Why care about ghosts among the particles?
At 7:35 am UTC on February 23, 1987, the Kamiokande II detector in Japan suddenly detected more than 10 anti-neutrinos. It was designed to detect proton decay, so this was not a regular event at the detector, and scientists were alarmed (It was confirmed by proton decay detectors in the US and Russia). Then a grad student named Ian Shelton announced his observation of a supernova in the Large Magellanic Cloud. It was evident then that the neutrinos were a by-product of a core-collapse supernova*.
Being inert and small has its benefits for the neutrino. When a star is at the end of its life it turns into a supernova and explodes. Even light cannot escape the core of the supernova as it interacts with matter inside the supernova, if it hasn’t already disintegrated to give out electrons and positrons(anti-electrons) but neutrinos can and in fact they are the first messengers of such cosmic events. Moreover, light from these stellar events are deflected by magnetic fields due to their electromagnetic nature but neutrinos aren’t, which makes them reliable sources of information about events in the universe. As an evidence to such utility, an extragalactic (out of the Milky Way galaxy) blazar** was identified as the source of high-energy neutrino by ice-cube in 2017.
Besides supernovas, neutrinos can also be used as a means towards more ambitious goals. Answering the age-old question of evolution of the universe is one of them. Also, these particles are expected to provide the answer to the questions like why is everything made up of matter (and not antimatter)? Wait, how can these tiny things tell us so much about the universe? I’m glad you asked.
The largest source of neutrinos in the universe today are the remnants of the first 0.0001 seconds after the big bang. Billions of them are moving near you, touching you and are penetrating your body as you read this. These neutrinos exist as the cosmic background radiation and can help understand the early universe, its formation and evolution. Also, according to the standard model of physics, equal matter and antimatter must have been created and hence they must have annihilated each other until none remained. However, both matter and antimatter exist today. In fact, there is more matter than antimatter in the universe. The reason for this imbalance is speculated to be hidden in the neutrino because despite being charge less, neutrinos and anti-neutrinos have been observed to show different behaviours. Studying this may lead to an answer to those big questions.
For us to get the answer to these giants of questions, we need to detect these neutrinos, but they are a ghost in the particles, and the most sensitive of our instruments find it hard to observe. We want to ‘see’ these things, we want to study them, and we want our questions answered. But the first question we need to tackle is, ‘How do we do it?’
*A supernova is formed when a star runs out of fuel and cannot support itself against its own gravity. A core collapse supernova is one in which the whole surface of a star moves towards its own core, due to gravity, until there is no space to move further inwards and the speed has to be zero suddenly. This creates a super-huge outward force that leads to the explosion. If the star however manages to push inwards, ta-da, a black hole is formed.
** Blazars are fast rotating discs of matter that throw out jets of ionized matter. These are found in the center of galaxies and are thought to be created due to black holes in the center of the parent galaxy.
In case you haven’t read the first article. Read the first article <<Neutrino, a ghost in the particle>>.