Particle physics

Strangeness in the proton

The proton can contain pairs of elementary particles known as strange quarks. The contribution of these particles to the proton's electric-charge distribution and magnetic moment has been determined.

Elementary particles called quarks come in six flavours: up, down, charm, strange, top and bottom. This year marks the seventieth anniversary of the discovery of strange quarks in matter. This discovery set the foundation on which the quark model was developed1,2,3, providing us with the simple picture that the proton — and all conventional atomic nuclei — comprises only up and down quarks. On small scales, deep inside the proton, this simple picture has been superseded by one consisting of a range of quarks, antiquarks (the antiparticles of quarks) and gluons (the particles that bind quarks together). In principle, all six flavours of quark can be present. But viewing the proton from afar, the influence of the four 'hidden' flavours has been difficult to resolve. Writing in Physical Review Letters, Sufian et al.4 report the precise computation of the electric-charge distribution and magnetization associated with strange quarks in the proton.

After the up and down quarks, the strange quark is the next lightest, and is therefore expected to be the dominant hidden flavour in the proton. To understand how strange quarks might affect the proton's properties, we first consider the electric-charge distribution. According to the theory that describes the interactions of quarks and gluons — known as quantum chromodynamics (QCD) — every time a strange quark is created, a strange antiquark must also emerge. In other words, the net amount of 'strangeness' must always be zero. However, the spatial distributions of these quarks need not be the same, and any difference would manifest as a contribution to the proton's total charge distribution. A simple analogy is the hydrogen atom: although it has zero net charge, positive and negative charges exist on different scales, with a negatively charged electron cloud at a distance from the atom's centre and a compact, positively charged nucleus at its core.

Sufian and colleagues use supercomputers to simulate QCD processes. They show that when a pair of strange quarks is created in the proton, the strange antiquark will, on average, be distributed slightly farther from the proton's centre than the strange quark. This asymmetry affects the proton's total charge distribution. The authors also determine the strange quarks' contribution to the proton's magnetic moment. An individual proton behaves like a tiny bar magnet. The contribution of hidden quark flavours to this magnetization can be thought of as being induced by the circulation of electric charge about the proton's polarization axis (Fig. 1). Sufian et al. show that strange quarks provide a small enhancement of (0.8 ± 0.2)% of the proton's total magnetic moment. They confirm the existence of a positive contribution at the 99.99% confidence level.

Figure 1: The proton's internal structure.

Elementary particles called quarks come in six flavours, including up (u), down (d) and strange (s). Inside the proton, in addition to two up quarks, one down quark and gluons (the particles that bind quarks together), pairs of quarks and their antiparticles (antiquarks; denoted by overbars) are frequently created and destroyed. The presence of these additional particles influences the proton's properties. Using supercomputer simulations, Sufian et al.4 show that when strange quarks are created in the proton, the strange antiquark is, on average, distributed slightly farther from the centre of the proton than its counterpart — this asymmetry affects the proton's total charge distribution. The authors also show that strange quarks increase the magnitude of the proton's magnetic moment by about 0.8%. This contribution can be thought of as being induced by a net circulation of negative electric charge of the strange quarks (indicated by the red arrow) about the proton's polarization axis.

The authors' simulation confirms the prediction of an earlier calculation5 of the behaviour of strange quarks in the proton, and allows a comprehensive assessment of systematic uncertainties to be made for the first time. The tremendous precision of Sufian and colleagues' magnetic-moment result is more than ten times better than that of the most precise experimental measurement6. This outcome contrasts starkly with results for most aspects of the proton's structure. For instance, experimental measurements of the proton's total magnetization are some tens of millions of times more precise than can be determined in numerical calculations of QCD7,8.

Isolating the strange-quark component of the proton's magnetization is challenging, because it requires precise parity-violation measurements (measurements of the difference between the scattering of left-handed and right-handed polarized electrons from protons). By contrast, isolating this component in a numerical calculation is relatively straightforward. As computational techniques improve, it will be both feasible and desirable to obtain precise results for the proton's total magnetization, which should provide further confirmation of our understanding of the interactions between quarks and gluons.

Beyond the study of quarks and gluons in QCD, Sufian and colleagues' findings could have an immediate impact on the analysis of the Qweak experiment in Virginia9. This experiment measures the 'weak' interaction between a proton and an electron, providing a test of whether quarks have an internal structure with a spatial resolution of less than 10−19 metres. One of the main components of background in this experiment arises from the magnetization of strange quarks in the proton. The authors' calculations could allow this background to be more easily identified, enabling the experiment to set stronger constraints.

As the precision of QCD studies of the proton improve, there is scope for these numerical calculations to be used in diverse areas of physics. For example, strange quarks in the proton play a key part in the ongoing search for dark matter, the 'missing' mass in the Universe. One of the favoured explanations for dark matter is that it consists of weakly interacting massive particles (WIMPs). The interactions of WIMPs with protons could occur through the Higgs force, which is associated with the famous Higgs boson. Because up and down quarks have minuscule masses, these interactions are likely to be governed by strange quarks and other hidden flavours in the proton. With strange quarks aptly named for the behaviour revealed in subatomic particles 70 years ago, it would make for a fascinating tale if they were again to be instrumental in unveiling new strange behaviour in nature.Footnote 1


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Correspondence to Ross D. Young.

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Young, R. Strangeness in the proton. Nature 544, 419–420 (2017).

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