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| CMS-FSQ-16-012 ; CERN-EP-2018-271 | ||
| Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 10 October 2018 | ||
| Phys. Lett. B 797 (2019) 134826 | ||
| Abstract: Evidence for the light-by-light scattering process, ${\gamma\gamma}\to{\gamma\gamma}$, 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 $E_{\mathrm{T}}^{\gamma} > $ 2 GeV, pseudorapidity $|{\eta^{\gamma}}| < $ 2.4, diphoton invariant mass ${m^{{\gamma\gamma}}} > $ 5 GeV, diphoton transverse momentum ${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 (theo) events for the signal and 4.0 $\pm$ 1.2 (stat) for the background processes. The excess observed in data relative to the background-only expectation corresponds to a significance of 4.1 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, $\sigma_\text{fid} ({\gamma\gamma} \to {\gamma\gamma})= $ 120 $\pm$ 46 (stat) $\pm$ 28 (syst) $\pm$ 4 (theo) nb, is consistent with the standard model prediction. The ${m^{{\gamma\gamma}}}$ distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles, via the ${\gamma\gamma} \to \mathrm{a} \to {\gamma\gamma}$ process, in the mass range $m_{\mathrm{a}} = $ 5-90 GeV. | ||
| Links: e-print arXiv:1810.04602 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Schematic diagrams of light-by-light scattering ($ {\gamma \gamma}\to {\gamma \gamma}$, left), QED dielectron ($ {\gamma \gamma}\to {{\mathrm {e}^+} {\mathrm {e}^-}}$, centre), and central exclusive diphoton ($ {\mathrm {g}} {\mathrm {g}} \to {\gamma \gamma}$, right) production in ultraperipheral PbPb collisions. The (*) superscript indicates a potential electromagnetic excitation of the outgoing ions. |
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Figure 2:
Acoplanarity distribution of exclusive $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ events measured in data (circles), compared to the expected QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ spectrum 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 (nonacoplanar) background pairs. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histogram include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. |
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Figure 3:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for the 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 $y$ (bottom right). Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panels. |
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Figure 3-a:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for the exclusive $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ events passing all selection criteria, as a function of dielectron acoplanarity. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panel. |
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Figure 3-b:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for the exclusive $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ events passing all selection criteria, as a function of mass. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panel. |
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Figure 3-c:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for the exclusive $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ events passing all selection criteria, as a function of ${p_{\mathrm {T}}}$. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panel. |
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Figure 3-d:
Comparison of data (circles) and STARLIGHT MC expectation (histogram, scaled as described in the text) for the exclusive $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ events passing all selection criteria, as a function of rapidity $y$. Error bars around the data points indicate statistical uncertainties, and hashed bands around the histograms include systematic and MC statistical uncertainties added in quadrature. The horizontal bars around the data symbols indicate the bin size. The ratio of the data to the MC expectation is shown in the bottom panel. |
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Figure 4:
Diphoton acoplanarity distribution for exclusive events measured in the data after selection criteria (squares), compared to the expected LbL scattering signal (orange histogram), QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ (yellow histogram), and the CEP+other (light blue histogram, scaled to match the data in the $\text {A}_{\phi} > $ 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. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5:
Distributions of the single photon ${E_{\mathrm {T}}}$, $\eta $, and $\phi $, as well as diphoton ${p_{\mathrm {T}}}$, rapidity, and invariant mass measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-a:
Distribution of the single photon ${E_{\mathrm {T}}}$, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-b:
Distribution of the single photon $\eta $, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-c:
Distribution of the single photon $\phi $, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-d:
Distribution of diphoton invariant mass, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-e:
Distribution of diphoton rapidity, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 5-f:
