Missing-mass search in forward-proton-tagged dilepton events with the ATLAS detector

Abstract

A search is conducted in proton-proton collisions at the Large Hadron Collider for photon-induced production $pp\rightarrow pp+ γγ, (γγ\rightarrow VX)$ of a visible particle $V$ decaying into a pair of same-flavour charged leptons ($e^+e^-$ or $μ^+μ^-$) and an undetected invisible component $X$. Measurements of the outgoing proton energies by the ATLAS forward proton spectrometer allow the full photon-photon four-momentum to be reconstructed. By subtracting the visible four-momentum of the central system measured with the ATLAS detector, the 'missing mass' of any event components not detected in the central region can be reconstructed, enabling the reconstruction of $X$ without knowing its properties, thus allowing the search to be model-independent. A search for a narrow resonance is performed in the missing-mass spectrum between 100 GeV and 900 GeV. The analysis uses data collected in 2017 from proton-proton collisions at a centre-of-mass energy of $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 14.7 fb$^{-1}$. No significant excess over the Standard Model expectation is observed and upper limits at 95% confidence level are set on the fiducial cross sections for three different signal models in the range between 128 and 2.5 fb. Additionally, model-independent limits are set on the visible cross section of BSM processes, for two sets of selection criteria. Both individual lepton flavour decay channels of the visible boson and a combination of the two channels are considered.

Figures (16)

• Figure 1: Feynman diagram of the elastic signal process, with a leptonically decaying visible boson $V$ produced along with an undetected massive particle or particle system $X$, in association with protons which remain intact, but are deflected away from the main LHC beam.
• Figure 2: Layout of the ATLAS forward proton spectrometer used to detect protons which remain intact following interactions in the central ATLAS detector and become deflected from the main LHC beam. The silicon tracker modules are brought to about $15\sigma$ from the beam centre, where $\sigma$ is the width of the Gaussian beam profile. The orange and blue structures represent quadrupole focusing and dipole bending magnets, respectively.
• Figure 3: Representative Feynman loop diagrams contributing to photon-induced $Z+H'$ production.
• Figure 4: Feynman diagram for photon-induced di-ALP ($S_1, S_2$) production.
• Figure 5: Feynman diagram for photon-induced $Z+X$ via a four-point interaction.