The intrinsic charm of the proton
A new piece of research by the NNPDF (Neural Networks Parton Distribution Functions) collaboration, led by the University of Milan and the INFN, National Institute of Nuclear Physics, sheds light on a surprising feature of proton structure. The study, published in Nature, has found that also the 'charm' quarks, together with the more well-known and lighter 'up' and 'down' quarks, is to be counted among the intrinsic components of the proton, confirming a hypothesis proposed over 40 years ago. The result was obtained using innovative machine learning techniques, thanks to which it was possible to analyze and process the large amount of data produced by particle colliders. In addition to improving the understanding of the structure of protons, still only partly known, the study will help to improve the accuracy of the description of collisions between these particles, enabling the possible observation of signals of new physics.
A simple representation of the proton as an object consisting of only two 'up' quarks and one 'down, quark confined and bound together by means of the exchange of the bosons that mediate of the strong interaction, the gluons, ignores the presence inside the proton of an infinite number of pairs of quarks and antiquarks, their antimatter partners. In proton collisions, this complex composition changes as the energy of the collision increases, and thus so does the resolution of the measurement. This corresponds to the predictions of quantum chromodynamics, the current theory of the strong nuclear force.
“Starting from the experimental data", explains Stefano Forte (INFN researcher and professor at the Physics Department "Aldo Pontremoli" of the University of Milan) who leads the NNPDF collaboration, "and using quantum chromodynamics, we are able to indirectly resolve the internal structure of the proton at rest. The main difficulty in this procedure is due to the fact that experimental measurements and theoretical predictions are both uncertain: if these uncertainties are not treated correctly, final results may depend on subjective assumptions".
In 1980, physicists Stan Brodsky, Paul Hoyer, Carsten Peterson and Noriskue Sakai suggested that even the 'charm' quark, whose mass is greater than that of the proton, might be an intrinsic component of the proton at rest - as distinct from heavy quarks irradiated in collisions between protons at high energies. This suggestion remained controversial for over 40 years. The controversy has been resolved by the NNPDF collaboration, thanks to the development and use of cutting-edge machine learning methodologies.
"The result was made possible by the use of very recent new data" explains Forte. Felix Hekhorn and Alessandro Candido, respectively postdoc and doctoral student at the University of Milan stress that "a fundamental role was played by the use of a number of theoretical results obtained by our own collaboration. These allow us to unambiguously separate the intrinsic component from the radiative component". Juan Cruz Martinez, postdoctoral fellow at the University of Milan, observes that "most important of all was the development of a methodology based on machine learning that allows information to be extracted from data in a particularly effective way".
This result on the intrinsic charm component of the proton will have important implications in the study of the properties of the proton, and also on searches of physics beyond the Standard Model, the theory that describes elementary particles and their interactions. "On the one hand, the 'charm' component will have to be included in the computation of physical processes occurring in proton collisions, and it will affect the result. This will therefore have an impact on precision physics studies, such as the measurement of the parameters of the current theory, and consequently on the search for deviations from it, such as searches for possible candidate particles for dark matter. On the other hand, this knowledge provides us with important clues in the understanding of the proton structure starting from quantum chromodynamics, which remains difficult and elusive”, concludes Forte.
The work of the NNPDF collaboration was made possible thanks to the results achieved by N3PDF, a research project, also coordinated by Forte. This project has as its main goal the development of a methodology based on modern machine learning techniques for the determination of the structure of the proton. The project was funded in 2016 by the European Research Council through an ERC Advanced Grant. The methodology developed by N3PDF is now used systematically by NNPDF. All results, methods and codes of both projects are made freely available in open-source format.
The NNPDF international collaboration, which includes nine universities and research institutes from six countries, is one of three main international collaborations devoted to the determination of the structure of the proton.