Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

Abstract

Light-by-light scattering ($γγ\rightarrowγγ$) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Using 480 $μ$b$^{-1}$ of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the $γγ\rightarrowγγ$ reaction. A total of 13 candidate events are observed with an expected background of 2.6$\pm$0.7 events. After background subtraction and analysis corrections, the fiducial cross section of the process $\textrm{Pb+Pb}\,(γγ)\rightarrow \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,γγ$, for photon transverse energy $E_{\mathrm{T}}>$3 GeV, photon absolute pseudorapidity $|η|<$2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 $\pm$ 24 (stat.) $\pm$ 17 (syst.) nb, which is in agreement with Standard Model predictions.

Figures (6)

• Figure 1: Diagrams illustrating the QED LbyL interaction processes and the equivalent photon approximation. (a) Diagrams for Delbrück scattering (left), photon splitting (middle) and elastic LbyL scattering (right). Each cross denotes external field legs, e.g., an atomic Coulomb field or a strong background magnetic field. (b) Illustration of an ultra-peripheral collision of two lead ions. Electromagnetic interaction between the ions can be described as an exchange of photons that can couple to form a given final-state X. The flux of photons is determined from the Fourier transform of the electromagnetic field of the ion, taking into account the nuclear electromagnetic form factors.
• Figure 2: Photon identification and reconstruction efficiencies. (a) Photon PID efficiency as a function of photon extracted from FSR event candidates. (b) Photon reconstruction efficiency as a function of photon (approximated with $\eT^e - \pT^\textrm{trk2}$) extracted from $\gamma\gamma\rightarrow e^+e^-$ events with a hard-bremsstrahlung photon. Data (closed markers) are compared with MC simulations (open markers). The statistical uncertainties arising from the finite size of the data and simulation samples are indicated by vertical bars.
• Figure 3: Kinematic distributions for $\gamma\gamma\rightarrow \gamma\gamma$ event candidates. (a) Diphoton acoplanarity before applying $\textrm{Aco}<0.01$ requirement. (b) Diphoton invariant mass after applying $\textrm{Aco}<0.01$ requirement. Data (points) are compared to MC predictions (histograms). The statistical uncertainties on the data are shown as vertical bars.
• Figure 4: Kinematic distributions for $\textrm{Pb+Pb}\,(\gamma\gamma)\rightarrow \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,e^+e^-$ event candidates: (a) dielectron mass, (b) dielectron , (c) electron pseudorapidity and (d) electron transverse energy. Data (points) are compared to MC expectations (histograms). Electrons with $\et>2.5~\gev$ and $|\eta|<2.47$ excluding the calorimeter transition region $1.37<|\eta|<1.52$ are considered. The statistical uncertainties on the data are shown as vertical bars. The uncertainty on the integrated luminosity, used to estimate the number of expected MC events, is 6%.
• Figure 5: Schematic diagram for the CEP $gg\rightarrow\gamma\gamma$ process production mechanism.