Physicists surprised to discover the proton contains a charm quark

Alex Wilkins is a writer.

The structure of the protons appears to be more complex than is usually given in textbooks. It's possible that the find will affect sensitive particle physics experiments.

Experiments with particle colliders in the 1960s showed that protons contained three smaller particles called quarks. Two up quarks and one down quark are contained in the protons.

There is a chance that other quarks could crop up inside the protons in the form of matter-antimatter pairs. The results of an experiment at the European Muon Collaboration in 1980 were not unanimous.

Different groups found conflicting results when trying to separate the building blocks of a protons from the high energy environment of particle accelerators.

Juan Rojo and his colleagues at Vrije University Amsterdam have found evidence that shows the charm quark plays a small part in the strength of the protons. Rojo says it's remarkable that even after all these decades of study we're still finding new properties.

The charm component was isolated by using a machine learning model to come up with a hypothetical structure of all the different flavours of quarks.

Christine Aidala at the University of Washington said that the use of machine learning was important because it could generate models that physicists wouldn't necessarily think of by themselves.

There is a small chance that the results of the study will be seen. Physicists call this a "3-sigma" result, which is usually seen as a sign of something interesting. The threshold for a discovery is usually 1 in 3.5 million, so more work is needed to get the results to 5-sigma level.

The team looked at recent results from the Z-boson experiment and modeled the statistical distribution of the protons's momentum. If the protons are assumed to have a charm quark, the model would match the results better. They are more confident in their proposal of the presence of a charm quark. Rojo says that the fact that different studies converge on the same result made them confident.

There is still a lot we don't know about the substructure of this particle, so this is important.

The model of the substructure of the protons could have ramifications for other physics experiments. The IceCube Neutrino Observatory may need to take into account the new structure. The charm of the protons is sensitive to the probability of a Cosmic Ray hitting an atmosphere nucleus.

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