First neutrinos made by particle collider detected

Discovery promises to help physicists understand the nature of the universe's most abundant particle

22 March 2023

An international research team with participation from Johannes Gutenberg University Mainz (JGU) has for the first time detected neutrinos created by a particle collider at very high energies. For this purpose, the researchers evaluated measurements from the new experiment FASER, which took data for the first time in 2022 at CERN with the start of the third run of the Large Hadron Collider (LHC). The results were presented at the 57th Rencontres de Moriond conference.

Neutrinos are ubiquitous elementary particles that are produced, among other things, during fusion processes in the sun or radioactive decays in nuclear reactors. They were first discovered in 1956. "Neutrinos are the weakest interacting elementary particles. Billions of them pass through our bodies every second without us noticing. That is why neutrinos are also called ghost particles," explained Professor Matthias Schott, experimental physicist at the PRISMA+ Cluster of Excellence at Mainz University. "Neutrinos are also produced billions of times during particle collisions in accelerators, where you have two beams of particles smashing together at extremely high energy. But until now we have never been able to detect them. We have now succeeded in doing so for the first time with FASER. From these experiments, we hope to gain further insight into the nature of these mysterious particles, especially in terms of their mass – a great mystery of modern physics," said Schott.

A new source of neutrinos

With their experiment, the researchers have thus detected neutrinos from a completely new – artificial – source. The neutrinos detected by FASER are the most energetic ever produced in a laboratory; a total of 153 neutrino events were detected in the current study. The work could therefore shed light on cosmic neutrinos that travel great distances before hitting Earth, opening a window to distant parts of the cosmos that would otherwise be inaccessible.

However, scientists are taking an even broader approach with FASER, short for "ForwArd Search ExpeRiment", to generally search for new light and weakly interacting particles that might be produced when protons collide. "With a five-meter detector, FASER is a comparatively small experiment," said Professor Matthias Schott. "And yet a very valuable addition to the physics program at CERN."

This is because the four large LHC detectors cannot "see" such particles – including neutrinos – as they fly unimpeded through the detector material and thus escape undetected. Only hundreds of meters further on could they transform into detectable particles, such as electrons and positrons, and thus become visible to detectors.

For this reason, the FASER collaboration has set up its detector 480 meters away from the point where protons collide with great force in the ATLAS detector – in an unused service tunnel of the LHC. This is located in extension of the beam axis, i.e., in almost direct path to the collision point, because the light particles fly away mainly in this direction. The construction was completed in the record time of only about one year between May 2020 and April 2021. The first data was then to be taken at the beginning of the third LHC run last summer. Parts of the FASER detector were fabricated by Matthias Schott's group at the PRISMA+ detector laboratory and brought to the CERN research center for assembly in 2021.

"After only a short runtime, FASER has already delivered an impressive result", summarized Professor Matthias Schott, who is also chair of the Collaboration Board of FASER. "In addition to neutrinos, which occur in the Standard Model of particle physics, we want to use it mainly to search for particles that go beyond the Standard Model – first and foremost, the mysterious dark matter. It makes up by far the largest fraction of matter in the universe, but we have never been able to detect it directly."

The FASER collaboration consists of 85 members from 22 institutions and 9 countries.