The Standard Model of Particle Physics: Theory of the Subatomic World

The Standard Model is the most comprehensive description of the subatomic world that has ever been created in modern physics. The model was built throughout the 20th century on the foundations of Quantum mechanics, the strange theory that describes the behavior of particles at the smallest scales. The Standard Model explains three of the four forces of nature: electromagnetism, strong nuclear force and the weak nuclear force. The theory has been tested thousands of times with incredible accuracy and, despite its shortcomings, remains one of the most important achievements of modern science.

“It’s the dominant paradigm for thinking about how things interact at the most basic level,” and it’s been “tested to a phenomenal degree of accuracy,” said Union College physicist and author Chad Orzel. of several popular physics books, including “How to Teach Your Dog Quantum Physics” (Scribner, 2009), Live Science said in an email.

How was the standard model developed?

Physicists began developing the Standard Model in the 1950s, following a series of groundbreaking theoretical and experimental developments. On the theory side, physicists had just extended quantum mechanics – originally developed to understand only subatomic particles – to explain the electromagnetic force. On the experimental side, physicists had just developed the atomic bomb and knew the strong and weak nuclear forces but did not yet have a full description of them.

The Standard Model reached its modern form in the 1970s once a few key elements were in place: a quantum theory to explain the strong force, the realization that the weak electromagnetic and nuclear forces could be unified, and the discovery of the Higgs mechanism which gave rise to masses of particles, according to the US Department of Energy (opens in a new tab) (DOE).

“I think it is one of the greatest intellectual triumphs in the history of human civilization, both for the range of phenomena it encompasses and also for the degree of difficulty in putting it all together. “said Orzel.

How is the standard model organized?

Infographic showing the standard model subatomic particles. (Image credit: Karl Tate, Infographic Artist Livescience.com)

The Standard Model organizes the subatomic world into two broad classes of particles, called fermions and bosons, according to the University of Tennessee, Knoxville (opens in a new tab). Basically, fermions cannot share the same quantum state (e.g. the same energy level inside a atom). Fermions are the “building blocks” of ordinary matter, which combine in different ways to form some of the well-known subatomic particles, such as protons, electrons and neutrons.

There are two kinds of fermions: leptons, which respond to weak electromagnetic and nuclear forces, and quarks, which respond to the strong nuclear force. Leptons include the familiar electron, as well as its heavier cousins, the muon and tau. These two particles have exactly the same properties as the electron but are more massive.

Each of these leptons is paired with a corresponding neutrino. Neutrinos are ultralight particles that rarely interact with matter but are generated during nuclear reactions. So there are electron neutrinos, muon neutrinos and tau neutrinos.

In addition to these six leptons, there are quarks, which come in six types, or “flavors”: high, low, charm, strange, high, and low. Up and down quarks are the lightest and most stable, and they bind in triplets to form protons and neutrons.

On the other hand, bosons can share the same energy state. The best known boson is the photon, carrier force of the electromagnetic force. Other force-carrying bosons include the three carriers of the weak nuclear force (called W+, W-, and Z bosons) and the eight carriers of the strong nuclear force, called gluons, according to the DOE.

The last boson, called the the Higgs bosonis particular and plays a very important role in the standard model.

What is the role of the Higgs mechanism in the Standard Model?

Conceptual illustration of the Higgs particle produced by the collision of two protons. (Image credit: Mark Garlick/Science Photo Library via Getty Images)

The Higgs boson performs two important functions in the Standard Model. At high energies, the weak electromagnetic and nuclear forces merge into a common, unified force called the electroweak force. At low energies (i.e. energies typical of everyday life), both forces split into their familiar forms. The Higgs boson is responsible for separating these two forces at low energies, as weak nuclear and electromagnetic forces interact differently with the Higgs boson, according to the institute of physics (opens in a new tab).

All other quarks and leptons (except neutrinos) also interact with the Higgs boson. This interaction gives these particles their individual masses, which depend on how strongly the particle interacts with the Higgs. Thus, the presence of the Higgs boson allows many particles in our universe to acquire mass.

How is the standard model tested?

Here we see an image of the Large Hadron Collider. Physicists and engineers replaced the core of the CMS experiment in 2017 to improve its ability to make precise measurements. (Image credit: CERN)

Testing the Standard Model is extremely difficult, because all the particles involved are extremely small.

“None of these particles, except perhaps the electron, is directly observable, and yet their existence is proven almost incontrovertibly through the accumulation of the work of generations of physicists probing ever deeper into nature. reality,” Orzel said.

That said, the standard model has survived decades of high-precision experimentation. Almost all of these experiments incorporate the use of particle colliders, such as the Large Hadron Collider near Geneva, which slam particles at close to the speed of light. These collisions release huge amounts of energy, allowing physicists to study the fundamental interactions of nature, according to CERN (opens in a new tab)the European Organization for Nuclear Research, home to the Large Hadron Collider.

“For me, the most impressive feature is that it allows us to determine real-world parameters with astonishing precision – something like 13 to 14 decimal places in the case of something like the anomalous magnetic moment of the electron” , Orzel said.

What are the problems with the standard model?

Despite its enormous successes in explaining a wide variety of natural phenomena within a single mathematical framework, physicists know that the Standard Model is not complete. More importantly, attempts to incorporate gravity in the standard model have consistently failed.

“The inability to merge gravity with the Standard Model framework for the rest of fundamental physics is the the biggest challenge facing theoretical particle physicists, and pushed them into some pretty baroque realms of speculation,” Orzel said. “It is not at all clear how this will be solved, or even if it is possible to solve it with plausible technology in the near future. ”

Besides missing gravity, the model does not include a mechanism to give neutrinos their mass and does not incorporate black matter Where dark energywhich are the dominant forms of mass and energy in the universe.

However, even though the Standard Model is not complete, physicists do not have a widely accepted theory of how to extend it, and so it remains the best working description of subatomic physics ever devised.

Additional Resources

For more on the strong force in particular, see this podcast episode (opens in a new tab) by the author of the article Paul Sutter. Join Fermilab scientist Don Lincoln for a tour of the Standard Model in this video (opens in a new tab). For a popular overview on the subject, check out “The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics” (opens in a new tab) (Penguin Publishing Group, 2006), by physicist Robert Oerter.

Bibliography

Hoddeson, L. et al. “The Rise of the Standard Model: A History of Particle Physics from 1964 to 1979” (Cambridge University Press 1997)

Cottingham, WN and Greenwood, DA “An Introduction to the Standard Model of Particle Physics” (Cambridge University Press 2007)

Oerter, R. “The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics” (Pi Press 2006)

Bardin, D. and Passarino, G. “The Standard Model in the Making: Precision Study of Electroweak Interactions” (Clarendon Press 1999)

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