Search for new phenomena in events with three charged leptons at $\sqrt{s}=7\mathrm{TeV}$ with the ATLAS detector

A generic search for anomalous production of events with at least three charged leptons is presented. The search uses a $pp$-collision data sample at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$ corresponding to $4.6{\mathrm{fb}}^{-1}$ of integrated luminosity collected in 2011 by the ATLAS detector at the CERN Large Hadron Collider. Events are required to contain at least two electrons or muons, while the third lepton may either be an additional electron or muon, or a hadronically decaying tau lepton. Events are categorized by the presence or absence of a reconstructed tau-lepton or $Z$-boson candidate decaying to leptons. No significant excess above backgrounds expected from Standard Model processes is observed. Results are presented as upper limits on event yields from non-Standard-Model processes producing at least three prompt, isolated leptons, given as functions of lower bounds on several kinematic variables. Fiducial efficiencies for model testing are also provided. The use of the results is illustrated by setting upper limits on the production of doubly charged Higgs bosons decaying to same-sign lepton pairs.

Figure 1

The $m_{\mathrm{T}}$ distribution of the $E_{\mathrm{T}}^{\mathrm{miss}}$ and the lepton not associated with the $Z$-boson candidate decay in $\ge 3e/\mu$ events in the $Z+\mathrm{\text{jets}}$ control region. The last bin shows the integral of events above 90 GeV. The bottom panel shows the ratio of events observed in data to those expected from background sources for each bin.

Figure 2

The $p_{\mathrm{T}}$ distribution of the third lepton candidate in (a) $\ge 3e/\mu$ events and (b) $2e/\mu +\ge 1{\tau }_{\mathrm{had}}$ events in the $Z+\mathrm{\text{jets}}$ control region. The last bin in the left (right) plot shows the integral of events above 100 GeV (150 GeV). The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 3

The $p_{\mathrm{T}}$ distribution of the (a) subleading lepton in $2e/\mu$ events and (b) ${\tau }_{\mathrm{had}}$ in $1e/\mu +1{\tau }_{\mathrm{had}}$ events in the $t\overline{t}$ control region. The expected contribution from non-Standard-Model processes is illustrated in the left figure by events with fourth-generation down-type quarks ($b^{\prime }$). The contribution from $b^{\prime }$ events in the right figure is negligible. The last bin in each plot shows the integral of events above 100 GeV. The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 4

The $p_{\mathrm{T}}$ distribution of the (a) third lepton candidate in $\ge 3e/\mu$ events and (b) ${\tau }_{\mathrm{had}}$ in $2e/\mu +\ge 1{\tau }_{\mathrm{had}}$ events in the low-mass Drell-Yan control region. The last bin shows the integral of events above 50 GeV (100 GeV) in the left (right) figure. The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 5

The $H_{\mathrm{T}}^{\mathrm{leptons}}$ distribution for the off-$Z$ (a) $\ge 3e/\mu$ and (b) $2e/\mu +\ge 1{\tau }_{\mathrm{had}}$ signal channels. The dashed lines represent the expected contributions from events with pair-produced doubly charged Higgs bosons with masses of 300 GeV. The last bin in the left (right) figure shows the integral of events above 60 GeV (500 GeV). The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 6

The $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution for the off-$Z$ (a) $\ge 3e/\mu$ and (b) $2e/\mu +\ge 1{\tau }_{\mathrm{had}}$ signal channels. The dashed lines represent the expected contributions from events with fourth-generation down-type quarks with masses of 500 GeV. The last bin in the left (right) figure shows the integral of events above 300 GeV (200 GeV). The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 7

The $m_{\mathrm{eff}}$ distribution for the on-$Z$ (a) $\ge 3e/\mu$ and (b) $2e/\mu +\ge 1{\tau }_{\mathrm{had}}$ signal channels. The dashed lines represent the expected contributions from events with fourth-generation down-type quarks with masses of 500 GeV. The last bin in the left (right) figure shows the integral of events above 1.2 TeV (1 TeV). In the $\ge 3e/\mu$ channel, a total of 2.13 events are expected for $m_{\mathrm{eff}}>1\mathrm{TeV}$, and 4 events are observed. The bottom panels show the ratio of events observed in data to those expected from background sources for each bin.

Figure 8

The observed and median expected 95% CL limit on the visible cross section (${\sigma }_{95}^{\mathrm{vis}}$) in the different signal channels, as functions of increasing lower bounds on $H_{\mathrm{T}}^{\mathrm{leptons}}$. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the median expected limit are indicated by green and yellow bands, respectively.

Figure 9

The observed and median expected 95% CL limit on the visible cross section (${\sigma }_{95}^{\mathrm{vis}}$) in the different signal channels, as functions of increasing lower bounds on $E_{\mathrm{T}}^{\mathrm{miss}}$, for events with $H_{\mathrm{T}}^{\mathrm{jets}}<100\mathrm{GeV}$. The lowest bin boundary $X$ is 0 GeV for the off-$Z$ channels and 20 GeV for the on-$Z$ channels. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the median expected limit are indicated by green and yellow bands, respectively.

Figure 10

The observed and median expected 95% CL limit on the visible cross section (${\sigma }_{95}^{\mathrm{vis}}$) in the different signal channels, as functions of increasing lower bounds on $E_{\mathrm{T}}^{\mathrm{miss}}$, for events with $H_{\mathrm{T}}^{\mathrm{jets}}\ge 100\mathrm{GeV}$. The lowest bin boundary $X$ is 0 GeV for the off-$Z$ channels and 20 GeV for the on-$Z$ channels. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the median expected limit are indicated by green and yellow bands, respectively.

Figure 11

The observed and median expected 95% CL limit on the visible cross section (${\sigma }_{95}^{\mathrm{vis}}$) in the different signal channels, as functions of increasing lower bounds on $m_{\mathrm{eff}}$. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the median expected limit are indicated by green and yellow bands, respectively.

Figure 12

The observed and median expected 95% CL limit on the visible cross section (${\sigma }_{95}^{\mathrm{vis}}$) in the different signal channels, as functions of increasing lower bounds on $m_{\mathrm{eff}}$, for events with $E_{\mathrm{T}}^{\mathrm{miss}}>75\mathrm{GeV}$. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the median expected limit are indicated by green and yellow bands, respectively.

Figure 13

The expected and observed 95% confidence level upper limits on the cross-section times branching ratio of the (a) $H^{\pm \pm }\to {\mu }^{\pm }{\mu }^{\pm }$ and (b) $H^{\pm \pm }\to {\mu }^{\pm }{\tau }^{\pm }$ final states as a function of the $H^{\pm \pm }$ mass for the $\ge 3e/\mu$ channel. For $H^{\pm \pm }\to {\mu }^{\pm }{\mu }^{\pm }$, the median expected limit on the $H_L^{\pm \pm }$ mass is 319 GeV and the corresponding observed limit is 330 GeV; for $H^{\pm \pm }\to {\mu }^{\pm }{\tau }^{\pm }$, the median expected limit is 229 GeV and the corresponding observed limit is 237 GeV. Results from the dedicated ATLAS search for $H^{\pm \pm }\to {\mu }^{\pm }{\mu }^{\pm }$ [60] are also shown.