We report on the first measurement of Mutual Coulomb Dissociation in heavy ion collisions. We employ forward calorimeters to measure neutron multiplicity at beam rapidity. The cross-section for simultaneous electromagnetic breakup of Au nuclei at a nucleon-nucleon center of mass energy √ sNN = 130 GeV is σMCD = 3.67±0.26 barns which is comparable to the geometrical cross section. The ratio of the electromagnetic to the total cross section is in good agreement with calculations, as is the neutron multiplicity distribution.PACS numbers: 29.40.Vj, An interesting aspect of the Relativistic Heavy Ion Collider (RHIC) is the high rate of γ-hadron collisions it produces. Photons from the highly Lorentz contracted electro-magnetic field produced by the heavy ions of one beam collide with the nuclei of the other beam (for an overview see [1]). For example, the flux of equivalent photons with energies 2 GeV ≤ E γ ≤ 300 GeV in the target nucleus rest-frame corresponds to a γ-nucleus luminosity of 10 29 cm −2 × s −1 . With such a high flux, nuclear dissociation is highly probable. The calculated cross-section for Single Beam Dissociation of 95 barns limits the maximum beam lifetime at RHIC [2].We report the first measurement of Mutual Coulomb Dissociation (MCD) whereby both beams dissociate electromagnetically. The calculation of this cross-section uses an extension of the Weizsäcker-Williams formalism since it is dominated by 2nd order 2 photon exchange [3,4]. The total cross section σ tot , for the nuclear break up of both beams, due to either Coulomb or strong (hadronic) interactions, was calculated in [3]. There it was argued that uncertainties in the calculation of the two interactions partially cancel resulting in a theoretical error of 5%. One can exploit this to derive MCD and hadronic cross sections from the data presented here. The MCD process is also of interest because it can be used for luminosity monitoring at RHIC and at the Large Hadron Collider [5,6]. Finally an understanding of the photon flux generated in peripheral heavy ion collisions is necessary to understand the emerging field of γγ, γA and γ pomeron interactions [7,8].The presented data are collected from the 3 experiments BRAHMS, PHENIX and PHOBOS at RHIC with Au beams at √ s N N = 130 GeV. MCD results in emission of a few nucleons, dominantly neutrons, with small (few MeV) kinetic energy in the nucleus rest frame. In the laboratory frame they therefore have small angular spread with respect to the beam and close to 65 GeV energy. We measured neutron multiplicities close to the beam direction with Zero Degree Calorimeters (ZDCs) that are common to all RHIC experiments. The topological cross-sections Au+Au→ N Lef t neutrons +N Right neutrons +X+Y are measured and compared to calculations [3,4]. In addition, Beam-Beam Counters which differ for each experiment were used to detect produced particles at large angles.The ZDCs are small transverse-area hadron calorimeters with an angular acceptance of |θ| < 2 mrad with respect to the beam axis. They are lo...
