Breathtaking Neutrino Experiment at CERN

Breathtaking Neutrino Experiment at CERN
The SND@LHC experiment, located about 0,5 kilometers from where protons are smashed inside the much larger ATLAS detector, is located in a side tunnel of the Large Hadron Collider. Image courtesy of Maximilien Brice/CERN

Future observations of high-energy neutrinos at the LHC may reveal new information about tau neutrinos, the strong force, or possibly new physics.

Scientists have built multiple, massive underground detector arrays to detect the light, uncharged neutrino. Numerous research projects focus on fusion events occurring inside the Sun, neutrinos produced in a nuclear reactor, and powerful cosmic events such as supernovae. The IceCube Neutrino Observatory, a massive detector buried in the ice of Antarctica, has discovered the first definitive evidence of neutrinos originating from the Milky Way, which are billions of times more energetic than solar neutrinos.

Two studies published in the journal Physical Review Letters confirm the discovery of more than 160 neutrinos produced during tests at the Large Hadron Collider (LHC) at CERN, the world's most powerful particle accelerator. This represents both the highest energy recorded for neutrinos produced and detected in a laboratory setting and the first time neutrinos have been detected in a collider experiment.

“It's a fact that every time you see neutrinos in a new way, you learn something absolutely surprising about our universe,” says Jonathan Feng of the University of California, Irvine, who served as co-spokesperson for one of the teams that discovered neutrinos.

New information can be gained from studying the properties of LHC neutrinos, including a better understanding of the force that holds quarks together and more accurate measurements of the tau neutrino, which is extremely difficult to detect.

The two newest detectors at the LHC are responsible for the new neutrino observations. The Advanced Search Experiment (FASER) was inspired by a 2017 publication co-authored by Feng and three postdoctoral researchers from Irvine. They pointed out a flaw in the LHC's detection scheme: When two proton beams collide, detectors such as ATLAS and CMS almost completely surround the collision area. The detectors' openings, known as cavities, allow the rays to pass through. The collision produces particles known as forward direction, which pass through these openings and travel down the beam pipe.

The goal of FASER, which started collecting data last year, is to find hitherto undiscovered particles such as neutrinos that are not affected by the LHC's powerful guiding magnets. An aluminum container carrying a ton of tungsten has been placed in an LHC side tunnel to capture neutrinos that continue on their way after colliding with the ATLAS experiment. A charged particle arises from the interaction between a neutrino and a tungsten atom. Which of the three types of neutrinos a particle is produced by (muon, electron, or tau) determines its identity.

All of the neutrinos confirmed by FASER to date are of the muon and electron types. (Electron neutrino detections were reported at a neutrino workshop held in August.) In later stages of the experiment, the team expects to find tau neutrinos. Since the mass of the tau particle is about 17 times that of a muon and 3500 times that of an electron, a strong source neutrino is required to detect a neutrino. According to Feng, in some tests “the incoming neutrino doesn't have enough energy for you to actually produce the tau particle.” FASER, on the other hand, “doesn't really have that problem” in light of the energy of the LHC.

The Scattering and Neutrino Detector at the LHC also performed its first research run last year. SND@LHC, unlike FASER, is located slightly off the collision axis. According to collaboration spokesman Giovanni De Lellis of the University of Naples, this location is suitable for the detection of extremely heavy particles, especially neutrinos that arise in the decay of the charm quark. The group hopes to use neutrino measurements to better understand the strong forces that hold charm quarks together.

De Lellis suggests that astrophysics will also benefit from the LHC's measurements. The neutrinos produced at the LHC are similar to most of the neutrinos produced when high-energy cosmic rays collide with molecules in Earth's atmosphere, as both have energies exceeding one trillion electron volts.

Charm quarks that decay and produce neutrinos are among the intermediate byproducts of these encounters.

According to Soldin, a University of Utah particle physicist who was not involved in the LHC project, the results from the collider experiment are related to Dennis Soldin's work with IceCube. “We have backgrounds from neutrinos produced in the atmosphere, and these neutrinos are produced at exactly the same energies that the LHC is investigating,” says Soldin. Measurements from the LHC will help Soldin and his colleagues at IceCube identify astronomical sources producing high-energy neutrinos, reducing the uncertainty in their measurements of the neutrino background.

The FASER team and the SND@LHC collaboration reported in their Physical Review Letters paper that they found approximately 153 and 8 neutrinos, respectively. Juan Rojo, a particle physicist at Vrije Universiteit Amsterdam who was not involved in the study, claims that current experiments are “too small to realize their full physics potential.” With the LHC's upcoming high-luminosity upgrade, researchers could increase neutrino detection rates to thousands per day.

LHC experiments may produce findings that point to physics outside the standard model, perhaps providing an explanation for dark matter with suitably robust statistics. De Lellis claims that FASER and SND@LHC are capable of discovering strange particles that escape detection by moving forward. According to Soldin, “This is the beginning of a completely new type of program at the LHC that opens a window into a wide variety of physics.”

Source: Physics Today

📩 12/09/2023 21:16