To make sense of cosmic rays, CERN team tracks a fragile nucleus

The hydrogen atom is the lightest in the universe and it consists of the simplest nucleus: a single proton. But while helium is the second-lightest element, its nucleus isn’t the second simplest. That distinction belongs to the deuteron, the nucleus of the deuterium atom, which contains one proton and one neutron. (Deuterium is an isotope of hydrogen.)

However, the two particles are bound with a very low binding energy, making deuterons seem fragile relative to the energetic, messy environment created when particles collide at nearly the speed of light at the Large Hadron Collider (LHC). Yet experiments have repeatedly observed deuterons (and anti-deuterons) emerging from these collisions. How do they survive this environment?

Coalescence scenario

Physicists have come up with two broad explanations. One, called direct emission, assumes the deuterons are produced directly from a hot source, meaning their formation doesn’t involve a different particle (or particles) condensing from a soup of energy, then decaying to form deuterons. The other idea, called coalescence, holds that the proton and neutron are produced first, then they stick together later if they get close enough.

The problem is a proton and a neutron can’t fuse if they have too much energy, so there has to be a third particle to carry away the excess energy. That third participant can be a type of particle called pion that acts like a catalyst, i.e. it will enable the reaction without becoming part of the final deuteron.

Finding out whether this is possible matters beyond collider physics. If scientists have to predict how light nuclei and antinuclei form in high-energy collisions in space — such as when cosmic rays strike interstellar gas — they need to know which formation mechanisms are possible and which ones nature doesn’t ‘allow’.

Delta resonance

A new study out of the ALICE collaboration at the LHC has provided the answer. The collaboration works with the ALICE detector, one of nine detectors on the LHC. At four points on its ring, the LHC smashes together opposing beams of protons at high energy. The collisions produce a morass of particles and reactions between them. The detectors have computers and sensors that are triggered when they identify reactions of interest and which they record and analyse.

For the new study, the ALICE collaboration used a technique called femtoscopy to infer how and when particles were produced by checking whether two particles come out with very similar velocities more often than chance would predict. Its basic object was a ratio called the correlation function. The numerator was how often a given pair (a pion and a deuteron) is seen with a small relative momentum. The denominator was how often such pairs would form assuming they had no affinity for each other.

The team was looking specifically for a particle called Δ(1232) resonance. (‘Δ’ is pronounced ‘delta’.) The resonance is a very short-lived excited version of a proton or a neutron that quickly decays. Δ(1232) in particular decays into a pion and a proton or neutron. If a deuteron is later formed using that same proton (or neutron), then the pion and the deuteron would appear to be ‘linked’ in the data by having a correlated momentum.

ALICE reported just this signal in the pion-deuteron data, meaning many of the deuterons are formed after the Δ decays, rather than directly at the start.

Where a nucleus is born

If the pions were only bumping into deuterons that already existed, some of those collisions should have broken the deuteron apart (which is fragile, remember). In that case, the data should’ve shown a dip around the Δ region. ALICE, however, found a positive signal, meaning deuterons were forming after Δ particles decayed and the pions from the same decay were correlated with the new deuterons.

Norwegian University of Science and Technology physics professor Michael Kachelriess called the finding a “great achievement” to Physics World.

From the size of the Δ signal, the team estimated that around 62% of deuterons were produced following Δ decays. They figured if they included other short-lived resonances as well, about 80% of deuterons would be formed by coalescence.

This seems to be the answer to why deuterons can survive the LHC’s high-energy collisions. Resonances usually travel a short distance before they decay, so the coalescence that forms deuterons happens slightly later and slightly away from the most violent part of the collision. Thus the deuterons are also ‘born’ into a less extreme environment.

Cosmic rays, dark matter

In sum, most deuterons aren’t born as readymade nuclei at the instant of collision but are instead assembled shortly afterwards from existing protons and neutrons, with help from pions nearby. This should change how theorists model the way nuclei (and anti-nuclei) are produced in high-energy particulate reactions.

In fact the ALICE team’s paper, published on December 10, noted that scientists can use this insight to build more realistic models of reactions induced by cosmic rays.

Cosmic rays are very energetic protons and atomic nuclei hurtling through space and which often collide with other nuclei in outer space. When scientists are studying telescope data of these collisions or are modelling them in the lab, e.g. for astronomy research or because they’re sending a probe to that part of space, they’ll need to know which signals are coming from which sources.

“These findings not only explain a long-standing puzzle in nuclear physics but could have far-reaching implications for astrophysics and cosmology,” the ALICE team said in a statement.

“Light nuclei and antinuclei are also produced in interactions between cosmic rays and the interstellar medium, and theymay be created in processes involving the dark matter that pervades the universe. By building reliable models for the production of light nuclei and anti-nuclei, physicists can better interpret cosmic-ray data and look for possible dark-matter signals.”

mukunth.v@thehindu.co.in