FASER Poised to Further Our Understanding of Neutrinos, Dark Matter


By: Hannah Pell

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

Illustration of the FASER experiment. Image Credit: FASER/CERN.

“These neutrinos will have the highest energies yet of man-made neutrinos, and their detection and study at the LHC will be a milestone in particle physics, allowing researchers to make highly complementary measurements in neutrino physics,” Jamie Boyd, co-spokesperson for FASER, said in December 2019.

Physicists are hopeful that FASER will capture new light particles that have previously evaded detection and could potentially help explain dark matter. These particles are long-lived, traveling far beyond an interaction point before further decaying into particles that FASER will detect based on its location and its size. Surprisingly, FASER is relatively tiny — only 25 cm wide, 25 cm tall, and 5 meters long — but weighs a staggering 1.2 tonnes (more than 2600 pounds). Comparatively, the Super-K neutrino detector weighs a whopping 50,000 tonnes!

The LHC is currently amid another shutdown period for maintenance and upgrades, but FASER is expected to be fully online and taking data by the next LHC Run 3 from 2021-2023. The FASER collaboration consists of 70 members from 19 institutions and 8 countries.

“We are extremely excited to see this project come to life so quickly and smoothly,” said Boyd. “Of course, this would not have been possible without the expert help of the many CERN teams involved!”

Map showing the location of FASER. Image Credit: CERN/FASER.

The FASERν collaboration posted the first results from the pilot detector to the arXiv on May 13, 2021, including observation of the first neutrino interaction candidates.

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