Search for long-lived particles that decay into final states containing two electrons or two muons in proton-proton collisions at $\sqrt{s} = 8$ TeV

A search is performed for long-lived particles that decay into final states that include a pair of electrons or a pair of muons. The experimental signature is a distinctive topology consisting of a pair of charge dleptons originating from a displaced secondary vertex. Events corresponding to an integrated luminosity of $19.6\,(20.5)~\mathrm{fb}^{-1}$ in the electron (muon) channel were collected with the CMS detector at the CERN LHC in proton-proton collisions at $\sqrt{s} = 8~\mathrm{TeV}$. No significant excess is observed above standard model expectations. Upper limits on the product of the cross section and branching fraction of such a signal are presented as a function of the long-lived particle's mean proper decay length. The limits are presented in an approximately model-independent way, allowing them to be applied to a wide class of models yielding the above topology. Over much of the investigated parameter space, the limits obtained are the most stringent to date. In the specific case of a model in which a Higgs boson in the mass range $125-1000~\mathrm{GeV}/c^2$ decays into a pair of long-lived neutral bosons in the mass range $20-350~\mathrm{GeV}/c^2$, each of which can then decay to dileptons, the upper limits obtained are typically in the range $0.2-10~\mathrm{fb}$ for mean proper decay lengths of the long-lived particles in the range $0.01-100~\mathrm{cm}$. In the case of the lowest Higgs mass considered ($125~\mathrm{GeV}/c^2$), the limits are in the range $2-50~\mathrm{fb}$. These limits are sensitive to Higgs boson branching fractions as low as $10^{-4}$