Fourth-generation particles

Scientists have built a well-tested model of the building blocks of the universe. But experiments at the Large Hadron Collider could reveal additional types of subatomic particles that make up our world.

Everything we see – from coffee cups to computers to human beings – is made up of just three basic particles: two types of quarks that make up the nucleus of an atom and an electron that orbits that nucleus.

But scientists have found the particles most familiar to us have heavier cousins, other elementary particles that share some of their properties but have different masses. Over the past century, physicists have discovered 12 elementary particles, which they organized into the Standard Model of particle physics. It serves as a kind of periodic table of elements for particle physics.

Quarks, the particles that make up a nucleus, come in six varieties, called up, down, charm, strange, top and bottom, also known as beauty. Up and down quarks make up most of the matter around us. They are the lightest and are known as the first generation of quarks. Heavier charm and strange quarks make up the second generation, and heavier still, the top and bottom quarks form the third generation.

Similarly, scientists have found more massive copies of electrons: a second generation of particles they call muons and a third generation they call taus. These second- and third-generation particles are highly unstable and decay into members of the first generation.

Even elementary particles called neutrinos, which are almost massless and barely interact with other matter, come in three generations.

Theorists wonder why nature stopped there. Perhaps a fourth generation of particles exists. It is possible that experiments at the Large Hadron Collider will find fourth-generation particles too massive to have been created at previous particle accelerators. From experiment, scientists know for example that only three generations of light neutrinos can exist; a fourth generation neutrino would need to have at least about 50 times the mass of a proton.

Fourth-generation models could offer insight into some of the biggest mysteries in physics today, from the imbalance of matter and antimatter to the effects of dark matter to the origin of mass.

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