Mechanism of color confinement in Quantum Chromodynamics

Determine the fundamental mechanism of color confinement within Quantum Chromodynamics that prevents the observation of free quarks and gluons as asymptotic states, and characterize how this mechanism manifests in hadron properties and spectra.

Background

Quantum Chromodynamics is the established theory of strong interactions, describing quarks and gluons as colored degrees of freedom. Despite decades of experimental validation and theoretical progress, the phenomenon of color confinement—the empirical fact that quarks and gluons are never observed in isolation—lacks a definitive, universally accepted theoretical explanation. This gap in understanding is central to hadron physics and underpins many open directions in nonperturbative QCD.

Resolving the confinement mechanism would clarify how observable hadrons emerge from elementary colored constituents and connect to related phenomena such as dynamical chiral symmetry breaking. The Electron-Ion Collider program is motivated, in part, by the need to probe the nonperturbative regime where confinement operates, linking theory to precise measurements of hadron structure.

References

At the core of this theory, however, lies the phenomenon of color confinement which is still not understood.

— The glue that binds us all -- Latin America and the Electron-Ion Collider  (2409.18407 - Aguilar et al., 2024) in Section 1, Hadron Physics in the wake of the EIC

Although the connection has not yet been proven to everybody's satisfaction, this is presumably related to the phenomenon of ``confinement'': quarks and gluons can only exist within bound states, i.e. in hadrons like the proton or pion; they do not exist as free particles, unlike the electron or photon which can travel over macroscopic distances.

— Resolved Photon Processes: A Tribute to Rohini Godbole  (2608.17864 - Drees, 18 Aug 2026) in Section 1, Introduction: What Are Resolved Photons?

The result was negative , but given that other searches for quarks had already been conducted for nearly five years prior, the question arose: do they even exist in free form? This question remains unanswered and is known as the "confinement problem," the supposed solution of which states that the physical spectrum does not directly correspond to the system of elementary gauge fields.

— Hadron interactions in the U-70 energy range: results and problems  (2608.30605 - Petrov et al., 31 Aug 2026) in Section “Quest for quarks”