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. 2022 Aug;608(7923):483-487.
doi: 10.1038/s41586-022-04998-2. Epub 2022 Aug 17.

Evidence for intrinsic charm quarks in the proton

NNPDF Collaboration
Collaborators

Evidence for intrinsic charm quarks in the proton

NNPDF Collaboration. Nature. 2022 Aug.

Abstract

The theory of the strong force, quantum chromodynamics, describes the proton in terms of quarks and gluons. The proton is a state of two up quarks and one down quark bound by gluons, but quantum theory predicts that in addition there is an infinite number of quark-antiquark pairs. Both light and heavy quarks, whose mass is respectively smaller or bigger than the mass of the proton, are revealed inside the proton in high-energy collisions. However, it is unclear whether heavy quarks also exist as a part of the proton wavefunction, which is determined by non-perturbative dynamics and accordingly unknown: so-called intrinsic heavy quarks1. It has been argued for a long time that the proton could have a sizable intrinsic component of the lightest heavy quark, the charm quark. Innumerable efforts to establish intrinsic charm in the proton2 have remained inconclusive. Here we provide evidence for intrinsic charm by exploiting a high-precision determination of the quark-gluon content of the nucleon3 based on machine learning and a large experimental dataset. We disentangle the intrinsic charm component from charm-anticharm pairs arising from high-energy radiation4. We establish the existence of intrinsic charm at the 3-standard-deviation level, with a momentum distribution in remarkable agreement with model predictions1,5.We confirm these findings by comparing them to very recent data on Z-boson production with charm jets from the Large Hadron Collider beauty (LHCb) experiment6.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. The intrinsic charm PDF and comparison with models.
Left, the purely intrinsic (3FNS) result (blue) with PDFU alone, compared to the 4FNS PDF, which includes both an intrinsic and a radiative component, at Q = mc = 1.51 GeV (orange). The purely intrinsic (3FNS) result obtained using N3LO matching is also shown (green). Right, the purely intrinsic (3FNS) final result with total uncertainty (PDFU + MHOU), with the PDFU indicated as a dark shaded band; the predictions from the original BHPS model and from the more recent meson/baryon cloud model are also shown for comparison (dotted and dot-dashed curves, respectively).
Fig. 2
Fig. 2. Intrinsic charm and Z + charm production at LHCb.
Top left, the LHCb measurements of Z-boson production in association with charm-tagged jets, Rjc, at s=13TeV, compared with our default prediction, which includes an intrinsic charm component, as well as with a variant in which we impose the vanishing of the intrinsic charm component. The thicker (thinner) bands in the LHCb data indicate the statistical (total) uncertainty, while the theory predictions include both PDFU and MHOU. Top right, the correlation coefficient between the charm PDF at Q = 100 GeV in NNPDF4.0 and the LHCb measurements of Rjc for the three yZ bins. The dotted horizonal line indicates the maximum possible correlation. Centre, the charm PDF in the 4FNS (right) and the intrinsic (3FNS) charm PDF (left) before and after inclusion of the LHCb Z + charm (c) data. Results are shown for both experimental correlation models discussed in the text. Bottom left, the intrinsic charm PDF before and after inclusion of the EMC charm structure function data. Bottom right, the statistical significance of the intrinsic charm PDF in our baseline analysis, compared to the results obtained also including the LHCb Z + charm (with uncorrelated systematics) or the EMC structure function data, or both. The dotted horizontal line indicates the 3σ threshold.
Extended Data Fig. 1
Extended Data Fig. 1. Evaluation of the charm PDF in the 3FNS.
The 4FNS charm PDF is parametrized at Q0 and evolved to all Q, where it is constrained by the NNPDF4.0 global dataset. Subsequently, it is transformed to the 3FNS where (if nonzero) it provides the intrinsic charm component.
Extended Data Fig. 2
Extended Data Fig. 2. Kinematic coverage of the NNPDF4.0 determination.
The kinematic coverage in the (x, Q) plane covered by the 4,618 cross-sections used for the determination of the charm PDF in the present work. These cross-sections have been classified into the main different types of processes entering the global analysis.
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Comment in

References

    1. Brodsky SJ, Hoyer P, Peterson C, Sakai N. The intrinsic charm of the proton. Phys. Lett. B. 1980;93:451–455. doi: 10.1016/0370-2693(80)90364-0. - DOI
    1. Brodsky SJ, et al. A review of the intrinsic heavy quark content of the nucleon. Adv. High Energy Phys. 2015;2015:231547. doi: 10.1155/2015/231547. - DOI
    1. Ball, R. D. et al. The path to proton structure at 1% accuracy. Eur. Phys. J. C82, 428 (2022).
    1. Ball RD, et al. Intrinsic charm in a matched general-mass scheme. Phys. Lett. B. 2016;754:49–58. doi: 10.1016/j.physletb.2015.12.077. - DOI
    1. Hobbs TJ, Londergan JT, Melnitchouk W. Phenomenology of nonperturbative charm in the nucleon. Phys. Rev. D. 2014;89:074008. doi: 10.1103/PhysRevD.89.074008. - DOI

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