Two researchers who separately discovered that neutrinos have mass won the Nobel Prize in Physics

For particle physics this was the historical discovery. The Standard Model of the Innermost Engine of Matter has worked successfully, surviving all experimental challenges for over twenty years, but the model may only cover part of the picture of the universe

A fundamental discovery in astrophysics earned its discoverer a Nobel Prize. The 2015 Nobel Prize in Physics was jointly awarded to Takaki
Kajita of Japan and Arthur MacDonald of Canada, for their contributions in conducting experiments that demonstrated that neutrino particles change identity. This change requires that the neutrino particles have mass. The discovery changed our understanding of the most basic operation of matter and may be essential to our understanding of the universe.

Over the years there have been quite a few proposals to solve the riddle - how neutrino particles are created in the sun. One of them was that they change their identity. According to the standard model of particle physics, there are three types of neutrinos - electro-neutrino, muon-neutrino and tau-neutrino. Each type of neutrino has a corresponding electrically charged particle - the electron and its heavy and short-lived relatives the muon and the tau. The sun produces only electron-neutrinos, but if they change identity and become muon-neutrinos or tau neutrinos, the mystery of the electron-neutrino's disappearance from the sun will be solved.

Around 1990, Kajita presented a discovery that neutrino particles in the atmosphere switch between two identities on their way to the Kamiokanda detector in Japan.

Meanwhile, a group of researchers in Canada led by Arthur MacDonald managed to show that neutrinos from the Sun do not disappear on their way to Earth. Instead they simply take on a different identity when they arrive at the Sudbury Neutrino Observatory.
The neutrino puzzle that physicists have struggled with for decades has been solved. Compared to the theoretical calculations of the number of neutrino particles, up to two-thirds of the neutrinos were missing in the observation made on Earth. Now these two experiments revealed that these particles do not disappear but simply change identity. The discovery led to far-reaching conclusions according to which the neutrinos, which for many years were considered massless, must have a mass, even a very small one.

For particle physics this was a historic discovery. The standard model of the innermost engine of matter has been operating successfully, surviving all experimental challenges for over twenty years. However, because neutrinos must have mass, the new observations showed that the Standard Model cannot be a complete theory that covers all the fundamental particles in the universe.
Neutrinos, difficult to detect due to the lack of electric charge, are the second most abundant particle in the universe after photons - the particles of light. The earth is bombarded with thousands of billions of them that pass, among other things, through our bodies without coming into contact with the material.

Most of the neutrino particles were created in reactions between cosmic radiation and the Earth's atmosphere. Others are produced in the nuclear reactions within the Sun. Almost nothing stops their journey. Neutrinos are one of the most elusive elementary particles.
Experiments are now ongoing around the world with the aim of capturing neutrinos and studying their properties. New discoveries about the deepest secrets are expected to change our understanding of the history, structure, and future fate of the universe.

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• Science on ice
• Ghost lighthouses of new physics
• The dark matter puzzle