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Particle physics

The undemocratic proton

Nature volume 397, pages 472–473 (1999)Cite this article

Since the proton is the lightest stable composite object in the Universe we might expect it to be the simplest. Yet its structure is much more complicated than we can explain with current theories, as reported in Physical Review Letters by the HERMES collaboration1 at the HERA accelerator in Hamburg and by the NuSea collaboration2 at the Fermilab Tevatron near Chicago. HERMES used semi-inclusive deep-inelastic scattering of a positron beam to identify the contribution of individual quarks to the momentum of the proton. Both groups see an excess of virtual down-antiquarks over up-antiquarks, contradicting predictions that the two flavours of antiquark should be present inside the proton in equal numbers.

Elastic scattering is what happens with billiard balls — they collide and bounce without changing their identity and with no significant loss of kinetic energy. Inelastic scattering usually means that one or both of the colliding objects has been changed by the collision, in that they may be deformed or broken into pieces like a billiard ball shattered by a bullet. Deep-inelastic scattering is a subtle amalgam of the two, as if a bullet shatters a billiard ball but in the process makes a quasi-elastic collision with a heavy, compact ball-bearing buried inside the target. Particle physicists began to believe in the reality of quarks in the early 1970s (ref. 3), when deep-inelastic scattering experiments with electrons (bullets) on protons (billiard balls) gave far more large-angle electron scatters than expected. The results can be explained only if there are heavy, compact objects (the quarks, equivalent to the buried ball-bearings) inside the protons.

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Figure 1: Principle of the HERMES experiment1.
Figure 2: The excess of down-antiquarks in the proton sea.

References

  1. Ackerstaff, K.et al. (HERMES collaboration) Phys. Rev. Lett. 81, 5519–5523 (1998).

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  2. Hawker, E. A.et al. (NuSea collaboration) Phys. Rev. Lett. 80, 3715–3718 (1998).

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  3. Miller, G.et al. (SLAC-MIT collaboration) Phys. Rev. D 5, 528–544 (1972).

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  5. Ellis, R. K., Stirling, W. J. & Webber, B. R. QCD and Collider Physics (Cambridge Univ. Press, 1996).

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  1. the Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT, London, UK

    David J. Miller

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  1. David J. Miller
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Miller, D. The undemocratic proton. Nature 397, 472–473 (1999). https://doi.org/10.1038/17211

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