Higher order QED in high-masse+e−pair production at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

Abstract

Lowest order and higher order QED calculations have been carried out for the RHIC high mass e + e − pairs observed by PHENIX with single ZDC triggers. The lowest order QED results for the experimental acceptance are about two standard deviations larger than the PHENIX data. Corresponding higher order QED calculations are within one standard deviation of the data. PACS: 25.75.-q, 34.90.+q A recent publication of the PHENIX collaboration at RHIC has presented data on ultraperipheral Au + Au photoproduction of the J/ψ and of continuum e + e − pairs in the invariant mass range of 2.0 -2.8 GeV/c 2 [1]. The present paper concerns itself not with the J/ψ but with the continuum pairs. A previous measurement of e + e − pairs at RHIC was carried out by the STAR colaboration in the somewhat lower invariant mass range 140 -265 MeV/c 2 [2], and it was subsequently argued that these data exhibited evidence for higher order QED effects [3]. This paper presents results of a higher order QED calculation for the PHENIX data using the same methodology as the calculations[3] previously published for the STAR data.The PHENIX data was presented as a differential cross section with respect to the pair equivalent mass m e + e − and the pair rapidity y pair with the constraint of at least one neutron detected in one of the zero degree calorimeters (ZDC): d 2 σ/dm e + e − dy pair (e + e − + Xn, y pair = 0). The differential cross section at y pair = 0 was taken from the sample |y pair | < 0.35 but with no constraint on pseudorapidity η of individual electrons and positrons. However, since the events measured by PHENIX detector had both the electron and positron pseudorapidities within |η| < 0.35, the stated results without the individual |η| < 0.35 were necessarily constructed from a model calculation. The PHENIX publication states, "The fraction of events with |(y pair )| < 0.35 and 2.0 < m e + e − < 2.8 GeV/c 2 , where both electron and positron are within |η| < 0.35 is 1.10%. The corresponding numbers for 2.0 < m e + e − < 2.3 GeV/c 2 and 2.3 < m e + e − < 2.8 GeV/c 2 are 1.11% and 1.08%, respectively." Since we wanted to carry out calculations closest to what PHENIX actually measured, here we calculate the cross sections with the electron and positron constraints |η| < 0.35 and compared with the cross sections in the PHENIX publication multiplied by 0.011, etc. (see also Ref. [4]).Following the method of the higher order calculations previously used for the STAR data, the cross sections here are computed from the product of the pair production probability P ee (b), the probability at least one of the ions Coulomb dissociating P 1x (b), and a survival factor exp[−P nn (b)] to exclude events where hadronic interactions occur(1)Unlike the STAR Coulomb dissociation factor P xx (b), which corresponds to neutrons detected in both ZDCs,here the corresponding Coulomb dissociation factor P 1x (b) is the unitarized probability that requires only that at least one of the colliding nuclei suffers Coulomb dissociationP C (b) the non-unitarized...

Discovery of higher-order quantum electrodynamics effect for the vacuum pair production

“…However, with the multiple pair production taken into account [24], the higher-order effect for total cross section became small. Following this strategy, the hint of higher-order effect [25,26] had been observed in comparison with the RHIC measurements. Over the decades, various experimental measurements of lepton pair production [27][28][29][30][31][32][33][34] has been made in ultra-peripheral collisions (UPC) at RHIC and LHC.…”