires new theories for an explanation. Scientists have recorded an anomaly in the decay of the composite particle called the B meson. The observed decay breaks the law of lepton universality as predicted by the Standard Model of particle physics.
I know most of you are not an expert in particle physics, but you are here thanks to your curiosity. So, I am not going to disappoint you by complicating things. I will start with the very basic and explain what’s really happening in Geneva, the home of the Large Hadron Collider. Before we jump on to the decay of the B meson, there are a few concepts that you must know. So let’s begin!
The Standard Model
According to physicists, everything you see around is a result of four fundamental forces. The first is gravity. It is responsible for the motion of astronomical objects. The second is the electromagnetic force responsible for the motion of the electrons around the nucleus of an atom. The third on the list is the weak nuclear force responsible for the radioactive decay of atoms. Finally, we have the strong nuclear force that binds the nucleus (and ultimately matter) together.
After years of brainstorming and hard work, physicists successfully combined three of the four fundamental forces into a complex theory called the Standard Model. The three forces are the electromagnetic force, the weak force, and the strong force. The elementary particles of the standard model are given below.
Look at the above chart carefully. It contains two types of particles: Fermions (yellow and cyan) and bosons (purple and blue). All the fermions have a half-integer spin, and the bosons have an integral spin. If you don’t know the concept of spin, don’t worry. It’s not required here. But you can read this article for a detailed explanation of spin in quantum mechanics.
Now, look at the fermions. The ones colored yellow are called quarks. There are six types of quarks. Different combinations of quarks and anti-quarks make up different composite particles. For example, two up quarks (uu) and a down quark (d) make up a proton, while two down quarks (dd) and one up quark make up a neutron, as shown below. The B meson is composed of a down quark and a beauty (or bottom) antiquark.
The cyan-colored elementary particles are the six leptons: electron, muon, tau, and the three neutrinos. Muon and tau particles are the cousins of electrons that differ in mass. A muon is about 200 times more massive than an electron. Quarks make up the protons and the neutrons, which further make up the nucleus. Electrons revolving around the nucleus form atoms. So, quarks and leptons are basically the building blocks of the universe.
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The decay of B Meson
We are now equipped with the basic tools to understand the observed anomaly in B meson decay. There are different ways in which composite particles can decay into elementary particles. Particle physicists refer to them as decay channels. We calculate the probabilities of decay channels. When physicists measured the decay of the B meson, they found something unusual. According to the law of lepton universality, as predicted by the Standard Model, the probability of B mesons decaying into electrons and muons should be equal. But, it was observed that nature favors the decay channel of electrons more than the muons. B mesons seem to decay to muons 15 percent less often than they do to electrons.
Physicists are intrigued by this unusual observation because there could be a hidden force in nature that is hindering the decay of the B mesons into muons. “It’s certainly intriguing, this new measurement,” says Monika Blanke, a theoretical physicist at the Karlsruhe Institute of Technology in Germany, who was not involved with the new research. “If it’s eventually confirmed experimentally, then there actually is something beyond the Standard Model that treats the lepton flavors differently.”
The finding has a statistical significance of 3.1 sigma, which meets the standard baseline for evidence in particle physics. This means there’s a probability of 1 in 1000 that it’s a statistical fluctuation. Although it may seem convincing enough, it is well short of the standard of discovery of 5 sigma, a probability of 1 in 3.4 million.
Particle physics is littered with 3 sigma discoveries. Most of them do not hold up as more data is collected, leaving the Standard Model triumphant. But physicists are excited because this result follows a pattern of other measurements that also hint at differences between electrons and muons.
Beyond the Standard Model
It’s a matter of irony that in the same month, we have reasonable evidence against and in favor of the Standard Model. While the Large Hadron Collider in the north has come up with an observation that goes against this theory, the IceCube Neutrino Observatory at the South Pole has found something that proves a 50-year-old theory related to the Standard Model.
In December 2016, a highly energetic electron anti-neutrino (6.3 PeV) from a distant galaxy crashed into the Antarctic ice. It interacted with the electrons and created the heavyweight of the model, the W boson, which further created a shower of energetic particles. This event called the Glashow resonance was predicted 50 years ago, but no accelerator in the world can create such energetic particles to prove it. You can watch the video for a detailed explanation below
One of the several goals of the largest machine in the world, the Large Hadron collider, is to search for physics beyond the Standard Model. Although it has stood several tests of time, it is not the final word on the universe’s composition. It does not explain dark matter and dark energy that makeup 96% of the universe. Besides, the Standard Model does not unify gravity with the other three fundamental forces. So yeah! It’s indeed an exciting discovery and may lead to something altogether new.
You can read the preprint of the research paper on the B meson decay here. The original paper has been published in the journal Nature.