Evidence for Higher Order QED Effects ine+e−Pair Production at the BNL Relativistic Heavy Ion Collider

Abstract

A new lowest order QED calculation for RHIC e + e − pair production has been carried out with a phenomenological treatment of the Coulomb dissociation of the heavy ion nuclei observed in the STAR ZDC triggers. The lowest order QED result for the experimental acceptance is nearly two standard deviations larger than the STAR data. A corresponding higher order QED calculation is consistent with the data. PACS: 25.75.-q, 34.90.+q In a recent publication data were presented by the RHIC STAR collaboration on "Production of e + e − pairs accompanied by nuclear dissociation in ultraperipheral heavy-ion collisions" [1]. In this publication the authors compared the experimental cross section σ with a lowest order QED calculation σ QED and concluded that "At a 90% confidence level, higher-order corrections to the cross section, ∆σ = σ − σ QED must be within the range −0.5σ QED < ∆σ < 0.2σ QED ".In this Letter, I will present a new lowest order QED cross section calculation result significantly larger than that presented in the STAR publication. This new lowest order QED cross section is now two standard deviations above the STAR experimental result. Furthermore, I find that a higher order QED calculation gives good agreement with the experimental result.The STAR experimental cross sections are for a final state with an e + e − pair in combination with mutual nuclear excitation of both ions by the ions' Coulomb fields observed in the Zero Degree Calorimeters (ZDC). Thus impact parameter dependent probabilities have to be calculated both for e + e − pair production and for mutual Coulomb dissociation of the ions.The calculations presented here make use of the method of calculating the impact parameter dependent probability of producing e + e − pairs previously described [2]. Use of an appropriate gauge allowed the electromagnetic fields of colliding ions to be represented as colliding delta functions in the ultrarelativisitic limit. The solution of the time-dependent Dirac equation for lepton pair production was thereby obtained in closed form. This solution contains higher order QED effects and goes to the known perturbative (lowest order) result in that limit. It is well known that photoproduction of e + e − pairs on a heavy target shows a negative (Coulomb) correction proportional to Z 2 that is well described by the Bethe-Maximon theory [3]. The calculated higher order heavy ion pair production cross sections show analogous reductions from perturbation theory.The impact parameter (b) dependent amplitude presents a particular numerical challenge since it involves the a rapidly oscillating phase exp(ik · b) in the integral over the transverse momentum k transfered from the ion to the lepton pair. The usual method of evaluating the perturbative impact parameter dependent probability is to first square the amplitude and then integrate over the sum and difference of k and k ′ . Here I have integrated before squaring, and I deal with the rapid oscillations with the piecewise analytical integration method previously describe...

Two-photon interactions with nuclear breakup in relativistic heavy ion collisions

“…The cross-section measured by STAR is larger than lowest-order QED calculation; the difference can be explained by higher order corrections [11].…”

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

“…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 perturbative and higher order QED pair production probabilities P ee (b) were calculated using the methods previously described [3,7]. In the P ee (b) calculations an analytical elastic form factor was employed f (q) = 3 (qr) 3 [sin(qr) − qr cos(qr)] 1 1 + a 2 q 2…”

Bound-free electron-positron pair production in relativistic heavy-ion collisions

“…For the total electron-positron pair production, the effect of the 'Coulomb correction' is known to play an important role [21] due to multi-photon exchange of the produced electron-positron with the colliding nuclei. This Coulomb correction for the free pair production has a negative value and it is proportional to Z 2 which is obtained by Bethe-Maximon [22].…”