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@TECHREPORT{Collaboration:599325,
      key          = {599325},
      collaboration = {{ATLAS Collaboration}},
      title        = {{S}earch for narrow low-mass resonances in the four-muon
                      final state with the {ATLAS} detector at the {LHC}},
      number       = {ATLAS-CONF-2023-041},
      reportid     = {PUBDB-2023-07321, ATLAS-CONF-2023-041},
      pages        = {21},
      year         = {2023},
      note         = {Preliminary results. All figures including auxiliary
                      figures are available at
                      https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2023-041.
                      The authors list may be incomplete!},
      abstract     = {A search for new low-mass resonances in the four-muon final
                      state in the four-muon invariant mass range from $10-50$~GeV
                      is performed using $pp$ collision data recorded with the
                      ATLAS detector at the Large Hadron Collider. The data
                      correspond to an integrated luminosity of 20.3 fb$^{-1}$ at
                      a centre-of-mass energy ($\sqrt{s}$) of 8T eV collected in
                      2012, and 51.5 fb$^{-1}$ and 58.5 fb$^{-1}$ collected at
                      $\sqrt{s}=13$ TeV in 2015--2017 and 2018, respectively. An
                      excess at 18 GeV in the invariant mass spectrum of the
                      four-muon system of the selected
                      $\Upsilon(1\mathrm{S})+\mu^+\mu^-\to\mu^+\mu^-\,\mu^+\mu^-$
                      final state is observed in the 8 TeV data. Although not
                      blinded, typical selection criteria are used and variations
                      studied to explore the stability of the excess, resulting in
                      local significances of the excess of between be
                      $3.6\,\sigma$ and $6.3\,\sigma$ corresponding to global
                      significances of $1.9\,\sigma$ to $5.4\,\sigma$. A baseline
                      choice of selection cuts defined for the analysis at 8~TeV
                      was defined in the 8~TeV data to be further probed at Run 2.
                      Subsequent analysis of 13 TeV data finds a smaller excess at
                      the same four-muon invariant mass with a significance of
                      $1.9\,\sigma$ in early 13~TeV data runs, while data
                      collected with more restrictive trigger selections in the
                      2018 dataset finds no evidence of a signal. LHCb and CMS
                      have observed no excess at this mass. Signal MC and
                      di-$\Upsilon(1\mathrm{S})$ production studies in data are
                      used to quantify performance across the three data-taking
                      periods, finding reductions in expected sensitivity in the
                      13 TeV dataset compared to that at 8 TeV. The lack of
                      observation of a signal in the 13 TeV is assessed to be in
                      tension with the structure observed at an invariant mass of
                      18 GeV in the 8 TeV dataset, up to a maximum of $2.7\sigma$.
                      We subsequently set and report limits on the total
                      production cross section times branching fraction to a
                      four-muon final state for a new state at a mass of 18 GeV
                      for a variety of models.},
      keywords     = {four-muon (autogen) / resonance (autogen) / Upsilon+mumu
                      (autogen) / BPHYS (autogen)},
      cin          = {ATLAS},
      cid          = {I:(DE-H253)ATLAS-20120731},
      pnm          = {611 - Fundamental Particles and Forces (POF4-611)},
      pid          = {G:(DE-HGF)POF4-611},
      experiment   = {EXP:(DE-H253)LHC-Exp-ATLAS-20150101},
      typ          = {PUB:(DE-HGF)29},
      doi          = {10.3204/PUBDB-2023-07321},
      url          = {https://bib-pubdb1.desy.de/record/599325},
}