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

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.

Figures (7)

• Figure 1: Feynman diagram of a generic Higgs-like diphoton resonance $\upphi\xspace$ produced via gluon fusion decaying into a pair of photons.
• Figure 2: Distribution of the NN score for signal events produced via gluon fusion at different mass hypotheses, normalized to 100$\text{\,nb}$ 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.
• 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$\,\text{Ge\spaceV}$ (left) and 60$\,\text{Ge\spaceV}$ (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.
• 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$\,\text{Ge\spaceV}$ in the SR. The background fits include one (light blue) and two (yellow) standard deviation ($\upsigma$) 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.
• Figure 5: Expected (dashed blue) and observed (solid black) limits at 95% $\text{CL}$ on the gluon-fusion production cross section times the diphoton branching fraction $\upsigma ({{ \mathup{{{g}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{g}}{} _{ {}} ^{ {}}} }\xspace \to \upphi\xspace\xspace}) \mathcal{B}(\upphi\xspace \to { \mathup{{{\upgamma}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{\upgamma}}{} _{ {}} ^{ {}}} }\xspace\xspace)$ of a narrow diphoton resonance, as functions of the mass in the range 10--70$\,\text{Ge\spaceV}$ with minimal model-dependent assumptions, along with $1\,\upsigma$ (blue) and $2\,\upsigma$ (yellow) uncertainty bands from the expected limits. The four subranges spanning the diphoton spectrum, as employed for the background modeling, are labeled W1--W4.