CMS-PAS-HIG-24-014

CMS-PAS-HIG-24-014
Search for a narrow resonance with mass between 10 and 70 GeV decaying to a pair of photons in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
20 March 2025

Abstract: The existence of a new spin-zero particle with a mass lower than the electroweak scale is predicted by several theoretical models, and searches for resonant production of photon pairs at the LHC are able to probe these models. We present a search for a narrow resonance produced through gluon-fusion that decays into a pair of photons in the invariant mass range between 10 and 70 GeV, using a proton-proton collision data set from the CMS experiment. This data set was recorded in 2018 at a center-of-mass energy of $\sqrt{s}=$ 13 TeV and corresponds to an integrated luminosity of 54.4 fb $^{-1}$ . No significant excess above the expected background is observed. Upper limits are set on the cross section times branching fraction for diphoton resonance production via gluon-fusion. An interpretation of these limits in the context of an axion-like-particle effective field theory model is also provided.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of a generic Higgs-like diphoton resonance $\phi$ produced via gluon fusion decaying into a pair of photons.
png pdf	Figure 2: Distribution of the NN score for signal events at different mass hypotheses generated via gluon fusion, assuming a cross section times branching fraction of 1 pb. A scaling of the signals by a factor 10000 is applied to facilitate a clearer comparison of the signal and background shapes. The observed distribution of data in the preselection region is shown as black dots. The NN score threshold of 0.8 used to define the signal region (SR) is indicated by the gray dashed line.
png pdf	Figure 3: The parametric signal model (blue solid line), derived from simulation of Higgs-like diphoton signals (square markers), is evaluated at two mass hypotheses: 25 GeV (left) and 60 GeV (right). The typical experimental resolution for the signal is also displayed in terms of the effective $\sigma$ ( $\sigma_{\textrm {eff}}$ ), corresponding to the shaded region. The $\sigma_{\textrm {eff}}$ is defined as the smallest interval that contains 68% of the total probability density.
png pdf	Figure 3-a: The parametric signal model (blue solid line), derived from simulation of Higgs-like diphoton signals (square markers), is evaluated at two mass hypotheses: 25 GeV (left) and 60 GeV (right). The typical experimental resolution for the signal is also displayed in terms of the effective $\sigma$ ( $\sigma_{\textrm {eff}}$ ), corresponding to the shaded region. The $\sigma_{\textrm {eff}}$ is defined as the smallest interval that contains 68% of the total probability density.
png pdf	Figure 3-b: The parametric signal model (blue solid line), derived from simulation of Higgs-like diphoton signals (square markers), is evaluated at two mass hypotheses: 25 GeV (left) and 60 GeV (right). The typical experimental resolution for the signal is also displayed in terms of the effective $\sigma$ ( $\sigma_{\textrm {eff}}$ ), corresponding to the shaded region. The $\sigma_{\textrm {eff}}$ is defined as the smallest interval that contains 68% of the total probability density.
png pdf	Figure 4: Upper panel: The data and background component of the signal-plus-background model are shown for each of the four sub-range windows (W1--W4) spanning the diphoton spectrum by using the full dataset from 10 to 70 GeV in the SR. The background fits include one (light blue) and two (yellow) standard deviation uncertainty bands. Lower panel: Residuals in data and uncertainty bands after subtracting the background fit. To illustrate the continuity of the data and fit models across the mass spectrum, event counts in both panels are scaled by the bin width of each sub-range window, leading to an average of event frequency computed for each bin throughout the mass spectrum.
png pdf	Figure 5: Expected (dashed blue) and observed (solid black) limits at 95% CL on the gluon fusion production cross section of a narrow diphoton resonance, $\sigma_{\textrm{gg}\rightarrow\phi} \times$ BR( $\phi \rightarrow\gamma\gamma$ ), as a function of the mass in the range 10--70 GeV with minimal model-dependent assumptions, along with 1 $\sigma$ (blue) and 2 $\sigma$ (yellow) uncertainty bands from the expected limits. The four sub-range windows spanning the diphoton spectrum as employed for the background modeling are labeled W1--W4.
png pdf	Figure 6: Signal plus background fits (dark blue) are shown for the most significant excess in the SR at a mass hypothesis of 13.6 GeV, with signal yield freely floating. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 7: Recasting of the expected (dashed pink line) and observed (solid black line) upper limits on the gluon fusion production cross section of a diphoton resonance as a function of its mass in the range 10--70 GeV into the ALP parameter space as constraints on the decay constant $f_a$ , assuming $c_1 = c_2 = c_3 =$ 10. The upper limit on the production cross section corresponds to a lower bound on $f_a$ . The theoretical framework under consideration is the Kim-Shifman-Vainshtein-Zakharov (KSVZ) [83,84] model. The right y-axis shows the corresponding conversion of the limits on $f_a$ to upper limits on $g_{a\gamma\gamma}$ .
