The bottom quark, also called the beauty quark, is the second-heaviest of the six kinds of quark. Quarks are elementary particles, not made of anything smaller as far as experiments can tell, and they are the building blocks of protons, neutrons and many other particles. No experiment has measured a size for the bottom quark, so the 300 yoctometers shown above is not a measurement. A yoctometer is 10⁻²⁴ meters, a trillionth of a trillionth of a meter.

Size

In every experiment so far, quarks behave like points. Physicists look for a particle's size by firing other particles at it and watching how they bounce off. The ZEUS experiment did this at the HERA accelerator, colliding electrons with protons, and saw no sign that the quarks inside had any size.

In 2016, ZEUS reported that the radius of those quarks, if they have one, must be smaller than 4.3 × 10⁻¹⁹ meters. That makes them less than 8.6 × 10⁻¹⁹ meters across. It is an upper limit, not a size, and it comes from quarks inside protons rather than from bottom quarks on their own. The explorer's 300 yoctometers is far below this limit, so no experiment could confirm or rule it out.

Size comparisons

A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 2,000 times as wide as the upper limit for a quark.
If a proton were enlarged to the width of a basketball, about 24 centimeters (9.4 inches), a quark could be no more than about 0.12 millimeters across. That is a little wider than a human hair.
The upper limit for a quark is about 50 million billion (5 × 10¹⁶) times the Planck length, one of the bottom quark's nearest neighbors in the explorer.

Discovery

On June 30, 1977, scientists at the Fermilab accelerator laboratory announced a new particle called the upsilon. Their experiment, E288, was led by Leon Lederman. The upsilon is a bottom quark bound to a bottom antiquark, its antimatter partner. It was about three times as heavy as any particle found before.

The bottom quark was the first quark of a third family of particles. Its partner in that family, the top quark, was not found until 1995.

Heavy, and never alone

The Particle Data Group, which collects particle measurements from around the world, gives the bottom quark's mass as 4.183 GeV. Particle physicists measure mass in units of energy, and a GeV is a billion electronvolts. That makes the bottom quark about 4.5 times as heavy as a whole proton. Its electric charge is minus one-third of a proton's.

Quarks are never found alone: every search for a free quark has come up empty. Bottom quarks are always bound inside larger particles. At CERN, the LHCb experiment studies them to learn why the universe seems to be made almost entirely of matter, with almost no antimatter.

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