Catching neutrinos at the LHC

CERN physicist Jamie Boyd enters a tunnel near the ATLAS detector, an experiment at the world’s largest particle accelerator. From there it turns into an underground room labeled TI12.

“This is a very special tunnel,” says Boyd, “because this is where the old transfer line for the Large Electron-Positron Collider used to be, before the Large Hadron Collider.” After the LHC was built, a new transfer line was added, “and this one Tunnel was then abandoned.”

The tunnel is no longer exited. Its new occupant is an experiment much more modest than the neighboring ATLAS detector. The 5 meter long ForwArd Search Experiment or FASER detector sits in a shallow excavated trench in the ground, surrounded by low railings and cables.

Scientists – including Boyd, who serves as FASER’s co-spokesperson – installed the relatively small detector in 2021. Just in time for the LHC to restart in April, physicists built another small experiment called the Scattering and Neutrino Detector, or [email protected], on top of the other Page of ATLAS.

Both detectors are now running and have started collecting data. Scientists hope the two detectors will mark the beginning of a new attempt to capture and study particles that the LHC’s four main detectors can’t see.

Hide in plain sight

Both FASER and [email protected] detect particles called neutrinos. Not to be confused with neutrons – particles in the nucleus of atoms made up of quarks – neutrinos cannot be broken down into smaller components. Along with quarks, electrons, muons, and taus, neutrinos are fundamental particles of matter in the Standard Model of physics.

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These light, neutral particles are abundant throughout the galaxy. Some have been around since the Big Bang; others are produced in particle collisions, such as when cosmic rays strike the atoms that make up Earth’s atmosphere. Every second, trillions of neutrinos pass us without leaving a trace – because they rarely interact with other matter.

Neutrinos are also produced in collisions at the LHC. Scientists are aware of their presence, but for more than a decade of LHC physics, neutrinos went undetected as the ATLAS, CMS, LHCb, and ALICE detectors were developed for other types of particles.

The four largest LHC experiments fail to detect neutrinos directly, says Milind Diwan, a senior scientist at the US Department of Energy’s Brookhaven National Laboratory. Diwan was an original proponent and spokesperson for today’s Deep Underground Neutrino Experiment hosted by the Fermi National Accelerator Laboratory.

In 2021, FASER was the first detector to capture neutrinos at the LHC – or any other particle accelerator.

A new way of looking at neutrinos

Neutrinos are the chameleons of the particle world. They come in three flavors, called muon, electron, and tau neutrinos for the particles associated with them. As they travel through the universe at nearly the speed of light, neutrinos alternate between the three flavors. Both FASER and [email protected] can detect all three types of neutrinos.

The detectors will only capture a small fraction of the neutrinos that pass through them, but the LHC’s high-energy collisions should produce a staggering number of particles. For example, during the current run of the LHC, which will last until the end of 2025, physicists appreciate FASER and its new subdetector called FASERv (pronounced FIBERnu) will experience a flux of 200 billion electron neutrinos, 6 trillion muon neutrinos, and 4 billion tau neutrinos, along with a comparable number of anti-neutrinos of each type.

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“We are now guaranteed to see thousands of neutrinos at the LHC for the first time,” says Jonathan Feng, co-speaker of the FASER collaboration.

These neutrinos will have the highest energies ever seen from a man-made source, says Tomoko Ariga, project leader for FASERv, who previously worked on the DONUT neutrino experiment. “At such extreme energies, FASERv be able to study the properties of neutrinos in new ways.”

The experiments will provide a new way to study other particles as well, says Giovanni De Lellis, spokesperson for both the [email protected] and the OPERA neutrino experiment.

Since a large fraction of the neutrinos produced in the region accessible to [email protected] will come from the decay of charm quark particles, [email protected] can be used to monitor the production of charm quark particles in a region to investigate that other LHC experiments cannot explore. This will help both physicists studying collisions at future colliders and physicists studying neutrinos from astrophysical sources.

FASER and [email protected] could also be used to detect dark matter, says Diwan. If dark matter particles are formed during collisions at the LHC, they could slide away from the ATLAS detector along the beamline – directly into FASER and [email protected].

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A suggestion for the future

These experiments could only be the beginning. Physicists have proposed building five more experiments – including advanced versions of the FASER and [email protected] detectors – near the ATLAS detector. The experiments – FASTERv2, Advanced SND, FASER2, FORMOSA and FLArE – could be stationed at a planned Forward Physics Facility during the next phase of the LHC, the High-Luminosity LHC.

The advanced FIBERv and [email protected] detectors would increase the experiments’ detection of neutrinos by a factor of 100, says Feng. “This means, for example, that instead of tens of tau neutrinos, they will detect thousands, which allows us to separate tau neutrinos from anti-tau neutrinos and, for the first time, carry out precision studies on these two independently.”

The FLArE experiment, which would detect neutrinos in a different way than FASER and [email protected], could also be sensitive to bright dark matter.

Even without the proposed future experiments, scientists are poised to learn more about neutrinos from their studies at the LHC. MORE QUICKLYv and [email protected] have already started collecting physical data and are expected to present new results in 2023.

“Neutrinos are amazing,” says Feng. “Every time we look at it from a new source, be it a nuclear reactor, the sun or the atmosphere, we learn something new. I’m excited to see what surprises nature has in store.”

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