Physicists detect neutrino signals on large particle accelerators

In a publication published in the journal Physical Review D, the researchers describe how they observed six neutrino interactions in an experiment conducted with the compact emulsion detector installed at the LHC in 2018. .

'Prior to this project, no signal of neutrinos had been detected on a particle accelerator,' said co-author Jonathan Feng, UCI professor of astrophysics and co-leader of the Collaboration Group FASER, said. 'This remarkable breakthrough is a step towards a deeper understanding of elusive particles and their role in the universe'.

The discovery, he said, was made in an experiment that his team had obtained two important pieces of information. 'First, it identifies the location of the ATLAS interaction point as the right site for neutrino detection,' Feng said. 'Second, our attempt to demonstrate the efficiency of using a soft detector to observe those kinds of neutrino interactions'.

The experimental device was made of lead and tungsten sheets interspersed with emulsion layers. During particle collisions at the LHC, several neutrinos collided with nuclei in metals, thereby forming particles that moved through the emulsion and created visible traces. during processing. Those traces provide clues about the energies of the particles, their flavors -tau, muons or electrons - and whether they are neutrinos or antineutrinos.

According to Feng, the emulsion was 'required' in a similar way to photography in the pre-digital camera era. When the 35-millimeter-thick film is exposed, the photons leave, revealing as patterns. FASER researchers were able to see neutrino interactions after removing and growing emulsions in the detector.

The FASER experiment was located 480 meters from the ATLAS site in the LHC. According to Jonathan Feng, this is a reasonable site for neutrino detection, which is the result of particle collisions at the LHC.

'To take advantage of the emulsion detector approach to observing the interactions of neutrinos produced on a particle accelerator, the FASER team is currently preparing a new series of experiments with a new set of experiments. the complete device, which is not only larger, but also significantly more sensitive,' said Feng.

As of 2019, he and his colleagues have been working on an experiment with FASER instruments to learn about dark matter at the LHC. They are hoping to detect dark photons, which could give researchers the first chance to look at how dark matter interacts with ordinary atoms and other matter in the universe through particles. non-gravitational force.

With the success of their work on neutrinos over the past few years, the FASER team - comprising 76 physicists in 21 research institutes from nine countries - is combining a new emulsion detector with the machine. by FASER. While the experimental detector weighs about 64 pounds, the FASERnu instrument will be around 2,400 pounds, and it will add more responsiveness and be able to distinguish between neutrinos.

'Given the right detector and site at CERN, we expect to be able to record more than 10,000 neutrino interactions in a new experiment at the LHC, starting in 2022,' said co-author David Casper, co-author leads the FASER project and is an associate professor of physics and astronomy at UCI. 'We will detect the most energetic neutrinos ever produced from a man-made source'.

What makes FASERnu unique, he says, is that while other experiments are capable of distinguishing between two or three types of neutrinos, it will be able to observe all three in addition to the reflections their particle – the antineutrino. There have only been about 10 observations of tau neutrinos in human history, Casper said, but he hopes his team will be able to double or triple this number in the next few years.

'It's a very nice connection to tradition in the physics department at UCI,' says Feng, 'because it seamlessly aligns with the legacy of Frederick Reines, a UCI founding faculty member and Nobel laureate in physics. due to the discovery of the neutrino'.

'We performed a world-class experiment at the world's pioneering particle physics laboratory in record time and with non-traditional sources,' Casper said. 'We have received large grants from the Heising-Simons Foundation, the Simons Foundation, as well as the Japan Society for the Promotion of Science and CERN'.

Savannah Shively and Jason Arakawa, UCI biophysics and astrophysicists, jointly contributed to this work.