Searches for supersymmetry with the ATLAS detector using final states with two leptons and missing transverse momentum in $\sqrt{s}=7$ TeV proton-proton collisions

Results of three searches are presented for the production of supersymmetric particles decaying into final states with missing transverse momentum and exactly two isolated leptons, e or mu. The analysis uses a data sample collected during the first half of 2011 that corresponds to a total integrated luminosity of 1 fb^-1 of sqrt{s} = 7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Opposite-sign and same-sign dilepton events are separately studied, with no deviations from the Standard Model expectation observed. Additionally, in opposite- sign events, a search is made for an excess of same-flavour over different-flavour lepton pairs. Effective production cross sections in excess of 9.9 fb for opposite-sign events containing supersymmetric particles with missing transverse momentum greater than 250 GeV are excluded at 95% CL. For same-sign events containing supersymmetric particles with missing transverse momentum greater than 100 GeV, effective production cross sections in excess of 14.8 fb are excluded at 95% CL. The latter limit is interpreted in a simplified weak gaugino production model excluding chargino masses up to 200 GeV.

28 October 2011

Contact: SUSY conveners internal

Phys. Lett. B 709 (2012) 137-157

e-print arXiv:1110.6189 - pdf from arXiv

Inspire record

Data points

Figures | Auxiliary Material

Figures

Figure 01a

The $E^{\mathrm{miss}}_{\mathrm{T}}$ distributions of same-sign dilepton events before any jet requirement~(a), and after requiring two high-$p_\mathrm{T}$ jets (b) and the $E^{\mathrm{miss}}_{\mathrm{T}}$ distributions of all opposite-sign dilepton events before any jet requirement~(c), after requiring 3 high-$p_\mathrm{T}$ jets~(d) and after the 4 jet requirement~(e). Errors on data points are statistical, while the error band on the SM background represents the total uncertainty. The lower inserts show the ratio between the data and the SM expectation. The component labelled ``Fake leptons'' is evaluated using data as described in the text. The remaining background contributions are from MC, normalised to their respective cross sections and the luminosity of the data sample.

png (48kB) eps (28kB) pdf (8kB)

Figure 01b

Figure 01c

Figure 01d

Figure 01e

Figure 02

Predicted number of background events, observed number of events and the corresponding 95$\%$ CL upper limit on A$\times\epsilon\times\sigma$, calculated using the CL$_{s}$ technique, for each opposite-sign and same-sign signal region.

png (70kB) eps (32kB) pdf (9kB)

Figure 03a

Distributions of the invariant mass in data together with the SM expectation for same-flavour (SF) dilepton events with $E^{\mathrm{miss}}_{\mathrm{T}}>80$~GeV after a Z-veto requirement (FS-SR1)~(a) and 2-jet requirement (FS-SR2)~(b). Also shown are the different-flavour (DF) distributiosn. Errors on data points are statistical, while the error bands on the SM predictions represent the total uncertainties.

png (75kB) eps (38kB) pdf (16kB)

Figure 03b

Auxiliary material

Figure 01

Criteria defining each of the three signal regions for the opposite-sign analysis (OS-SRx), each of the two signal regions for the same-sign analysis (SS-SRx) and each of the three regions for the flavour subtraction analysis (FS-SRx). Regions OS-SR1 and FS-SR3 are identical.

png (100kB) eps (574kB) pdf (158kB)

Figure 02

Figure 03

The observed values of $\mathcal{S}$ ($\mathcal{S}_{obs}$, left column), mean (middle column) and root-mean-squared (RMS, right column) of the distributions of the expected $\mathcal{S}_{b}$ from one million hypothetical signal-free pseudo-experiments.

png (46kB) eps (251kB) pdf (77kB)

Figure 04

Consistency of the observation with the SM expectation (middle column), computed as the percentage of signal-free pseudo-experiments giving values of $\mathcal{S}$ greater than the observation, $\mathcal{S}_{obs}$. Observed limit (right column) on the numbers of same-flavour events from new phenomena multiplied by detector acceptances and efficiencies in each signal region.

png (71kB) eps (339kB) pdf (97kB)

Figure 05

The predicted and observed numbers of events in each opposite-sign signal region. These background contributions are evaluated using the techniques described in Section~\ref{sec:backgrounds}. Entries marked \emph{neg.} are negligible.

png (288kB) eps (2MB) pdf (482kB)

Figure 06

The predicted and observed numbers of events in each same-sign signal region. These background contributions are evaluated using the techniques described in Section~\ref{sec:backgrounds}. Entries marked \emph{neg.} are negligible.

png (147kB) eps (881kB) pdf (251kB)

Figure 07

The predicted and observed numbers of events in each opposite-sign flavour subtraction signal region. These background contributions are evaluated using the techniques described in Section~\ref{sec:backgrounds}.

png (159kB) eps (980kB) pdf (275kB)

Figure 08

Observed limit on $\bar{\mathcal{S}}_{S}$ obtained for FS-SR3, with different fractions of uncorrelated SUSY contributions to the identical and different flavour channels, using the data-driven estimations of the contributions in each channel from the SM background and the purely MC estimates of the contributions in each channel from SM background.

png (35kB) eps (181kB) pdf (53kB)

