- New research shows that protons contain intrinsic charm quarks.
- This is despite the fact that subatomic charm quarks are about 1.5 times heavier than the proton itself.
- When charm quarks are present, they carry about half the mass of the proton.
Protons are particles that exist in the nucleus of all atoms, their number defining the elements themselves. However, protons are not elementary particles. Rather, they are composite particles made up of smaller subatomic particles, namely two “up quarks” and one “down quark” held together by force-carrying particles (bosons) called “gluons”.
However, this structure is not certain, and quantum physics suggests that other particles should “appear” and disappear along with these three quarks at any time, affecting the mass of the proton. This includes other quarks and even quark-antiquark pairs.
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After all, the deeper scientists studied the structure of the proton using high-energy particle collisions, the more complicated the situation became. As a result, physicists have speculated for about four decades that protons might harbor a heavier form of quarks than up and down quarks called “intrinsic charm quarks,” but confirmation of this has been elusive.
Using a high-precision determination of the quark-gluon content of the proton and examining 35 years of data, particle physics data scientists have uncovered evidence that the proton contains intrinsic charm quarks.
What makes this result even more extraordinary is that this quark flavor is one and a half times more massive than the proton itself. However, as part of the proton, the charm quark is still only about half the mass of the composite particle.
The weirdness of quantum mechanics
This counter-intuitive setup is a consequence of the weirdness of quantum mechanics, the physics that governs the subatomic world. This requires an idea of the structure of a particle and what can be found in it as being probabilistic in nature.
“There are six types of quarks in nature, three of which are lighter than the proton [up, down, and strange quarks] and three are heavier [charm, up, and down quarks]says Stefano Forte, team leader of the NNPDF collaboration and Professor of Theoretical Physics at the University of Milan Podcast “Nature Briefing”.. “You might think that only the lighter quarks are in the proton, but in fact the laws of quantum physics allow the heavier quarks to be in the proton as well.”
Forte – the lead author of a paper published in the journal earlier this month Nature, describe the research — and his team wanted to find out whether the lightest of these heavier quarks, the charm quark, is present in the proton.
When the Large Hadron Collider (LHC) and other particle accelerators hurl protons (and other particles, such as electrons) at each other at high energies, a particle shower is created. This can be used to ‘reconstruct’ the composition of the original particle and the particles that make it up, collectively known as ‘partons’.
Each of these partons carries a portion of the system’s total momentum – the momentum distribution – with that momentum fraction known as momentum fraction.
Forte and colleagues fed 35 years of data from particle accelerators, including the world’s largest and most powerful machine of its kind, the LHC, into a computer algorithm that reassembles the proton structure by looking for a “best fit” for its structure at high-energies . From here, the team calculated the structure of the proton at rest.
This led to the first evidence that protons do indeed sometimes have charm quarks. These are called “intrinsic” because they are part of the proton for a long time and are still present when the proton is at rest, i.e. not resulting from the high-energy interaction with another particle.
“You have a small but not negligible chance of finding a charm quark in the proton, and when you find one, what happens is that typically that charm quark carries about half the mass of the proton,” Forte says on the podcast. “This is quantum physics, so everything is probabilistic.”
The “intrinsic” charm quark scenario
Romona Vogt is a high-energy physicist at Lawrence Livermore National Laboratory (LLNL) in California who wrote a News and Views article for Nature to accompany the new research work.
she explainsPopular mechanicshow charm quarks could be connected to the proton structure and how the intrinsic charm quark scenario differs from the standard scenario in which protons consist of only two up and one down quarks connected by gluons.
“Charm quarks occur in quark-antiquark pairs in both the standard and intrinsic charm scenarios,” says Vogt. “In the standard scenario, a gluon emits this pairing at a high-energy interaction. Because of the charm quark’s mass, it is too heavy to be part of the “sea” of light up, down, and strange quarks.”
This means that the charm quark doesn’t play a huge role when physicists compute standard parton momentum distribution functions until momentum reaches a threshold above mass.
“This is very different from the intrinsic charm scenario, where the charm distribution carries a large part of the proton momentum,” adds Vogt. “Because in the intrinsic charm quark scenario, the quark-antiquark pair is bound to more than one of the up and down quarks in the proton they travel with. That is why the charm quarks appear at large momentum fractions.
“The proton in this scenario is more or less ’empty’ or has a small configuration because the proton is just up, up, down quarks and charm quark pairs with no other quarks at low momentum fractions in the minimal intrinsic charm model . ”
Vogt suggests that the results of the NNPDF collaboration could lead other researchers to wonder whether other quarks might play a role in proton composition.
“One question these results might raise is whether or not there are other intrinsic quark scenarios, like intrinsic underside and intrinsic strangeness,” she says.