Distribution of diphoton ${p_{\mathrm {T}}}$, measured for the fourteen exclusive events passing all selection criteria (squares), compared to the expectations of LbL scattering signal (orange 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 {A}_{\phi} > $ 0.02 region). Signal and QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ MC samples are scaled according to their theoretical cross sections and integrated luminosity. The error bars around the data points indicate statistical uncertainties. The horizontal bars around the data symbols indicate the bin size. |
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Figure 6:
Observed (full line) and expected (dotted line) 95% CL limits on the production cross section $\sigma ({\gamma \gamma}\to {\mathrm {a}} \to {\gamma \gamma})$ as a function of the ALP mass $m_ {\mathrm {a}} $ in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The inner (green)and outer (yellow) bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. |
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Figure 7:
Exclusion limits at 95% CL in the ALP-photon coupling $g_{{\mathrm {a}} \gamma}$ versus ALP mass $m_ {\mathrm {a}} $ plane, for the operators $ {\mathrm {a}} F\tilde{F}/4\Lambda $ (left, assuming ALP coupling to photons only) and $ {\mathrm {a}} B\tilde{B}/4\Lambda \cos^2\theta _\textrm {W}$ (right, including also the hypercharge coupling, thus processes involving the ${{\mathrm {Z}}}$ boson) derived in Refs. [30,55] from measurements at beam dumps [59], in ${\mathrm {e}^+} {\mathrm {e}^-}$ collisions at LEP-I [55] and LEP-II [56], and in pp collisions at the LHC [13,57,58], and compared to the present PbPb limits. |
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Figure 7-a:
Exclusion limits at 95% CL in the ALP-photon coupling $g_{{\mathrm {a}} \gamma}$ versus ALP mass $m_ {\mathrm {a}} $ plane, for the operators $ {\mathrm {a}} F\tilde{F}/4\Lambda $, assuming ALP coupling to photons only, derived in Refs. [30,55] from measurements at beam dumps [59], in ${\mathrm {e}^+} {\mathrm {e}^-}$ collisions at LEP-I [55] and LEP-II [56], and in pp collisions at the LHC [13,57,58], and compared to the present PbPb limits. |
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Figure 7-b:
Exclusion limits at 95% CL in the ALP-photon coupling $g_{{\mathrm {a}} \gamma}$ versus ALP mass $m_ {\mathrm {a}} $ plane, for the operators $ {\mathrm {a}} B\tilde{B}/4\Lambda \cos^2\theta _\textrm {W}$, including also the hypercharge coupling, thus processes involving the ${{\mathrm {Z}}}$ boson, derived in Refs. [30,55] from measurements at beam dumps [59], in ${\mathrm {e}^+} {\mathrm {e}^-}$ collisions at LEP-I [55] and LEP-II [56], and in pp collisions at the LHC [13,57,58], and compared to the present PbPb limits. |
| Tables | |
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Table 1:
Number of diphoton candidates measured in data and expected from MC simulation for LbL scattering, QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ production, and from the CEP+other contributions, after each event selection step (cumulative) described in the text. 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 LbL scattering simulation uncertainty quoted is that of the theoretical uncertainty of the prediction, whereas the uncertainties in the QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ and CEP+others yields are statistical. |
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Table 2:
Summary of the overall cross section measurement efficiencies $C^{{\gamma \gamma}, {\mathrm {e}} {\mathrm {e}}}$, efficiencies from simulation $\varepsilon ^{{\gamma \gamma}, {\mathrm {e}} {\mathrm {e}}}$, and individual data-to-simulation scale factors $\text {SF}^{{\gamma \gamma}, {\mathrm {e}} {\mathrm {e}}}$, obtained for the diphoton and dielectron analyses. "Reco. and ID'' stands for reconstruction and identification. All quoted uncertainties are systematic. |
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Table 3:
Summary of the systematic uncertainties in the ratio of the fiducial LbL scattering to total QED $ {{\mathrm {e}^+} {\mathrm {e}^-}}$ cross sections. |
| Summary |
| Evidence for light-by-light (LbL) scattering, ${\gamma\gamma} \to {\gamma\gamma}$, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV has been reported, 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 LbL-scattering candidate events passing all selection requirements have been observed, with photon transverse energy above $2 GeV$ 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 measured total yields and kinematic distributions are in accord with the expectations for the LbL scattering signal plus small residual backgrounds that are mostly from misidentified exclusive dielectron (${\gamma\gamma}\to{\mathrm{e^{+}}\mathrm{e^{-}}}$) and gluon-induced central exclusive ($\mathrm{g}\mathrm{g}\to {\gamma\gamma}$) processes. The observed (expected) significance of the LbL scattering signal over the background-only expectation is 4.1 (4.4) standard deviations. The measured fiducial light-by-light scattering cross section, $\sigma_\text{fid} ({\gamma\gamma} \to {\gamma\gamma}) = $ 120 $\pm$ 46 (stat) $\pm$ 28 (syst) $\pm$ 4 (theo) nb, is consistent with the standard model expectation. The measured exclusive diphoton invariant mass distribution is used to set new exclusion limits on the production of pseudoscalar axion-like particles (ALPs), via the process ${\gamma\gamma} \to {\mathrm{a}} \to {\gamma\gamma}$, over the $m_{\mathrm{a}} = $ 5-90 GeV mass range. For ALPs coupling to the electromagnetic (and electroweak) current, the derived exclusion limits are currently the best over the $m_{\mathrm{a}} = $ 5-50 GeV (5-10 GeV) mass range. |
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