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How do you weigh a ghost? If you are a cosmologist, you could use … the universe. Combine vast cosmological data with information from particle accelerators. You have a good enough scale to measure the mass of a neutrino, also called "ghost particle".
Thus, for the first time, a team of scientists set an upper limit on the mass of the lightest of the three types of neutrinos.
Neutrinos are special little things. They are among the most abundant subatomic particles in the universe, similar to electrons, but without charge and almost without mass. This means that they very rarely interact with normal matter; In fact, billions of people are currently passing through your body.
That's why they earned the ghost particle nickname. It also makes them incredibly difficult to detect. We have some detection methods, such as the Cherenkov neutrino detectors, but they are indirect and capture the effects of passing neutrinos, rather than the neutrinos themselves.
All of this means that measuring the near zero mass of these particles is a particularly difficult challenge.
"We used information from various sources, including space and ground telescopes observing the first light of the Universe (cosmic microwave background radiation), explosive stars, the largest 3D map of Universe galaxies, particle accelerators, nuclear reactors and more, "said cosmologist Arthur Loureiro of University College London in the UK.
"Since neutrinos are abundant but tiny and elusive, we needed all the knowledge available to calculate their mass, and our method could be applied to other big questions that are of interest to both cosmologists and particle physicists. . "
We know that neutrinos have a mass because they come in three types, or "flavors": the electron, the muon and the tau. But it's not as simple as it's three separate entities; the particles really oscillate between these aromas (and what aroma to which mass is still a mystery).
"The three flavors can be compared to ice cream where you have a ball containing strawberry, chocolate and vanilla," explained Loureiro.
"Three flavors are always present, but in different ratios.The changing ratio, as well as the strange behavior of the particle, can only be explained by the neutrinos having a mass."
Upper limits were previously fixed to the combined mass of the three neutrinos using cosmological measurements. We can use data on the Universe because the mass of neutrinos can actually modify some of the cosmological observables.
Thus, once we have calculated the neutrino density, we can use it to set an upper bound on the collective effect produced by neutrinos.
This time, the researchers went further by using the data they had collected as a framework for mathematically modeling the neutrino mass. This was introduced into a powerful supercomputer called Grace to perform the calculations.
"We have used more than half a million hours of computation to process the data, which equates to nearly 60 years with a single processor," said cosmologist Andrei Cuceu of the company. University College London.
But Grace did it: the supercomputer returned a mass corresponding to the lightest of the three neutrons of 0.086 electron volts (with a lower limit of zero), about 1.5 × 10.-37 kilograms. The team also calculated a combined mass for the three neutrinos – 0.26 electronvolt. Both results have a 95% confidence interval.
By context, a stationary electron has a mass of 511 000 electrons-volts, ie 9.109 10-31 kilograms.
We still have no constraints for the other two masses, nor have we mapped these masses to neutrino aromas. And there is still uncertainty about the rate of expansion of the Universe (which cosmologists are trying to solve). But it's a pretty amazing step.
"It is impressive that the clustering of galaxies on large scales can inform us about the mass of the lightest neutrino, a result of fundamental importance for physics," said astronomer Ofer Lahav of the United States. University College London.
"This new study demonstrates that we are on the right track to truly measure neutrino masses with the next generation of large spectroscopic galaxy surveys, such as DESI, Euclid and others."
The research must appear in Letters of physical examination and is available on arXiv.
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