CMS looks deep inside quarks

17 April 2026

Photo: A.Iqbal/CMS

Protons were shown to consist of quarks in 1968, but the question of whether quarks are made of even smaller particles is as yet unanswered.

The CMS Collaboration has probed inside quarks to the scale of 10^(-20) metres to search for potential building blocks within them

According to our current understanding of the Universe, quarks are considered to be  fundamental, point-like particles: basic building blocks which are not made up of smaller particles. A recent paper from the CMS Collaboration describes how it probed quarks to the scale of 10^-20 m to test this premise.

At this scale, no evidence of constituent particles was identified, but history has demonstrated that structures once considered fundamental can reveal deeper layers. Matter was found to consist of molecules, which were shown to be made of atoms, which were then found to consist of a dense nucleus surrounded by a cloud of electrons.

Rutherford discovered the nucleus by sending a beam of helium nuclei towards a gold foil target. These nuclei scattered off the gold atoms that made up the foil at a range of angles, which Rutherford then measured. By studying the distribution of the scattering angles, he was able to prove that atoms contained a point-like nucleus at the centre. This was possible because the helium beam in the experimental setup had enough energy to probe the inside of the atoms.

The nucleus was then shown to be made of protons and neutrons, which were later found to consist of quarks. LHC experiments, including CMS, now continue this quest, colliding particles at extremely high energies to probe the potential inner structure of quarks.

When two beams of protons collide within CMS, they break apart into their constituent quarks. These outgoing quarks become two jets – sprays of particles – that can be measured and used to reconstruct the scattering angle between the quarks.

The distribution of the scattering angle between the two jets can be compared to the distribution that would be expected if the quark was a point-like particle. The recent results from the CMS collaboration, which were based on data from the second run of the LHC,  showed no significant disagreement from the scattering distribution of a point-like quark. This means that quarks are not likely to be larger than 10^-20metres  if they are composite structures..

This size estimate is derived from the constraints on the energy scale where quark ‘compositeness’ reveals itself. For the benchmark model of the recent CMS paper, which assumed quarks were composite , the recent results set the most stringent limit to date at 37 TeV.

Similarly to how Rutherford could only identify the components of the atom because his beam of particles had enough energy, studying particle collisions with higher energies could help us resolve smaller potential structures within quarks. Data from the third run of the LHC and the upcoming High-Luminosity LHC could help reduce the uncertainties on the measurement of the scattering angle,  allowing us to identify even smaller structures and continue the search for the smallest building blocks of matter.

A leading role in the analysis was taken by QU key researcher Andreas Hinzmann from DESY, engaged in the quest for quark compositeness since the start of the LHC.
This work addresses one of the most fundamental questions regarding the composition of matter in the early phase of the quantum universe.