Peripheral nuclear collisions

Brief history

In 1924, Enrico Fermi, then 23 years old, submitted a paper entitled "On the Theory of Collisions Between Atoms and Elastically Charged Particles" to Zeitschrift für Physik.[1] 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.[2] 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 and E. J. Williams extended this approach to include ultra-relativistic particles, and the method is often known as the Weizsäcker–Williams method.[3][4]

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 \omega, n(\omega), 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 collide, two types of electromagnetic processes can occur. A photon from one ion can strike the other, or, photons from each nucleus can collide, in a photon-photon collision.

Ultra-peripheral hadron–hadron collisions provide unique opportunities for studying electromagnetic processes in a much simpler environment without any background. At the Large Hadron Collider in Geneva/Switzerland, photon–proton 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 production in γ+A collisions which will give information on gluon distribution functions in nuclei/ Coulomb excitation 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.[5][6][7]

Multiple, independent interactions among a single ion pair are also possible. Reactions like multiple vector meson production can be used for studies involving polarized photons. The high photon energies can be used to study the gluon density in heavy nuclei[8] at low Feynman-x.

References

  1. ^ E. Fermi (1924). "On the Theory of Collisions Between Atoms and Elastically Charged Particles". Zeitschrift für Physik 29: 315. Bibcode 1924ZPhy...29..315F. doi:10.1007/BF03184853. 
  2. ^ W. Marciano, and S. White, ed (2003). Electromagnetic Probes of Fundamental Physics. World Scientific. 
  3. ^ C. F. Weizsaecker (1934). "Ausstrahlung bei Stößen sehr schneller Elektronen". Zeitschrift für Physik 88 (9-10): 612. Bibcode 1934ZPhy...88..612W. doi:10.1007/BF01333110. 
  4. ^ E. J. Williams (1934). "Nature of the High Energy Particles of Penetrating Radiation and Status of Ionization and Radiation Formulae". Physical Review 45 (10): 729. Bibcode 1934PhRv...45..729W. doi:10.1103/PhysRev.45.729. 
  5. ^ G. Baur and C. A. Bertulani (1986). "Multiple Electromagnetic Excitation of Giant Dipole Phonons in Relativistic Heavy Ion Collisions". Physics Letters B 174: 23. Bibcode 1986PhLB..174...23B. doi:10.1016/0370-2693(86)91121-4. 
  6. ^ J. L. Ritmann ;et al. (1993). "First observation of the Coulomb-excited double giant dipole resonance in 208Pb via double-γ decay". Physical Review Letters 70 (5): 533. Bibcode 1993PhRvL..70..533R. doi:10.1103/PhysRevLett.70.533. PMID 10054138. 
  7. ^ R. Schmidt et al. (1993). "Electromagnetic excitation of the double giant dipole resonance in 136Xe". Physical Review Letters 70 (12): 1767. Bibcode 1993PhRvL..70.1767S. doi:10.1103/PhysRevLett.70.1767. PMID 10053381. 
  8. ^ V.P. Goncalves and C.A. Bertulani (2002). "Peripheral heavy ion collisions as a probe of the nuclear gluon distribution". Physical Review C 65 (5): 054905. arXiv:hep-ph/0110370. Bibcode 2002PhRvC..65e4905G. doi:10.1103/PhysRevC.65.054905.