How Deuterons Survive High-Energy LHC Collisions

The deuteron, the nucleus of deuterium, is one of the simplest bound nuclear systems, consisting of just one proton and one neutron. Despite its very low binding energy, experiments at the Large Hadron Collider have repeatedly observed deuterons and anti-deuterons emerging intact from extremely energetic particle collisions. This long-standing puzzle has now been addressed by new experimental evidence.

Why Deuterons Seem Too Fragile

In high-energy proton–proton collisions, such as those at the LHC, matter is briefly converted into a dense, hot environment filled with strongly interacting particles. The deuteron’s weak binding makes it appear unlikely to survive such conditions. This led physicists to question whether deuterons are produced directly during the collision or assembled later from their constituent particles.

Direct Emission Versus Coalescence

Two main theories have been proposed. The direct emission model suggests deuterons are created immediately from the hot collision zone. The alternative coalescence scenario argues that protons and neutrons form first and later bind together if they are sufficiently close in space and momentum. However, coalescence requires a third particle, typically a pion, to remove excess energy and enable binding.

ALICE Experiment and Delta Resonance Evidence

A new study by the ALICE collaboration, using the ALICE detector at the LHC, has provided strong evidence in favour of coalescence. Using femtoscopy, researchers analysed correlations between pions and deuterons. They identified signatures of the short-lived Δ(1232) resonance, an excited state of a proton or neutron that decays into a pion and a nucleon. The observed momentum correlations indicate that many deuterons form after these resonances decay, not directly at the collision instant.

Important Facts for Exams

• Deuteron is the nucleus of deuterium, an isotope of hydrogen with one proton and one neutron.
• The Large Hadron Collider is located at CERN near Geneva and studies high-energy particle collisions.
• Δ(1232) resonance is a short-lived excited state of nucleons that decays into pions.
• Femtoscopy studies particle production using momentum correlations at very small scales.

Implications for Astrophysics and Cosmology

The ALICE team estimates that about 62% of deuterons form following Δ decays, rising to nearly 80% when other resonances are included. Because resonances decay slightly away from the most violent collision region, deuterons are effectively ‘born’ in a calmer environment. This insight reshapes models of light-nuclei formation, with implications for understanding cosmic-ray interactions and potential dark-matter signals in astrophysical observations.