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| CMS-HIN-21-015 ; CERN-EP-2024-284 | ||
| Measurement of light-by-light scattering and the Breit-Wheeler process, and search for axion-like particles in ultraperipheral PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 19 December 2020 | ||
| Submitted to J. High Energy Phys. | ||
| Abstract: Measurements of light-by-light scattering (LbL, $ \gamma\gamma\to\gamma\gamma $) and the Breit-Wheeler process (BW, $ \gamma\gamma\to\mathrm{e}^+\mathrm{e}^- $) are reported in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV. The data sample, corresponding to an integrated luminosity of 1.7 nb$^{-1}$, was collected by the CMS experiment at the CERN LHC in 2018. Events with an exclusively produced $ \gamma\gamma $ or $ \mathrm{e}^+\mathrm{e}^- $ pair with invariant masses $ m^{\gamma\gamma,\mathrm{e}\mathrm{e}} > $ 5 GeV, along with other fiducial criteria, are selected. The measured BW fiducial production cross section, $ \sigma_\text{fid} (\gamma\gamma\to\mathrm{e}^+\mathrm{e}^-)= $ 263.5 $ \pm $ 1.8 (stat) $ \pm $ 17.8 (syst) $ \mu $b, as well as the differential distributions for various kinematic observables, are in agreement with leading-order quantum electrodynamics predictions complemented with final-state photon radiation. The measured differential BW cross sections allow discriminating between different theoretical descriptions of the photon flux of the lead ion. In the LbL final state, 26 exclusive diphoton candidate events are observed compared with 12.0 $ \pm $ 2.9 expected for the background. Combined with previous results, the observed significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section, $ \sigma_\text{fid} (\gamma\gamma\to\gamma\gamma)= $ 107 $ \pm $ 24 (stat) $ \pm $ 13 (syst) nb, is in agreement with next-to-leading-order predictions. Limits on the production of axion-like particles coupled to photons are set over the mass range 5-100 GeV, including the most stringent limits to date in the range of 5-10 GeV. | ||
| Links: e-print arXiv:2412.15413 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Schematic diagrams of light-by-light scattering ($ \gamma\gamma\to\gamma\gamma $, upper left), the Breit-Wheeler process ($ \gamma\gamma\to\mathrm{e}^+\mathrm{e}^- $, upper right), central exclusive diphoton production ($ \mathrm{g}\mathrm{g}\to\gamma\gamma $, lower left), and axion- or graviton-like particle production ($ \gamma\gamma\to \mathrm{a},\mathrm{G} \to\gamma\gamma $, lower right) in ultraperipheral PbPb collisions. The $ (*) $ superscript indicates a potential electromagnetic excitation of the outgoing Pb ions. |
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Figure 2:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-a:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-b:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-c:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-d:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-e:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-f:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-g:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 2-h:
Detector-level kinematic distributions for exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the analysis requirements (Table 1) in the data (black points), and in superchic$+$FSR(PHOTOS++) and STARLIGHT simulations (histograms). The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. The $ p_{\mathrm{T}}^\mathrm{e} $ and $ m^{\mathrm{e}\mathrm{e}} $ distributions display the number of events per bin, divided by the bin width. Ratios of the data to MC expectation are shown in the lower panels. Error bars around the data points (hatched bands) indicate statistical (quadrature sum of MC statistical and systematic) uncertainties. |
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Figure 3:
Probability for different neutron multiplicity classes (0n, 1n, and $ X $n with $ X \geq $ 1) measured on each ZDC side for the exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the fiducial phase space of Table 1. The measured ratios are compared with SUPERCHIC 4.2, STARLIGHT 3.13, and gamma-UPC 1.6 predictions. |
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Figure 4:
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $ p_{\mathrm{T}} $ (upper left), rapidity (upper right), invariant mass (lower left), and $ |\cos\theta^{*}| $ (lower right). Data (black dots) are compared with superchic$+$FSR(PHOTOS++), starlight$+$FSR(PY8), and gamma-upc$+$FSR(PY8) predictions. Vertical bars (hatched bands) show statistical (systematic) uncertainties. |
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Figure 4-a:
