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2021 – A year in which physicists asked, “What is beyond the standard model?”

LHC ATLAS calorimeter

Experiments with the Large Hadron Collider in Europe, like the ATLAS calorimeter seen here, provide more accurate measurements of fundamental particles. Credit: Maximilien Brice, CERN

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 basic physics of how the universe works. It has endured over 50 trips around the Sun despite the fact that experimental physicists are constantly searching for cracks in the model's foundation.

With few exceptions, it has stood for this control, passing experimental test after experimental test with gloss. But this wildly successful model has conceptual gaps that suggest there is a little more to learn about how the universe works.

I'm a neutrino physicist. Neutrinos represent three of the 17 fundamental particles in the standard model. They flash through all the people on Earth at all times of the day. I study the properties of interactions between neutrinos and normal substance particles.

In 2021, physicists around the world ran a series of experiments examining the standard model. Keep the basic parameters of the measured model more accurate than ever before. Others examined the fringes of knowledge where the best experimental measurements do not quite match the predictions from the standard model. And finally, groups built more powerful technologies designed to push the model to its limits and potentially discover new particles and fields. If these efforts fail, they could lead to a more complete theory of the universe in the future.

Standard Model of Physics Quarks Leptons Higgs Boson

The standard model of physics allows scientists to make incredibly accurate predictions about how the world works, but it does not explain everything. Credit: CERN

Filling holes in standard model

In 1897, JJ Thomson discovered the first fundamental particle, the electron, using nothing but 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. It is things like electrons and quarks that make up protons and neutrons. Second, it predicts how these substance particles 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 first discovered in 2012 after decades of work CERN, the huge particle colliding in Europe.

The standard model is incredibly good at predicting many aspects of how the world works, but it has some gaps.

In particular, it contains no description of gravity. While Einstein's theory of general relativity describes how gravity works, physicists have not yet discovered a particle that conveys gravity. A proper "Theory of Everything" would do everything the standard model can, but also include the messenger particles that communicate how gravity interacts with other particles.

Another thing that the standard model cannot do is explain why every particle has a certain mass - physicists have to measure the mass of particles directly using experiments. Only after experiments have given physicists these exact masses can they be used for predictions. The better the measurements, the better predictions can be made.

Recently, physicists on a team at CERN measured how strongly the Higgs boson feels. Another CERN team also measured the mass of the Higgs boson more accurately than ever before. And finally, there was also progress in measuring the mass of neutrinos. Physicists know that neutrinos have more than zero mass, but less than the amount that can currently be detected. A team in Germany has continued to refine the techniques that could allow them to directly measure the mass of neutrinos.

Muon g-2 experiment on Fermilab

Projects like the Muon g-2 experiment highlight inconsistencies between experimental measurements and predictions of the standard model that point to problems somewhere in physics. Credit: Reidar Hahn, Fermilab

Hints 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 basic particles in the standard model, and this measurement of one of its properties is the most accurate to date. The reason why this experiment was important was that the measurement did not fit perfectly with the standard model's prediction of the magnetic moment. Basically, muons do not behave as they should. This finding could point to undetected particles interacting with muons.

But at the same time, in April 2021, physicist Zoltan Fodor and his colleagues showed how they used a mathematical method called the Lattice QCD to accurately calculate the magnetic moment of a muon. Their theoretical prediction differs from old predictions, still works within the standard model and, importantly, matches experimental measurements of the muon.

The disagreement between the previously accepted predictions, this new result, and the new prediction must be reconciled before physicists will know whether the experimental result is really beyond the standard model.

Celestial spiral galaxy

New tools will help physicists search for dark matter and other things that can help explain the mysteries of the universe. Credit: Mark Garlick

Upgrading the tools of physics

Physicists must alternate between creating the feeble-minded ideas of reality that constitute theories and advancing technologies to the point where new experiments can test these theories. 2021 was a great year to advance the experimental tools of physics.

First, the world's largest particle accelerator, the Large Hadron Collider at CERN, was shut down and underwent some significant upgrades. Physicists have just restarted the facility in October, and they plan to begin the next data collection run in May 2022. The upgrades have increased the power of the collider so it can produce collisions at 14 TeV, up from the previous limit of 13 TeV. This means that the batches of tiny protons moving in rays around the circular accelerator together carry the same amount of energy as an 800,000 pound (360,000 kilogram) passenger train running at 100 mph (160 km / h). At these incredible energies, physicists can discover new particles that were too heavy to see at lower energies.

Some other technological advances were made to help search for dark matter. Many astrophysicists believe that dark matter particles that do not currently fit into the standard model could answer some outstanding questions about the way gravity bends around stars - called gravitational lenses - as well as the speed at which stars rotate in spiral galaxies. Projects like Cryogenic Dark Matter Search have not yet found dark matter particles, but the teams are developing larger and more sensitive detectors to be implemented in the near future.

Particularly relevant to my work with neutrinos is the development of huge new detectors such as Hyper-Kamiokande and DUNE. By using these detectors, researchers will hopefully be able to answer questions about a fundamental asymmetry in how neutrinos oscillate. They will also be used to monitor proton decay, a proposed phenomenon that certain theories predict should occur.

2021 highlighted some of the ways in which the Standard Model fails to explain every mystery in the universe. But new measurements and new technology are helping physicists move forward in their search for Theory of Everything.

Written by Aaron McGowan, Associate Professor of Physics and Astronomy, Rochester Institute of Technology.

This article was first published in The Conversation.The conversation

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