The Beauty Experiment
By: Ella Alderson
Tags: Space, Science, Quantum Physics, Physics, Universe

The three forces described by the Standard Model — electromagnetism, weak nuclear, and strong nuclear — are each carried by their respective particles. Electromagnetism is carried by the photon, the weak nuclear force is carried by W and Z bosons, and the strong nuclear force has the gluon. In addition, the Higgs boson is a particle that emerges from the Higgs field. Pushing against certain particles causes them to interact with the Higgs field. This resistance, in turn, becomes the mass of particles like electrons and quarks. Within the Standard Model, particles are divided into two groups: quarks and leptons. There are six of each, with quarks gathering together into what are known as beauty hadrons. These hadrons are naturally made of beauty quarks.

The Standard Model of particle physics. Among other riddles, this highly successful model can’t answer our question on the mass of the Higgs boson. It is observed to have a mass 10,000 trillion times lighter than expected.

There are a number of many different ways in which beauty hadrons can decay into new particles. After they’re created at the cavernous and cylindrical LHCb detector, scientists make predictions about what they will observe. The hadrons themselves, of which we’ve already produced in the billions, are very light — only 5 giga electron volts — which means that the extra energy is able to give the hadrons a push into the detector. Anytime there is a difference between the predictions and observations of these decays, it’s possible the difference is due to unknown particles. The particles will show up at internal decay points or perhaps connecting the initial and final states of the reaction in some as yet unseen way.

During the process, something incredible happens. As the beauty quarks are decaying and transitioning into particles like charm quarks or strange quarks, we see W and Z bosons appearing right at the point of transition. These are carriers of the weak force, but they can also be many times heavier than the original particles set for decay. This means that the law of conservation of energy, which says that the overall energy in an isolated system has to remain the same, is violated during these reactions. This is because the increase in mass during the decay is equal to an increase in energy, according to E=mc². The strange and unpredictable world of quantum mechanics allows for this as long as it happens quickly enough. When these heavier particles appear during the points of transformation, they’re referred to as virtual particles.

As opposed to the ATLAS and CMS experiments that discovered the Higgs boson, the point of the beauty experiments is not to try to create the undiscovered particles directly but to use beauty hadrons to then see the effects of these mysterious and elusive new particles. That is, it is the interference of virtual particles and their effects on matter which scientists hope will guide them to a better understanding of the subatomic world. And not only the subatomic world but, because it underlies everything, the nature of reality itself. Already, there have been some strange findings in studies of the decays.

During the decay of a B zero bar meson, for example, a W boson appears at the point of the beauty hadron’s transition into a charm quark. According to the Standard Model, the W boson has an equal probability of decaying into a muon, an electron, or a tau lepton (this equal probability is known as lepton universality). And yet several experiments from different scientists around the world have shown that the W boson prefers to decay into a tau lepton (and its antineutrino) at a much higher rate than expected. In fact, it’s such an important anomaly that it’s been described as one of the most notable discrepancies in particle physics, and one that poses a persistent challenge to our current laws.

In experiments involving ratios RK and RK*, measurements found that decays naturally preferred electrons over muons, even though the Standard Model says both should occur at the same rate, again violating lepton universality. These preferences are at the heart of the beauty experiment.

There are already a few theories for why these discrepancies exist. Since Higgs bosons do not adhere to lepton universality and prefer to decay into tau particles that have higher mass, a new charged Higgs particle could be to blame. But this is a tentative fit and one that doesn’t necessarily match with our observations. Other possibilities include a Z prime particle which would be a heavier relative of the Z boson, and a leptoquark which, as the name suggests, would allow interactions between quarks and leptons. Further data from the LHC might even reveal that the Standard Model can account for our observations after all. But it’s a growing puzzle. One that reveals just how difficult it is to find beautiful, simple laws of the universe.

The above image shows the LHCb detector that focuses on gathering information (trajectories, momentum, energy, identities) on B mesons. Image by Kamell.

The ultimate goal of the beauty experiments is to understand the imbalance of matter and antimatter in the universe. There should have been an equal amount of each created after the Big Bang, and yet matter outnumbered antimatter enough to give birth to our incandescent star systems, the messy webs of human beings, and the world on which we live. We use beauty quarks to help solve the mystery because they were abundant in the early stages of the universe. They exist now only in manmade processes. Comparing their decays can help us understand a higher physics, an answer to why any of this exists at all.