CMS-HIG-24-014

CMS-HIG-24-014 ; CERN-EP-2026-007
Search for a narrow resonance with a mass between 10 and 70 GeV decaying to a pair of photons in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
3 March 2026
Submitted to Physical Review D
Abstract: The existence of a new spin-zero particle with a mass below the electroweak scale is predicted by several theoretical models. 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 with an invariant mass between 10 and 70 GeV, using a proton-proton collision data set from the CMS experiment. This data set, corresponding to an integrated luminosity of 54.4 fb$ ^{-1} $, was recorded in 2018 at a center-of-mass energy of 13 TeV using a newly introduced diphoton trigger that enabled exploration of the low-mass diphoton spectrum. No significant excess above the expected background is observed. Upper limits are set on the product of the gluon fusion production cross section and the branching fraction of the diphoton decay of a narrow resonance. An interpretation of these limits within an effective field theory framework for axion-like particles is also provided.
Links: e-print arXiv:2603.0325 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; 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 produced via gluon fusion at different mass hypotheses, normalized to 100nb for illustration purposes. The observed distribution of data in the preselection region, serving as a representation of the background events, is shown as black dots. The NN score threshold of 0.8 used to define the 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 width ($ \sigma_{\text{eff}} $), corresponding to the shaded region. The $ \sigma_{\text{eff}} $ is defined as half of the smallest interval that contains 68% of the total probability density distribution.
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 width ($ \sigma_{\text{eff}} $), corresponding to the shaded region. The $ \sigma_{\text{eff}} $ is defined as half of the smallest interval that contains 68% of the total probability density distribution.
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 width ($ \sigma_{\text{eff}} $), corresponding to the shaded region. The $ \sigma_{\text{eff}} $ is defined as half of the smallest interval that contains 68% of the total probability density distribution.
png pdf	Figure 4: Upper panel: The data (filled points) and background (dashed line) component of the signal-plus-background model are shown for each of the four windows (W1--W4) spanning the diphoton invariant mass spectrum by using the full data set from 10 to 70 GeV in the SR. The background fits include one (light blue) and two (yellow) standard deviation ($ \sigma $) uncertainties. Lower panel: Residuals in data and the 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 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 times the diphoton branching fraction $ \sigma ({\mathrm{g}\mathrm{g} \to \phi}) \mathcal{B}(\phi \to \gamma\gamma) $ of a narrow diphoton resonance, as functions 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 subranges 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 pseudo-experiments generated separately for each window.
png pdf	Figure 7: Recasting of the expected (dashed blue 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} $.

Summary

A novel search for a narrow-width resonance has been presented in the diphoton invariant mass range of 10--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 analyzed. The gluon-fusion production cross section times diphoton branching fraction of a narrow and prompt resonance is studied across a broad mass range with minimal model-dependent assumptions. No significant diphoton signal is observed. The 95% confidence level upper limit is set on the production cross section times diphoton branching fraction, as a function of the resonance mass. The main sources of uncertainty stem from the limited number of collected data and from background modeling. Finally, an interpretation of the limits within an effective field theory framework for axion-like particles is also provided, setting lower limits on its decay constant parameter within 4--15 TeV, mildly dependent on the mass of the particle.

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