Trillions of neutrinos are currently passing through you, all other people on Earth, and the whole planet itself as you read this page. According to Diego Alejandro Restrepo, a professor at the Institute of Physics of the University of Antioquia and a physicist with a PhD from the University of Valencia, this occurs because these particles are among the most prevalent in nature. The abundance of neutrinos is comparable to that of photons, which are ten trillion times more abundant than any other particle and are the building blocks of light. Neutrinos have been present for 13 billion years, which means they are constantly passing through us. They have existed “since almost the big bang itself, which makes them some of the oldest particles in the universe,” according to Deywis Moreno López, PhD in Particle Physics from the Johannes Gutenberg University, Germany, and director of the high energy experimental physics group at the Antonio Nario University (UAN).

Neutrinos are paradoxically particles with very few known properties, considering their age and abundance, which is partially explained by the fact that they are highly “shy.” They interact with other particles or atoms relatively little because of their shyness, to use technical jargon. Even though billions of neutrinos pass through us every second, only two are thought to interact with the body during a person’s lifetime, according to Restrepo. They were given the name “ghost particles” by the physics community because of this. Italian Enrico Fermi, one of the most significant physicists of the 20th century and recipient of the 1938 Nobel Prize in Physics, was the first to discuss neutrinos. By building on the work of his Austrian colleague Wolfgang Pauli, he achieved this while giving the particle this name because, according to the calculations of the Italian physicist, it lacked both mass and charge. The particles Fermi was referring to were eventually found by an experiment a few decades later, around the middle of the century. This is how neutrinos were included to the standard model of particle physics, which is the most accurate theory to date regarding the nature and behavior of particles.

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Two separate tests, however, were able to demonstrate the reverse at the turn of the millennium, disproving Fermi’s theory and upending the accepted model by demonstrating that the neutrino has mass. Scientists at the Sudbury Neutrino Observatory (SNO) in Canada and the Super-Kamiokande detector in Japan made this significant discovery. They did so by identifying one of neutrinos’ unique characteristics—their oscillation—which proved the Italian researcher incorrect. Which does this imply? Three different forms of neutrinos—electron, muonic, and tauonic—have so far been identified, according to researcher Alberto Zapata from the University of Antioquia’s Institute of Physics. “These particles are electronic neutrinos when they leave the Sun, one of their primary sources, but tauonic or muonic when they come to the Earth. They alter their identity as they reproduce. Amalia Betancur, a physics PhD candidate and professor at the EIA University of Medellin, compares this oscillation to having cats, panthers, and tigers. And the cat develops into a panther, then a tiger. Oscillation is the term for this shift in the “identity” of these particles, and despite its identification, scientists are still baffled as to why it happens. The Japanese Takaaki Kajita and the Canadian Arthur McDonald, from Super-Kamiokande and SNO, respectively, earned the Nobel Prize in Physics in 2015 for proving that they did have the mass of neutrinos, but they are unable to estimate it.

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