Fundamental Particles of Matter


By Robert Hazen, Ph.D.George Mason University

The structure of matter at its tiniest scale is much smaller than the atom. Atoms are composed of even smaller particles, called quarks and leptons. Particle physicists have invented lots of names for the fundamental particles of matter. There are four major names, four major groups: leptons, hadrons, quarks and antimatter.

Diagram of fundamental particles which compose an atom.
Each atom is composed of many different fundamental particles. (Image: Emir Kaan/Shutterstock)

Leptons: Electrons and Neutrinos

Leptons are a class of six different particles that can’t exist in the nucleus, that fly away from the nucleus when they’re produced. The negatively charged electron flies out of the nucleus during beta decay. The electrically neutral neutrino also flies out of the nucleus when you have a beta decay or other kinds of nuclear reactions. 

These are particles—the electron and neutrino—that cannot exist in the nucleus. The remaining two particles are other brands of neutrinos, the tau neutrino and the mu neutrino, also electrically neutral particles. There’s strong evidence now that the neutrinos actually do have mass because it appears that they spontaneously oscillate, or switch, from one kind of neutrino to another. 

According to the solar neutrino problem, you only see one-third of the neutrinos you expect. It’s believed that it is because the normal, ordinary neutrinos oscillate to tau and mu. So you have one-third of each kind, and so we only see a third as many neutrinos as we’d expect, suggesting that neutrinos have mass.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Hadrons and Quarks

An image of a hadron collider.
Hadrons consist of neutrons and protons as well as many other particles. (Image: Yurchanka Siarhei/Shutterstock)

The second group is hadrons. Hadrons are all the varied particles that reside in the atom’s nucleus. It includes protons, neutrons and lots of other stuff.

Indeed, there are hundreds of other hadrons, some of which are extremely short-lived, only a few milliseconds, or fractions of a second. This diversity suggested to many physicists that there are more fundamental particles that must make up those hadrons. 

That’s where the third group, quarks come in.

Quarks were first proposed in the late 1960s by the American physicist Murray Gell-Mann. Quarks are a curious collection of six particles. They combine, always in twos or in threes, to form the hadrons. 

Quarks have fractional charge; no other particles seem to have that. Quarks have charges, for example, of minus one-third or plus two-thirds. These six quarks differ from each other in their mass, their charge and other properties. And they’re given fanciful names, like the top and the bottom quark, and the up and the down quark.

Learn more about the ultimate structure of matter.

Demonstrating the Existence of Quarks

The existence of quarks is demonstrated experimentally by scattering electrons off hadrons. The electrons are repelled by the quarks, and so you see some very interesting scattering patterns; some electrons actually bounce back at you. This indicates the point charges, the quarks, are there.

This discovery was described beautifully by Michael Riordan in a book called The Hunting of the Quark. In May 1994, the sixth, the last of the quarks and the most massive, called the top quark, was discovered; and this picture of our universe with six leptons and six quarks seemed to have solidified. 

Positrons and Symmetry Breaking

In 1932, a young graduate student at CalTech, Carl Anderson, performed a cosmic-ray experiment in which collisions produced streams of particles. A few of these particles had the exact same mass as the electron. They looked for all the world like electrons; but when you turned on a magnetic field, the electrons spun off one way, and these particles spun off the opposite way, indicating that instead of having a minus one charge like an electron, they had a positive one charge. These were called positrons. 

This is an example of what’s called symmetry breaking in particle physics. When you have two particles that look identical, but when you apply some external force or stimulus and they go off and do different things, that’s symmetry breaking. You’ve broken the symmetry, and therefore, revealed a new kind of particle. Another attribute of this is when the positron combines with an electron, you get a complete annihilation of both particles, and only energy remains.

These are the examples of the fourth group, antimatter.

Learn more about semiconductors and modern microelectronics.

How Positron-Emission Tomography Work

Antimatter is not just an obscure and arcane subject. We use antimatter in modern technology; for example, in the medical technique of positron-emission tomography.

Set-up for Positron-emission tomography in an isolated room.
Tomography can measure the areas of the brain that positrons are being emitted from. (Image: helloabc/Shutterstock)

This is a diagnostic technique where patients are fed a glucose solution, a sugar solution with energy rich molecules that have been synthesized with an unstable isotope of oxygen; an oxygen isotope that emits positrons. 

Glucose is an energy supply and is carried to cells in your body, particularly cells in your brain that are active and are working. Different parts of your brain work at different times; when you’re doing mathematical calculations, or when you’re reading, or when you’re sleeping, different parts of the brain are active. 

Positron-emission tomography measures the places in your brain where positrons are being emitted from, and therefore it can test brain function; it can look for brain abnormalities. It’s a really important way of testing how the brain is functioning.

Common Questions about the Fundamental Particles of Matter

Q: What are leptons? 

Leptons are a class of six distinct particles that can’t exist in the nucleus. They fly away from the nucleus as soon as they’re produced.

Q: What are hadrons? 

Hadrons form a group of fundamental particles. Hadrons are all the varied particles that reside in the atom’s nucleus, including protons, neutrons and lots of other stuff.

Q: What are quarks? 

Quarks are a curious collection of six particles. They combine, always in twos or in threes, to form the hadrons. Quarks have fractional charge; no other particles seem to have that.

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