Finally, the "ghost particle" collided

The concept of neutrinos was proposed with a dramatic flash of inspiration, and the discovery of neutrinos and the detection of their basic physical properties are the result of long-term and arduous efforts by particle physics experimentalists. However, the production and detection of cosmic ray neutrinos or solar neutrinos have always been passive, and the energy of neutrinos produced by reactors or fixed target experiments is relatively low. In March 2023, the FASER experiment announced the first direct observation of higher energy neutrinos at a collider.

Written by Chen Xin, Hu Zhen, and Wang Qing (Department of Physics, Tsinghua University)

In March this year, at the 57th Rencontres de Moriond Conference on Electroweak Interactions and Unified Theory held in Italy, FASER (Forward Search Experiment) [1] announced the first direct observation of neutrinos at the Large Hadron Collider. This is very important for our understanding of the basic properties of neutrinos and observations in particle astrophysics.

Neutrinos are uncharged, very light (less than one millionth of an electron), move at speeds close to the speed of light, participate in only very weak interactions, and have extremely strong penetrating power. Every moment, hundreds of billions of neutrinos flow through our bodies, but we are completely unaware of them. Therefore, neutrinos have earned the nickname "ghost particles." Neutrino detection is very difficult. Even if they pass through a material as thick as the diameter of the earth, only about one out of 10 billion neutrinos will react with the material. In fact, most particle physics and nuclear physics processes are accompanied by the production and absorption of neutrinos, such as nuclear reactor power generation (nuclear fission), solar luminescence (nuclear fusion), natural radioactivity (beta decay), supernova explosions, cosmic rays, etc., but our means of detecting neutrinos are quite limited. Moreover, most neutrino experiments are located underground or under ice, and require the deployment of detectors of sufficient volume in ultrapure water or liquid scintillator environments, such as China's Daya Bay Reactor Neutrino Experiment, Japan's Super-Kamiokande Neutrino Detection Experiment, the United States' IceCube experiment, and the Jiangmen Neutrino Experiment Station in China, which is under construction and has 20,000 tons of liquid scintillators. They are either used to detect reactor neutrinos or to capture traces of cosmic ray neutrinos or solar neutrinos.

Of course, neutrinos can also be produced in large quantities at the Large Hadron Collider (LHC) in Europe, but the four main large detectors at the LHC, ALICE, ATLAS, CMS and LHCb, are not suitable for detecting signals of light and extremely weakly interacting particles produced parallel to the particle beam line, such as neutrinos and dark photons (note: dark photons have not yet been confirmed to exist). Although the momentum of neutrinos perpendicular to the beam line can be calculated, this is only an indirect measurement based on the principle of conservation of momentum.

In 2019, some far-sighted theorists and a group of experimentalists working on these four detectors saw this limitation and submitted a proposal to CERN to build a small detector to detect long-lived particles (LLP) and neutrinos beyond the standard model, which was quickly approved. This detector, named FASER, is located 480 meters away from the front of ATLAS along the tangent direction of the proton beam line to detect the decay products of LLP produced in the collision center of ATLAS. This particle is usually very light and has a very low coupling strength with the standard model particles, so it is easy to escape the eyes of ordinary detectors. If the mass is not too small, it will decay into leptons or photon pairs. Placing a detector on its flight path can detect the decay products of these LLP particles, thereby confirming its existence. LLP particles can be dark photons, axion-like particles, or scalar particles with odd CP parity. Generally speaking, the yield of LLP and neutrinos is the highest along the tangent direction of the beam line, and it is also the most likely to detect LLP and neutrinos.

FASER is placed in the TI12 tunnel of the European Organization for Nuclear Research, which connects the LHC and the nearby Super Proton Synchrotron (SPS). After the LLP is produced in the ATLAS detector, it flies 480 meters in a straight line and enters FASER. Its decay products along the way will be detected by FASER. During this process, the LLP passes through 10 meters of cement and 90 meters of rock before reaching FASER. Many models other than the Standard Model predict the existence of LLP particles. These models attempt to solve some of the biggest problems in physics, such as the nature of dark matter, the origin of neutrino mass, and the huge difference in the amount of matter and antimatter.

Figure 1: Inside the TI12 tunnel

Figure 2: FASER installed in the TI12 tunnel

FASER also includes a subdetector called FASERν, which is specifically designed to detect neutrinos from the ATLAS collision center. The interactions in the energy region where these neutrinos are located have not been studied in detail, and their reaction cross sections have not yet been measured. FASER's electronic detectors cannot detect neutrinos produced by the collider because it lacks sufficient target material to form very weak interactions between neutrinos and matter. FASERν is composed of thousands of tungsten absorption plates and nuclear latex alternating, which can be used as both a target material and a detector to observe the interaction between neutrinos and matter. In 2021, the FASERν Pilot detector used for verification announced the results of data collected in 2018, announcing the first detection of 6.1 neutrino candidate events from the collider [2].

