Ultra-Peripheral Collisions at RHIC

This presentation summarizes the results on ultra-peripheral collisions obtained at RHIC. It also discusses some aspects of the corresponding electromagnetic interactions in pp and pp collisions. Ultra-peripheral nucleus-nucleus collisions are defined as collisions in which the distance between the nuclei is large enough that no purely hadronic interactions can occur. This roughly means impact parameters larger than the sum of the nuclear radii. The interaction is then instead mediated by the electromagnetic field. For a recent review of ultra-peripheral collisions, see [1]. 1 Ultra-peripheral collisions at RHIC The Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory began operating in the year 2000. This meant an increase in the maximum center of mass energies for heavy-ion collisions by more than an order of magnitude compared with the earlier fixed-target experiments. At very high collision energies, the electromagnetic field surrounding a nucleus contains photons energetic enough to produce new particles in ultra-peripheral collisions. This can happen in a purely electromagnetic process through a two-photon interactions or in an interaction between a photon from one of the nuclei and the other (“target”) nucleus. The photon spectrum for a minimum impact parameter, bmin, extends to ∼ γ/bmin, which corresponds to about 300 GeV in the rest frame of the target nucleus in a gold on gold collision at RHIC. These photon energies are thus far above the threshold for particle production. The coherent contribution from the Z protons in the nucleus, furthermore, enhances the number of equivalent photons by a factor Z. The high photon energies and fluxes lead to large cross sections for several photon-induced reactions; some have cross sections much larger than the total hadronic cross section and are major sources of beam-loss at heavy-ion colliders [2]. For example, the cross section for breaking up one of the nuclei in an Au+Au collision at RHIC through a photonuclear interaction is 95 b. The cross section for exchanging two photons and thereby simultaneously breaking up both nuclei in the same event is also large, about 4 b. The dominating fragmentation mechanism is excitation to a Giant Dipole Resonance followed by emission of one or a few neutrons. The mutual Coulomb dissociation has been studied at RHIC by detecting the forward going neutrons in Zero-Degree Calorimeters [3]. These are located 18 m downstream from the interaction point and have an angular acceptance of θ < 2 mrad with respect to the beam axis. The relative contribution of the photon-induced fragmentation to the total cross section was found to be in good agreement with calculations based on the method of equivalent photons combined † talk presented at EDS07 with measured γ+Au cross sections. The calculations also reproduced the neutron multiplicity distribution for photon-induced events reasonably well. Another ultra-peripheral process with very high cross section is two-photon production of electron-positron pairs. Of particular interest is the sub-class of events where the produced electron binds to one of the beam nuclei. The captured electron changes the charge and thus the rigidity of the ion, leading to a different deflection by the guiding magnets in the accelerator ring and eventual loss. Under certain conditions, the ion with an attached electron will hit the wall of the beam-pipe enclosure at a well-defined spot down stream of the interaction point. At the Large Hadron Collider at CERN, because of the high beam flux and energy, this has the potential to heat and quench the superconducting magnets near this area. The phenomenon was recently observed for the first time at RHIC with Cu–beams [4]. The location of the point of incidence (≈140 m downstream from the interaction point) and the multiplicity of secondary particles resulting from the interaction of the 100 A GeV Cu beam with the beam-pipe and the surrounding magnets were found to be in good agreement with theoretical calculations, although the experimental uncertainties were large. Particle production in ultra-peripheral collisions has been studied by both of the two large experiments at RHIC, STAR and PHENIX. Some of these results will be discussed in the following two sections. 2 Results from STAR The first results on particle production in ultra-peripheral collisions at RHIC were studies of coherent production of ρ mesons in Au+Au interaction by the STAR collaboration [5]. The cross section to produce a ρ in an Au+Au collision at RHIC is about 10% of the total inelastic, hadronic cross section. STAR has also published final results on two-photon production of free ee–pairs [6] and preliminary results on photo-production of ρ in d+Au collisions [7] and coherent production of four pions in Au+Au collisions [8]. In d+Au collisions more than 90% of the photo-produced ρ mesons come from events where the gold nucleus emitted the photon. The interactions can leave the deuteron intact γ+d → ρ + d or lead to break-up γ+ d → ρ +n+ p. Two triggers were implemented to study the two cases. Both were based on triggering on low multiplicity combined with a “topology” cut to reject cosmic rays. The multiplicity was measured in the STAR Central Trigger Barrel, which consists of 240 scintillators covering the full azimuth in the pseudo-rapidity range |η| < 1. To trigger on interactions where the deuteron breaks up, it was in addition required that the forward going neutron should be detected in the Zero Degree Calorimeter. Examples of the ππ invariant mass distributions for the two samples are shown in Fig. 1. The invariant mass distribution is well described by the sum of the amplitudes for a resonant ρ term and a non-resonant (Söding) term: