Search for short- and long-lived axion-like particles in $H\rightarrow a a \rightarrow 4\gamma$ decays with the ATLAS experiment at the LHC

The ATLAS collaboration
Eur.Phys.J.C 84 (2024) 742, 2024.

Abstract (data abstract)
A search for anomalous Higgs boson decays into two axion-like particles (ALPs) using the full Run~2 data set recorded by the ATLAS experiment is presented. The ALPs are assumed to decay into two photons, providing sensitivity to recently proposed models that could explain the $(g-2)_{\mu}$ discrepancy. This analysis covers an ALP mass range from $100\,\text{MeV}$ to $62\,\text{GeV}$ and ALP-photon couplings in the range $10^{-5}\,\text{TeV}^{-1}<C_{a\gamma\gamma}/\Lambda<1 \,\text{TeV}^{-1}$, and therefore includes signatures with significantly displaced vertices and highly collinear photons. No significant excess of events above the Standard Model background is observed. Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to two ALPs in the four-photon final state, and are in the range of $ 10^{-5}$ to $3\times 10^{-2}$, depending on the hypothesized ALP mass and ALP-photon coupling strength.

  • Signal cut-flow

    Data from AuxTables 1-6.

    10.17182/hepdata.144534.v1/t1

    Cut-flow and number of events in a region with $110 \text{ GeV} <m_{aa}<130 \text{ GeV}$ for selected signal samples

  • Systematic uncertainties

    Data from AuxTable 7.

    10.17182/hepdata.144534.v1/t2

    Overview of relative systematic uncertainties on the signal normalization for selected couplings and mass points in the dominant categories. All...

  • Exclusion limits for $C_{a\gamma\gamma}=1$

    Data from Figure 6a.

    10.17182/hepdata.144534.v1/t3

    Upper limits on $\mathcal{B}(H\rightarrow aa\rightarrow 4\gamma)$ at 95% CL as a function of the axion mass and for ALP-photon coupling...

  • Exclusion limits for $C_{a\gamma\gamma}=0.01$

    Data from Figure 6b.

    10.17182/hepdata.144534.v1/t4

    Upper limits on $\mathcal{B}(H\rightarrow aa\rightarrow 4\gamma)$ at 95% CL as a function of the axion mass and for ALP-photon coupling...

  • Exclusion limits for $C_{a\gamma\gamma}=5\times10^{-4}$

    Data from Figure 6c.

    10.17182/hepdata.144534.v1/t5

    Upper limits on $\mathcal{B}(H\rightarrow aa\rightarrow 4\gamma)$ at 95% CL as a function of the axion mass and for ALP-photon coupling...

  • Exclusion limits for $C_{a\gamma\gamma}=10^{-5}$

    Data from Figure 6d.

    10.17182/hepdata.144534.v1/t6

    Upper limits on $\mathcal{B}(H\rightarrow aa\rightarrow 4\gamma)$ at 95% CL as a function of the axion mass and for ALP-photon coupling...

  • Exclusion limits for prompt analysis

    Data from Figure 8.

    10.17182/hepdata.144534.v1/t7

    Upper limits on $B(H\rightarrow 4\gamma)$ at 95% CL as a function of the signal mass hypothesis and for the assumption...

  • Tight efficiency

    Data from Auxiliary Figure 5a.

    10.17182/hepdata.144534.v1/t8

    Identification efficiency for isolated photons that pass the tight photon ID requirements in dependence of the opening angle in $\Delta...

  • Merged efficiency

    Data from Auxiliary Figure 5b.

    10.17182/hepdata.144534.v1/t9

    Efficiency for classified merged isolated photons in dependence of the opening angle in $\Delta R$ of both decay photons of...

  • 2D exclusion limits

    Data from Figure 9.

    10.17182/hepdata.144534.v1/t10

    Limits on the ALP mass and coupling to photons at 95\%~CL, assuming $\mathcal{B}(a\rightarrow\gamma\gamma) = 1$, $\Lambda = 1\,TeV$ with $|C_{aH}^{\text{eff}}|$...

  • Data background comparison

    Data from Figure 5.

    10.17182/hepdata.144534.v1/t11

    The number of data and estimated background events in the signal region of the most sensitive categories..

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