⚛️ Probing the inside of quarks

According to our current understanding of the Universe, quarks are point-like particles, known as "fundamental," meaning they are not themselves made up of smaller particles.

A recent article published by the CMS collaboration at the LHC describes how its teams probed quarks at a scale of 10^-20 meter (3.9 × 10^-19 inches) to test this hypothesis.


At this scale, no hint of possible basic constituents was observed, but history shows that structures once thought fundamental can reveal deep substructures: we thus discovered that matter is made of molecules, themselves composed of atoms, themselves consisting of a dense nucleus surrounded by a cloud of electrons.

Rutherford discovered the atomic nucleus by firing a beam of particles (helium nuclei) at a target made of a gold leaf. He found that these particles were deflected at different angles due to the structure of gold atoms. Rutherford then measured the angles of the deflected particle paths.

By studying the distribution of these angles (the scattering angles), he was able to prove that atoms contain a point-like nucleus at their center. This experiment could be performed because the helium beam used in the experimental setup had sufficient energy to probe the inside of atoms.

Later, it was shown that the nucleus consists of protons and neutrons, which themselves are made of quarks. The LHC experiments, including CMS, continue this research today by colliding particles at extremely high energies to probe the possible internal structure of quarks.

When two proton beams collide in CMS, the quarks composing them scatter into two jets—or particle showers—that can be measured and used to reconstruct the scattering angle between the quarks.

The distribution of scattering angles between the two jets can then be compared to the distribution that would be expected if the quark were indeed a point-like particle. The recent results from the CMS collaboration, based on data from the second run of the LHC, showed no significant discrepancy with the scattering distribution of a point-like particle. This means that, if quarks are composite structures, their size is unlikely to exceed 10^-20 meter (3.9 × 10^-19 inches).

The size is estimated from constraints on the energy scale at which the composite nature of the quark might reveal itself. Regarding the reference model used for the recent CMS paper, which assumed a composite quark, the latest results set the most stringent limit to date at 37 TeV.

Just as Rutherford was able to identify the building blocks of the atom because his particle beam had sufficient energy, studying particle collisions at higher energies could allow us to detect potential smaller structures inside quarks.

Data from the third run of the LHC and the future High-Luminosity LHC could help reduce uncertainties in the measurement of the scattering angle, thereby enabling the detection of even smaller structures and continuing the quest for the smallest "building blocks" of matter.