Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb-1 of Pb+Pb data with the ATLAS detector

ATLAS-CONF-2020-010

25 May 2020

These preliminary results are superseded by the following paper:

HION-2019-08
ATLAS recommends to use the results from the paper.

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Abstract
This note describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during the Run 2 of the LHC. The study is using 2.2 $\mathrm{nb^{-1}}$ of integrated luminosity collected in 2015 and 2018 at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Light-by-light scattering candidates are selected in events with two photons exclusively produced, each with transverse energy $E_{\textrm{T}}^{\gamma} > 2.5$ GeV, pseudorapidity $|\eta_{\gamma}| < 2.37$, diphoton invariant mass $m_{\gamma\gamma} > 5$ GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The fiducial integrated and differential cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits for diphoton masses of 6-100 GeV compared to previous results.
Figures
Figure 01a:
Schematic diagrams of (left) SM LbyL scattering and (right) axion-like particle production in PbPb UPC. A potential electromagnetic excitation of the outgoing Pb ions is denoted by (*).

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Figure 01b:
Schematic diagrams of (left) SM LbyL scattering and (right) axion-like particle production in PbPb UPC. A potential electromagnetic excitation of the outgoing Pb ions is denoted by (*).

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Figure 02:
The Level-1 trigger efficiency extracted from γγ→e+e- events that pass the supporting triggers. Data are shown as points, separately for two data-taking periods: 2015 (open squares) and 2018 (full circles). The efficiency is parameterised using the error function fit, shown as a dashed (2015) or solid (2018) line. Shaded bands denote total (statistical and systematic) uncertainty.

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Figure 03:
Photon reconstruction efficiency as a function of photon ETγ (approximated with ETe-pTtrk2) extracted from γγ→e+e- events with a hard-bremsstrahlung photon. Data (full symbols) are compared with γγ→e+e- MC simulation (open symbols). The error bars denote statistical uncertainties.

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Figure 04a:
Photon PID efficiency as a function of photon ET extracted from FSR event candidates in 2015 (left) and 2018 (right) data (full symbols) and signal MC sample (open symbols). The error bars denote statistical uncertainties.

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Figure 04b:
Photon PID efficiency as a function of photon ET extracted from FSR event candidates in 2015 (left) and 2018 (right) data (full symbols) and signal MC sample (open symbols). The error bars denote statistical uncertainties.

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Figure 05a:
Kinematic distributions for Pb+Pb (γγ)→ Pb(*)+Pb(*) e+e- event candidates in the 2018 data set: dielectron mass (top-left), dielectron rapidity (top-right), dielectron pT (bottom-left) and electron transverse energy (bottom-right). Data (points) are compared to MC expectations (histograms). Systematic uncertainties due to electron energy scale and resolution, electron reconstruction and identification, and trigger efficiency, are shown as shaded bands.

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Figure 05b:
Kinematic distributions for Pb+Pb (γγ)→ Pb(*)+Pb(*) e+e- event candidates in the 2018 data set: dielectron mass (top-left), dielectron rapidity (top-right), dielectron pT (bottom-left) and electron transverse energy (bottom-right). Data (points) are compared to MC expectations (histograms). Systematic uncertainties due to electron energy scale and resolution, electron reconstruction and identification, and trigger efficiency, are shown as shaded bands.

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Figure 05c:
Kinematic distributions for Pb+Pb (γγ)→ Pb(*)+Pb(*) e+e- event candidates in the 2018 data set: dielectron mass (top-left), dielectron rapidity (top-right), dielectron pT (bottom-left) and electron transverse energy (bottom-right). Data (points) are compared to MC expectations (histograms). Systematic uncertainties due to electron energy scale and resolution, electron reconstruction and identification, and trigger efficiency, are shown as shaded bands.

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Figure 05d:
Kinematic distributions for Pb+Pb (γγ)→ Pb(*)+Pb(*) e+e- event candidates in the 2018 data set: dielectron mass (top-left), dielectron rapidity (top-right), dielectron pT (bottom-left) and electron transverse energy (bottom-right). Data (points) are compared to MC expectations (histograms). Systematic uncertainties due to electron energy scale and resolution, electron reconstruction and identification, and trigger efficiency, are shown as shaded bands.

