Team detects first neutrinos made by particle collider

The FASER particle detector, located deep underground at CERN's Large Hadron Collider and built in large part out of spare parts from other experiments.CERN

An international team of scientists has for the first time detected neutrinos created by a particle collider.

The discovery, announced March 19 by the Forward Search Experiment - or FASER collaboration - at the 57th Rencontres de Moriond Electroweak and Unified Theories conference in Italy, promises to deepen scientists' understanding of the nature of neutrinos, which are the most abundant particle in the cosmos. FASER's detector picked up neutrinos generated by the Large Hadron Collider, which is based at CERN - the European Council for Nuclear Research - in Geneva, Switzerland.

The work promises to shed light on the nature of neutrinos near and far. It could unlock insights about cosmic neutrinos that travel large distances and collide with the Earth, providing a window on distant parts of the cosmos. In addition, neutrinos were critical in developing the Standard Model of particle physics - the current scientific framework for fundamental particles and forces in the universe. Studying neutrinos from different sources could help scientists understand if the model needs tweaking, or more.

"This is new territory," said FASER scientist Shih-Chieh Hsu, a University of Washington associate professor of physics. "Direct observation of neutrinos originating from the Large Hadron Collider has revealed a new pathway to study the deep mysteries of the Standard Model."

Hsu was a founding member of the FASER collaboration, which was launched by particle physicist Jonathan Feng of the University of California, Irvine. The team now includes researchers at 24 partner institutions. FASER scientists designed, built and operate a particle detector installed at the LHC site.

"We've discovered neutrinos from a brand new source, from particle colliders, where you have two beams of particles smashing together at extremely high energy to make the neutrinos," said Feng.

Since their discovery in 1956, the majority of neutrinos studied by physicists have been low-energy neutrinos. But the neutrinos detected by FASER are the highest energy ever produced in a laboratory setting, and are similar to the neutrinos found when deep-space particles trigger dramatic particle showers in our atmosphere.

"They can tell us about deep space that we can't learn in other ways," said FASER co-spokesperson Jamie Boyd, a particle physicist at CERN. "These very high-energy neutrinos in the LHC are important for understanding really exciting observations in particle astrophysics."

"This is a historical milestone for neutrino experiments, and will fill the gap between studies of neutrinos from other sources, including reactors and cosmic events," said UW research scientist Ke Li, a member of the FASER team. "In the future, FASER will have the largest dataset of tau neutrinos, which are the least-understood particles in Standard Model."

Li led efforts to integrate the tracking software used in the FASER detector, and has helped commission the first set of data generated by the experiment. Other UW scientists involved in the FASER neutrino detection are physics doctoral student Ali Garabaglu and undergraduate student David Lai. UW involvement in the FASER collaboration is funded by the National Science Foundation, the Simons Foundation and the Heising-Simons Foundation.

FASER itself is unique among particle-detecting experiments. Compared to other detectors at CERN like ATLAS, which is several stories tall and weighs thousands of tons, FASER is only about one ton and fits neatly into a small side-tunnel at CERN. It took only a few years to design and construct, using spare parts from other experiments.

Beyond neutrinos, one of FASER's other chief objectives is to help identify the particles that make up dark matter, which physicists think comprises most of the matter in the universe, but which they've never directly observed before.

FASER has yet to find signs of dark matter, but with the LHC set to begin a new round of particle collisions in a few months, the detector stands ready to record them, should they appear.

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