Scientists have learned more about the subatomic particles known as neutrinos, sometimes called ghost particles. They're known for traveling at near lightspeeds and being nearly massless, but a new study has shown the masslessness of the most massless neutrino.
It's least six million times lighter than the mass of an electron.
There are three types of neutrinos, referred to as the three flavors: electron, muon, and tau neutrinos. But there's so much that's still unknown about them. Scientists suspect electron neutrinos could be the lightest, but that honor could also go to tau neutrinos. Understanding them could help explain how the universe is held together, the reasons behind its continual expansion, and even the mysteries of dark matter.
“A hundred billion neutrinos fly through your thumb from the sun every second, even at night. These are very weakly interactive ghosts that we know little about. What we do know is that as they move, they can change between their three flavors, and this can only happen if at least two of their masses are non-zero,” says Dr. Arthur Loureiro, of University of College London Physics & Astronomy, in a press statement.
“The three flavors can be compared to ice cream where you have one scoop containing strawberry, chocolate, and vanilla. Three flavors are always present but in different ratios, and the changing ratio–and the weird behavior of the particle–can only be explained by neutrinos having a mass.”
That neutrinos even have mass is a somewhat new idea. That discovery in 1998 eventually earned Takaaki Kajita a Nobel Prize in 2015. But even after scientists learned of their mass, they still had lots of questions, like which one is the heaviest and which one is the lightest.
Neutrinos might have what's called a “normal mass ordering,” in which they are similar to the masses of the particles with which they're associated. Or it could be exactly the reverse: an “inverted mass ordering," in which their expected masses are flipped.
A new paper, published in Physical Review Letters by researchers from UCL, Universidade Federal do Rio de Janeiro, Institut d'Astrophysique de Paris and Universidade de Sao Paulo, sets an upper limit for the lightest neutrino, regardless of the flavor. No lower limit could be determined.
While there's a lot unknown about neutrinos, the scientists did have a big pool from which to study. They collected data from the Baryon Oscillation Spectroscopic Survey (BOSS) in New Mexico, which has examined 1 million galaxies to study the universe's expansion by "mapping the distribution of luminous red galaxies and quasars," according its to website.
“We used information from a variety of sources including space- and ground-based telescopes observing the first light of the universe (the cosmic microwave background radiation), exploding stars, the largest 3D map of galaxies in the Universe, particle accelerators, nuclear reactors, and more,” says Dr. Loureiro.
“As neutrinos are abundant but tiny and elusive, we needed every piece of knowledge available to calculate their mass and our method could be applied to other big questions puzzling cosmologists and particle physicists alike.”
All of that data required a tremendous amount of computing power.
“We used more than half a million computing hours to process the data; this is equivalent to almost 60 years on a single processor. This project pushed the limits for big data analysis in cosmology,” says Ph.D. student Andrei Cuceu, the paper's second author.
Neutrino mass will be crucial to cosmological studies to come, which is why the paper's authors are excited about future research projects like the Dark Energy Spectroscopic Instrument (DESI), which, when completed, will build a 3D map spanning the nearby universe to 11 billion light years.
"It is impressive that the clustering of galaxies on huge scales can tell us about the mass of the lightest neutrino, a result of fundamental importance to physics. This new study demonstrates that we are on the path to actually measuring the neutrino masses with the next generation of large spectroscopic galaxy surveys, such as DESI, Euclid and others,” says UCL Professor Ofer Lahav, coauthor of the study and chair of the UK Consortiums of the Dark Energy Survey and DESI.
And if studying neutrinos in space doesn't work out, scientists can always try building them here on Earth.
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