Ghost in the machine: First neutrino observation at Large Hadron Collider

Physicists at CERN’s Large Hadron Collider (LHC) have made the first ever direct observation of neutrinos in a particle accelerator.

Neutrinos are tiny, near massless and chargeless particles. They are among the elementary particles that make up the Standard Model of particle physics. Of all the particles in the Standard Model, neutrinos are among the least understood.

Even seeing a neutrino is extremely difficult, despite the fact they are among the most numerous particles in the universe. An estimated 100 trillion (100 million million) neutrinos pass through your body every second!

Detectors have picked up the traces of neutrinos from known sources such as the Sun, nuclear reactors, cosmic rays and supernovae since they were first observed in 1956.

But they should show up in particle accelerators too. Until now, physicists have been unable to confirm observations of neutrinos in their colliders.

Now, two large research collaborations namely FASER (Forward Search Experiment) and SND (Scattering and Neutrino Detector)@LHC, have observed collider neutrinos for the very first time. Their results are published in two papers published in Physical Review Letters.

The measurements were obtained from detectors at CERN’s LHC in Switzerland.

“Neutrinos are produced very abundantly in proton colliders such as the LHC,” Cristovao Vilela, part of the SND@LHC Collaboration, says in an article on Phys.org. “However, up to now, these neutrinos had never been directly observed. The very weak interaction of neutrinos with other particles makes their detection very challenging and because of this they are the least well studied particles in the Standard Model of particle physics.”

“Particle colliders have existed for over 50 years, and have detected every known particle except for neutrinos,” Jonathan Lee Feng of FASER tells Phys.org. “Every time neutrinos have been discovered from a new source, whether it is a nuclear reactor, the sun, the Earth, or supernovae, we have learned something extremely important about the universe.”

FASER researchers found neutrinos by placing a detector along the line of the particle beam, following the colliding particles’ trajectories.

They found 153 distinct neutrino signatures using their “very small, inexpensive” detector.

The FASER neutrinos have the highest energy recorded for the particles in a laboratory environment. They could prove useful in gaining insight into neutrino properties as well as the search for other elusive particles.

SND@LHC reported their findings shortly after showing a further 8 neutrino events. Their 2-metre-long detector was placed at a site expected to see high neutrino flux. It was shielded from other debris caused by proton collisions by about 100 metres of concrete and rock.

“The observation of collider neutrinos opens the door to novel measurements which will help us understand some of the more fundamental puzzles of the Standard Model of particle physics, such as why there are three generations of matter particles (fermions) that seem to be exact copies of each other in all aspects except for their mass,” notes Vilela.

Both experiments will continue collecting data and may lead to further developments including in the search for dark matter.

“We will be running the FASER detector for many more years, and expect to collect at least 10 times more data,” Feng comments. “A particularly exciting fact is that this initial discovery only used part of the detector. In the coming years, we will be able to use the full power of FASER to map out these high energy neutrino interactions in exquisite detail.”