quark charm proton

Study confirms that the proton has an intrinsic charm quark


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While physicists have been debating this for almost forty years, a new data analysis by the NNPDF collaboration reveals that the proton does indeed have another elementary particle called a charm quark. The intrinsic nature of this quark could have important implications in the search for new physics.

All the matter that surrounds us is made up of atoms, which themselves contain subatomic particles: protons and neutrons make up the nucleus, around which electrons gravitate. According to the standard model of particle physics, the proton is a so-called composite particle: experimental evidence shows that it is made up of at least three particles (two quarks at the top and a quark down), bound by gluons. Quantum theory, however, predicts that the proton can contain several other quark-antiquark pairs, including quarks charm — which are more massive than the proton itself.

Theorists believe that these quarks charm are “intrinsic” to the proton, which means that they are part of the proton on long time scales and are not the result of interactions with an external particle. However, no experiment has succeeded in proving the existence of this quark. charm intrinsic at the moment. Thanks to the analysis of huge amounts of collision data via machine learning techniques, the NNPDF collaboration finally provides the long-awaited proof.

Data from over 500,000 collisions analyzed

The NNPDF collaboration (for neural network particle distribution function) conducts research in the field of high energy physics. Its objective is to determine the precise structure of the proton (ie the distribution of its constituents, quarks and gluons) using artificial intelligence methods. This knowledge is a crucial part of CERN’s Large Hadron Collider (LHC) research programme.

Concretely, the group used a machine learning model to construct different hypothetical structures of protons, with different flavors of quarks; remember in passing that these flavors are six in number: at the top, down, High, bottom, odd and charm. They then compared these different proton structures to the results obtained during more than 500,000 real collisions, implemented in particle accelerators over the past ten years.

charm quark distribution function
Parton distribution functions of the intrinsic charm quark and comparison with models. © NNPDF Collaboration

They thus discovered that a tiny part (0.5%) of the momentum of a proton is attributable to a quark. charm. The latter is much heavier than quarks at the top and down (a few thousand times heavier than a quark at the top !). This discovery is due in particular to an LHCb (Large Hadron Collider beauty) experiment carried out last year on the Z boson, which revealed the presence of quarks charm in protons. According to their calculations, the team estimates that in the proton – whose mass is slightly less than 1 GeV – quarks charm and their antiparticle, having a mass of about 1.5 GeV each, sometimes appear spontaneously.

A level of confidence still too low

Thus, as incredible as it may seem, the proton can be composed of a particle more massive than itself! ” This goes against all common sense. It’s like buying a one-kilo packet of salt, and two kilos of sand come out. But in quantum mechanics, such a thing is quite possible. says Juan Rojo, a theoretical physicist at the Free University of Amsterdam and lead author of the paper describing the discovery.

The researchers also claim that if the proton did not have a pair of charm-anticharm quarks, there would be only a 0.3% chance of obtaining the values ​​observed experimentally. This gives their results a confidence level of 3 sigma. ” This is what we call a serious index in particle physics “Says Rojo. However, a level of 5 sigma is necessary for a result to be considered truly significant. Other research will therefore have to be carried out to move from the status of “proof” to the status of “discovery”.

In particle accelerators, the motion of colliding protons provides such an amount of energy that heavy quarks and their antiparticles can sometimes form from this energy — these “extrinsic” quarks are not fundamental to the identity of the proton. On the other hand, it is a question here of quarks which appear naturally, from time to time, in an undisturbed proton, therefore at low energy.

The phenomenon is rare, but can be of great importance for the experiments carried out at the LHC. ” In CERN experiments, we create collisions between protons and look for subtle anomalies that could indicate new particles or forces. This is only possible if one fully understands their nature. “, concludes the physicist.

Source: NNPDF Collaboration, Nature

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