How Deuterons Survive Extreme Particle Collisions

Why in the News ?

A recent study by the ALICE collaboration at CERN’s Large Hadron Collider (LHC) has resolved a long-standing puzzle about how fragile deuterons survive ultra-high-energy particle collisions, with implications for cosmic ray physics, astrophysics, and dark matter research.

Fragile Deuterons in a Violent Collider Environment:

• The deuteron, nucleus of deuterium (hydrogen isotope), consists of one proton and one neutron, bound by very low binding energy.
• Despite this fragility, deuterons and anti-deuterons are repeatedly observed in collisions at the Large Hadron Collider (LHC), where particles move close to the speed of light.
• This created a paradox: such weakly bound nuclei should break apart in the extremely energetic collision environment.
• Physicists proposed two explanations — direct emission, where deuterons form instantly from a hot source, and coalescence, where protons and neutrons combine later.
• Understanding the correct mechanism is crucial not only for nuclear physics, but also for modelling high-energy reactions in outer space, such as cosmic ray–interstellar gas collisions.

ALICE Experiment and the Coalescence Breakthrough

• Scientists from the ALICE Collaboration used the ALICE detector, one of the major experiments at the Large Hadron Collider, operated by CERN.
• Using a technique called femtoscopy, researchers studied momentum correlations between pions and deuterons.
• They searched for signals of the Δ(1232) resonance, a short-lived excited state of a proton or neutron that decays into a pion and nucleon.
• The data showed a positive correlation, indicating that deuterons form after these decays, not during the initial collision.
• The study estimated that ~62% of deuterons form after resonance decays, rising to ~80% when other short-lived resonances are included — strongly supporting the coalescence model.