Search for new physics in final states with a lepton and missing transverse energy in $pp$ collisions at the LHC

This Letter describes the search for an enhanced production rate of events with a charged lepton and a neutrino in high-energy $pp$ collisions at the LHC. The analysis uses data collected with the CMS detector, with an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$ at $\sqrt{s}=7\mathrm{TeV}$, and a further $3.7{\mathrm{fb}}^{-1}$ at $\sqrt{s}=8\mathrm{TeV}$. No evidence is found for an excess. The results are interpreted in terms of limits on a heavy charged gauge boson ($W^{\prime }$) in the sequential standard model, a split universal extra dimension model, and contact interactions in the helicity-nonconserving model. For the last, values of the binding energy below 10.5 (8.8) TeV in the electron (muon) channel are excluded at a 95% confidence level. Interpreting the $\ell \nu$ final state in terms of a heavy $W^{\prime }$ with standard model couplings, masses below 2.90 TeV are excluded.

Figure 1

Observed lepton-$E_{\mathrm{T}}^{\mathrm{miss}}$ transverse mass distributions in the electron (top) and the muon (bottom) channel. The dashed lines show the parametrization of the background as described in the text (labeled as BG parametrization in the legend). Simulated signal distributions for a SSM $W^{\prime }$ are also shown, including detector resolution effects. The simulated background labeled as “diboson” includes $WW$, $ZZ$, and $WZ$ contributions. The last bin contains the contributions of all bins above the displayed range.

Figure 2

Simulated transverse mass distribution shown for contact interaction (HNC model), in the $\mathrm{\text{muon}}+E_{\mathrm{T}}^{\mathrm{miss}}$ channel at generator level for CI signals with $\Lambda =4$, 7, and 9 TeV, and the SM background ($W\to \mu \nu$).

Figure 3

Limits on the cross section times the single channel branching fraction ($\sigma \times \mathcal{B}$) for heavy bosons based on the 2012 data for the electron and the muon channels. For the individual channels, only the observed limits are shown. For the combination, the observed limit, the expected limit, the $1\sigma$, and the $2\sigma$ bands are displayed. The model assumes equal branching fractions for the electron and the muon channel, hence the combination corresponds to doubling the number of events. All limits are displayed for the single channel branching fraction. The $W^{\prime }$ mass limits are derived with a Bayesian method for the models of a SSM $W^{\prime }$ and $W_{\mathrm{KK}}^{n=2}$ in split UED. The $W_{\mathrm{KK}}^{n=2}$ is the lowest mass state that can couple to SM fermions and has the same final state as the SM-like $W^{\prime }$. Because it has even KK parity, it can be produced singly.

Figure 4

Combination of 2011 and 2012 data for the electron and the muon channels using $5.0{\mathrm{fb}}^{-1}$ of data at $\sqrt{s}=7\mathrm{TeV}$ from 2011 and $3.7{\mathrm{fb}}^{-1}$ of data at $\sqrt{s}=8\mathrm{TeV}$ from 2012. The left and middle plots show the individual combinations for the electron (left) and muon channel (right). To the right the combination of both is displayed. The limits are derived with a Bayesian method. Plotted is the signal strength modifier ${\sigma }_{\mathrm{excl}}/{\sigma }_{\mathrm{SSM}{W}^{\prime }}$ as a function of the $W^{\prime }$ mass, where ${\sigma }_{\mathrm{excl}}$ is the cross section excluded at a 95% C.L. and ${\sigma }_{\mathrm{SSM}{W}^{\prime }}$ is the cross section predicted by the SSM. All $W^{\prime }$ mass points below the ratio ${\sigma }_{\mathrm{excl}}/{\sigma }_{\mathrm{SSM}{W}^{\prime }}=1$, shown as a red dashed line, are excluded in the sequential standard model.

Figure 5

The 95% C.L. limits on the split-UED parameters $\mu$ and $1/R$ derived from the $W^{\prime }$ mass limits taking into account the corresponding width of the $W_{\mathrm{KK}}^{n=2}$. For the $3.7{\mathrm{fb}}^{-1}$ of 2012 data, the individual limits for the electron and muon channels are shown together with their combination, improving the excluded parameter space based on the 2011 data shown in yellow.

Figure 6

Bayesian limits on the cross section times the single channel branching fraction ($\sigma \times \mathcal{B}$) for contact interactions (HNC model) in the $\mathrm{\text{electron}}+E_{\mathrm{T}}^{\mathrm{miss}}$ (top) and the $\mathrm{\text{muon}}+E_{\mathrm{T}}^{\mathrm{miss}}$ channel (bottom) based on an integrated luminosity of $3.7{\mathrm{fb}}^{-1}$ of $\sqrt{s}=8\mathrm{TeV}$ data. The expected limit in either channel excludes values of the new interaction scale $\Lambda <9\mathrm{TeV}$. The observed limit, driven by the data, excludes $\Lambda <10.5\mathrm{TeV}$ (8.8) in the electron (muon) channel. The signal efficiency is independent of $\Lambda$.