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 space labeled TI12.
“It’s a very special tunnel,” says Boyd, “because that’s where the old transfer line for the Large Electron-Positron Collider existed, before the Large Hadron Collider.” After the construction of the LHC, a new transfer line was added, “and this tunnel was then abandoned”.
The tunnel is no longer abandoned. Its new resident is a much smaller experiment than the neighboring ATLAS detector. Five meters long, the ForwArd Search ExpeRiment, or FASER, detector sits in a shallow trench dug into the ground, surrounded by low railings and cables.
Scientists including Boyd, who is co-spokesperson for FASER, installed the relatively small detector in 2021. Just in time before the LHC restart in April, physicists installed another small experiment, called Scattering and Neutrino Detector or SND@LHC, on the other side of ATLAS.
Both detectors are now working and have started collecting data. The scientists say they hope the two detectors represent the start of a new effort to capture and study particles that the LHC’s four main detectors cannot see.
Hiding in plain sight
FASER and SND@LHC 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 constituents. Along with quarks, electrons, muons and tau, neutrinos are fundamental particles of matter in the standard model of physics.
These light, neutral particles are abundant throughout the galaxy. Some have been around since the Big Bang; others are produced during particle collisions, such as those that occur when cosmic rays strike the atoms that make up the Earth’s atmosphere. Every second, trillions of neutrinos pass through 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 physics at the LHC, neutrinos have not been detected, as the ATLAS, CMS, LHCb and ALICE detectors were designed with other types of particles at the mind.
The four largest LHC experiments cannot detect neutrinos directly, says Milind Diwan, senior scientist at the US Department of Energy’s Brookhaven National Laboratory. Diwan was an early proponent and spokesperson for what is now the Deep Underground Neutrino Experiment hosted by the Fermi National Accelerator Laboratory.
In 2021 FASER became the first detector to capture neutrinos at the LHC or any particle collider.
A new way to see 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 traverse the universe at almost the speed of light, neutrinos switch from one flavor to another. FASER and SND@LHC can detect all three types of neutrinos.
The detectors will only pick up a small fraction of the neutrinos that pass through them, but the LHC’s high-energy collisions are expected to produce an impressive number of particles. For example, during the current LHC run, which will last until the end of 2025, physicists estimate that 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 antineutrinos of each flavor.
“We are now guaranteed to see thousands of neutrinos at the LHC for the first time,” says Jonathan Feng, co-spokesperson for the FASER collaboration.
These neutrinos will be at the highest energies ever seen from an artificial source, says Tomoko Ariga, project manager for FASERv, who previously worked on the neutrino DONUT experiment. “At such extreme energies, FASERv will be able to probe the properties of neutrinos in new ways.
The experiments will also offer a new way to study other particles, says Giovanni De Lellis, spokesperson for the SND@LHC and the OPERA neutrino experiment.
Since a large part of the neutrinos produced in the range accessible to SND@LHC will come from the decay of particles consisting of charmed quarks, SND@LHC can be used to study the production of charmed quark particles in a region only other LHC experiments can not 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 travel away from the ATLAS detector along the beamline, directly into FASER and SND@LHC.
A proposal for the future
These experiments could be just the beginning. The physicists have proposed five more experiments, including advanced versions of the FASER and SND@LHC detectors, to be built near the ATLAS detector. Experiences—FASERv2, Advanced SND, FASER2, FORMOSA and FLArE – could sit in a proposed advanced physics facility during the next phase of the LHC, the High-Luminosity LHC.
The advanced FASERv and the SND@LHC detectors would increase the detection of neutrinos by the experiments by 100, according to Feng. “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 do precision studies of these two independently for the first time.”
The FLArE experiment, which would detect neutrinos differently from 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. EASIERv and SND@LHC have already started collecting physics data and are expected to present new results in 2023.
“Neutrinos are amazing,” Feng said. “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 can’t wait to see the surprises nature has in store for us.
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