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $ p_{\mathrm{T}} $ (upper left), rapidity (upper right), invariant mass (lower left), and $ |\cos\theta^{*}| $ (lower right). Data (black dots) are compared with superchic$+$FSR(PHOTOS++), starlight$+$FSR(PY8), and gamma-upc$+$FSR(PY8) predictions. Vertical bars (hatched bands) show statistical (systematic) uncertainties. |
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Figure 4-b:
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $ p_{\mathrm{T}} $ (upper left), rapidity (upper right), invariant mass (lower left), and $ |\cos\theta^{*}| $ (lower right). Data (black dots) are compared with superchic$+$FSR(PHOTOS++), starlight$+$FSR(PY8), and gamma-upc$+$FSR(PY8) predictions. Vertical bars (hatched bands) show statistical (systematic) uncertainties. |
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Figure 4-c:
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $ p_{\mathrm{T}} $ (upper left), rapidity (upper right), invariant mass (lower left), and $ |\cos\theta^{*}| $ (lower right). Data (black dots) are compared with superchic$+$FSR(PHOTOS++), starlight$+$FSR(PY8), and gamma-upc$+$FSR(PY8) predictions. Vertical bars (hatched bands) show statistical (systematic) uncertainties. |
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Figure 4-d:
Differential cross sections for exclusive dielectron production, in the fiducial phase space of Table 1, as functions of the pair $ p_{\mathrm{T}} $ (upper left), rapidity (upper right), invariant mass (lower left), and $ |\cos\theta^{*}| $ (lower right). Data (black dots) are compared with superchic$+$FSR(PHOTOS++), starlight$+$FSR(PY8), and gamma-upc$+$FSR(PY8) predictions. Vertical bars (hatched bands) show statistical (systematic) uncertainties. |
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Figure 5:
Diphoton acoplanarity distribution over $ A_{\phi}^{\gamma\gamma} = $ 0-0.1 in events passing the fiducial criteria of Table 1 (except the $ A_{\phi}^{\gamma\gamma} < $ 0.01 one) measured in data (black dots) compared with the predictions for the LbL signal (orange histogram), the BW process (yellow histogram), and the CEP (blue histogram, normalized to data in the region $ A_{\phi}^{\gamma\gamma} > $ 0.015 as explained in the text) backgrounds. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower data/MC ratio) represent systematic uncertainties. |
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Figure 6:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-a:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-b:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-c:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-d:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-e:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-f:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-g:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 6-h:
Detector-level kinematic distributions for candidate exclusive diphoton events passing all selection criteria (Table 1) in the data (black dots) compared with the simulated LbL scattering signal (orange histogram) and backgrounds from the BW (yellow histogram) and CEP (blue histogram, scaled as described in the text) processes. The MC simulations are normalized to match $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and corrected with the SFs listed in Table 2. Error bars on the data points show statistical uncertainties, and dashed bands on the stacked histograms (and at unity in the lower-panel data/MC ratios) represent systematic uncertainties. |
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Figure 7:
Differential exclusive diphoton cross sections in the fiducial phase space of Table 1 as a function of the diphoton rapidity (left) and invariant mass (right) measured in data (black dots) compared with SUPERCHIC and gamma-UPC@NLO predictions. The lower panels show the corresponding data/MC ratios. Vertical bars (hatched bands) indicate statistical (systematic) uncertainties. |
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Figure 7-a:
Differential exclusive diphoton cross sections in the fiducial phase space of Table 1 as a function of the diphoton rapidity (left) and invariant mass (right) measured in data (black dots) compared with SUPERCHIC and gamma-UPC@NLO predictions. The lower panels show the corresponding data/MC ratios. Vertical bars (hatched bands) indicate statistical (systematic) uncertainties. |
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Figure 7-b:
Differential exclusive diphoton cross sections in the fiducial phase space of Table 1 as a function of the diphoton rapidity (left) and invariant mass (right) measured in data (black dots) compared with SUPERCHIC and gamma-UPC@NLO predictions. The lower panels show the corresponding data/MC ratios. Vertical bars (hatched bands) indicate statistical (systematic) uncertainties. |
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Figure 8:
