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

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 charged leptons 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 fb for mean proper decay lengths of the long-lived particles in the range 0.01–100 cm. In the case of the lowest Higgs mass considered ($125\mathrm{GeV}/c^2$), the limits are in the range 2–50 fb. These limits are sensitive to Higgs boson branching fractions as low as $10^{-4}$.

Figure 1

The $|d_0|/{\sigma }_d$ distribution for the electron (left) and muon (right) channels, shown in the top row for events in the control region ($|\mathrm{\Delta }\mathrm{\Phi }|>\pi /2$) and in the bottom row for events in the signal region ($|\mathrm{\Delta }\mathrm{\Phi }|<\pi /2$). Of the two leptons forming a candidate, the distribution of the one with the smallest $|d_0|/{\sigma }_d$ is plotted. The solid points indicate the data, the shaded histograms are the simulated background, and the hashed histograms show the simulated signal. The histogram corresponding to the $H\to XX$ model is shown for $m_H=1000\mathrm{GeV}/c^2$ and $m_X=350\mathrm{GeV}/c^2$. The histogram corresponding to the ${\tilde{\chi }}^0\to {\ell }^{+}{\ell }^{-}\nu$ model is shown for $m_{\tilde{q}}=350\mathrm{GeV}/c^2$ and $m_{{\tilde{\chi }}^0}=140\mathrm{GeV}/c^2$. The background histograms are stacked, and each simulated signal sample is independently stacked on top of the total simulated background. The $d_0$ corrections for residual tracker misalignment, discussed in the text, have been applied. The vertical dashed line shows the selection requirement $|d_0|/{\sigma }_d>12$. Any entries beyond the right-hand side of a histogram are shown in the last visible bin of the histogram.

Figure 2

Comparison of the tail-cumulative distributions of $|d_0|/{\sigma }_d$ for data in the signal region ($|\mathrm{\Delta }\mathrm{\Phi }|<\pi /2$) and the control region ($|\mathrm{\Delta }\mathrm{\Phi }|>\pi /2$) for the electron channel (left) and the muon channel (right). The $d_0$ corrections for residual tracker misalignment, discussed in the text, have been applied. Of the two leptons forming a candidate, the distribution of the one with the smallest $|d_0|/{\sigma }_d$ is plotted. The bottom panels show the statistical significance of the difference between the distributions in the signal and control regions.

Figure 3

Efficiency to find a track in the tracker, measured using cosmic ray muons reconstructed in the muon detectors, as a function of the transverse (left) and longitudinal (right) impact parameters (relative to the nominal interaction point of CMS). The efficiency is plotted in bins of 2 cm width. For the left plot, the longitudinal impact parameter $|z_0|$ is required to be less than 10 cm, and for the right plot, the transverse impact parameter $|d_0|$ must be less than 4 cm. The bottom panels show the ratio of the efficiency in data to that in simulation. The uncertainties in the simulation are smaller than the size of the markers and are not visible.

Figure 4

The 95% C.L. upper limits on $\sigma (H\to XX)\mathcal{B}(X\to e^{+}{e}^{-})$, as a function of the mean proper decay length of the $X$ boson, for Higgs boson masses of 125 (top left), 200 (top right), 400 (bottom left), and $1000\mathrm{GeV}/c^2$ (bottom right). In each plot, results are shown for several $X$ boson mass hypotheses. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $20\mathrm{GeV}/c^2$ $X$ boson mass. Corresponding bands for the other $X$ boson masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.

Figure 5

The 95% C.L. upper limits on $\sigma (H\to XX)\mathcal{B}(X\to {\mu }^{+}{\mu }^{-})$, as a function of the mean proper decay length of the $X$ boson, for Higgs boson masses of 125 (top left), 200 (top right), 400 (bottom left), and $1000\mathrm{GeV}/c^2$ (bottom right). In each plot, results are shown for several $X$ boson mass hypotheses. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $20\mathrm{GeV}/c^2$ $X$ boson mass. Corresponding bands for the other $X$ boson masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.

Figure 6

The 95% C.L. upper limits on $\sigma (\tilde{q}{\tilde{q}}^{*}+\tilde{q}\tilde{q})\mathcal{B}(\tilde{q}\to q{\tilde{\chi }}^0,{\tilde{\chi }}^0\to {\ell }^{+}{\ell }^{-}\nu )$ for the electron (left), and muon (right) channels, as a function of the mean proper decay length of the neutralino. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $120\mathrm{GeV}/c^2$ squark and a $48\mathrm{GeV}/c^2$ neutralino mass. Corresponding bands for the other squark and neutralino masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.

Figure 7

The 95% C.L. upper limits on $\sigma (H\to XX)\mathcal{B}(X\to e^{+}{e}^{-})A(X\to e^{+}{e}^{-})$, as a function of the mean proper decay length of the $X$ boson, for Higgs boson masses of 125 (top left), 200 (top right), 400 (bottom left), and $1000\mathrm{GeV}/c^2$ (bottom right). In each plot, results are shown for several $X$ boson mass hypotheses. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $20\mathrm{GeV}/c^2$ $X$ boson mass. Corresponding bands for the other $X$ boson masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.

Figure 8

The 95% C.L. upper limits on $\sigma (H\to XX)\mathcal{B}(X\to {\mu }^{+}{\mu }^{-})A(X\to {\mu }^{+}{\mu }^{-})$, as a function of the mean proper decay length of the $X$ boson, for Higgs boson masses of 125 (top left), 200 (top right), 400 (bottom left), and $1000\mathrm{GeV}/c^2$ (bottom right). In each plot, results are shown for several $X$ boson mass hypotheses. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $20\mathrm{GeV}/c^2$ $X$ boson mass. Corresponding bands for the other $X$ boson masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.

Figure 9

The 95% C.L. upper limits on $\sigma (\tilde{q}{\tilde{q}}^{*}+\tilde{q}\tilde{q})\mathcal{B}(\tilde{q}\to q{\tilde{\chi }}^0,{\tilde{\chi }}^0\to {\ell }^{+}{\ell }^{-}\nu )A(\tilde{q}\to q{\tilde{\chi }}^0,{\tilde{\chi }}^0\to {\ell }^{+}{\ell }^{-}\nu )$ for the electron (left), and muon (right) channels, as a function of the mean proper decay length of the neutralino. The shaded band shows the $\pm 1\sigma$ range of variation of the expected 95% C.L. limits for the case of a $120\mathrm{GeV}/c^2$ squark and a $48\mathrm{GeV}/c^2$ neutralino mass. Corresponding bands for the other squark and neutralino masses, omitted for clarity of presentation, show similar agreement with the respective observed limits.