The charm quark is one 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 charm quark, so the 1 zeptometer shown above is not a measurement. A zeptometer is 10⁻²¹ meters, a billionth 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 charm quarks themselves. The explorer's 1 zeptometer 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 the upper limit for a quark were enlarged to the width of a tennis ball, about 6.7 centimeters (2.6 inches), a proton would be about 130 meters (430 feet) across. A hydrogen atom would be about 8,200 kilometers (5,100 miles) across, about two-thirds the width of Earth.
The upper limit for a quark is about 50 million billion (5 × 10¹⁶) times the Planck length, the nearest smaller entry in the explorer with a defined value.

The November Revolution

In November 1974, two teams working separately found the same new particle. Burton Richter's team at the Stanford Linear Accelerator Center called it psi, and Samuel Ting's team at Brookhaven National Laboratory called it J. On November 11, the two leaders met and realized they had made the same discovery. Richter and Ting shared the 1976 Nobel Prize in Physics.

The J/psi, as it is now called, is a charm quark bound to a charm antiquark, its antimatter partner. It lasted about a thousand times longer than physicists expected, and many concluded that explaining it needed a fourth kind of quark. Charm had already been predicted, and its discovery brought the known quarks to four.

Mass and charge

The Particle Data Group, which collects particle measurements from around the world, gives the charm quark's mass as 1.273 GeV. Particle physicists measure mass in units of energy, and a GeV is a billion electronvolts. That makes a charm quark about 1.4 times as heavy as a whole proton, and nearly 600 times as heavy as an up quark. Its electric charge is two-thirds of a proton's.

Like other quarks, charm quarks are never found alone. Every search for a free quark since 1977 has come up empty.

Sources

Summary Tables: Quarks. Particle Data Group, 2025.
Limits on the effective quark radius from inclusive ep scattering at HERA. ZEUS Collaboration, Physics Letters B, 2016.
The Nobel Prize in Physics 1976: Press release. Royal Swedish Academy of Sciences, 1976.
Revisiting the b revolution. CERN Courier, 2017.
January 1976: From the "Oops-Leon" to the Upsilon Particle. Dan Garisto, APS News, American Physical Society, 2022.
The Nobel Prize in Physics 1995: Press release. Royal Swedish Academy of Sciences, 1995.
SI prefixes. International Bureau of Weights and Measures.
Proton rms charge radius. NIST CODATA, 2022.
Proton mass energy equivalent in MeV. NIST CODATA, 2022.
Bohr radius. NIST CODATA, 2022.
Planck length. NIST CODATA, 2022.
Rules of Tennis 2026. International Tennis Federation, 2026.
Earth Fact Sheet. NASA.

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