A search for a dark photon, a new light neutral particle, which decays promptly into collimated pairs of electrons or muons is presented. The search targets dark photons resulting from the exotic decay of the Standard Model Higgs boson, assuming its production via the dominant gluon-gluon fusion mode. The analysis is based on 140 fb$^{-1}$ of data collected with the ATLAS detector at the Large Hadron Collider from proton-proton collisions at a center-of-mass energy of 13 TeV. Events with collimated pairs of electrons or muons are analysed and background contributions are estimated using data-driven techniques. No significant excess in the data above the Standard Model background is observed. Upper limits are set at 95% confidence level on the branching ratio of the Higgs boson decay into dark photons between 0.001% and 5%, depending on the assumed dark photon mass and signal model.
Simulated distributions of the number of μLJ candidates for a selection of γ<sub>d</sub> mass values. The shape and normalisation of the distributions are extracted from the parameterisation obtained for μLJ-μLJ SR, using the FRVZ model and assuming a branching ratio of the Higgs boson decay to dark photons of 5%.
The background-only fit (with its components) of the μLJ mass distributions for the μLJ–μLJ region, where both the μLJs are included. A signal distribution for a dark photon mass of 1 GeV is overlaid, assuming the HAHM model and a branching ratio of the Higgs boson to dark photons of 0.5%. The points reported in the table correspond to the μLJ mass distribution in data. The background pdf is defined in Eq. 1 in the paper. The corresponding fitted parameters in the Signal Region are N<sub>exp1</sub>=54, N<sub>exp2</sub>=137, τ<sub>1</sub>=3.2 GeV, τ<sub>2</sub>=1.3 GeV, N<sub>J/ψ</sub>=34. The parameter σ<sub>J/ψ</sub> is fixed from the Control Region fit to 0.033 GeV.
The background-only fit (with its components) of the μLJ mass distributions for the eLJ–μLJ region. A signal distribution for a dark photon mass of 1 GeV is overlaid, assuming the HAHM model and a branching ratio of the Higgs boson to dark photons of 0.5%. The points reported in the table correspond to the μLJ mass distribution in data. The background pdf is defined in Eq. 1 in the paper. The corresponding fitted parameters in the Signal Region are N<sub>exp1</sub>=168, N<sub>exp2</sub>=26, τ<sub>1</sub>=0.50 GeV, τ<sub>2</sub>=0.34 GeV, N<sub>J/ψ</sub>=26. The parameter σ<sub>J/ψ</sub> is fixed from the Control Region fit to 0.033 GeV.
Forum