Neutrinos from the extra-solar system
The direct detection of neutrinos in 1956 from a nuclear reactor showed that the probability of neutrino interactions with matter is very small. This indicated that the neutrinos could travel cosmic distances without a large amount of interactions, and that they could escape from very dense environments hidden from traditional methods of observations. As people started to search for extraterrestrial neutrinos, the neutrinos from the Sun were detected in 1968. In 1987, the first detection of neutrinos from a supernova was reported (SN 1987A in the Large Magellanic Cloud). These were neutrinos with energies lower than 1 GeV (109 eV). Based on cosmic-ray measurements at Earth, we know that there are high energy particles coming from outside of our Galaxy with energies up to 1021 eV. Observations of high-energy neutrinos provide unique information about the accelerators of these high-energy particles in our Universe. Cosmic rays bend inside magnetic fields, which makes it practically impossible to use the information from them to find out their origins. Gamma rays can be generated by both leptonic (e.g. electrons) and hadronic (e.g. protons) particles, and cannot travel from more distant galaxies due to their interactions with the radiative environment in the Universe. High-energy neutrinos are thus the most reliable messenger to trace the dynamics of hadronic particles, which is the dominant component of cosmic rays measured at Earth, and to probe particle acceleration across the widest volume of the Universe.
Detection of high-energy neutrinos
The probability of neutrino interaction with material is much smaller than for cosmic rays or gamma rays. Because of this, neutrino detectors requires a large instrumented volume to increase the chance of interaction. The IceCube observatory, located under the South Pole, has an active detection volume of 1 cubic kilometer. A 3-D array of 5160 photo-detectors is embedded inside this large volume to detect the Cherenkov light emitted by secondary particles generated by the interaction of neutrinos in the ice. As the interactions of cosmic rays with the atmosphere generate secondary particles similar to the byproduct of neutrino interactions, cosmic rays are the major background for neutrino detection. To reduce the background, the high-energy neutrino detectors are all located lower than 1 kilometer below sea level. Since the IceCube detection of 28 high-energy neutrinos events, first reported in 2013, the flux of high-energy neutrinos coming from outside of our solar system is well established. However, the origin of these high-energy neutrinos remains uncertain. In 2017, IceCube reported the detection of high-energy neutrino event, tagged as IceCube-170922A, in the direction of a blazar TXS 0506+056 located at a redshift of 0.3. Various observatories around the world pursued follow-up observations covering the electromagnetic spectrum from radio to very-high-energy gamma ray energies. These observations found a coincident gamma-ray flare from TXS 0506+056. While this event cannot provide a full description for the origin of the astrophysical neutrino flux, it certainly demonstrates the importance of multi-messenger observations and provides the first evidence of the nature of the astrophysical neutrino sources.
My research in high-energy neutrinos
My research interest in high-energy neutrinos is to find their astrophysical sources, both within and outside of our own Galaxy, and to understand the hadronic acceleration at the various source sites by combining the information from high-energy neutrino and very-high-energy gamma-ray observations. I am also interested in developing detectors for future high-energy neutrino observatories, such as P-ONE and IceCube-Gen2.
- IceCube collaboration: Located at South Pole, IceCube is a neutrino detector with an active volume of a cubic kilometer.
- P-ONE (Pacific Ocean Neutrino Experiment): an initiative towards constructing a multi-cubic-kilometre neutrino telescope in the Pacific Ocean. Cascadia Basin is selected as a host site for P-ONE.