Equivalent photons in proton–proton and ion–ion collisions at the Large Hadron Collider

Abstract

Equivalent photon approximation is used to calculate fiducial cross sections for dimuon production in ultraperipheral proton-proton and lead-lead collisions. Analytical formulae taking into account experimental cuts are derived. The results are compared with the measurements reported by the ATLAS collaboration. IntroductionThis year we celebrate the 111th anniversary of L. D. Landau. This paper is devoted to the modern state of the problem first considered by L. D. Landau and E. M. Lifshitz in 1934 when they calculated the production cross section of e + e − pair in ultrarelativistic heavy ions collisions [1]. We will demonstrate that this problem is still of great interest.In spite of many efforts, no New Physics has been found at the LHC so far. It might be a good time to consider scenarios of appearance of New Physics at the LHC that were less attractive at the time when the LHC was under construction. Although the LHC was conceived as a hadron-hadron collider, it also acts as a photon-photon collider with the photons appearing in ultraperipheral collisions of hadrons. This idea is quite old, and it was thoroughly considered during the construction and operation of the RHIC and the LHC [2-16]. However, since hadronic interactions were more likely to deliver the signal of New Physics, particularly in Higgs boson properties, they received more attention in the literature and were given higher priority in the LHC schedule. With the long shutdown of the LHC beginning at the end of 2018, it might be a good time to reconsider photon-photon collisions at the LHC as a source of possible New Physics events so that the necessary detectors adjustments could be made and, perhaps, more time for heavy ions collisions could be negotiated in the LHC schedule.The leading order Feynman diagram for an ultraperipheral collision is presented in Fig. 1 where instead of lead nuclei there could be any charged particles. The distinctive signature of an ultraperipheral collision is that the charged particles remain intact after the collision. These particles won't have high transverse momentum, so they are difficult to detect with just the main detectors of the ATLAS and CMS experiments, but there exist additional detectors at low scattering angles (the ATLAS forward proton detector [17] and the CMS-TOTEM precision proton spectrometer [18]). However, even without the forward detectors, ultraperipheral collisions manifest through production of particles.Let us compare proton-proton and lead-lead ultraperipheral collisions as possible sources of New Physics events. Integrated luminosity delivered by the LHC in Run 2 in proton-proton collisions is over 150 fb −1 both for the ATLAS and the CMS experiments [19,20]. Integrated luminosity delivered by the LHC in lead-lead collisions in the heavy ions run was 0.7 nb −1 in 2015 [21] and 1.8 nb −1 in 2018 [19]. Cross section for an ultraperipheral collision is proportional to Z 4 where Z is the particle charge. For Pb, Z = 82, so we get that if there exists New Physics that appears in γγ collisi...

Ultraperipheral vs. Ordinary Nuclear Interactions

“…It is small in pp and quite noticeable in P bP b processes due to the Z 4 -factor. Using the parameters of preasymptotic estimates borrowed from [17], it is argued that the ultraperipheral processes AA → AAπ 0 and AA → AAe + e − can be observed at pernucleon energies higher than 3.6 GeV.…”

“…The optimism is supported by studies [17] of ultraperipheral production of µ + µ − pairs. The ultraperipheral cross section (6) at 13 TeV is equal 3 to 0.22 µb.…”

“…The parameter u is the least precisely determined element of the whole approach. The careful treatment of formfactors of protons and nuclei with account of the photon virtuality (see also Refs [12,14] where the problem was treated in more detail) and the suppression factors [17] lead to its values u pp ≈ 0.2 for pp and u P bP b ≈ 0.02 for P bP b-collisions within the factors about 1.5 which depend on the particular shape of the formfactors (see [17]). In what follows, motivated by these results I use the parameters u obtained in Ref.…”

Quasistable charginos in ultraperipheral proton-proton collisions at the LHC

“…Photons with higher energy are produced as well, but their production is suppressed by the nucleus form factor, thus greatly reducing the benefits from higher photon flux in a collision of heavy ions. Nevertheless, the production cross section for a system with invariant mass about 100 GeV is several orders of magnitude larger in lead-lead collisions than in proton-proton collisions [12].…”

“…However, photon energy is limited by the breaking energy of its source particle. In the case of proton, this energy should be at the level of Λ QCD ; calculation based on its electromagnetic form factor results in q = 0.20 GeV [12], whereq is the maximum momentum of a virtual photon in the proton rest frame. In the laboratory reference frame the maximum photon energy isqγ where γ is the Lorentz γ-factor of the proton; for a 6.5 TeV protonqγ = 1.4 TeV.…”