Left: Exclusive diphoton invariant mass distribution measured in data (black points) with the expected LbL, BW, and CEP backgrounds (orange, yellow, and blue histograms, respectively), and two arbitrary ALP signals injected at masses $ m_{\mathrm{a}} = $ 14 and 30 GeV with $ g_{\mathrm{a}\gamma}=$ 0.25 TeV$^{-1}$ (gray and red histograms, respectively). Right: Observed (solid black line) and expected (dotted black line) 95% CL limits on the ALP production cross section $ \sigma(\gamma\gamma\to\mathrm{a}\to\gamma\gamma) $ as a function of mass $ m_{\mathrm{a}} $. 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. The red curves indicate the expected ALP cross sections as a function of $ m_{\mathrm{a}} $ for decreasing photon couplings ($ g_{\mathrm{a}\gamma} = $ 0.3, 0.1, 0.05 TeV$^{-1}$, upper to lower). |
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Figure 8-a:
Left: Exclusive diphoton invariant mass distribution measured in data (black points) with the expected LbL, BW, and CEP backgrounds (orange, yellow, and blue histograms, respectively), and two arbitrary ALP signals injected at masses $ m_{\mathrm{a}} = $ 14 and 30 GeV with $ g_{\mathrm{a}\gamma}=$ 0.25 TeV$^{-1}$ (gray and red histograms, respectively). Right: Observed (solid black line) and expected (dotted black line) 95% CL limits on the ALP production cross section $ \sigma(\gamma\gamma\to\mathrm{a}\to\gamma\gamma) $ as a function of mass $ m_{\mathrm{a}} $. 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. The red curves indicate the expected ALP cross sections as a function of $ m_{\mathrm{a}} $ for decreasing photon couplings ($ g_{\mathrm{a}\gamma} = $ 0.3, 0.1, 0.05 TeV$^{-1}$, upper to lower). |
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Figure 8-b:
Left: Exclusive diphoton invariant mass distribution measured in data (black points) with the expected LbL, BW, and CEP backgrounds (orange, yellow, and blue histograms, respectively), and two arbitrary ALP signals injected at masses $ m_{\mathrm{a}} = $ 14 and 30 GeV with $ g_{\mathrm{a}\gamma}=$ 0.25 TeV$^{-1}$ (gray and red histograms, respectively). Right: Observed (solid black line) and expected (dotted black line) 95% CL limits on the ALP production cross section $ \sigma(\gamma\gamma\to\mathrm{a}\to\gamma\gamma) $ as a function of mass $ m_{\mathrm{a}} $. 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. The red curves indicate the expected ALP cross sections as a function of $ m_{\mathrm{a}} $ for decreasing photon couplings ($ g_{\mathrm{a}\gamma} = $ 0.3, 0.1, 0.05 TeV$^{-1}$, upper to lower). |
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Figure 9:
Exclusion limits at 95% CL in the axion-photon coupling $ g_{\mathrm{a}\gamma} $ versus axion mass $ m_{\mathrm{a}} $ plane, for the operator $ \frac{1}{4\Lambda}aF\widetilde{F} $ (assuming ALPs coupled only to photons) derived from multiple measurements (gray areas) compared with the limits extracted in this analysis (red area, the corresponding expected limits are indicated with a dashed line). Previous limits have been obtained from data from LHC PbPb [18,83], LEP [84,85,22], PrimEx [86,87], BELLE II [88], BES-III [89], LHC (pp) [90,91,92,93,94], and beam dumps [95,96,97,98,99,100], as well as from SN1897A supernova constraints [101]. |
| Tables | |
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Table 1:
Definition of the fiducial phase space for the BW and LbL scattering processes, used in their respective cross section measurements. |
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Table 2:
Summary of the overall efficiencies from simulation ($ \varepsilon^{\gamma\gamma,\mathrm{e}\mathrm{e}} $), individual data-to-simulation SFs, and final corrected efficiency factors ($ C^{\gamma\gamma,\mathrm{e}\mathrm{e}} $) obtained for the exclusive diphoton and dielectron analyses. The quoted uncertainties in $ \varepsilon^{\gamma\gamma,\mathrm{e}\mathrm{e}} $, SF, and $ C^{\gamma\gamma\mathrm{e}\mathrm{e}} $ are statistical, systematic, and statistical and systematic added in quadrature, respectively. |
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Table 3:
Exclusive dielectron yields after applying each selection criteria in data and MC simulations. The MC simulation yields match the integrated luminosity of the measurement, $ \sigma_\text{fid,MC}\mathcal{L}_\text{int} $, and are corrected by the SFs listed in Table 2. The (%) column indicates the percentage of events remaining after applying the selection with respect to the previous row. |
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Table 4:
Summary of relative systematic uncertainties in the measurement of exclusive dielectron cross sections. |
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Table 5:
