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Neutrino

For almost 20 years, the Standard Model of elementary particle physics, comprising quantum chromodynamics and the Glashow-Weinberg-Salam theory of electroweak processes, has been accepted as the theory that describes all elementary interactions except gravity. There have been no convincing experimental findings that require a modification of the current Standard Model. The discovery of the Higgs boson (H), the origin of mass, in the ATLAS and CMS experiments at the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) in 2012 confirmed the existence of all particles represented in the current Standard Model of elementary particles (see Figure). Some questions remain unanswered, such as the nature of dark matter, the whereabouts of antimatter after the Big Bang, the three generations of quarks and leptons, and the differences in mass scales between generations. One of these is the nature of neutrinos, for which physics beyond the Standard Model is needed. Moreover, another connection needs to be mentioned.

When the Nobel Foundation awarded the 2002 Nobel Prize in Physics to Ray Davis and Masatoshi Koshiba, it could have chosen to highlight any one of their many achievements. R. Davis had become famous for detecting neutrinos from the Sun, the first extraterrestrial examples of these elusive particles; Dr. Koshiba had detected others from the great supernova explosion of 1987. Their work helped establish that neutrinos, thought by physicists to have zero mass, in fact had a small mass. But the Nobel Foundation honored R. Davis and M. Koshiba above all for having inaugurated a new scientific discipline: neutrino astrophysics.

Neutrino astrophysics is the branch of astrophysics that observes celestial objects using detectors of neutrinos, low-mass neutral leptons described by electroweak theory. Given their very weak interaction with matter, neutrinos have the ability to cross cosmological distances without deviating from their initial trajectory, making them excellent astronomical messengers that can be directly traced back to their place of production.

Observing cosmic neutrinos enables us to better study the workings of the most energetic and distant phenomena in the Universe. However, the difficulty of detecting these particles currently limits our ability to detect celestial objects emitting neutrinos. Before 2022, only three associations of neutrinos with celestial objects had been established: the Sun, supernova 1987A, and the active galaxy TXS0506+056.

The first experiments to observe solar neutrinos were carried out in 1967 – 1968 by scientists Raymond Davis Jr. and John N. Bahcall in the Homestake experiment. A neutrino detector, set up underground at a depth of 1,480 m to block the cosmic ray background and containing 610 tons of liquid perchloroethylene (C2Cl4), was used at Brookhaven National Laboratory continuously from 1968 to 1973. Researchers soon noticed that the number of neutrinos detected was lower than predicted by theory.

In July 2018, the IceCube observatory announced that it had been able to determine the origin of a high-energy neutrino in the TXS0506+056 blazar, located 3.7 billion light- years from Earth. This is the first detection to locate an object in the sky, and the first source of cosmic neutrinos to be identified.

In November 2022, IceCube made another important neutrino detection, identifying 79 neutrinos from the M77 galaxy, just 47 million light-years away. This first detection in a little distant and much-studied object should serve as a benchmark for future observations, and enable us to learn more about the active core of this galaxy.


Figure: Elementary particles in the Standard Model

Unveiling the Universe through Neutrinos: Illuminating Core-Collapse Supernovae, One Signal at a Time 

IntroductionNeutrino OscillationCore-Collapse Supernova Neutrino

Introduction

Neutrinos were first proposed as unknown neutral particles by W. Pauli in 1930 to explain the fact that the kinetic energy spectrum of electrons emitted in nuclear beta decay is a continuous spectrum. In 1954, Reines and Cowan succeeded in directly detecting antielectron neutrinos from nuclear reactors using a 300-liter liquid scintillator, establishing experimentally the existence of neutrinos. In 1962, the mu neutrino was discovered, and in 1997, the tau neutrino was discovered. Thus far, experiments have confirmed the existence of three generations of neutrinos and the existence of various neutrino sources in nature. In the Standard Model, neutrinos were thought to be massless, but the discovery of neutrino oscillations in SK in 1998 strongly suggested that neutrinos have finite masses. The HK experiment, scheduled to begin in 2027, will be able to study neutrinos with large statistics. Below, we discuss the properties of neutrinos, and more precisely neutrino oscillation, which is the focus of much attention in the search for theories beyond the Standard Model. 

Neutrino Oscillation

To learn more about neutrino oscillation, see Master 1 report.


Core-Collapse Supernova Neutrino

To learn more about neutrino from core-collapse supernova, see Master thesis.

Neutrino Detector


Some major neutrino detectors are DUNE, JUNO, HK, SK, IceCube, and KM3NeT. Here is a summary of those major neutrino detectors.


Each detector accordingly contributes to a wider neutrino property, oscillation, astrophysical phenomenon, and fundamental physics beyond the Standard Model.