Figure 09a

Opposite-Sign distributions of the, (a) invariant mass distribution, (b) Number of jets with \pt\ $>$ 20 GeV, (c) \pt\ of the highest $p_\mathrm{T}$ lepton, (d) \pt\ of the next-highest $p_\mathrm{T}$ lepton, (e) \pt\ of the highest $p_\mathrm{T}$ jet and (f) \pt\ of the next-highest $p_\mathrm{T}$ jet. Errors on data points are statistical, while the error band on the MC represents the total uncertainty. In the bottom histogram the black data points, and the uncertainty band, have been divided by the total MC to show whether the fractional deviation of the data from the MC lies within the uncertainty band. The red MC line is the sum of all the SM backgrounds. The component labelled ``Fake leptons'' is evaluated using data as described in the text. The remaining background contributions are from MC, normalised to their respective cross sections and the luminosity of the data sample.

png (48kB) eps (37kB) pdf (12kB)

Figure 09b

Figure 09c

Figure 09d

Figure 09e

Figure 09f

Figure 10a

Same-Sign distributions of the, (a) the invariant mass, (b) Number of jets with \pt\ $>$ 20 GeV, (c) \pt\ of the highest $p_\mathrm{T}$ lepton, (d) \pt\ of the next-highest $p_\mathrm{T}$ lepton, (e) \pt\ of the highest $p_\mathrm{T}$ jet and (f) \pt\ of the highest $p_\mathrm{T}$ jet. Errors on data points are statistical, while the error band on the MC represents the total uncertainty. In the bottom histogram the black data points, and the uncertainty band, have been divided by the total MC to show whether the fractional deviation of the data from the MC lies within the uncertainty band. The red MC line is the sum of all the SM backgrounds. The component labelled ``Fake leptons'' is evaluated using data as described in the text. The remaining background contributions are from MC, normalised to their respective cross sections and the luminosity of the data sample.

png (44kB) eps (28kB) pdf (8kB)

Figure 10b

Figure 10c

Figure 10d

Figure 10e

Figure 10f

Figure 11a

Electron reconstruction efficiency as a function of lepton pT.

png (35kB) eps (13kB) pdf (6kB)

Figure 11b

Muon reconstruction efficiency as a function of lepton pT.

png (37kB) eps (14kB) pdf (7kB)

Figure 11c

Ratio of electron to muon reconstruction efficiencies as a function of pT.

png (30kB) eps (10kB) pdf (6kB)

Figure 12a

Flavour subtracted invariant mass distribution for events in data and MC in FS-SR1. Errors on data points are statistical Poisson limits, while the error band on the MC represents the total uncertainty.

png (42kB) eps (17kB) pdf (8kB)

Figure 12b

Flavour subtracted invariant mass distribution for events in data and MC in FS-SR2. Errors on data points are statistical Poisson limits, while the error band on the MC represents the total uncertainty.

png (47kB) eps (22kB) pdf (8kB)

Figure 13

Distribution of $\mathcal{S}$ values from one-million hypothetical signal-free experiments in FS-SR3. The width is dominated by Poisson fluctuations in the number of $t\bar{t}$ events. The mean of this distribution is near-zero from the expected cancellation of the identical-flavour and different-flavour contributions.

png (18kB) eps (8kB) pdf (4kB)

Figure 14

A summary of the dominant systematic uncertainties on the estimates of the fully-leptonic $t \bar t$ event yields in each opposite-sign signal region. The uncertainties are different in each signal region, because each has a different control region.

png (67kB) eps (390kB) pdf (115kB)

Figure 15a

95$\%$ CL upper cross section limits (CL$_{s}$) in pb and observed and expected limit contours for $\tilde{\chi}^{\pm}_{1}\tilde{\chi}^{0}_{2}$ production in direct gaugino simplified models for (a) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$ and (b) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+3/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$.

png (72kB) eps (31kB) pdf (9kB)

Figure 15b

Figure 16a

95$\%$ CL upper cross section limits (CL$_{s}$) in pb and observed and expected limit contours for $\tilde{\chi}^{\pm}_{1}\tilde{\chi}^{0}_{2}$ production in direct gaugino simplified models for (a) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$, (b) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/2(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$ and (c) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+3/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$.

png (103kB) eps (34kB) pdf (8kB)

Figure 16b

95$\%$ CL upper cross section limits (CL$_{s}$) in pb and observed and expected limit contours for $\tilde{\chi}^{\pm}_{1}\tilde{\chi}^{0}_{2}$ production in direct gaugino simplified models for (a) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$, (b) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/2(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$ and (c) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+3/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$.

png (99kB) eps (34kB) pdf (8kB)

Figure 16c

95$\%$ CL upper cross section limits (CL$_{s}$) in pb and observed and expected limit contours for $\tilde{\chi}^{\pm}_{1}\tilde{\chi}^{0}_{2}$ production in direct gaugino simplified models for (a) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$, (b) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/2(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$ and (c) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+3/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}_{1}})$.

png (100kB) eps (34kB) pdf (7kB)

Figure 17a

Selection efficiency (left) and the detector acceptance (right) for the direct gaugino simplified models for cases, (a) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}})$, © $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+1/2(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}})$ and (e) $m_{\tilde{l}}=m_{\tilde{\chi}^{0}_{1}}+3/4(m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^{0}})$.

png (50kB) eps (37kB) pdf (9kB)

Figure 17b

Figure 17c

Figure 17d

Figure 17e

Figure 17f