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Physicists surprised to discover the proton contains a charm quark

The textbook description of a proton says it contains three smaller particles – two up quarks and a down quark – but a new analysis has found strong evidence that it also holds a charm quark

Physics 17 August 2022


An artist’s impression of a proton – the large red spheres are up quarks and the large blue sphere a down quark


The proton, a particle found at the heart of every atom, appears to have a more complicated structure than is traditionally given in textbooks. The find could have ramifications for sensitive particle physics experiments like the Large Hadron Collider (LHC).

While protons were once thought to be indivisible, experiments with particle accelerators in the 1960s revealed that they contained three smaller particles, called quarks. Quarks come in six types, or flavours, and the proton contains two up quarks and one down quark.

But in quantum mechanics, a particle’s structure is governed by probabilities, meaning there is theoretically a chance that other quarks could crop up inside the proton in the form of matter-antimatter pairs. An experiment at the European Muon Collaboration at CERN in 1980 hinted the proton might contain a charm quark and its antimatter equivalent, an anticharm, but the results were inconclusive and hotly debated.


There were further attempts to identify the proton’s charm component, but different groups found conflicting results and had difficulty separating out the intrinsic building blocks of a proton from the high energy environment of particle accelerators, where every kind of quark is created and destroyed in rapid succession.

Now, Juan Rojo at Vrije University Amsterdam in the Netherlands and his colleagues have found evidence that a small part of the proton’s momentum, around 0.5 per cent, comes from the charm quark. “It’s remarkable that even after all these decades of study, we’re still finding new properties of the proton and, in particular, new constituents,” says Rojo.

To isolate the charm component, Rojo and his team used a machine learning model to come up with hypothetical proton structures consisting of all the different flavours of quarks and then compared them with more than 500,000 real-world collisions from decades of particle accelerator experiments, including at the LHC.

This use of machine learning was especially important, says Christine Aidala at the University of Washington, because it could generate models that physicists wouldn’t necessarily think of by themselves, reducing the chance of biased measurements.

The researchers found that, if the proton doesn’t contain a charm-anticharm quark pair, there is only a 0.3 per cent chance of seeing the results they examined. Physicists call this a “3-sigma” result, which is normally seen as a potential sign of something interesting. More work is needed to boost the results to 5-sigma level, meaning about a 1 in 3.5 million chance of a fluke result, which is traditionally the threshold for a discovery.

The team looked at recent results from the LHCb Z-boson experiment and modelled the statistical distribution of the proton’s momentum both with and without a charm quark. They found the model better matched the results if the proton is assumed to have a charm quark. This means they are more confident in proposing the presence of a charm quark than the sigma level by itself suggests. “The fact that very different studies converge on the same result made us especially confident that our results were solid,” says Rojo.

“Given how ubiquitous this particle is and how long we’ve known about it, there’s still a lot we don’t actually understand about its substructure, so this is definitely important,” says Harry Cliffe at the University of Cambridge.

The proton’s charm component could also have ramifications for other physics experiments at the LHC, says Cliffe, as they rely on accurate models of proton substructure. The IceCube Neutrino Observatory in Antarctica, which looks for rare neutrinos produced when cosmic rays hit particles in Earth’s atmosphere, might also need to take this new structure into account, says Rojo. “The probability of a cosmic ray impacting an atmosphere nucleus and producing neutrinos is quite sensitive to the charm content of the proton,” he says.

Journal reference: Nature, DOI: 10.1038/s41586-022-04998-2

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