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Evidence at last that the proton has intrinsic charm
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17 August 2022 .
Evidence at last that the proton has intrinsic charm .
An analysis of the distribution of the elementary particles that make up the proton provides evidence that it contains a type of quark known as an intrinsic charm quark — verifying a proposal made four decades ago.
Ramona Vogt 0 .
Ramona Vogt Ramona Vogt is at Lawrence Livermore National Laboratory, Livermore, California 94550, USA, and in the Department of Physics and Astronomy, University of California, Davis, Davis, California, USA.
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Physics textbooks describe the proton as a subatomic particle that contains three quarks, bound together by elementary particles known as gluons. But quantum theory predicts that the proton can contain other quark–antiquark pairs, including some quarks — known as charm quarks — that are more massive than the proton itself. These charm quarks are thought to be ‘intrinsic’, meaning that they are part of the proton over long timescales and not produced by interactions with a particle that is external to the proton 1 , 2 . However, attempts to confirm intrinsic charm in experiments have so far fallen short. In a paper in Nature , the NNPDF Collaboration 3 reports an analysis of collision data that acts as evidence — if not the discovery — of intrinsic charm in the proton.
Read the paper: Evidence for intrinsic charm quarks in the proton
The intrinsic-charm content of the proton is expected to differ from the charm–anticharm quark pairs that are generated by high-energy radiation — for example, when a photon fuses with a gluon in an electron–proton collision. In such a collision, ‘extrinsic’ charm quarks appear in the centre of the collision, so the statistical distribution of their position, averaged over many experiments, dies away farther from the centre. Instead, in the intrinsic-charm picture that was first proposed in the early 1980s, the charm quark continues moving along the direction of travel of the parent proton in a collision between, for example, an electron and a proton, or between two protons 1 , 2 . The intrinsic charm quark is therefore expected to appear in the ‘forward region’ with respect to the centre of mass of the collision. The distribution of charm quarks in the proton determined by the NNPDF Collaboration’s analysis is, in fact, similar to that first predicted four decades ago 1 .
At the most basic level, a proton with intrinsic charm would be made up of a charm–anticharm pair, together with two ‘up’ quarks and one ‘down’ quark. Theoretically, it should be easy to tell the difference between this intrinsic-charm state and the radiatively generated extrinsic state. In practice, however, charm quarks that are produced in the forward region can be difficult to detect and are thus statistically less likely to be observed than are those that appear in the centre.
In the early 1980s, the European Muon Collaboration measured the charm-quark structure function for the proton, which describes the momentum distribution of charm quarks in the proton 4 . The team found indications that could be attributed to intrinsic charm, but low statistical precision made the result inconclusive. Subsequent experiments in the 1990s extended the range of particle momenta over which the measurements were taken, but it was not possible to fit the whole data set in terms of a single intrinsic-charm scenario, and this was taken as evidence against the idea that the proton contains intrinsic charm 5 , 6 . However, these measurements were not considered definitive.
Antimatter in the proton is more down than up
The presence of intrinsic charm was also hinted at by other types of data, such as those showing differences in the distributions of particles known as mesons. A proton with intrinsic charm (containing two up quarks, one down quark and a charm–anticharm pair) can make a meson — known as a leading meson — that contains either an up–anticharm pair or a down–anticharm pair. The corresponding antiparticles (antiup–charm or antidown–charm) cannot be made from an intrinsic state and can be generated only through the extrinsic scenario. These mesons are known as non-leading mesons.
Leading mesons are produced in the forward region, and their non-leading counterparts are produced close to the collision centre. Their distributions are therefore different, and this difference is referred to as an asymmetry. Asymmetries that have been detected in the distributions of mesons 7 – 9 could arise from intrinsic charm in the proton, but there are other models 10 of particle production based on connections between quarks that can explain this discrepancy, so these data cannot be taken as definitive proof of intrinsic charm.
Because no clear evidence had surfaced over several decades, interest in intrinsic charm waxed and waned. However, there has been something of a renaissance in the topic in the past 10 years or so, with several alternative models of intrinsic charm being put forward 11 – 14 . These various models have been used in analyses 15 – 18 of data from many different collision experiments. Such analyses have attempted to set limits on the intrinsic-charm content of the proton by assuming that it takes one or all of the forms set out in the models, but, so far, they have returned contradictory or inconclusive results.
Strangeness in the proton
The NNPDF Collaboration’s study is unique among these analyses in that the authors did not make any assumptions about the way the constituents of the proton are distributed. Instead, they used machine-learning techniques that are agnostic to any particular model. A previous paper 18 by members of the same team found some evidence to support the postulate that intrinsic charm can be detected in the forward region, but this evidence was not definitive. In the present work, the authors distinguished the distributions of momenta that include the charm quark as part of the proton from those that would be expected if charm is generated only radiatively — and found compelling evidence that intrinsic charm is indeed present in the proton.
The authors found that the resulting distribution of charm quarks peaked at a maximum fraction of the proton momentum that is close to that predicted in the 1980s. The effect is small, as expected, because the intrinsic charm carries less than 1% of the total momentum of the proton. However, the result is robust with respect to the mass of the charm quarks, the methodology and the data sets that are included in the analysis. The NNPDF Collaboration concludes that intrinsic charm is present in the proton with a statistical significance of three times the standard deviation (Fig. 1). This is considered evidence of an effect in particle physics — but not a discovery, which is a term reserved for a significance of five times the standard deviation.
Figure 1 Figure 1 Evidence of intrinsic charm in the proton. The NNPDF Collaboration 3 used machine-learning techniques to analyse data from experiments designed to determine whether the proton contains an elementary particle, known as a charm quark, that is ‘intrinsic’ to the proton. The authors found that intrinsic charm is present in the proton with a statistical significance of 2.5 times the standard deviation ( σ ). The baseline analysis excluded measurements 4 made by the European Muon Collaboration (EMC) in the early 1980s — data that are generally thought to be too imprecise to be conclusive — and those 19 announced in July 2021 by the LHCb collaboration at the Large Hadron Collider at CERN. Including these data in the analysis had the effect of increasing the statistical significance to 3 σ , which is considered evidence of an effect in particle physics. (Adapted from Fig. 2 of ref. 3.)
In July 2021, data 19 were announced by researchers working at the Large Hadron Collider at CERN, Europe’s particle-physics laboratory near Geneva, Switzerland (see go.nature.com/3p2tswa ). In these experiments, the particles that were produced in proton–proton collisions seemed to suggest the presence of intrinsic charm in the proton — providing some of the clearest experimental indicators so far for intrinsic charm. Including these data in its analysis helped the NNPDF Collaboration to confirm its results, but its evidence does not rely on these data. Other experiments have been planned at lower energies 20 than those previously used to study intrinsic charm in the proton, and these experiments could provide insight into the conditions under which intrinsic charm is expected to appear.
Nature 608 , 477-479 (2022)
doi: https://doi.org/10.1038/d41586-022-02186-w
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