A new study has discovered strong evidence that a proton contains a charm quark, in addition to the two up and one down quarks listed in textbook descriptions.
A fundamental component of every atom, the proton appears to have a more complex structure than is typically described in textbooks. A delicate particle physics experiment like the Large Hadron Collider could be affected by the discovery (LHC).
It was once believed that protons were indestructible, but experiments with particle accelerators in the 1960s showed that they were indeed composed of three tiny particles called quarks. A proton contains two up quarks and one down quark from six different types or varieties of quark.
However, since the structure of a particle in quantum mechanics is determined by probabilities, it is theoretically possible for more quarks to appear as matter-antimatter pairs inside the proton. According to a 1980 experiment at the European Muon Collaboration at CERN, the proton could contain a charm quark and its antimatter counterpart, an anti-talisman, but the results were uncertain and the subject of intense debate.
More attempts have been made to detect the proton's charm element, but various teams have found conflicting findings and have had trouble separating a proton's intrinsic components from the high-energy particle accelerator environment, where all kinds of quarks are constantly being produced and destroyed.
Now, Juan Rojo of the Vrije University of Amsterdam in the Netherlands and colleagues have discovered evidence that the charm quark contributes only a tiny fraction of the proton's momentum, about 0.5%. Despite decades of research, Rojo finds it surprising that we are "constantly uncovering new properties of the proton, and especially new components."
Rojo and his team used a machine learning model to create hypothetical proton structures made up of all quark flavors to identify the charm component.
They then compared these structures to more than 500.000 real collisions from decades of particle accelerator experiments, including those at the LHC.
According to Christine Aidala of the University of Washington, this application of machine learning was crucial because it produces models that physicists wouldn't be able to think of on their own, reducing the likelihood of skewed measurements.
The researchers discovered that if the proton did not have the charm-anti-charm quark pair, they were only 0.3% likely to see the results they were looking at. This is what physicists call the "3-sigma" result, which is typically interpreted as a possible indication of something intriguing. More work is needed to elevate results to the traditional benchmark for a discovery – a 5 sigma level, or roughly 3,5 in 1 million chance of bad luck.
Taking into account the latest findings from the LHCb Z-boson experiment, the researchers modeled the statistical distribution of proton momentum, both with and without an attractive quark. They discovered that the model more closely matched the results, given that the proton contains a charm quark.
This indicates that they have more confidence in their proposition for the existence of a charm quark than does the sigma level alone. According to Rojo, the fact that other studies have come to the same conclusion "convinced us in particular that our results are reliable."
This is very important, according to Harry Cliffe of Cambridge University. "Given how common this particle is and how long we've known about it, there's a lot we don't really understand about its infrastructure," he adds.
According to Cliffe, additional physics research based on precise models of proton infrastructure at the LHC could be influenced by the proton's gravitational component.
According to Rojo, this new structure may also need to be considered by the IceCube Neutrino Observatory in Antarctica, which studies rare neutrinos formed when cosmic rays hit particles in Earth's atmosphere. According to him, the attraction level of the proton has a significant effect on the probability that a cosmic ray will strike a nucleus in the atmosphere and produce neutrinos.