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The new analysis of the experimental data collected from the LHC particle collider shows that nucleons – protons and neutrons which form the nuclei of all atoms – are nearly half as big as it was assumed until now.
The structure of an atom with nucleons – protons and neutrons – visible in the center (artistic impression). Image credit: Wikimedia Commons, CC0 Public Domain
Scientists use a powerful machine called the Large Hadron Collider (LHC) to study tiny particles contained in the nuclei of atoms. With this equipment, researchers smash together heavy ions, which are atoms with their outer electrons removed. When these collisions happen, nucleons fall apart into their constituent parts.
These building blocks of protons and neutrons, called quarks and gluons, mix together to create a strange new form of matter called quark-gluon plasma (QGP). To understand this new form of matter, scientists compare a theoretical model to lots of experimental data. One important thing they look at is the size of the particles inside the colliding nuclei – usually, using lead ions.
Now, here’s the interesting part. In experiments done at lower energies, scientists thought the size of protons and neutrons was around 0.5 femtometers (that’s really tiny – about 5×10-16 meters). But when they looked at the collisions using heavy ions, they got a different answer.
The LHC collider. Here and at similar facilities, scientists investigate collisions of sub-atomic particles. Image Credit: CERN
Previous analyses suggested that the particles were actually bigger, around 1 femtometer. It was a bit confusing. This upper limit, however, was until now considered as the “standard” size of protons and neutrons.
Last year, an analysis done at the University of Bonn brought this “standard” size down to 0.84 femtometers.
But in the most recent analysis performed by scientists from Massachusetts Institute of Technology, Utrecht University and CERT, researchers looked at the rate at which the collisions of heavy ions happened.
They found that the size of the particles needed to be smaller to match the reaction rate. Including this rate in their analysis, they found that the preferred size of nucleons was around 0.6 femtometers. That also essentially solves the theoretical discrepancy that existed until now.
To measure the collision rate, scientists use something called the total hadronic cross-section. It’s a way to calculate how often atoms collide based on their density. The calculations are probably the easier part of the research, but actually measuring this parameter in experiments is harder.
The structure of an atom – artistic impression. Image credit: Geralt via Pixabay, free license
In 2022, researchers at ALICE made a better series of measurements, reducing the uncertainties. These measurements helped scientists compare their original model with more than 600 pieces of newly-acquired experimental data.
The original model explaining the size of protons and neutrons didn’t include the collision rate measurement, which led scientists to assume that the particle size was slightly smaller than 1 femtometer. But after the model was corrected using extended and more precise data, it now shows that the size of neutrons and protons is closer to 0.6 femtometers.
Explainer: How big is one femtometer?
The femtometer is an incredibly small unit of measurement. To give you an idea, it is equal to one quadrillionth (or one-millionth of one billionth) of a meter. That’s a mind-bogglingly tiny scale!
To put it into context, imagine shrinking down an object to the size of a femtometer—it would be comparable to reducing something from the size of our entire solar system down to the size of a ball with a radius of roughly one centimeter.
Or, you can also imagine shrinking our Earth down to a sphere with a radius of roughly 6.4 nanometers – which is equal to a size of a few nanotransistors in a modern microchip used in your phone.
Written by Alius Noreika
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