Illustration of a neutrino in a fly trap

Neutrino trapping 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, he turns into an underground space labeled TI12.

“This is a very special tunnel,” says Boyd, “because here was the old transfer line for the large electron-positron collider, before the large hadron collider.” After the LHC was built, a new transfer line was added, “and this tunnel was then abandoned.”

The tunnel is no longer abandoned. Its new resident is an experiment much more modest in size than the neighboring ATLAS detector. The five-meter-long ForwArd Search ExeRiment detector, or FASER, sits in a shallow trench excavated from the floor, surrounded by railings and low cables.

Scientists, including Boyd, who is a co-spokesperson for FASER, installed the relatively small detector in 2021. Just in time before restarting the LHC in April, physicists installed another small experiment, called the Scattering and Neutrino Detector, or SND@LHC, on the other side of ATLAS.

Both detectors are now operational and have started collecting data. Scientists say they hope the two detectors are the start of a new effort to capture and study particles that the LHC’s four main detectors cannot see.

Hiding in plain sight

Both FASER and SND@LHC detect particles called neutrinos. Not to be confused with neutrons – particles in the nuclei of atoms that are made up of quarks – neutrinos cannot be broken down into smaller constituents. Along with quarks, electrons, muons and taus, neutrinos are fundamental particles of matter in the standard model of physics.

These neutral, light particles are abundant throughout the galaxy. Some have existed since the Big Bang; others are produced in particle collisions, such as those that occur when cosmic rays strike the atoms that make up Earth’s atmosphere. Every second, neutrinos pass through us by the trillions without leaving a trace – because they only 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 have gone undetected because the ATLAS, CMS, LHCb and ALICE detectors were designed with other types of particles in mind.

The four largest LHC experiments cannot detect neutrinos directly, says Milind Diwan, a senior scientist at the US Department of Energy’s Brookhaven National Laboratory. Diwan was an early supporter and spokesperson for what is now the Deep Underground Neutrino Experiment hosted by Fermi National Accelerator Laboratory.

In 2021, FASER became the first detector to capture neutrinos at the LHC – or at any particle collider.

A new way to look at neutrinos

Neutrinos are the chameleons of the particle world. They come in three flavors, called muon neutrinos, electrons, and tau for the particles associated with them. As they travel through the universe at nearly the speed of light, neutrinos switch between the three flavors. Both FASER and SND@LHC can detect all three flavors of neutrinos.

The detectors will catch only a tiny fraction of the neutrinos that pass through them, but the LHC’s high-energy collisions should produce an astonishing number of particles. For example, during the LHC’s current run, which will last until the end of 2025, physicists estimate FASER and its new subdetector, called FASERV (pronounced FASERnu), 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 flavor.

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

Those neutrinos will be at the highest energies ever seen from a man-made source, says Tomoko Ariga, project leader for FASER.V, who previously worked on the DONUT neutrino experiment. “At such extreme energies, FASERV will be able to probe 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 SND@LHC and the OPERA neutrino experiment.

Since a large fraction of the neutrinos produced in the range accessible to the SND@LHC will come from the decay of charm-quark particles, the SND@LHC can be used to study the production of charm-quark particles in a region that other LHC experiments do not i can explore . This will help both physicists studying collisions in future colliders and physicists studying neutrinos from astrophysical sources.

FASER and SND@LHC could also be used to detect dark matter, Diwan says. If dark matter particles are produced in collisions at the LHC, they could slip from the ATLAS detector along the beamline – right into FASER and SND@LHC.

A proposal for the future

These experiments could be just the beginning. Physicists have proposed five more experiments – including advanced versions of the FASER and SND@LHC detectors – to be built next to the ATLAS detector. Experiments—FASERV2, Advanced SND, FASER2, FORMOSA and FLArE — could sit at a proposed advanced physics facility in the next phase of the LHC, the high-luminosity LHC.

The advanced FASERV and the SND@LHC detectors would boost the experiments’ detection of neutrinos by a factor of 100, Feng says. “This means, for example, that instead of tens of tau neutrinos, they will detect thousands, allowing us to separate tau neutrinos from anti-tau neutrinos and make precision studies of the two independently for the first time.”

The FLArE experiment, which would detect neutrinos in a different way than FASER and SND@LHC, could also be sensitive to light dark matter.

Even without the proposed future experiments, scientists are poised to learn more about neutrinos from their studies at the LHC. PHASEV and SND@LHC have already started taking physics data and are expected to present new results in 2023.

“Neutrinos are amazing,” says Feng. “Every time we look at them from a new source, whether it’s a nuclear reactor, the sun or the atmosphere, we learn something new. I look forward to seeing what surprises nature has in store for us.”

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