Figure 3: Image of a neutrino candidate event in a nuclear latex detector

Last year, the FASER experiment was launched simultaneously with the LHC Run 3 data collection. At the Rencontres de Moriond electroweak conference in March this year, FASER announced the first direct observation of collider neutrinos [3, 4, 5]. In particular, FASER observed candidate events for muon neutrinos and electron neutrinos. "Our statistical significance is about 16 standard deviations, far exceeding the threshold of 5 standard deviations for declaring a discovery," explained Jamie Boyd, co-spokesperson for FASER. The neutrinos detected in this analysis first interact with the FASER detector with electric current to produce muons, which are then detected by the FASER electronics silicon microstrip detector. The FASER experiment uses about a few dozen silicon microstrip modules for the detection and reconstruction of charged particle track points. These modules are spare modules of the ATLAS silicon detector and are directly used in the FASER detector with the consent of the ATLAS collaboration. In the future, these signal events above the background will be converted into scattering cross-sections of neutrino-matter interactions, which can be compared with theoretical calculations.

The FASER results also marked the first time that neutrinos have been unambiguously detected in a particle collider. "We found neutrinos from a completely new source, a particle collider, where two beams of particles collide at extremely high energies to form neutrinos," said Xiaoren Feng, co-spokesperson of the FASER collaboration and initiator of the project, and a particle physicist at the University of California, Irvine.[6] Another experiment, SND@LHC, also reported its first results at the Moriond conference, showing eight muon neutrino candidates. "We are still working on assessing the systematic uncertainty of the background. As a very preliminary result, our observations are at a level of 5 standard deviations," added Giovanni De Lellis, spokesperson for SND@LHC.[4] The SND@LHC detector was installed in the LHC tunnel, just in time for the start of LHC Run 3.

So far, neutrino experiments have only studied neutrinos from the sun, supernova explosions, the atmosphere, the earth, nuclear reactors or fixed target experiments. Among them, the energy of neutrinos from outer space celestial bodies is often very high. For example, the energy of neutrinos detected by the IceCube experiment in Antarctica can reach 10 PeV; the energy of electron neutrinos produced by the sun and reactors is usually below 10 MeV; the neutrinos in fixed target experiments can reach hundreds of GeV. The FASER experiment just fills the blank energy range between them - between hundreds of GeV and several TeV.

Chen Shaomin, professor of the Department of Engineering Physics and director of the Institute of Modern Physics at Tsinghua University, said: “The FASER experiment used the collider to observe statistically significant neutrinos with energies ranging from several hundred GeV to several TeV, bringing the TeV high-energy neutrinos observed by the IceCube experiment from the universe back to the laboratory. This not only provides an opportunity to study the properties of such high-energy neutrinos, but also makes the study of the origin of cosmic rays through high-energy cosmic ray neutrinos a step closer to the era of precision measurement.”[7]

FASER's research is also very important for understanding the atmospheric background of astrophysical neutrinos. The collisions of cosmic rays with atmospheric molecules and atoms produce a large amount of neutrino background. The energy of these collisions converted to the center-of-mass reference frame of the incident high-energy particles and the collided particles is similar to the collision energy of the LHC. FASER's research on neutrinos in this energy region will pave the way for the observation of astrophysical neutrinos.

Figure 4: Schematic diagram of FASER detecting collider neutrino events

The generation and detection of ghost particles at the highest energy frontier created by humans has opened up a new direction for basic science. The experimental results of FASER have opened a door for us to understand the mechanism of neutrino production, the behavior of low-momentum partons in protons, and the physics in the forward zone of the collision point. In the future, we look forward to FASER's detection of other types of neutrinos from the collider, the precise determination of the proportional relationship between different types of neutrino events (this will be an important test of the Standard Model in the field of neutrinos), and the search for possible new physical signals such as sterile neutrinos and dark matter particles.

The FASER collaboration now has more than 80 researchers from 22 partner institutions around the world. Tsinghua University in China was one of the 16 founding members of the FASER collaboration when it was first established, and has contributed to the construction, operation, and data analysis of the FASER experiment[7].

References

[1] https://faser.web.cern.ch/

[2] https://doi.org/10.1103/PhysRevD.104.L091101

[3] https://arxiv.org/abs/2303.14185

[4] https://home.cern/news/news/experiments/new-lhc-experiments-enter-uncharted-territory

[5] https://indico.cern.ch/event/1227016/contributions/5314959/attachments/2614023/4517266/FaserPhysicsResults.pdf

[6] https://news.uci.edu/2023/03/20/uc-irvine-led-team-is-first-to-detect-neutrinos-made-by-particle-collider/

[7] https://www.phys.tsinghua.edu.cn/info/1229/5480.htm

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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