The down quark is one of the two kinds of quark that make up protons and neutrons, along with the up quark. No experiment has measured a size for it, so the figure shown is not a measurement. Experiments have found only that a quark, if it has any size at all, is no more than about a two-thousandth of the width of a proton.

Size

The size line reads 8 zeptometers. A zeptometer is 10⁻²¹ meters, a thousandth of a billionth of a billionth of a meter. The notation 10⁻²¹ means 1 divided by a 1 followed by 21 zeros. No experiment has measured a size for any kind of quark.

To look for a size, physicists fire particles at a target and study how they bounce off. At the HERA accelerator, the ZEUS experiment collided electrons with protons and saw no sign that the quarks inside had any size. In 2016 it reported that a quark's radius must be less than 4.3 × 10⁻¹⁹ meters. That makes a quark less than about 8.6 × 10⁻¹⁹ meters across.

That figure is an upper limit, not a size. It is about 110 times the explorer's figure of 8 zeptometers. Experiments therefore do not rule out the explorer's figure, but nothing supports it either.

Size comparisons

A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 2,000 times as wide as the largest size experiments allow for a quark.
If a quark at that limit were enlarged to the size of a basketball, about 24 centimeters (9.4 inches) across, a proton would be about 470 meters (1,500 feet) across.
At the same scale, a hydrogen atom would be about 30,000 kilometers (18,000 miles) across, more than twice the width of Earth.
About 120 million quarks at that limit, lined up side by side, would span a hydrogen atom, which is about 0.1 nanometers (a tenth of a billionth of a meter) across.

How a down quark changes

A neutron on its own, outside an atomic nucleus, is unstable. On average it lasts about 15 minutes. Then it decays into a proton, an electron and an antineutrino, the antimatter partner of a neutrino. This process is called beta decay.

A neutron is made of one up quark and two down quarks, and a proton of two up quarks and one down quark. In beta decay, the weak force turns one of the neutron's down quarks into an up quark. The neutron gives off a W boson, a particle that carries the weak force, and the W boson turns into the electron and the antineutrino.

A down quark carries −1/3 of a proton's electric charge, and an up quark carries +2/3. The change raises the quark's charge by 1, and the electron's charge of −1 balances it.

How quarks were found

During the 1950s, physicists discovered a large number of new particles and searched for a pattern among them. The idea of quarks was proposed in 1964, with three kinds: up, down and strange. The physicist Murray Gell-Mann gave quarks their name. For years none could be found, and many physicists regarded them as mathematical ideas only.

The first traces came from experiments begun in 1967 by physicists from SLAC (the Stanford Linear Accelerator Center) and MIT. They fired electrons along an accelerator about 3.2 kilometers (2 miles) long at protons and neutrons. Many more electrons bounced off at large angles than expected, a sign of small, hard grains inside. Jerome Friedman, Henry Kendall and Richard Taylor shared the 1990 Nobel Prize in Physics for this work.

Sources

Summary Tables: Quarks. Particle Data Group, 2025.
Summary Tables: Baryons. Particle Data Group, 2025.
Limits on the effective quark radius from inclusive ep scattering at HERA. H. Abramowicz et al. (ZEUS Collaboration), Physics Letters B, 2016.
Introduction to Particle Physics. In University Physics Volume 3. OpenStax, 2016.
Quarks. In University Physics Volume 3. OpenStax, 2016.
The Standard Model. In University Physics Volume 3. OpenStax, 2016.
The Z boson. CERN.
DOE Explains: Quarks and Gluons. U.S. Department of Energy, Office of Science.
The Nobel Prize in Physics 1990: Press release. Royal Swedish Academy of Sciences, 1990.
SI prefixes. International Bureau of Weights and Measures.
Proton rms charge radius. NIST CODATA, 2022.
Bohr radius. NIST CODATA, 2022.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.
Earth Fact Sheet. NASA, 2024.

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