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Figure 06:
The diphoton acoplanarity distribution for events satisfying the signal region selection, but before applying the Aco < 0.01 requirement. The CEP gg→γγ background is normalised in the Aco > 0.01 control region. Data are shown as points with statistical error bars, while the histograms represent the expected signal and background levels. The shaded band represents the uncertainties on signal and background predictions, excluding the uncertainty on the luminosity.

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Figure 07a:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 07b:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 07c:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 07d:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 07e:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 07f:
Kinematic distributions for γγ→γγ event candidates: diphoton invariant mass (top-left), diphoton rapidity (top-right), diphoton transverse momentum (mid-left), diphoton |cos(θ*)| (mid-right) leading photon transverse energy (bottom-left) and photon pseudorapidity (bottom-left). Data (points) are compared to the sum of signal and background expectations (histograms). Systematic uncertainties on the signal and background processes, excluding that on the luminosity, are denoted as shaded bands.

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Figure 08a:
Measured differential cross sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton absolute rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.

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Figure 08b:
Measured differential cross sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton absolute rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.

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Figure 08c:
Measured differential cross sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton absolute rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.

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Figure 08d:
Measured differential cross sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton absolute rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.

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Figure 09a:
The 95% CL upper limit on the ALP cross section σγγ→ a →γγ (left) and ALP coupling 1/Λa (right) for the γγ→ a → γγ process as a function of ALP mass ma. The observed upper limit is shown as a solid black line and the expected upper limit is shown by the dashed black line, with a green 1σ and a yellow 2σ band.

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Figure 09b:
The 95% CL upper limit on the ALP cross section σγγ→ a →γγ (left) and ALP coupling 1/Λa (right) for the γγ→ a → γγ process as a function of ALP mass ma. The observed upper limit is shown as a solid black line and the expected upper limit is shown by the dashed black line, with a green 1σ and a yellow 2σ band.

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Figure 10a:
Compilation of exclusion limits at 95% CL in the ALP-photon coupling (1/Λa) versus ALP mass (ma) plane obtained by different experiments. The existing limits, derived from Refs. [60,61] are compared to the limits extracted from this measurement. The exclusion limits denoted as ``LHC" are based on pp collision data from ATLAS and CMS. The plot on the right is a zoomed version over the range 1<ma<120 GeV.

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Figure 10b:
Compilation of exclusion limits at 95% CL in the ALP-photon coupling (1/Λa) versus ALP mass (ma) plane obtained by different experiments. The existing limits, derived from Refs. [60,61] are compared to the limits extracted from this measurement. The exclusion limits denoted as ``LHC" are based on pp collision data from ATLAS and CMS. The plot on the right is a zoomed version over the range 1<ma<120 GeV.

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Figure 11a:
Measured normalised fiducial cross-sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The results are compared with the prediction from SuperChic v3.0 MC generator (solid line).

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Figure 11b:
Measured normalised fiducial cross-sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The results are compared with the prediction from SuperChic v3.0 MC generator (solid line).

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Figure 11c:
Measured normalised fiducial cross-sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The results are compared with the prediction from SuperChic v3.0 MC generator (solid line).

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Figure 11d:
Measured normalised fiducial cross-sections of γγ→γγ production in Pb+Pb collisions at sqn=5.02 TeV for four observables (from left to right and top to bottom): diphoton invariant mass, diphoton rapidity, average photon transverse momentum and diphoton |cos(θ*)|. The measured cross-section values are shown as points with error bars giving the statistical uncertainty and solid bands indicating the size of the total uncertainty. The results are compared with the prediction from SuperChic v3.0 MC generator (solid line).

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Figure 12a:
Transverse energy difference of electromagnetic clusters associated with identified electrons in γγ→e+e- events, normalised to the sum of transverse energies of these clusters. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy resolution uncertainty.

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Figure 12b:
Transverse energy difference of electromagnetic clusters associated with identified electrons in γγ→e+e- events, normalised to the sum of transverse energies of these clusters. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy resolution uncertainty.

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Figure 12c:
Transverse energy difference of electromagnetic clusters associated with identified electrons in γγ→e+e- events, normalised to the sum of transverse energies of these clusters. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy resolution uncertainty.

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Figure 12d:
Transverse energy difference of electromagnetic clusters associated with identified electrons in γγ→e+e- events, normalised to the sum of transverse energies of these clusters. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy resolution uncertainty.

