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Aaron McGowan, Senior Lecturer in Physics and Astronomy, Rochester Institute of Technology
If you ask a physicist like me to explain how the world works, my lazy answer might be, “It follows the Standard Model.
The Standard Model explains the fundamental physics of how the universe works. It endured more than 50 trips around the sun despite experimental physicists constantly searching for cracks in the model’s foundations.
With few exceptions, it has withstood this scrutiny, passing experimental test after experimental test with flying colors. But this hugely successful model has conceptual flaws that suggest there is a bit more to be learned about how the universe works.
I am a neutrino physicist. Neutrinos represent three of the 17 fundamental particles of the Standard Model. They pass through every person on Earth at all times of the day. I study the properties of interactions between neutrinos and particles of normal matter.
Related: Massive Simulation Of The Universe Explores The Mystery Of Ghostly Neutrinos
In 2021, physicists around the world conducted a number of experiments that probed the Standard Model. The teams measured the basic parameters of the model more precisely than ever. Others have studied the fringes of knowledge where the best experimental measures do not quite match the predictions made by the Standard Model. And finally, groups have built more powerful technologies designed to push the model to its limits and potentially discover new particles and fields. If these efforts are successful, they could lead to a more comprehensive theory of the universe in the future.
Filling holes in the standard model
In 1897, JJ Thomson discovered the first fundamental particle, the electron, using nothing more than glass vacuum tubes and wires. More than 100 years later, physicists are still discovering new parts of the Standard Model.
The Standard Model is a predictive framework that does two things. First, it explains what the basic particles of matter are. These are things like electrons and quarks that make up protons and neutrons. Second, it predicts how these particles of matter interact with each other using “messenger particles”. These are called bosons – they include photons and the famous Higgs boson – and they communicate the fundamental forces of nature. The Higgs boson was only discovered in 2012 after decades of work at CERN, the huge particle collider in Europe.
The Standard Model is incredibly good at predicting many aspects of how the world will work, but it does have a few holes.
Notably, it does not include any description of the severity. While Einstein’s general theory of relativity describes how gravity works, physicists have yet to discover a particle that transmits the force of gravity. A proper “theory of everything” would do everything the Standard Model can do, but also include the messenger particles that communicate how gravity interacts with other particles.
Another thing the Standard Model cannot do is explain why a particle has a certain mass – physicists have to measure the mass of particles directly using experiments. It is only after the experiments give physicists these exact masses that they can be used for predictions. The better the measurements, the better the predictions that can be made.
Recently, physicists from a CERN team measured the force felt by the Higgs boson. Another CERN team also measured the mass of the Higgs boson more accurately than ever. And finally, there have also been advances in measuring the mass of neutrinos. Physicists know that neutrinos have a mass greater than zero but less than what is currently detectable. A team in Germany continued to refine techniques that could allow them to directly measure the mass of neutrinos.
Signs of new forces or particles
In April 2021, members of the Muon g-2 experiment at Fermilab announced their first measurement of the muon’s magnetic moment. The muon is one of the fundamental particles of the Standard Model, and this measurement of one of its properties is the most accurate to date. The reason this experiment was important was that the measurement did not quite match the Standard Model’s prediction of magnetic moment. Basically muons don’t behave the way they should. This finding could point to undiscovered particles that interact with muons.
But simultaneously, in April 2021, physicist Zoltan Fodor and his colleagues showed how they used a mathematical method called Lattice QCD to accurately calculate the muon’s magnetic moment. Their theoretical prediction is different from the old predictions, still works in the Standard Model and, most importantly, matches experimental muon measurements.
The disagreement between previously accepted predictions, this new result, and the new prediction must be reconciled before physicists know if the experimental result is truly beyond the Standard Model.
Physicists must oscillate between developing the mind-boggling ideas about reality that make up theories and advancing technologies to the point where new experiments can test those theories. 2021 has been a great year for advancing the experimental tools of physics.
First, the world’s largest particle accelerator, CERN’s Large Hadron Collider, has been shut down and has undergone substantial upgrades. Physicists just restarted the facility in October and plan to start the next data collection in May 2022. The upgrades have increased the power of the collider so that it can produce collisions at 14 TeV, down from 13 TeV previously. This means that the batches of tiny protons traveling in beams around the circular accelerator together carry the same amount of energy as an 800,000 pound (360,000 kilogram) passenger train traveling at 100 mph (160 km / h). ). At these incredible energies, physicists could discover new particles too heavy to be seen at lower energies.
Other technological advances have been made to aid in the search for dark matter. Many astrophysicists believe that dark matter particles, which currently do not fit the Standard Model, could answer some unanswered questions about how gravity curves around stars – called the gravitational lens – as well as the speed at which them stars rotate in spiral galaxies. Projects like cryogenic dark matter research have yet to find dark matter particles, but teams are developing larger, more sensitive detectors that will be deployed in the near future.
The development of huge new detectors like Hyper-Kamiokande and DUNE is particularly relevant to my work on neutrinos. Using these detectors, scientists will hopefully be able to answer questions about a fundamental asymmetry in the way neutrinos oscillate. They will also be used to monitor proton decay, a proposed phenomenon that some theories predict is expected to occur.
2021 has highlighted some of the ways the Standard Model fails to explain every mystery in the universe. But new measurements and new technologies are helping physicists advance in the search for the theory of everything.
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