Study confirms that the proton has an intrinsic charm quark

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While physicists have debated this for nearly forty years, a new data analysis by the NNPDF collaboration reveals that the proton actually has another elementary particle called a charm quark. The intrinsic nature of this quark may have important implications in the search for new physics.

All the matter that surrounds us consists of atoms, which themselves contain subatomic particles: protons and neutrons form 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 consists of at least three particles (two quarks) up and a quark down), bound by gluons. However, quantum theory predicts that the proton may 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, meaning that they are part of the proton on long time scales and are not the result of interactions with an external particle. However, no attempt has succeeded in proving the existence of this quark. charm inherent in 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 parton distribution function) conducts research in high-energy physics. Its purpose 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 program.

Specifically, 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: up, down, top, bottom, strange and charm. They then compared these different proton structures with the results obtained during more than 500,000 real collisions, implemented in particle accelerators over the past ten years.

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

They discovered that a small fraction (0.5%) of a proton’s momentum can be attributed to a quark. charm. The latter are much heavier than quarks up and down (a few thousand times heavier than a quark up !). This discovery is mainly due 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 – there are quarks charm and their antiparticle, each having a mass of about 1.5 GeV, sometimes appears spontaneously.

A level of trust that is still too low

As incredible as it may seem, the proton can thus 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 achieving the experimentally observed values. 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. Therefore, other research will need to be carried out to move from the status of “evidence” 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 be formed from this energy – these “outer” quarks are not fundamental to the proton’s identity. On the other hand, we are talking about 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 to 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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