Production of $e^{+}{e}^{-}$ pairs accompanied by nuclear dissociation in ultraperipheral heavy-ion collisions

We present data on $e^{+}{e}^{-}$ pair production accompanied by nuclear breakup in ultraperipheral gold-gold collisions at a center of mass energy of $200\mathrm{GeV}$ per nucleon pair. The nuclear breakup requirement selects events at small impact parameters, where higher-order diagrams for pair production should be enhanced. We compare the data with two calculations: one based on the equivalent photon approximation, and the other using lowest-order quantum electrodynamics (QED). The data distributions agree with both calculations, except that the pair transverse momentum spectrum disagrees with the equivalent photon approach. We set limits on higher-order contributions to the cross section.

Figure 1

Schematic QED lowest-order diagram for $e^{+}{e}^{-}$ production accompanied by mutual Coulomb excitation. The dashed line shows the factorization into mutual Coulomb excitation and $e^{+}{e}^{-}$ production.

Figure 2

(Color online) (a) The pair-mass distribution, (b) pair $p_T$, (c) pair rapidity, and (d) pair $\cos \left({\theta }^{\prime }\right)$ distributions. The data (points) are compared with predictions from the EPA (solid histogram) and lowest-order QED (dashed histogram) calculations. The error bars include both statistical and systematic errors.

Figure 3

The $p_T$ spectra of the produced electrons and positrons, along with the comparable EPA and QED calculations. In both calculations, the electron and positron spectra are identical. Spectra from the two calculations are similar; the data agree with both of them.