Peripheral nuclear collisions

of two
relativistic colliding charges is equivalent to a collision between swarming photons. ]

Brief history

In 1924, Enrico Fermi, then 23 years old, submitted a paper ``On theTheory of Collisions Between Atoms and Elastically Charged Particles"to Zeitschrift fuer Physik. [cite journal | author=E. Fermi
title=On the Theory of Collisions Between Atoms and Elastically Charged Particles
journal=Zeitschrift fuer Physik | year=1924 | volume=29 | pages=315 ] This paper does not appearin his ``Collected Works". Nevertheless, it is said that this was oneof Fermi's favorite ideas and that he often used it later in life. [ cite journal | author=W.Marciano W and S. White S, editors
title=Electromagnetic Probes of Fundamental Physics
journal=World Scientific, Singapore
year=2003] In this publication, Fermi devised a method known as the"equivalent" (or virtual) "photon method", where he treated theelectromagnetic fields of a charged particle as a flux of virtualphotons (see figure 1). 10 years later, Weiszsaecker and Williams extended thisapproach to include ultra-relativistic particles, and the method isoften known as the "Weizsaecker-Williams method". [ cite journal
author=C.F. Weizsaecker
journal=Zeitschrift fuer Physik
title=Ausstrahlung bei Stößen sehr schneller Elektronen
year=1934
volume=88
pages=612
doi=10.1007/BF01333110 ] [cite journal
author=E.J. Williams
title=Nature of the High Energy Particles of Penetrating Radiation and Status of Ionization and Radiation Formulae
journal=Phys. Rev.
year= 1934
volume=45
pages=729
doi=10.1103/PhysRev.45.729]

A fast-moving charged particle has electric field vectors pointingradially outward and magnetic fields circling it. The field at a pointsome distance away from the trajectory of the particle will resemble thatof a real photon (see figure 1). Thus, Fermi replaced theelectromagnetic fields from a fast particle with an "equivalentflux of photons". The number of photons with energy $omega$,$n(omega)$, is given by the Fourier transform of the time-dependentelectromagnetic field. The virtual photon approach used in quantumelectrodynamics (QED) to describe, e.g. atomic ionization or nuclearexcitation by a charged particle can be simply described using Fermi'sapproach.

When two nuclei collide, two types of electromagnetic processes canoccur. A photon from one ion can strike the other, or, photons from each nucleus can collide, in aphoton-photon collision.

Ultra-peripheral hadron-hadron collisions provide uniqueopportunities for studying electromagnetic processes in a much simplerenviourment without any background. At the Large HadronCollider (LHC) at the Nuclear Research Center CERN in Geneva/Switzerland,photon-proton collisions occur at center of mass energies anorder of magnitude higher than are available at previous accelerators,and photon-heavy ion collisions reach 30 times the energies availableat fixed target accelerators. The electromagnetic fields of heavy-ionsare very strong, so reactions involving multi-photon excitations canbe studied.

Ultra-relativistic heavy-ion interactionshave been used to study nuclear photoexcitation(e.g. to a Giant Dipole Resonance), and photoproduction of hadrons.it can be used to study heavy meson production in gamma+A collisions whichwill give information on gluon disstribution functions in nucleous.Coulomb excitation is a traditional tool in low energy nuclearphysics. The strong electromagnetic fields from a heavy ion allow forthe study of multi-photon excitation of nuclear targets.This allows the study of high-lying states in nuclei, e.g. thedouble-giant resonance [cite journal
author=G. Baur G and C.A. Bertulani
title=Multiple Electromagnetic Excitation of Giant Dipole Phonons in Relativistic Heavy Ion Collisions
journal=Phys. Lett. B
year=1986
volume=174
pages=23
doi=10.1016/0370-2693(86)91121-4] [cite journal
author=J.L. Ritmann et al.
title=First observation of the Coulomb-excited double giant dipole resonance in 208Pb via double-γ decay
journal=Phys. Rev. Lett.
year=1993
volume=70
pages=533
doi=10.1103/PhysRevLett.70.533] [cite journal
author=R. Schmidt et al.
journal=Phys. Rev. Lett.
title=Electromagnetic excitation of the double giant dipole resonance in 136Xe
year=1993
volume=70
pages=1767
doi=10.1103/PhysRevLett.70.1767] .

Multiple, independentinteractions among a single ion pair are also possible. Reactionslike multiple vector meson production can be used for studiesinvolving polarized photons. The high photon energies can be used tostudy the gluon density in heavy nuclei [cite journal
author=V.P. Goncalves and C.A. Bertulani
title=Peripheral heavy ion collisions as a probe of the nuclear gluon distribution
journal=Phys. Rev. C
year=2002
volume=65
pages=054905
doi=10.1103/PhysRevC.65.054905] at lowFeynman-$x$.

References