Abstract
Evidence for the light-by-light scattering process, γγ→γγ, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390μb−1 recorded by the CMS experiment at the LHC. Light-by-light scattering processes are selected in events with two photons exclusively produced, each with transverse energy ETγ>2GeV, pseudorapidity |ηγ|<2.4, diphoton invariant mass mγγ>5GeV, diphoton transverse momentum pTγγ<1GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 9.0±0.9(theo) events for the signal and 4.0±1.2(stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 3.7 standard deviations, and has properties consistent with those expected for the light-by-light scattering signal. The measured fiducial light-by-light scattering cross section, σfid(γγ→γγ)=120±46(stat)±28(syst)±12(theo)nb, is consistent with the standard model prediction. The mγγ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the ▪ process, in the mass range ▪.
Highlights
Elastic light-by-light (LbL) scattering, γ γ → γ γ, is a pure quantum mechanical process that proceeds, at leading order in the quantum electrodynamics (QED) coupling α, via virtual box diagrams containing charged particles (Fig. 1, left)
Since each photon flux scales as the square of the ion charge Z 2, γ γ scattering cross sections in PbPb collisions are enhanced by a factor of Z 4 5 × 107 compared to similar proton-proton or electron-positron interactions
In order to have a full control of the QED e+e− background in the LbL scattering signal region, the same analysis carried out for the LbL events is done on exclusive dielectron candidates, applying the same criteria as described above for diphoton events, with the exception that exactly two opposite-sign electrons are reconstructed, instead of exactly two photons, and no additional track with pT > 0.1 GeV should be present in addition to the two tracks corresponding to the electrons
Summary
Elastic light-by-light (LbL) scattering, γ γ → γ γ , is a pure quantum mechanical process that proceeds, at leading order in the quantum electrodynamics (QED) coupling α, via virtual box diagrams containing charged particles (Fig. 1, left). Ions accelerated at high energies generate strong electromagnetic fields, which, in the equivalent photon approximation [9,10,11], can be considered as γ beams of virtuality Q 2 < 1/R2, where R is the effective radius of the charge distribution. Since each photon flux scales as the square of the ion charge Z 2, γ γ scattering cross sections in PbPb collisions are enhanced by a factor of Z 4 5 × 107 compared to similar proton-proton or electron-positron interactions.
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