png pdf	Figure 8: Signal plus background fits (dark blue) are shown for events entering the signal region (SR) within the four sub-range windows which encompass the low mass range under study in this search. Four mass hypotheses are shown as an example: 12 GeV (top left), 35 GeV (top right), 46 GeV (bottom left), and 64.5 GeV (bottom right), with signal yield freely floating. The best fitting background function in the envelope is displayed, and the signal fit is added at the tested mass point in each window. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 8-a: Signal plus background fits (dark blue) are shown for events entering the signal region (SR) within the four sub-range windows which encompass the low mass range under study in this search. Four mass hypotheses are shown as an example: 12 GeV (top left), 35 GeV (top right), 46 GeV (bottom left), and 64.5 GeV (bottom right), with signal yield freely floating. The best fitting background function in the envelope is displayed, and the signal fit is added at the tested mass point in each window. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 8-b: Signal plus background fits (dark blue) are shown for events entering the signal region (SR) within the four sub-range windows which encompass the low mass range under study in this search. Four mass hypotheses are shown as an example: 12 GeV (top left), 35 GeV (top right), 46 GeV (bottom left), and 64.5 GeV (bottom right), with signal yield freely floating. The best fitting background function in the envelope is displayed, and the signal fit is added at the tested mass point in each window. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 8-c: Signal plus background fits (dark blue) are shown for events entering the signal region (SR) within the four sub-range windows which encompass the low mass range under study in this search. Four mass hypotheses are shown as an example: 12 GeV (top left), 35 GeV (top right), 46 GeV (bottom left), and 64.5 GeV (bottom right), with signal yield freely floating. The best fitting background function in the envelope is displayed, and the signal fit is added at the tested mass point in each window. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 8-d: Signal plus background fits (dark blue) are shown for events entering the signal region (SR) within the four sub-range windows which encompass the low mass range under study in this search. Four mass hypotheses are shown as an example: 12 GeV (top left), 35 GeV (top right), 46 GeV (bottom left), and 64.5 GeV (bottom right), with signal yield freely floating. The best fitting background function in the envelope is displayed, and the signal fit is added at the tested mass point in each window. In the ratio pad, the background model is subtracted from the data. Uncertainty bands at $\pm 1 \sigma$ (light blue) and $\pm 2 \sigma$ (yellow) are obtained from toys generated separately for each window.
png pdf	Figure 9: Efficiency times acceptance for events entering the SR, evaluated using reference simulated signal samples and interpolated with a spline across the mass range 10--70 GeV. The 1 $\sigma$ uncertainty band is built by combining statistical and main experimental systematic uncertainties which affect the signal model.
png pdf	Figure 10: The gradual variation of the signal experimental mass resolution in the mass range between 10--70 GeV is shown in terms of the standard deviation characterizing the Gaussian core of the distribution ( $\sigma$ ). A linear parametrization well describes the variation of the mass resolution in the mass range of interest, found to be stable at around 1.5% of the resonance mass.
png pdf	Figure 11: Local p-value, the probability for a background fluctuation to be at least as large as the observed excess, as a function of the mass. The scan is produced with the same granularity employed for the limit computation. Amongst 130 tested mass hypotheses, six are found to exceed a 2 $\sigma$ local significance. A calculation of the global significance based on toys is provided for the most significant excess observed at a mass hypothesis of 13.6 GeV and it amounts to 1.9 $\pm$ 0.1 $\sigma$ .

Summary

A novel search for a narrow-width diphoton resonance is conducted in the diphoton invariant mass range between 10 and 70 GeV, enabled for the first time by a new trigger introduced for the CMS data-taking in 2018. An integrated luminosity corresponding to 54.4 fb

$^{-1}$ of proton-proton collision data at a center of mass energy of 13 TeV is exploited. In this work, the cross-section times branching ratio of a narrow and prompt diphoton resonance is studied across a broad mass range with minimal model-dependent assumptions. Since no significant diphoton signal is observed, 95% CL upper limits are set on the production cross section times branching ratio as a function of the resonance mass. The main sources of uncertainty stem from the limited number of collected pp collisions and from background modeling. The CMS search, based on the 2018 data set only, is able to achieve competitive results per unit of integrated luminosity with respect to ATLAS, and almost comparable limits in the higher mass range. Finally, an interpretation of the limits has been made in the context of ALPs, leading to the first contribution by CMS at such low masses in the picture of diphoton resonance searches.

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