The ALICE experiment at CERN has unravelled the formation of light atomic nuclei that are occasionally produced in collisions in the LHC accelerator. They are publishing their findings today in the scientific journal Nature.
Particle collisions in the Large Hadron Collider (LHC) can reach temperatures more than a hundred thousand times higher than those at the centre of the sun. Yet light atomic nuclei and their antimatter counterparts somehow emerge unscathed from this scorching environment, even though one would expect the bonds holding the nuclei together to break at much lower temperatures.
Physicists have wondered for decades how this is possible, but now the ALICE collaboration has provided experimental evidence for how it happens.
ALICE researchers studied deuterons (a proton and a neutron bound together) and antideuterons (an antiproton and an antineutron) produced in high-energy proton collisions in the LHC. They found evidence that nearly 90 percent of the deuterons and antideuterons did not originate directly from the collisions, but were formed by the nuclear fusion of particles resulting from the collision, with one of the constituent particles originating from the decay of a short-lived particle.
Important gap
‘These results are a milestone for the field,’ said Marco van Leeuwen, spokesperson for the ALICE experiment and affiliated with Nikhef. ‘They fill an important gap in our understanding of how nuclei are formed from quarks and gluons and provide essential input for the next generation of theoretical models.’ Nikhef is one of the major partners in the ALICE experiment.
These findings not only explain a long-standing puzzle in nuclear physics, but may also have far-reaching consequences for astrophysics and cosmology. Light nuclei and antinuclei are also produced in interactions between cosmic rays and the interstellar medium, and they can be created in processes involving dark matter that permeates the universe.
By building reliable models for the production of light nuclei and antinuclei, physicists can better interpret cosmic ray data and search for possible signals of dark matter.
Decay processes
‘This observation by ALICE provides a solid experimental basis for modelling the formation of light nuclei in space,’ said Maximilian Mahlein, a researcher with the ALICE collaboration at the Technical University of Munich. ‘It tells us that most of the light nuclei we observe are not formed in a single thermal burst, but through a series of decay processes and fusions that take place as the system cools down.’
The ALICE collaboration reached these conclusions by analysing the deuterons produced by high-energy proton collisions recorded during the second run of the LHC. The researchers measured the momenta of deuterons and pions, another type of particle consisting of a quark-antiquark pair.
They discovered a correlation between the momenta of pions and deuterons, suggesting that the pions and the protons or neutrons of the deuterons actually originate from the decay of a short-lived particle.
Delta resonance
This short-lived particle, known as the delta resonance, decays in about one trillionth of a trillionth of a second into a pion and a nucleon, i.e. a proton or a neutron. The nucleon can then fuse with other nearby nucleons to produce light nuclei, such as a deuteron. This nuclear fusion takes place at a short distance from the main collision point, in a cooler environment, giving the newly formed nuclei a much greater chance of survival.
These results were observed for both particles and antiparticles, indicating that the same mechanism governs the formation of deuterons and antideuterons.
‘The discovery illustrates the unique capabilities of the ALICE experiment to study the strong nuclear force under extreme conditions,’ said Laura Fabbietti, ALICE physicist and professor at the Technical University of Munich.
Source: CERN
