CMS-PAS-FSQ-16-012 | ||

Measurement of light-by-light scattering in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||

CMS Collaboration | ||

May 2018 | ||

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Abstract:
A measurement of light-by-light scattering, $\gamma\gamma\to\gamma\gamma$, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390 $\mu$b$^{-1}$ 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 $\mathrm{E}_{\mathrm{T}}^{\gamma} > $ 2 GeV, pseudorapidity $|\eta^{\gamma}| < $ 2.4, diphoton invariant mass $\mathrm{m}^{\gamma\gamma} > $ 5 GeV, diphoton transverse momentum $\mathrm{p}_{\mathrm{T}}^{\gamma\gamma} < $ 1 GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 11.1 $\pm$ 1.1 (th) events for the signal and 3.8 $\pm$ 1.3 (stat) for the background processes. The significance of the light-by-light signal against the background-only hypothesis is 4.1 standard deviations. The measured fiducial light-by-light scattering cross section, $\sigma_{\mathrm{fid}} (\gamma\gamma\to\gamma\gamma)=$ 122 $\pm$ 46 (stat) $ \pm $ 29 (syst) $ \pm $ 4 (th) is consistent with the standard model prediction.
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Links:
CDS record (PDF) ;
inSPIRE record ;
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These preliminary results are superseded in this paper, PLB 797 (2019) 134826.The superseded preliminary plots can be found here. |

Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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Figures | |

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Figure 1:
Schematic diagrams of light-by-light scattering ($\gamma \gamma \to \gamma \gamma $, left), QED dielectron production ($\gamma \gamma \to {\mathrm {e}}^+ {\mathrm {e}}^-$, centre), and central exclusive diphoton production ($ {\mathrm {g}} {\mathrm {g}} \to \gamma \gamma $, right) in ultraperipheral PbPb collisions (where the * superindex indicates a potential electromagnetic excitation of the outgoing ions). |

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Figure 2:
Acoplanarity distribution of QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events measured in data (circles), compared to the expected distribution in the STARLIGHT MC simulation (histogram), scaled as described in the text. The curve shows a $\chi ^2$ fit to the sum of two exponential distributions corresponding to exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ plus any residual (non acoplanar) background pairs. Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histogram include systematic and MC statistical uncertainties added in quadrature. |

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Figure 3:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events passing all selection criteria, as a function of dielectron acoplanarity (top left), mass (top right), $ {p_{\mathrm {T}}} $ (bottom left), and rapidity (bottom right). Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. |

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Figure 3-a:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events passing all selection criteria, as a function of dielectron acoplanarity. Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. |

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Figure 3-b:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events passing all selection criteria, as a function of dielectron mass. Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. |

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Figure 3-c:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events passing all selection criteria, as a function of dielectron $ {p_{\mathrm {T}}} $. Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. |

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Figure 3-d:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for exclusive $ {\mathrm {e}}^+ {\mathrm {e}}^-$ events passing all selection criteria, as a function of dielectron rapidity. Data and simulations are uncorrected for detector effects. Error bars around the data points indicate statistical uncertainties, hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. |

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Figure 4:
Diphoton acoplanarity distribution for exclusive events measured in the data after selection criteria (squares), compared to the expected LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ (yellow histogram), and the CEP+other (light blue histogram, scaled to match the data in the $\text {Aco} > $ 0.02 region as described in the text) backgrounds. Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

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Figure 5:
Distributions of the photon ${E_{\mathrm {T}}}$, $\eta $ and $\phi $, as well as diphoton $ {p_{\mathrm {T}}} $, rapidity and invariant mass measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

png pdf |
Figure 5-a:
Distribution of the photon ${E_{\mathrm {T}}}$ measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

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Figure 5-b:
Distribution of the photon $\eta $ measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

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Figure 5-c:
Distribution of the photon $\phi $ measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

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Figure 5-d:
Distribution of the diphoton $ {p_{\mathrm {T}}} $ measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

png pdf |
Figure 5-e:
Distribution of the diphoton rapidity measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

png pdf |
Figure 5-f:
Distribution of the diphoton invariant mass measured for exclusive events passing all selection criteria (squares), compared to the expectations of LbyL signal (red histogram), QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC predictions (yellow histogram), and the CEP plus other backgrounds (light blue histogram, scaled to match the data in the $\text {Aco} > 0.02$ region). Signal and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. Data and simulations are uncorrected for detector effects. The error bars around the data points indicate statistical uncertainties. |

Tables | |

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Table 1:
Number of diphoton candidates measured in data and expected from the LbyL and QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ MC generators, and from the CEP+other contributions, after each event selection step. The yields of the simulated processes are scaled according to their theoretical cross sections and the integrated luminosity of the analysed data set. The "CEP+other'' values are normalised from the high-acoplanarity tail with a scale factor estimated from the data as described in the text. The LbyL MC uncertainty quoted is that of the theoretical uncertainty of the prediction, whereas the uncertainties in the QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ and CEP+others counts are statistical. |

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Table 2:
Summary of the overall cross section efficiencies $C^{{\gamma \gamma},\text {ee}}$, MC-driven efficiencies $\text {Eff}^{{\gamma \gamma},\text {ee}}$, and individual data-to-MC scale factors $\text {SF}^{{\gamma \gamma},\text {ee}}$, obtained for the diphoton and dielectron analyses. |

png pdf |
Table 3:
Summary of the systematic uncertainties on the ratio of the fiducial LbyL to total QED $ {\mathrm {e}}^+ {\mathrm {e}}^-$ cross sections. |

Summary |

A study of elastic light-by-light (LbyL) scattering, $\gamma \gamma \rightarrow \gamma \gamma$, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV has been presented, based on a data sample corresponding to an integrated luminosity of 390 $\mu$b$^{-1}$ recorded by the CMS experiment at the LHC in 2015. Fourteen LbyL candidates passing all selection requirements are observed, with photon $ E_{\mathrm{T}} > $ 2 GeV and pseudorapidity $| \eta | < $ 2.4, diphoton invariant mass greater than 5 GeV, diphoton transverse momentum lower than 1 GeV and diphoton acoplanarity below 0.01. Both the yield and the kinematic distributions are in accord with the expectations from the expected LbyL signal plus small residual backgrounds, mostly from misidentified exclusive dielectron, ${\gamma\gamma}\to\mathrm{e}^+\mathrm{e}^-$, and central exclusive diphoton, $\mathrm{gg}\to {\gamma\gamma}$, production. The observed (expected) significance of the light-by-light signal over the background-only expectation is of 4.1 (4.4) standard deviations. The measured fiducial light-by-light scattering cross section, $\sigma_\text{fid} (\gamma\gamma \to \gamma\gamma) = $ 122 $\pm$ 46 (stat) $\pm$ 29 (syst) $\pm$ 4 (th) nb, is consistent with the standard model theoretical prediction. |

Additional Figures | |

png pdf |
Additional Figure 1:
Event display for a candidate light-by-light event, passing all analysis cuts, in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. |

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Compact Muon Solenoid LHC, CERN |