Brief history

In 1924, Enrico Fermi

Enrico Fermi

Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

, then 23 years old, submitted a paper entitled "On the Theory of Collisions Between Atoms and Elastically Charged Particles" to Zeitschrift für Physik

Zeitschrift für Physik

The European Physical Journal is a joint publication of EDP Sciences, Springer Science+Business Media, and the Società Italiana di Fisica...

. This paper does not appear in his "Collected Works", but it is nevertheless said that this was one of Fermi's favorite ideas and that he often used it later in life. In this publication, Fermi devised a method known as the equivalent (or virtual) photon method, where he treated the electromagnetic fields of a charged particle as a flux of virtual photons (see figure 1). 10 years later, Carl Friedrich von Weizsäcker

Carl Friedrich von Weizsäcker

Carl Friedrich Freiherr von Weizsäcker was a German physicist and philosopher. He was the longest-living member of the research team which performed nuclear research in Germany during the Second World War, under Werner Heisenberg's leadership...

 and E. J. Williams extended this approach to include ultra-relativistic particles, and the method is often known as the Weizsäcker–Williams method.

A fast-moving charged particle has electric field vectors pointing radially outward and magnetic fields circling it. The field at a point some distance away from the trajectory of the particle will resemble that of a real photon (see figure 1). Thus, Fermi replaced the electromagnetic fields from a fast particle with an equivalent flux of photons. The number of photons with energy , , is given by the Fourier transform of the time-dependent electromagnetic field. The virtual photon approach used in quantum electrodynamics (QED) to describe, e.g. atomic ionization or nuclear excitation by a charged particle can be simply described using Fermi's approach.

When two nuclei

Atomic nucleus

The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 collide, two types of electromagnetic processes

Quantum electrodynamics

Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...

 can occur. A photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 from one ion can strike the other, or, photons from each nucleus can collide, in a photon-photon collision.

Ultra-peripheral hadron

Hadron

In particle physics, a hadron is a composite particle made of quarks held together by the strong force...

–hadron collisions provide unique opportunities for studying electromagnetic processes in a much simpler environment without any background. At the Large Hadron Collider

Large Hadron Collider

The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....

 in Geneva/Switzerland, photon–proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 collisions occur at center-of-mass energies an order of magnitude higher than are available at previous accelerators, and photon–heavy-ion collisions reach 30 times the energies available at fixed target accelerators. The electromagnetic fields of heavy-ions are very strong, so reactions involving multiphoton excitations can be studied.

Ultra-relativistic heavy-ion interactions have been used to study nuclear photo-excitation (e.g. to a giant dipole resonance), and photoproduction of hadrons.
it can be used to study heavy meson

Meson

In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...

 production in γ+A collisions which will give information on gluon

Gluon

Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....

 distribution functions in nuclei/ Coulomb excitation

Coulomb excitation

Coulomb excitation is a technique in experimental nuclear physics to probe the electromagnetic aspect of nuclear structure. In coulomb excitation, a nucleus is excited by an inelastic collision with another nucleus through the electromagnetic interaction...

 is a traditional tool in low energy nuclear physics. The strong electromagnetic fields from a heavy ion allow for the study of multiphoton excitation of nuclear targets. This allows the study of high-lying states in nuclei, e.g. the double-giant resonance.

Multiple, independent interactions among a single ion pair are also possible. Reactions like multiple vector meson

Vector meson

In high energy physics, a vector meson is a meson with total spin 1 and odd parity . Compare to a pseudovector meson, which has a total spin 1 and even parity....

 production can be used for studies involving polarized photons

Photon polarization

Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photons are completely polarized...

. The high photon energies can be used to study the gluon density in heavy nuclei at low Feynman-.