The first observation of neutrinos at CERN’s Large Hadron Collider

Neutrinos are tiny neutrally charged particles, represented by the Standard Model of particle physics. Although they are believed to be among the most abundant particles in the universe, observing them has so far proven to be very difficult, as the likelihood of them interacting with other matter is low.

To detect these particles, physicists use advanced detectors and equipment to examine known sources of neutrinos. Their efforts eventually resulted in the observation of neutrinos from the sun, cosmic rays, supernovae and other cosmic objects, as well as particle accelerators and nuclear reactors.

A long-standing goal in this field of study has been to observe neutrinos inside colliders, particle accelerators in which two beams of particles collide. Two major research collaborations, namely FASER (Forward Search Experiment) and SND (Scattering and Neutrino Detector)@LHC, have observed these neutrinos in a collider for the very first time, using detectors located at the Large Hadron Collider (LHC) at CERN in Switzerland. The results of their two studies were recently published in Physical examination letters.

“Neutrinos are produced in very large quantities in proton colliders such as the LHC,” Cristovao Vilela, a member of the SND@LHC collaboration, told Phys.org. “However, until now, these neutrinos had never been observed directly. The very weak interaction of neutrinos with other particles makes their detection very difficult and, as a result, they are the least well studied particles in the world. standard model of particle physics.

The FASER and SND@LHC collaboration are two separate research efforts, both using CERN’s LHC. Recently, these two efforts independently observed the collider’s first neutrinos, which could open important new avenues for experimental research in particle physics.

The FASER collaboration is a large research effort established with the goal of observing light and weakly interacting particles. FASER was the first research group to observe neutrinos at the LHC, using the FASER detector, located more than 400 m from the famous ATLAS experiment, in a separate tunnel. FASER (and SND@LHC) observe neutrinos produced in the same “interaction region” inside the LHC as ATLAS.

“Particle colliders have been around for more than 50 years and have detected every known particle except neutrinos,” Jonathan Lee Feng, co-spokesperson for the FASER collaboration, told Phys.org. “At the same time, every time neutrinos have been discovered from a new source, whether it’s a nuclear reactor, the Sun, the Earth, or supernovae, we’ve learned something new. extremely important on the universe.As part of our recent work, we decided to detect for the first time the neutrinos produced in a particle collider.

The FASER collaboration observed collider neutrinos by placing their detector along the beam line, following their trajectories. It is known that high-energy neutrinos are mainly produced at this site, but other LHC detectors have blind spots in this direction and have therefore not been able to observe them in the past.

“Because these neutrinos have high fluxes and energies, making them much more likely to interact, we were able to detect 153 of them with a very small, inexpensive detector that was built in a very short time,” Feng explained. . “Previously, particle physics was thought to be divided into two parts: high-energy experiments, needed to study heavy particles, like top quarks and Higgs bosons, and high-intensity experiments, needed to “The study of neutrinos. This work has shown that high-energy experiments can also study neutrinos, which has made it possible to bring the frontiers of high energies and high intensities closer together.”

The neutrinos detected by Feng and the rest of the FASER collaboration have the highest energy ever recorded in a laboratory. They could thus pave the way for in-depth studies of the properties of neutrinos, as well as the search for other elusive particles.

Shortly after FASER reported the first sighting of neutrinos at a collider, the SND@LHC collaboration finalized its analysis, with eight additional LHC events involving neutrinos. The SND@LHC experiment was specifically created to detect neutrinos, using a two-meter-long detector, strategically placed at an LHC site where the neutrino flux is high, but protected from collision debris of protons through about 100 meters of concrete. and rock.

“Even with its strategic positioning, the most energetic muons produced during collisions reach our detector at a speed tens of millions of times faster than neutrino interactions,” explained Vilela. “These muons generate neutral hadrons in their interactions with the material surrounding our experiment, which in turn produce neutrino-like signals in the detector. Overcoming this background has been the biggest challenge in the analysis, which used the distinctive pattern of a muon track associated with a hadronic shower and no charged particles entering the detector to identify neutrino interactions.

As part of their recent study, the SND@LHC collaboration analyzed data collected by their detector between July and November 2022, which was its first cycle of operation. This first data collection proved to be very successful, as the team eventually recorded 95% of the collision data provided to them and finally observed the collider’s neutrino events.

“Observing neutrinos in a collider opens the door to new measurements that will help us understand some of the most fundamental puzzles of the Standard Model of particle physics, such as why there are three generations of particles of matter ( fermions) that appear to be exact copies of each other in every way except their mass,” Vilela said. “In addition, our detector is placed in a location that is a blind spot for large LHC experiments. Therefore, our measurements will also contribute to a better understanding of the structure of colliding protons.”

These recent studies carried out by the FASER and SND@LHC collaborations contribute significantly to the ongoing experimental research in particle physics and could soon pave the way for new advances in this field. Now that the presence of neutrinos at the LHC has been confirmed, these two experiments will continue to collect data, potentially leading to more meaningful observations.

“We will use the FASER detector for many more years and hope to collect at least 10 times more data,” Feng added. “A particularly interesting fact is that this first discovery only used part of the detector. In the years to come, we will be able to use the full power of FASER to map these interactions of high energy neutrinos with exquisite detail.In addition, we are working on the Forward Physics Facility, a proposal to build a new underground cavern at the LHC, which will allow us to detect millions of high-energy neutrinos, as well as search for milli-charged particles and other phenomena associated with dark matter.