Abstract
A search is performed for a long-lived particle decaying into a final state that includes a pair of muons of opposite-sign electric charge, using proton-proton collision data collected at $\sqrt{s}=$ 13 TeV by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 32.9 fb$^{-1}$. No significant excess over the Standard Model expectation is observed. Limits at 95% confidence level on the lifetime of the long-lived particle are presented in models of new phenomena including gauge-mediated supersymmetry or decay of the Higgs boson, $H$, to a pair of dark photons, $Z_D$. Lifetimes in the range c$\tau$ = 1-2400 cm are excluded, depending on the parameters of the model. In the supersymmetric model, the lightest neutralino is the next-to-lightest supersymmetric particle, with a relatively long lifetime due to its weak coupling to the gravitino, the lightest supersymmetric particle. The lifetime limits are determined for very light gravitino mass and various assumptions for the neutralino mass in the range 300 GeV to 1000 GeV. In the dark photon model, the lifetime limits are interpreted as exclusion contours in the plane of the coupling between the $Z_D$ and the Standard Model $Z$ boson versus the $Z_D$ mass (in the range 20-60 GeV), for various assumptions for the $H\rightarrow Z_D Z_D$ branching fraction.
Highlights
The ATLAS and CMS experiments at the Large Hadron Collider (LHC) were conceived to address a variety of questions not fully explained within the Standard Model (SM) of particle physics
The data collected by the LHC experiments have not yet revealed evidence of physics beyond the Standard Model (BSM)
In many models of BSM physics there are free parameters that influence the lifetimes of the new particle states, with no strong motivation for assuming that all the particles decay promptly1 and give final states investigated with standard analysis techniques
Summary
In many models of BSM physics there are free parameters that influence the lifetimes of the new particle states, with no strong motivation for assuming that all the particles decay promptly and give final states investigated with standard analysis techniques. Supersymmetry breaking [12,13,14], and split supersymmetry [15,16] are examples where small couplings, mass scales associated with the BSM physics, or heavy mediator particles, respectively, lead to high-mass (greater than a few hundred GeV) LLPs. Scenarios with low-mass LLPs include hidden-valley models [17], stealth supersymmetry [18], and dark-sector gauge bosons [3,19]. Previous searches by the ATLAS Collaboration for highmass LLPs that decay within the inner detector to give displaced dilepton vertices excluded LLP lifetimes of cτ 1⁄4 0.1–100 cm [22]. Several other LLP searches targeting a wide range of lifetimes and signatures have been conducted by the ATLAS [24,25,26,27,28,29,30,31,32,33], CMS [34,35,36,37,38,39,40], LHCb [41,42,43,44], CDF [45], D0 [46,47], BABAR [48], Belle [49], and ALEPH [50] collaborations
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