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  • Review Article
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Imaging the nucleus with high-energy photons

Nature Reviews Physics volume 1, pages 662–674 (2019)Cite this article

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Abstract

Understanding how quarks and gluons behave in the nuclear environment is an important focus of modern nuclear physics. Recent measurements have provided an improved understanding of how quark and gluon densities are altered in heavy nuclei. It has also become possible to make multi-dimensional pictures of the nucleus, exploring how these alterations are distributed within heavy nuclei. The modifications are naturally expected to be largest in the core of a nucleus and smaller near its periphery; this variation can change the effective shape of the nucleus. Experiments have also started to explore the transverse momentum distribution of the partons in the nuclei, and, by using incoherent photoproduction as a probe, measure event-by-event fluctuations in nucleon and nuclei parton densities. This Review explores recent progress in measurements of nuclear structure at high energy, emphasizing these multi-dimensional pictures. We also discuss how a future electron–ion collider with high luminosity and centre-of-mass energy will make exquisitely detailed images of partons in a nucleus.

Key points

  • The partonic (quark and gluon) content of the nucleus is different from what one would get by simply combining protons and neutrons.

  • The density of partons carrying only a small fraction of the nucleon momentum (partons with small Bjorken-x) is very high. The resulting increase in parton–parton interactions modifies the parton momentum distribution and may lead to qualitatively different behaviour.

  • The high-energy photons that are produced in ultra-peripheral collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) can be used to study nuclear structure at small Bjorken-x.

  • Coherent vector meson photoproduction can be used to determine the location of interactions within the nucleus, thereby probing the spatial dependence of nuclear modifications to the parton distribution.

  • Incoherent vector meson photoproduction can be used to study event-by-event fluctuations in nuclear parton distributions and thereby determine whether nuclei are smooth or clumpy.

  • Studies of photoproduction and electroproduction at a future electron–ion collider should make it possible to make detailed three-dimensional maps of parton distributions.

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Fig. 1: Schematic of the nuclear effects seen when using deep inelastic scattering with nuclear targets.
Fig. 2: Diagrams of collisions used to probe parton distributions.
Fig. 3: Coherent photoproduction cross section for the \({\boldsymbol{J}}/{\boldsymbol{\psi }}\) meson in Pb–Pb collisions at 2.76 TeV centre of mass energy.
Fig. 4: Effect of nucleon shape fluctuations on the nuclear structure.
Fig. 5: Determination of the 2D profile of interaction sites.
Fig. 6: Best-fit proton configurations.
Fig. 7: Coincidence probability C for the production of two nearly back-to-back jets.

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Acknowledgements

H.M. is supported by the Academy of Finland, project 314764, and by the European Research Council, grant ERC-2015-CoG-681707. S.K.’s work was funded by the US Department of Energy under contract number DE-AC02-05-CH11231.

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  1. Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Spencer R. Klein

  2. Department of Physics, University of Jyväskylä, Jyväskylä, Finland

    Heikki Mäntysaari

  3. Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland

    Heikki Mäntysaari

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Glossary

Virtuality

A measure of how far the squared four-momentum of a particle is from its mass. For photons, this is a measure of their mass. Owing to the uncertainty principle, virtual particles must be short-lived; the larger the virtuality, the shorter the lifetime.

Impact parameter

The perpendicular distance between the path of a particle and the centre of a target off which it scatters.

Transverse momentum

The component of a particle’s momentum that is transverse to the beam line, which arises from interactions.

Inclusive

Refers to processes in which some interaction products may not be measured.

Dijets

Events containing two jets, produced in collisions containing two energetic partons in the final state.

Jets

Showers of particles (with a typical opening angle of 45°) produced when a high-momentum parton emerges from a hadronic collision.

Mesons

Bound quark–antiquark pairs.

Exclusive

Refers to processes in which all decay products are measured.

Rapidity

A measure of the momentum of a particle along the beam line; ranges from 0 (no momentum along beam line) to infinity (particle travelling at speed c along the beam line).

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Klein, S.R., Mäntysaari, H. Imaging the nucleus with high-energy photons. Nat Rev Phys 1, 662–674 (2019). https://doi.org/10.1038/s42254-019-0107-6

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