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New Nature paper proves: the proton has some intrinsic charm

17 August 2022
Researchers at Nikhef, VU Amsterdam and others have found the first evidence that not only up- and down quarks exist inside the proton, but also medium-heavy charm quarks. Their analysis of the largest data set ever is in Nature this week.

Theoretical physicist Juan Rojo (Nikhef/ VU) admits it sounds crazy. The proton, the nucleus of a hydrogen atom, contains quarks that are heavier than the proton itself. “That goes against all common sense. It's like buying a one-kilogram pack of salt, which then comes out two kilograms of sand. But in quantum mechanics, such a thing is just possible.”

Triple of quarks
Protons, according to particle physics, are basically made up of three quarks, two up-quarks and one down-quark, held together by gluons. In the shadowy quantum world, however, it is not excluded that very briefly other quarks arise and disappear again, together with their antiparticle. For example, the charm quark, some one thousand times heavier than an up-quark.

Rojo is lead author of the Nature article by the NNPDF collaboration (neural network parton distribution function), an occasional group that uses artificial intelligence to combine and analyse measurements of protons from all sorts of experiments. This also requires the best theoretical models for the proton.

Last year, NNPDF already published suspicions that protons could also contain heavier quarks than up and down, but statistically this was not yet fully convincing. The new analysis has a probability of 3 sigma that there are charm quarks in the proton. “That’s what we call a serious clue in particle physics”, Rojo says excitedly.

That result is also due to a measurement from the LHCb experiment at the LHC accelerator at CERN, which also saw evidence of charm quarks in protons last year. The new result means that in the proton of about 1 GeV mass, charm quarks and their antiparticle occasionally spontaneously appear, which have a mass of 1.3 GeV each. That counterintuitive news, says Rojo, is also presumably why a general journal like Nature publishes the article. “Particle physics rarely makes it to Nature unless you have something very special for a broad audience.”

Intrinsic charm
Two effects are at play in the hunt for charm in the proton, NNPDF theorists know. Most data on the quarks in protons come from colliding protons in accelerators. In doing so, the motion of the colliding protons feeds so much energy that heavy quarks and their antiparticle can sometimes form from it.

Rojo and his team, however, are concerned with what is called the intrinsic charm presence in the proton: charm quarks that naturally pop up now and then in the undisturbed proton. This is a very rare phenomenon, though: less than one percent of the energy of the nuclear particle is attributable to charm, the analysis shows.

Still, the small effect could prove to be of great importance to particle physics itself, Rojo thinks. “In the experiments at CERN, we shoot protons at each other and look for subtle anomalies that could indicate new particles or forces. You can only do that if you perfectly understand what you are shooting at each other. Precisely in that, we have now taken a new step.”

Image: A graphic impression of quarks and gluons inside the proton. Credits: CERN.