The ghost is finally actually in the machine: Scientists have created neutrinos in a particle collider for the first time. These abundant but mysterious subatomic particles are so removed from the rest of matter that they glide through it like ghosts, earning them the nickname “ghost particles.”
Scientists say this work represents the first direct observation of accelerator neutrinos and will help us understand how these particles are formed, their properties and their role in the evolution of the universe.
The results achieved with the FASERn detector at the Large Hadron Collider were presented at the 57th Rencontres de Moriond Electroweak Interactions and Unified Theories in Italy.
“We discovered neutrinos from a brand new source – a particle accelerator – where two beams of particles are smashed together at extremely high energy,” says particle physicist Jonathan Feng of the University of California Irvine.
Neutrinos are among the most abundant subatomic particles in the universe, after photons. But they have no electric charge, their mass is almost zero, and they barely interact with the other particles they encounter. Hundreds of billions of neutrinos are streaming through your body right now.
Neutrinos are produced under energetic conditions such as nuclear fusion occurring inside stars or supernova explosions. And while we may not notice them every day, physicists believe that their mass—however small—probably affects the universe’s gravity (although neutrinos have been largely ruled out as dark matter).
Although their interaction with matter is small, it is not completely non-existent; every now and then a cosmic neutrino collides with another particle and produces a very faint flash of light.
Underground detectors, isolated from other sources of radiation, can detect these explosions. IceCube in Antarctica, Super-Kamiokande in Japan and MiniBooNE at Fermilab in Illinois are three such detectors.
However, neutrinos produced in particle accelerators have long been sought by physicists because their high energies are not as well studied as low-energy neutrinos.
“They can tell us about deep space in ways we won’t learn otherwise,” says particle physicist Jamie Boyd of CERN. “These very high-energy neutrinos at the LHC are important for understanding really exciting observations in particle astrophysics.”
FASERnu is an emulsion detector consisting of millimeter tungsten plates alternating with layers of emulsion film. Tungsten was chosen because of its high density, which increases the probability of neutrino interaction; the detector consists of 730 emulsion films and a total tungsten weight of around 1 ton.
During particle experiments at the LHC, neutrinos can collide with nuclei in tungsten plates, producing particles that leave tracks in emulsion layers, a bit like how ionizing radiation creates tracks in a cloud chamber.
These plates need to be developed as photographic film before physicists can analyze the particle tracks to determine what created them. Six neutrino candidates were identified and published as early as 2021. Now researchers have confirmed their discovery using data from the third run of the upgraded LHC, which began last year, with a significance level of 16 sigma.
This means that the probability that the signals were created by random chance is so low as to be next to nothing; a significance level of 5 sigma is sufficient to qualify as a discovery in particle physics.
The FASER team is still hard at work analyzing the data collected by the detector, and it seems likely that many more neutrino detections will follow. The LHC 3 run is expected to continue until 2026, and data collection and analysis is still ongoing.
In 2021, UC Irvine physicist David Casper predicted the run would produce around 10,000 neutrino interactions, meaning we’ve barely scratched the surface of what FASERn has to offer.
“Neutrinos are the only known particles that much larger experiments at the Large Hadron Collider cannot directly detect,” he says, “so a successful FASER observation means that the accelerator’s full physics potential is finally being exploited.”
The team’s results were presented at the 57th Rencontres de Moriond Electroweak Interactions .
