For the first time, neutrinos resulting from nuclear reactions caused by the Large Hadron Collider have been detected. Although physicists were sure that the reactions that occur when particles accelerate to near the speed of light and collide with each other produce neutrinos, getting evidence was a different matter. This achievement could help particle physicists solve some unknowns in the behavior of subatomic particles.
In the 1930s, physicists noticed that the products of many nuclear reactions carry less energy than the particles that preceded the reaction. This violated the law of conservation of energy, and the obvious explanation was that additional products were missing. Such particles, called neutrinos, must be very light — for a long time they were thought to be massless — and interact only very weakly with familiar objects. Otherwise, we would find them easier to spot.
Despite some disdain for the idea of undetectable objects that were only invented to solve a problem, it was confirmed that neutrinos were coming from a nuclear reactor in 1956, and the discovery won them a Nobel Prize. It has since been discovered to originate in the sun, interactions between cosmic rays and the upper atmosphere, and high-energy astronomical events such as supernovae.
“With every new source come new insights, with important implications for many fields, from particle physics to geophysics to astrophysics and cosmology,” writes the Collaboration for Advanced Research Experiment (FASER). Researchers have identified three types known as flavours. Although trillions pass through your body every second, it would take huge reservoirs of water buried underground to see the pathways created by the small percentage that produce or alter other molecules as it passes.
The numbers produced by machines like CERN’s Large Hadron Collider are naturally a fraction of those of astronomical origin, which makes the task of finding them much more difficult. However, this has now been done by two teams independently.
“Neutrinos are produced in abundance in proton colliders like the LHC,” said Christovao Vilela of the SND@LHC Collaboration. Phys.org. “However, until now, these neutrinos have never been directly observed. The very weak interaction of neutrinos with other particles makes them very difficult to detect, which is why they are the least well-studied particle in the Standard Model of particle physics.”
In fact, neutrinos are the only particles in the Standard Model, and thus confirmed to exist, that have not been detected by particle colliders.
The two teams took different approaches to neutrino capture. The FASER team positioned their detector along the beamline, so that the highest-energy neutrinos would pass through, and those that would continue along a similar path to the particles would pass through. Although they are still difficult to observe, high-energy neutrinos are much more likely to interact with matter than lower-energy ones.
The FASER detector consists of 730 sheets of tungsten, each 1.1 mm (0.044 in) thick, with emulsion films sandwiched between them. The team was rewarded with 153 detections above the background level, with energies of more than 200 billion electron volts, during five months of observations.
On the other hand, SND@LHC put its detector to one side and observed only eight candidate events. Both teams shielded their detectors with 100 meters of rock and concrete, which blocked out most of the other particles produced by the reaction. Neutrinos, with their low chance of interacting with all that mass, passed unharmed. However, Viella explained that for every neutrino. The SND@LHC detector picked up tens of millions of muons, which gave off very similar signals.
Tales of needles in haystacks do not do justice to trying to pick out neutrino interactions from those caused by muons.