Protons contain intrinsic charm quarks, machine-learning analysis suggests

23 Aug 2022

The Large Hadron Collider: evidence for intrinsic charm quarks in protons has been found in LHC data. (Courtesy: Maximilien Brice/CERN)

A 40-year-old debate about charm quarks in protons may have been settled by a new machine-learning analysis of data from the Large Hadron Collider (LHC) at CERN and other facilities. However, not all particle physicists agree with this assessment.

For decades, physicists have debated whether protons contain what are known as intrinsic charm quarks. Quantum chromodynamics (QCD), the theory of the strong nuclear force, tells us that protons consist of two up quarks and a down quark bound together by force carriers called gluons. But it also predicts that protons, like neutrons or any other hadron, contain a host of other quark–antiquark pairs.

Large numbers of these additional particles are known to be generated when gluons are accelerated during high-energy collisions between protons, just as electromagnetic theory tells us that photons are given off when charged particles accelerate. But what is less clear is the extent to which there could be additional quarks within the protons and neutrons to begin with – so-called intrinsic quarks that contribute to the hadrons’ quantum wavefunctions.

Heavier than protons

Scientists agree on the existence of intrinsic strange quarks, given that strange quarks have a far smaller mass than protons. However, there continues to be uncertainty about the existence and possible contribution of intrinsic charm quarks. These quarks are heavier than protons, but only by a small amount – leaving open the possibility that they provide a fairly small but nevertheless observable component to a proton’s mass.

While some researchers have concluded that charm quarks can provide no more than 0.5% of a proton’s momentum, others have instead found that a contribution of up to 2% is possible.

In the latest work, the NNPDF Collaboration – made up of physicists from the University of Milan, the Free University of Amsterdam and the University of Edinburgh – says it has found “unambiguous evidence” that intrinsic charm quarks do indeed exist. It has done so by drawing on reams of collision data from the LHC and elsewhere that it previously used to work out what are known as parton distribution functions (PDFs), which they call NNPDF4.0.

Point-like particles

Parton is a generic term to describe point-like particles within a hadron, proposed by Richard Feynman in the 1960s to analyse particle collisions and is now equivalent to a quark or gluon. Because the momentum, spin and other properties of partons are determined by the strong force under conditions of very large coupling, their values cannot be calculated using the approximations possible with perturbative QCD. However, by studying the kinematics of hadron collisions it is possible to build up probability distributions showing the odds that a parton will have a certain fraction of a hadron’s momentum at a particular scale.

The new research involved calculating a charm quark’s PDF by considering the momentum that it and the three lightest quarks – up, down and strange – contribute to a colliding proton in the scattering process. They then used perturbative QCD – approximating strong interactions by using either the first two or three terms in an expansion of the strong coupling expression – to convert this PDF into one consisting of radiative components from only the lightest three quarks. As they point out, stripped of the charm quark’s own radiative component this new PDF would comprise only intrinsic charm.

Doing so using neural networks to best match experimental data with the shape and magnitude of PDFs, they conclude that intrinsic charm quarks definitely exist. Although they work out that intrinsic charm contributes less than 1% of proton momentum, its associated PDF strongly resembles that expected from theory – a peak at a momentum fraction of around 0.4 (the tiny probabilities involved meaning integration yields a small total) while tailing off rapidly at small fractions. It also closely matches the PDFs worked out from other collision data – specifically, recent results involving the production of Z bosons at the LHCb experiment and much earlier data from CERN’s European Muon Collaboration (EMC).

NNPDF calculates that with the data from its 4.0 analysis alone the statistical significance of intrinsic charm being real is about 2.5σ, while the significance rises to around 3σ if the LHCb and EMC data are also included. A statistical significance of 5σ or greater is usually considered to be a discovery in particle physics.

“Our findings close a fundamental open question in the understanding of nucleon structure that has been hotly debated by particle and nuclear physicists for the past 40 years,” the collaboration writes in a paper in Nature describing its research.

Neutrino observations

The researchers say they look forward to further studies of intrinsic charm at experiments such as CERN’s LHCb and those at the Electron–Ion Collider (currently being built at the Brookhaven National Laboratory in the US). Observations at neutrino telescopes are also of interest because particles containing charm quarks can decay to generate neutrinos in Earth’s atmosphere. “These measurements can help to pin down the shape and magnitude of intrinsic charm, as well as probe any differences between intrinsic charm quarks and antiquarks,” according to group member Juan Rojo of the Free University of Amsterdam.READ MORE

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Other experts too welcome further data but disagree on the importance of the latest work. Stanley Brodsky at the SLAC National Accelerator Laboratory in the US says the result provides “convincing” evidence for intrinsic charm. However, Ramona Vogt of the Lawrence Livermore National Laboratory, also in the US, points out that its statistical significance falls short of that needed for discovery in particle physics. “This result is a step forward but it’s not the final word,” she says.

Wally Melnitchouk at the Thomas Jefferson National Accelerator Facility, again in the US, is more critical. Far from being definitive, he regards NNPDF’s evidence as contingent on how it defines intrinsic charm and the choices it makes for the perturbative calculation, arguing that definitions from other groups that have not found evidence are equally valid. He maintains that a much more compelling signal would be the observation of a difference between the charm and anticharm PDFs in the proton. “A non-zero difference between these is much less susceptible to choices of theoretical schemes and definitions,” he says.

Edwin Cartlidge is a science writer based in Rome.

from physicsworld.com   28/8/2022