CERN physicist Jimmy 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 called TI12.
“This is a very special tunnel,” Boyd says, “because that’s where the old transmission line of the Large Electron-Positron Collider used to be, before the Large Hadron Collider.” After the construction of the LHC, a new transmission line was added, “and that tunnel was abandoned.”
The tunnel is no longer deserted. Its new resident is an experiment more modest in scale than the neighboring ATLAS detector. The ForwArd Search ExpeRiment, or FASER, is five meters long and is located in a shallow trench dug into the floor, surrounded by handrails and low cables.
Scientists – including Boyd, who serves as a co-spokesperson for FASER – installed the relatively small detector in 2021. Just in time before the LHC restarted in April, physicists performed another small experiment, called the Scattering and Neutrino Detector or SND@LHC, On the other side of the Atlas.
Both detectors are now up and running and have started collecting data. Scientists say they hope the two detectors will mark the beginning of a new effort to capture and study particles that the LHC’s four main detectors cannot see.
Hiding on her finger
FASER and SND@LHC both detect particles called neutrinos. Not to be confused with neutrinos – particles in the nuclei of atoms that are made up of quarks – neutrinos cannot be broken down into smaller components. Besides quarks, electrons, muons, and taus, 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 result in particle collisions, such as those that occur when cosmic rays collide with the atoms that make up Earth’s atmosphere. Every second, trillions of neutrinos pass through us without a trace – because they rarely interact with other matter.
Neutrinos are also produced in collisions at the LHC. Scientists are aware of their existence, but for more than a decade of LHC physics, neutrinos have not been detected, as the ATLAS, CMS, LHCb, and ALICE detectors were designed with other types of particles in mind.
Milind Dewan, chief scientist at the US Department of Energy’s Brookhaven National Laboratory, says the four largest experiments of the Large Hadron Collider cannot detect neutrinos directly. Dewan was an original supporter and spokesperson for what is now the Deep Underground Neutrino Experiment hosted by the Fermi National Accelerator Laboratory.
In 2021, FASER becomes the first detector to capture neutrinos at the LHC – or 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, electron, and tau neutrinos for their associated particles. As they travel through the universe at nearly the speed of light, neutrinos travel between the three flavours. FASER and SND@LHC can both detect all three flavors of neutrinos.
The detectors will pick up only a small fraction of the neutrinos that pass through them, but the high-energy collisions of the LHC should produce an astonishing number of particles. For example, during the current operation of the LHC, which will continue until the end of 2025, physicists estimate FASER and its new sub-detector, called FASER.Fifth (pronounced FASERnu), it would see a flux of 200 billion electron neutrinos, 6 trillion muon neutrinos, and 4 billion tau neutrinos, along with a similar number of antineutrinos per flavor.
“We are now guaranteed to see thousands of neutrinos in the LHC for the first time,” says Jonathan Feng, spokesperson for the Pfizer collaboration.
These neutrinos will be at their highest energies ever from a man-made source, says Tomoko Ariga, head of the FASER project.Fifth, who previously worked on the DONUT neutrino experiment. “At such extreme energies, FASERFifth It will be able to examine the properties of neutrinos in new ways.”
The experiments will provide a new way to study other particles, too, says Giovanni de Lillis, a spokesperson for both the SND@LHC and the OPERA neutrino experiment.
Since a significant portion of the neutrinos produced in the available range of SND@LHC will come from decay of particles made of charm quarks, SND@LHC can be used to study the production of charm quark particles in an area that other LHC experiments cannot explore. This will help physicists study collisions at future collisions and physicists who study neutrinos from astrophysical sources.
Dewan says FASER and SND@LHC can also be used to detect dark matter. If dark matter particles are produced in collisions at the LHC, they can slip away from the ATLAS detector alongside the beamline—directly into FASER and SND@LHC.
Suggestion for the future
These experiences could be just the beginning. Physicists have proposed five more experiments—including advanced versions of the FASER and SND@LHC detectors—to be built near the ATLAS detector. Experiments – FASERFifth2, Advanced SND, FASER2, FORMOSA, FLArE – could sit in the proposed advanced physics facility during the next phase of the LHC, the high-brightness LHC.
Advanced FASERFifth And the SND@LHC detectors will boost experimental detection of neutrinos by a factor of 100, says Feng. “This means, for example, that instead of tens of tau neutrinos, they will be observing thousands, allowing us to separate tau neutrinos from antineutrinos and to conduct accurate studies of these two independently for the first time.”
The FLArE experiment, which will detect neutrinos differently than FASER and SND@LHC, could be sensitive to light dark matter.
Even without the proposed future experiments, scientists are preparing to learn more about neutrinos from their studies at the LHC. fasterFifth And SND@LHC have already started taking physical data and are expected to provide 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 look forward to seeing the surprises that nature has in store.”