The textbook description of a proton says it contains three smaller particles
- two up quarks and a down quark - but a new analysis has found strong
evidence that it also holds a charm quark
The proton, a particle found at the heart of every atom, appears to have a
more complicated structure than is traditionally given in textbooks. The
find could have ramifications for sensitive particle physics experiments
like the Large Hadron Collider (LHC).
While protons were once thought to be indivisible, experiments with particle
accelerators in the 1960s revealed that they contained three smaller
particles, called quarks. Quarks come in six types, or flavours, and the
proton contains two up quarks and one down quark.
But in quantum mechanics, a particle’s structure is governed by
probabilities, meaning there is theoretically a chance that other quarks
could crop up inside the proton in the form of matter-antimatter pairs. An
experiment at the European Muon Collaboration at CERN in 1980 hinted the
proton might contain a charm quark and its antimatter equivalent, an
anticharm, but the results were inconclusive and hotly debated.
There were further attempts to identify the proton’s charm component, but
different groups found conflicting results and had difficulty separating out
the intrinsic building blocks of a proton from the high energy environment
of particle accelerators, where every kind of quark is created and destroyed
in rapid succession.
Now, Juan Rojo at Vrije University Amsterdam in the Netherlands and his
colleagues have found evidence that a small part of the proton’s momentum,
around 0.5 per cent, comes from the charm quark. “It’s remarkable that even
after all these decades of study, we’re still finding new properties of the
proton and, in particular, new constituents,” says Rojo.
To isolate the charm component, Rojo and his team used a machine learning
model to come up with hypothetical proton structures consisting of all the
different flavours of quarks and then compared them with more than 500,000
real-world collisions from decades of particle accelerator experiments,
including at the LHC.
This use of machine learning was especially important, says Christine Aidala
at the University of Washington, because it could generate models that
physicists wouldn’t necessarily think of by themselves, reducing the chance
of biased measurements.
The researchers found that, if the proton doesn’t contain a charm-anticharm
quark pair, there is only a 0.3 per cent chance of seeing the results they
examined. Physicists call this a “3-sigma” result, which is normally seen as
a potential sign of something interesting. More work is needed to boost the
results to 5-sigma level, meaning about a 1 in 3.5 million chance of a fluke
result, which is traditionally the threshold for a discovery.
The team looked at recent results from the LHCb Z-boson experiment and
modelled the statistical distribution of the proton’s momentum both with and
without a charm quark. They found the model better matched the results if
the proton is assumed to have a charm quark. This means they are more
confident in proposing the presence of a charm quark than the sigma level by
itself suggests. “The fact that very different studies converge on the same
result made us especially confident that our results were solid,” says Rojo.
“Given how ubiquitous this particle is and how long we’ve known about it,
there’s still a lot we don’t actually understand about its substructure, so
this is definitely important,” says Harry Cliffe at the University of
Cambridge.
The proton’s charm component could also have ramifications for other physics
experiments at the LHC, says Cliffe, as they rely on accurate models of
proton substructure. The IceCube Neutrino Observatory in Antarctica, which
looks for rare neutrinos produced when cosmic rays hit particles in Earth’s
atmosphere, might also need to take this new structure into account, says
Rojo. “The probability of a cosmic ray impacting an atmosphere nucleus and
producing neutrinos is quite sensitive to the charm content of the proton,”
he says.
Reference:
The NNPDF Collaboration. Evidence for intrinsic charm quarks in the proton.
Nature 608, 483–487 (2022).
DOI: 10.1038/s41586-022-04998-2
Tags:
Physics