Abstract
We present an overview of processes induced by the coherent Weizsäcker-Williams (WW) photon cloud in heavy ion collisions. Due to the rather broad energy spectrum of photons a large variety of physics topics can be addressed. At the low-energy side, there are the electromagnetic dissociation processes. We discuss the few-neutron production via giant dipole resonance excitation. A typical high energy photonuclear process that has attracted much attention is the diffractive photoproduction of vector mesons. We review the color dipole approach to coherent and incoherent diffractive photoproduction of J/psi mesons. Finally we turn to peripheral and semicentral collisions and discuss the role of WW photons in the production of very low-P_T dileptons.
Highlights
Fast moving heavy ions are accompanied by their cloud of Weizsäcker-Williams (WW) photons, which offer the possibility to study a large range of photon-photon processes as well as photonuclear processes ranging from low to high energies [1–3]
The WW photons couple coherently to the whole nucleus, and photon fluxes in an ion are enhanced by the square of the large charge Z |e|
Cross sections involving the electromagnetic interaction in the collision of ions A and B, where A serves as the source of the WW photons can be calculated from the γ B cross section convoluted with the flux of photons: a e-mail: Wolfgang.Schafer@ifj.edu.pl
Summary
The measurement of ultraperipheral reactions at the high energies of present day colliders (RHIC and LHC) often requires special triggers. As an input we need the total photoabsorption cross section of a given nucleus, see Fig. 2 for the case of 208 Pb. The cross section for excitation of one of the ions is dominated by the absorption of a single photon: σtot( A1 A2 → A1 A∗2; Emax) ≈ d2bPsurv(b) exp[−n A2 (b)]. We need as an input crucial the fractions f (E, k) of final states with k neutrons coming from the decay of an excited nucleus at excitation energy E. We are confined to low-neutron multiplicities, as final states of large number of neutron (k) can be produced by processes in the energy region E > Emax which we do not model so far.
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