20. March 2023

Collider neutrinos observed for the first time Collider neutrinos observed for the first time

International study helps to better understand the nature of the nearly massless elementary particle

Neutrinos are among the most abundant particles in the cosmos, but still pose many mysteries to researchers. An international team with participation of the University of Bonn has now for the first time directly observed neutrinos produced in a particle accelerator. The physicists hope that their new discovery will enable them to better understand the nature of these almost massless elementary particles. The results were presented last weekend at the 57th Moriond Conference in Italy and will soon be submitted for scientific peer review in the journal Physical Review Letters.

FASER Particle detector
FASER Particle detector - The FASER particle detector located deep underground at CERN's Large Hadron Collider (LHC). © CERN
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Neutrinos were first discovered in 1956 and, among other things, play a key role in the process that makes stars burn. Scientists hope to gain new insights into energetic objects in the universe by studying neutrinos.

"Neutrinos are particles that we know exist," says Prof. Dr. Florian Bernlochner of the Physikalisches Institut of the University of Bonn. "They were very important for the development of the Standard Model of particle physics. But until now, no neutrino produced at a particle accelerator has ever been directly detected by an experiment."

In the current study, Bernlochner and his colleagues now detected neutrinos from an entirely new source - particle accelerators, in which two beams of particles collide at extremely high energy to produce the neutrinos. It is the latest result of the Forward Search Experiment, or FASER. This is a particle detector installed at the CERN nuclear research center in Geneva, Switzerland. There, FASER detects particles produced by CERN's Large Hadron Collider (LHC). Compared to typical particle detectors, FASER is distinguished by the fact that its setup is relatively small - it fits into a small side tunnel at CERN.

Accelerators like the LHC produce abundant neutrinos and antineutrinos of all types - even at very high energies, where neutrino interactions have not yet been observed. One reason why accelerator neutrinos have remained undetected until now is their extremely weak interactions. In addition, the neutrinos with the highest energy, where the interaction probability is greatest, are produced predominantly parallel to the beam axis. Accelerator detectors typically have holes there to allow the colliding particle beams to pass.

More than 150 neutrino events observed

For the current study, 153 neutrino events were detected in LHC collision data recorded between July and November 2022. The detected neutrinos interact with a so-called emulsion detector with tungsten plates, transforming into muons, other elementary particles. These in turn can be detected with the FASER detector and its spectrometer.

Most neutrinos studied by physicists to date are low-energy. "However, the neutrinos detected by FASER are the highest-energy neutrinos ever produced in a laboratory and are similar to the neutrinos found when particles from space trigger so-called particle showers in our atmosphere," says FASER group leader Florian Bernlochner. These very high-energy neutrinos in the LHC are important for understanding puzzling observations in particle astrophysics, he said. "They can tell us something about space that we can't learn in any other way," says Tobias Böckh of the Physikalisches Institut of the University of Bonn, who was involved in the study as part of his PhD thesis. The work could also shed light on cosmic neutrinos that travel great distances and collide with Earth.

In follow-up studies, the researchers now plan to investigate other properties of the neutrinos. They should provide new insights into the interactions of the ghostly particles at high energies.

Participating institutions:

The study involved 21 universities worldwide and CERN in Geneva.

Detected neutrino candidate in the FASER detector
Detected neutrino candidate in the FASER detector - In the rear part a muon can be seen (red line), which was produced by a neutrino in the tungsten/emulsion detector (yellow). Secondary particles are also produced (protons, electrons, positrons, photons), which are detected in the interface tracker (yellow lines). © FASER collaboration
FASER Group Leader
FASER Group Leader - FASER group leader Prof. Dr. Florian Bernlochner from the Physikalisches Institut at the University of Bonn. © Florian Bernlochner
Bonn Team
Bonn Team - The FASER team from Bonn University, which played a key role in the discovery: Tobias Blesgen (as part of his bachelor thesis), Tobias Böckh (as part of his PhD) and Dr. Markus Prim (from left to right). © Tobias Böckh

The study was supported by the Heising-Simons Foundation, the National Science Foundation, the European Research Council, and the Swiss National Science Foundation.

H. Abreu et al: First Observation of Collider Neutrino Events with the FASER Detector at the LHC FASER Collaboration. Preprint in Kürze verfügbar.

Prof. Dr. Florian Bernlochner
Physikalisches Institut der Universität Bonn
Tel.: +49 (0)228 73 6344
E-Mail: florian.bernlochner@uni-bonn.de

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