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Figure 12e:
Transverse energy difference of electromagnetic clusters associated with identified electrons in γγ→e+e- events, normalised to the sum of transverse energies of these clusters. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy resolution uncertainty.

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Figure 13a:
Ratio of transverse energy of the electron and transverse momentum of charged-particle track associated with this electron in γγ→e+e- events. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy scale uncertainty.

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Figure 13b:
Ratio of transverse energy of the electron and transverse momentum of charged-particle track associated with this electron in γγ→e+e- events. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy scale uncertainty.

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Figure 13c:
Ratio of transverse energy of the electron and transverse momentum of charged-particle track associated with this electron in γγ→e+e- events. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy scale uncertainty.

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Figure 13d:
Ratio of transverse energy of the electron and transverse momentum of charged-particle track associated with this electron in γγ→e+e- events. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy scale uncertainty.

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Figure 13e:
Ratio of transverse energy of the electron and transverse momentum of charged-particle track associated with this electron in γγ→e+e- events. Different electron ET ranges are shown (from left to right and top to bottom): 2.5 GeV<ETe<4 GeV, 4 GeV<ETe<5 GeV, 5 GeV<ETe<7 GeV, 7 GeV<ETe<10 GeV and 10 GeV<ETe<15 GeV. Data (points) are compared to ggee MC (histograms). Shaded bands denote the energy scale uncertainty.

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Figure 14a:
Breakdown of the relative systematic uncertainties on differential cross sections as a function of diphoton invariant mass (left) and diphoton rapidity (right). The relative statistical uncertainty as well as the total uncertainty are also shown.

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Figure 14b:
Breakdown of the relative systematic uncertainties on differential cross sections as a function of diphoton invariant mass (left) and diphoton rapidity (right). The relative statistical uncertainty as well as the total uncertainty are also shown.

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Figure 15a:
Detector response as a function of diphoton invariant mass (left) and diphoton rapidity (right).

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Figure 15b:
Detector response as a function of diphoton invariant mass (left) and diphoton rapidity (right).

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Figure 16a:
Efficiency and fiducial corrections as applied in the unfolding procedure as a function of diphoton invariant mass (left) and diphoton rapidity (right). The statistical uncertainties are given as bars while systematic uncertainties are shown as bands.

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Figure 16b:
Efficiency and fiducial corrections as applied in the unfolding procedure as a function of diphoton invariant mass (left) and diphoton rapidity (right). The statistical uncertainties are given as bars while systematic uncertainties are shown as bands.

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Figure 17a:
MC expectation of ALP signal for 1/Λa=1 TeV-1 and various masses ma after analysis selection. For every simulated sample a gauss function is fitted to the distribution after analysis selection. Left: linear x-axis scale. Right: logarithmic x-axis scale.

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Figure 17b:
MC expectation of ALP signal for 1/Λa=1 TeV-1 and various masses ma after analysis selection. For every simulated sample a gauss function is fitted to the distribution after analysis selection. Left: linear x-axis scale. Right: logarithmic x-axis scale.

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Figure 18:
Selection efficiency of ALP events within the fiducial volume (commonly referred to as C-factor) for simulated ALP signal.

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Figure 19:
Experimental systematic uncertainties on the ALP signal model for every mass bin used in the ALP limit setting. The steps in the energy resolution uncertainty correspond to steps in the bin-width of the mass bins used in the limit setting.

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Figure 20a:
The 95% CL upper limit on the ALP production cross section (left) and ALP coupling 1/Λa (right) for the γγ→ a → γγ process as a function of ALP mass ma. A fine bin spacing of 0.2 GeV in the high statistics region of ma < 22 GeV is used. The observed upper limit is shown as a solid black line and the expected upper limit is shown by the dashed black line, with a green 1σ and a yellow 2σ band. Limits are calculated using the SM background estimation from data.

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Figure 20b:
The 95% CL upper limit on the ALP production cross section (left) and ALP coupling 1/Λa (right) for the γγ→ a → γγ process as a function of ALP mass ma. A fine bin spacing of 0.2 GeV in the high statistics region of ma < 22 GeV is used. The observed upper limit is shown as a solid black line and the expected upper limit is shown by the dashed black line, with a green 1σ and a yellow 2σ band. Limits are calculated using the SM background estimation from data.

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Tables
Table 01:
The detector correction factor, C, and its uncertainties for the fiducial cross-section measurement.

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2024-03-29 01:19:51