Protons have more charm than we thought

Protons may have more "charm" than we thought, new research suggests. 

A proton is one of the subatomic particles that make up the nucleus of an atom. As small as protons are, they are composed of even tinier elementary particles known as quarks, which come in a variety of "flavors," or types: up, down, strange, charm, bottom and top. Typically, a proton is thought to be made of two up quarks and one down quark. 

But a new study finds it's more complicated than that. Protons can also contain a charm quark, an elementary particle that's 1.5 times the mass of the proton itself. Even weirder, when the proton does contain the charm quark, the heavy particle still only carries about half the proton's mass.

The finding all comes down to the probabilistic world of quantum physics. Though the charm quark is heavy, the chance of it popping into existence in a proton is fairly small, so the high mass and small chance basically cancel each other out. Put another way, the full mass of the charm quark doesn't get taken up by the proton, even if the charm quark is there, Science News reported.

Though protons are fundamental to the structure of atoms — which make up all matter — they're also very complicated. Physicists don't actually know protons' fundamental structure. Quantum physics holds that beyond the up and down quarks known to be present, other quarks might pop into protons now and then, Stefano Forte, a physicist at the University of Milan, told the podcast Nature Briefing(opens in new tab). Forte was a co-author of the new paper showing evidence for the charm quark in protons, published in the journal Nature(opens in new tab) Aug. 17.

There are six types of quarks. Three are heavier than protons and three are lighter than protons. The charm quark is the lightest of the heavy batch, so researchers wanted to start with that one to find out whether a proton could contain a quark heavier than itself. They did this by taking a new approach to 35 years of particle-smashing data.

Related: Why physicists are interested in the mysterious quirks of the heftiest quark

To learn about the structure of subatomic and elementary particles, researchers fling particles against each other at blistering speeds at particle accelerators such as the Large Hadron Collider, the world's largest atom smasher, located near Geneva. Scientists with the nonprofit NNPDF collaboration gathered this particle-smashing data going back to the 1980s, including examples of experiments in which photons, electrons, muons, neutrinos and even other protons were crashed into protons. By looking at the debris from these collisions, researchers can reconstruct the original state of the particles.

In the new study, the scientists handed over all of this collision data to a machine-learning algorithm designed to look for patterns without any preconceived notions of how the structures might look. The algorithm returned possible structures and the likelihood that they might actually exist. 

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