'Ghostly' neutrinos detected for the first time inside the world's largest particle accelerator

For the first time, physicists have created and detected high-energy 'ghost particles' inside the world's largest atom smasher. The findings could help unlock the secrets of how stars go supernova.

The tiny particles, called neutrinos, were detected by the FASER neutrino detector at the Large Hadron Collider (LHC) - the world's largest particle accelerator, located at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland.

Neutrinos get their spectral nickname because their non-existent electric charge and near-zero mass mean they barely interact with other types of matter.

Illustration of three neutrinos, ghostly particles that barely interact with other forms of matter.

True to its spooky nickname, neutrinos fly through ordinary matter at nearly the speed of light. Physicists presented their findings at the 57th Rencontres de Moriond Electroweak Interactives and Unified Theories conference in La Thuile, Italy on March 19.

'We detected neutrinos from a completely new source — a particle collider — where two beams of particles smash together at extremely high energies,' Jonathan Feng, a physicist at the University of California Irvine and co-spokesperson of the FASER collaboration, said in a statement.

Every second, about 100 billion neutrinos pass through every square centimeter of your body. These tiny particles are everywhere. They are created in the nuclear fires of stars, in giant supernova explosions, by cosmic rays and radioactive decay, and in particle accelerators and nuclear reactors on Earth.

In fact, neutrinos, first detected escaping from a nuclear reactor in 1956, are second only to photons as the most abundant subatomic particles in the universe.

Yet despite their ubiquity, the minimal interactions of these uncharged and nearly massless particles with other matter make them very difficult to detect.

Despite this, many famous neutrino-detecting experiments—such as Japan's Super-Kamiokande detector, Fermilab's MiniBooNE, and the IceCube detector at the South Pole—have been able to detect neutrinos produced by the Sun.

Neutrinos from supernova explosions

But the neutrinos that reach us from the Sun are just a small fraction of the ghostly particles out there. At the other end of the energy spectrum are the high-energy neutrinos created in giant supernova explosions and in showers of particles as they crash into Earth's atmosphere from deep space. These high-energy ghosts remain a mystery to scientists to this day.

'These very high-energy neutrinos in the LHC are crucial to understanding some really exciting observations in particle astrophysics, ' said Jamie Boyd, CERN particle physicist and FASER co-spokesperson. ' The new findings could help explain how stars burn and explode, and how high-energy neutrino interactions spark the production of other particles in space.'

Device to 'catch' neutrinos

To catch the specter of atoms, physicists built a particle detector: dense metal plates of lead and tungsten sandwiched between layers of a light-detecting liquid called an emulsion. As high-energy proton beams smash into each other inside the LHC, they create a shower of byproduct particles, a small fraction of which are neutrinos, which enter the LHC.

By "growing" this membrane-like emulsion and analyzing the particle tracks, the physicists discovered that some of the tracks were created by jets of particles produced by neutrinos passing through the plates.

The six neutrinos detected by the experiment were first identified in 2021. But it took physicists two years to collect enough data to confirm they were real. Now they hope to find more, and think they could be used to probe environments in the universe where high-energy ghost particles are created.