Probability of different neutron multiplicity classes (0n, 1n, and $ X $n with $ X \geq $ 1) measured on each ZDC side for the exclusive $ \mathrm{e}^+\mathrm{e}^- $ events passing the fiducial criteria (fourth first rows of Table 1), compared with the predictions of SUPERCHIC 4.2, STARLIGHT 3.13, and gamma-UPC 1.6 for the deexcitation of the Pb ions in EMD processes. The experimental (MC model) uncertainties quoted are the square sum of statistical and systematic (MC statistical) sources. |
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Table 6:
Exclusive diphoton yields after applying each selection criteria in data and MC simulations. The simulation yields are scaled by the integrated luminosity of the measurement and corrected by the SFs listed in Table 2. The (%) column indicates the percentage of events remaining after applying the selection with respect to the previous row. |
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Table 7:
Summary of relative systematic uncertainties in the measurement of the LbL scattering cross section. |
| Summary |
| Measurements of light-by-light scattering (LbL, $ \gamma\gamma\to\gamma\gamma $) and the Breit-Wheeler process (BW, $ \gamma\gamma\to\mathrm{e}^+\mathrm{e}^- $) are reported in ultraperipheral collisions of lead ions at the LHC. The data, corresponding to an integrated luminosity of 1.70 nb$^{-1}$, were collected in 2018 by the CMS experiment at a centre-of-mass energy per nucleon pair of 5.02 TeV. The LbL and BW processes are studied in events with exclusively produced $ \gamma\gamma $ and $ \mathrm{e}^+\mathrm{e}^- $ pairs, respectively. Each reconstructed particle is required to have a transverse energy of $ E_{\mathrm{T}}^{\gamma,\mathrm{e}} > $ 2 GeV, a pseudorapidity of $ |\eta^{\gamma,\mathrm{e}}| < $ 2.2, and the pairs to have an invariant mass of $ m^{\gamma\gamma,\mathrm{e}\mathrm{e}} > $ 5 GeV, a transverse momentum of $ p_{\mathrm{T}}^{\gamma\gamma,\mathrm{e}\mathrm{e}} < $ 1 GeV, and an azimuthal acoplanarity of $ (1-\Delta \phi^{\gamma\gamma,\mathrm{e}\mathrm{e}}/\pi) < $ 0.01. The selected events are required to have no additional neutral particles with $ E_{\mathrm{T}} > $ 1 GeV over $ |\eta| < $ 5.2, as well as no charged particles with $ p_{\mathrm{T}} > $ 0.3 GeV over $ |\eta| < $ 2.4. About 20 000 events pass the selection criteria for the BW process, and their detector-level kinematic distributions are consistent with simulated events generated with the SUPERCHIC 3.03 and STARLIGHT 3.13 Monte Carlo (MC) codes based on quantum electrodynamics calculations at leading order. A fiducial cross section of $ \sigma_\text{fid}(\gamma\gamma\to\mathrm{e}^+\mathrm{e}^-)= $ 263.5 $ \pm $ 1.8 (stat) $ \pm $ 17.8 (syst) $ \mu $b is measured. The BW fiducial cross section and unfolded $ \mathrm{e}^+\mathrm{e}^- $ transverse momentum, rapidity, and invariant mass distributions are compared with the predictions of the STARLIGHT, SUPERCHIC, and gamma-UPC/MadGraph-5_aMC@NLO MC event generators, including photon final-state radiation (FSR) simulated with the PHOTOS++ or PYTHIA8 codes. The addition of photon FSR leads to better agreement of the calculations with the measured dielectron differential distributions. The SUPERCHIC and gamma-UPC predictions, both based on the charged form factor photon flux of the lead ion, are in better agreement with the data than the STARLIGHT calculations, which are based on an electric-dipole form factor. The probabilities of different multiplicities of forward neutrons emitted due to the electromagnetic excitation of the ions in the BW process are also measured, showing best agreement with the gamma-UPC model expectations. In the LbL final state, 26 exclusive diphoton candidate events are observed after applying all selection criteria, compared with an expectation of 12.8 events predicted for the signal and 12.0 for the background, the latter dominated by contributions from central exclusive (gluon mediated) production (10.1 events) with some remaining counts from the BW process (1.9 events). Combined with previous results, the significance of the LbL signal with respect to the background-only hypothesis is above five standard deviations. The measured fiducial LbL scattering cross section, $ \sigma_\text{fid}(\gamma\gamma\to\gamma\gamma) = $ 107 $ \pm $ 24 (stat) $ \pm $ 13 (syst) nb, is consistent with theoretical predictions at next-to-leading order accuracy. The unfolded diphoton rapidity and invariant mass differential cross sections show good agreement with the theoretical expectations. Exploiting the measured invariant mass distribution of exclusive diphoton events, new limits on the resonant production of axion-like particles coupled to photons are set in the mass vs. axion-photon coupling plane. Couplings larger than $ g_{\mathrm{a}\gamma} \approx $ 0.1TeV $^{-1} $ can be excluded over $ m_{\mathrm{a}} = $ 5-100 GeV, including the most stringent constraints to date in the 5-10 GeV range. |
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