Search for pair production of first- and second-generation scalar leptoquarks in $pp$ collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$

Results are presented from a search for the pair production of first- and second-generation scalar leptoquarks in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The data sample corresponds to an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$, collected by the CMS detector at the LHC. The search signatures involve either two charged leptons of the same flavor (electrons or muons) and at least two jets or a single charged lepton (electron or muon), missing transverse energy, and at least two jets. If the branching fraction of the leptoquark decay into a charged lepton and a quark is assumed to be $\beta =1$, leptoquark pair production is excluded at the 95% confidence level for masses below 830 GeV and 840 GeV for the first and second generations, respectively. For $\beta =0.5$, masses below 640 GeV and 650 GeV are excluded. These limits are the most stringent to date.

Figure 1

Dominant leading order diagrams for the pair production of scalar leptoquarks.

Figure 2

$eejj$ channel: the distributions of $S_{\mathrm{T}}^{ee}$ (top) and of $M_{ej}$ for each of the two electron-jet pairs (bottom) for events that pass the initial selection level. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=400\mathrm{GeV}$ is also shown.

Figure 3

$e\nu jj$ channel: the distributions of $S_{\mathrm{T}}^{e\nu }$ (top) and of $M_{ej}$ (bottom) for events that pass the initial selection level. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=400\mathrm{GeV}$ is also shown.

Figure 4

$\mu \mu jj$ channel: the distributions of $S_{\mathrm{T}}^{\mu \mu }$ (top) and of $M_{\mu j}$ for each of the two muon-jet pairs (bottom) for events that pass the initial selection level. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=400\mathrm{GeV}$ is also shown.

Figure 5

$\mu \nu jj$ channel: the distributions of $S_{\mathrm{T}}^{\mu \nu }$ (top) and of $M_{\mu j}$ (bottom) for events that pass the initial selection level. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=400\mathrm{GeV}$ is also shown.

Figure 6

$eejj$ channel: the distributions of $S_{\mathrm{T}}^{ee}$ (top) and of $M_{ej}$ for each of the two electron-jet pairs (bottom) for events that pass the final selection criteria optimized for a signal leptoquark mass of 600 GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=600\mathrm{GeV}$ is also shown.

Figure 7

$e\nu jj$ channel: the distributions of $S_{\mathrm{T}}^{e\nu }$ (left) and of $M_{ej}$ (right) for events that pass the final selection criteria optimized for a signal leptoquark mass of 600 GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=600\mathrm{GeV}$ is also shown.

Figure 8

$\mu \mu jj$ channel: the distributions of $S_{\mathrm{T}}^{\mu \mu }$ (left) and of $M_{\mu j}$ for each of the two muon-jet pairs (right) for events that pass the final selection criteria optimized for a signal leptoquark mass of 600 GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=600\mathrm{GeV}$ is also shown.

Figure 9

$\mu \nu jj$ channel: the distributions of $S_{\mathrm{T}}^{\mu \nu }$ (left) and of $M_{\mu j}$ (right) for events that pass the final selection criteria optimized for a signal leptoquark mass of 600 GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative histograms. The expected contribution from a leptoquark signal with $M_{\mathrm{LQ}}=600\mathrm{GeV}$ is also shown.

Figure 10

Top (bottom) frame: the expected and observed upper limits at 95% CL on the leptoquark pair-production cross section times ${\beta }^2(2\beta (1-\beta ))$ as a function of the first-generation leptoquark mass obtained with the $eejj$ ($e\nu jj$) analysis. The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals. The systematic uncertainties reported in Table 8 are included in the calculation. The left and middle shaded regions are excluded by the current ATLAS limit [15] and CMS limits [12, 13] for $\beta =1(0.5)$ in the $eejj$ ($e\nu jj$) channel. The right shaded regions are excluded by these results. The ${\sigma }_{\mathrm{theory}}$ curves and their bands represent, respectively, the theoretical scalar leptoquark pair-production cross section, and the uncertainties attributable to the choice of PDF and renormalization/factorization scales.

Figure 11

Top (bottom) frame: the expected and observed upper limits at 95% CL on the leptoquark pair-production cross section times ${\beta }^2(2\beta (1-\beta ))$ as a function of the second-generation leptoquark mass obtained with the $\mu \mu jj$ ($\mu \nu jj$) analysis. The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals. The systematic uncertainties reported in Table 8 are included in the calculation. The left and middle shaded regions in the top frame are excluded by the current ATLAS limit [14] and CMS limit [11] for $\beta =1$ in the $\mu \mu jj$ channel only. The left shaded region in the bottom frame is excluded by the current ATLAS limit [14] for $\beta =0.5$ in the $\mu \nu jj$ channel only. The right shaded regions are excluded by these results. The ${\sigma }_{\mathrm{theory}}$ curves and their bands represent, respectively, the theoretical scalar leptoquark pair-production cross section, and the uncertainties attributable to the choice of PDF and renormalization/factorization scales.

Figure 12

Top (bottom) frame: the expected and observed exclusion limits at 95% CL on the first- (second-)generation leptoquark hypothesis in the $\beta$ versus mass plane using the central value of signal cross section for the individual $eejj$ and $e\nu jj$ ($\mu \mu jj$ and $\mu \nu jj$) channels and their combination. The dark green and light yellow expected limit uncertainty bands represent the 68% and 95% confidence intervals. Solid lines represent the observed limits in each channel, and dashed lines represent the expected limits. The systematic uncertainties reported in Table 8 are included in the calculation. The shaded region is excluded by the current ATLAS limits [14, 15].