Deep Dive into LHC: What Lies Inside a Quark?

Scientists at the Large Hadron Collider have probed the internal structure of quarks to unprecedented precision, testing whether these fundamental particles might be composed of even smaller constituents. Using data from the CMS experiment, researchers examined quark scattering at distances as small as 10⁻²⁰ meters, finding no evidence of internal structure and reinforcing the current view of quarks as point-like particles in the Standard Model of particle physics.

The investigation follows a historical pattern in physics where particles once considered fundamental were later found to have internal complexity. Just as atoms were shown to contain nuclei, and protons and neutrons were revealed to consist of quarks, the CMS Collaboration sought to determine if quarks themselves might harbor deeper layers of composition. To probe this possibility, scientists analyzed the scattering patterns of quarks produced in high-energy proton collisions at the LHC.

When protons collide within the CMS detector, they break apart into their constituent quarks. These outgoing quarks produce collimated sprays of particles known as jets, which can be measured and used to reconstruct the scattering angle between the original quarks. By studying the distribution of these scattering angles, researchers can infer whether quarks behave as point-like entities or reveal signs of internal structure through deviations in the expected pattern.

The CMS team reported that their measurements showed no significant disagreement with the scattering distribution expected for point-like quarks. This result holds across the tested range down to 10⁻²⁰ meters — a scale two orders of magnitude smaller than previous probes — indicating that if quarks do have internal structure, it must exist at even smaller distances or be too faint to detect with current techniques.

Despite the null result, the scientists emphasized that the absence of evidence does not constitute evidence of absence. Drawing a parallel to Ernest Rutherford’s gold foil experiment, which uncovered the atomic nucleus by detecting unexpected scattering at large angles, the CMS researchers noted that history has shown structures once deemed fundamental can later reveal hidden complexity. The current findings constrain theoretical models that predict quark substructure but do not rule them out entirely.

The study contributes to an ongoing effort at the LHC to test the limits of the Standard Model by examining the behavior of matter at the smallest accessible scales. While no deviation from point-like behavior was observed, the precision achieved represents a significant advancement in experimental capability, enabling future searches for phenomena beyond established physics, including potential signs of quark compositeness or new forces operating at ultra-short distances.