How big is Top Quark? | Scale of the Universe

The top quark is the heaviest known elementary particle, as of 2026. An elementary particle is not made of anything smaller, as far as experiments can tell. The top quark has no measured size, so the 1 yoctometer shown above is a placeholder rather than a measurement. It stands for a size far smaller than any experiment can detect.

Size

A yoctometer is 10⁻²⁴ meters, a trillionth of a trillionth of a meter. No experiment has measured a size for the top quark, or for any other quark. In every test 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 studied collisions between electrons and protons at the HERA accelerator, and saw no sign that the quarks inside the protons had any size. In 2016 it reported that the radius of those quarks, if they have one, must be smaller than 4.3 × 10⁻¹⁹ meters. That is an upper limit, not a size, and it comes from the quarks found inside protons rather than from the top quark.

Size comparisons

The upper limit for the quarks inside protons, about 8.6 × 10⁻¹⁹ meters across, is about 860,000 times the explorer's figure.
A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 2,000 times as wide as that upper limit for a quark.
The explorer's 1 yoctometer is about 60 billion times the Planck length, 1.6 × 10⁻³⁵ meters, the next smaller entry in the explorer.
If 1 yoctometer were enlarged to the width of a human hair, about 0.1 millimeters, a proton would be about 170 kilometers across, and the hair would be about a light-year wide.

The heaviest known particle

The Particle Data Group, which collects the world's particle measurements, gives the top quark's mass as 172.56 ± 0.31 GeV. Particle physicists measure mass in these energy units, and a GeV is a billion electronvolts. The top quark's mass is close to that of a whole atom of gold. It is about 40 times the mass of the bottom quark, the next heaviest quark.

Being heavy does not make the top quark large. Mass and size are separate properties, and no quark, heavy or light, has shown any size in experiments.

A particle that barely lasts

Scientists on the CDF and DZero experiments announced the discovery of the top quark on March 2, 1995. They found it at Fermilab's Tevatron accelerator, which collided protons with antiprotons. Today the Large Hadron Collider at CERN makes top quarks in pairs, mostly from collisions between gluons, the particles that hold quarks together.

A top quark decays in less than 10⁻²⁴ seconds, into a W boson and a bottom quark. Other quarks join together into larger particles called hadrons, such as protons, but that takes several times 10⁻²⁴ seconds. The top quark decays before it can join any. It is too short-lived to observe directly, so experiments identify it from the particles it decays into.