Scientists Trace the Source of Exotic Particles

In the universe, there’s an unseen flow of particles and energy that surrounds and passes through us. This phenomenon is akin to the force from Star Wars, though it is grounded in reality. This so-called “force” is a critical by-product of nuclear processes and high-energy particle interactions that maintain the universe, known as neutrinos.

Neutrinos are tiny subatomic particles that travel close to the speed of light without an electric charge, constantly flowing through us. As you read this, approximately 100 trillion neutrinos are passing through your body every second, yet you’re completely unaware of them! As fundamental components of the universe, neutrinos aren’t composed of smaller particles, making them elementary particles.

Neutrinos originate from nuclear and high-energy reactions. Most neutrinos reaching Earth come from nuclear reactors and various stars. These neutrinos are low-energy, about 400 kiloelectron volts (6 x 10^-14 Joules). To put that in perspective, it would take nine quarters to match the energy contained in a single 12-ounce soda can. Additionally, neutrinos from beyond our solar system can strike Earth, possessing billions to trillions of electron volts of energy, which would require about 4 trillion yen to equal the energy of the same soda can.

Astrophysicists are eager to discover the origins of high-energy neutrinos emitted from deep space. They proposed that these neutrinos are generated by rapidly moving protons, known as cosmic rays that collide with unstable particles called pions. Physicists theorize that these collisions can generate high-energy gamma-ray photons and sometimes ultra-high-energy neutrinos. According to this hypothesis, neutrino detectors may observe a spike in detections from the same areas in the universe where gamma rays have been identified by other scientists.

To test this theory, the team analyzed neutrino detection data from the IceCube Neutrino Observatory in Antarctica. They noted that detectors like IceCube are one of three methods for scientists to uncover activities occurring in space, alongside gravitational wave detectors and telescopes. However, this is a challenging task, as scientists must wait for neutrinos to collide with atomic nuclei in water molecules. Such collisions produce a distinct blue light known as Cherenkov radiation that is measurable by the detector, and by evaluating the patterns of Cherenkov emissions, researchers can assess the energy levels of the incoming neutrinos.

Once the neutrino detector was installed, the next task was to identify areas where gamma rays are typically found. To achieve this, astrophysicists utilized data from the Large High-Altitude Air Shower Observatory (LHAASO). This data revealed gamma rays originating from sections of the sky containing much of the Milky Way galaxy, known as the galactic plane. The research team created a sky map delineating areas where LHAASO scientists detected gamma rays and developed several model maps predicting potential neutrino events, comparing them against IceCube neutrino detection data. One model assumed neutrinos could emerge from anywhere on the galactic plane, while another suggested they would arise from regions with dense gas concentrations, and a third posited that neutrinos could be emitted from all directions in the sky.

Astrophysicists then evaluated these maps against 2,500 days of IceCube data collected between 2011 and 2018, during which approximately 900,000 high-energy neutrinos were identified. Statistical analysis revealed that slightly more neutrinos originated from the galactic plane, supporting the theory that these particles are produced when cosmic rays collide with pions. They focused on specific regions of the galaxy, particularly near the constellation Sagittarius, where the most significant neutrino detections occurred. They recommended that future research focus on this part of the sky to study high-energy particle collisions in the universe.

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