Ultraperipheral Pb+Pb reactions at LHC energies

The magnetic field midway between two Pb nuclei passing at 20 fm is 2 x 10$^{20}$ gauss at LHC energies (1144 TeV in PbPb center of mass). At these energies the Coulomb field of a passing Pb nucleus can be regarded as a cloud of real photons. The cross sections for $\gamma \gamma $ and $\gamma $A reactions are huge compared to 7 b for two Pb nuclei actually colliding. The reaction rate is limited by the 323 b cross section for breaking up the Pb nucleus or for the capture by a Pb ion of an e$^{-}$ from the many e$^{+}$ e$^{-}$ pairs that are formed. These products go down the beam pipe and eventually hit superconducting magnets. The $\gamma \gamma $ reactions can produce particles with mc$^{2}$ up to 100 GeV. The $\gamma $A reactions can produce particles with mc$^{2}$ more than 900 GeV. These ultraperipheral reactions are particularly clean. In proton-proton reactions, the reaction between two partons to produce something of interest is accompanied by a large background caused by many other parton-parton reactions. When $\gamma $A breaks up a Pb nucleus, the transverse energy is small so that the fragments continue in the original beam direction. A single neutron in the original beam direction provides a useful flag that shows that an ultraperipheral reaction has occurred.