Search for pair production of first and second generation leptoquarks in proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$

A search for pair production of first and second generation leptoquarks is performed in final states containing either two charged leptons and two jets, or one charged lepton, one neutrino and two jets, using proton-proton collision data at $\sqrt{s}=8\mathrm{TeV}$. The data, corresponding to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$, were recorded with the CMS detector at the LHC. First-generation scalar leptoquarks with masses less than 1010 (850) GeV are excluded for $\beta =1.0(0.5)$, where $\beta$ is the branching fraction of a leptoquark decaying to a charged lepton and a quark. Similarly, second-generation scalar leptoquarks with masses less than 1080 (760) GeV are excluded for $\beta =1.0(0.5)$. Mass limits are also set for vector leptoquark production scenarios with anomalous vector couplings, and for R-parity violating supersymmetric scenarios of top squark pair production resulting in similar final-state signatures. These are the most stringent limits placed on the masses of vector leptoquarks and RPV top squarks to date.

Figure 1

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

Figure 2

Diagram of the Higgsino-mediated top squark decay via the RPV ${\lambda }_{132}^{\prime }$ ($\ell =e$) or ${\lambda }_{232}^{\prime }$ ($\ell =\mu$) coupling.

Figure 3

Distributions of $S_{\mathrm{T}}$ (left) and $M_{\ell j}^{\min }$ (right) at the initial selection level in the $eejj$ (top) and $\mu \mu jj$ (bottom) channels. “Other background” includes: diboson, $W+\mathrm{jets}$, $\gamma +\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 4

Distributions of $S_{\mathrm{T}}$ (left), and $M_{\ell j}$ (right) at the initial selection level in the $e\nu jj$ (top) and $\mu \nu jj$ (bottom) channels. “Other background” includes: diboson, $Z/{\gamma }^{*}+\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 5

Distributions of $S_{\mathrm{T}}$ (left) and $M_{\mu j}^{\min }$ (right) for the final selection for a LQ mass of 450 GeV (top) and 650 GeV (bottom) in the $\mu \mu jj$ channel. The dark shaded region indicates the statistical and systematic uncertainty in the total background prediction. “Other background” includes diboson, $W+\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 6

Distributions of $S_{\mathrm{T}}$ (left) and $M_{\mu j}$ (right) for the final selection for a LQ mass of 450 GeV (top) and 650 GeV (bottom) in the $\mu \nu jj$ channel. The dark shaded region indicates the statistical and systematic uncertainty in the total background prediction. “Other background” includes diboson, $Z/{\gamma }^{*}+\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 7

The $S_{\mathrm{T}}$ (left) and $M_{ej}^{\min }$ (right) distributions for events passing the $eejj$ selection optimized for $M_{\mathrm{LQ}}=450\mathrm{GeV}$ (top) and $M_{\mathrm{LQ}}=650\mathrm{GeV}$ (bottom). The dark shaded region indicates the statistical and systematic uncertainty in the background total prediction. “Other background” includes diboson, $W+\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 8

The $S_{\mathrm{T}}$ (left) and $M_{ej}$ (right) distributions for events passing the full $e\nu jj$ selection optimized for $M_{\mathrm{LQ}}=450\mathrm{GeV}$ (top) and $M_{\mathrm{LQ}}=650\mathrm{GeV}$ (bottom). The dark shaded region indicates the statistical and systematic uncertainty in the total background prediction. “Other background” includes diboson, $Z/{\gamma }^{*}+\mathrm{jets}$, and single top quark contributions. The horizontal lines on the data points show the variable bin width.

Figure 9

The $M_{ej}^{\min }$ distribution for the $eejj$ channel (left) and the $M_{ej}$ distribution for the $e\nu jj$ channel (right) after the selection criteria optimized for a LQ mass of 650 GeV have been applied. The dark shaded region indicates the statistical and systematic uncertainty in the total background prediction. The signal corresponds to a LQ mass of 650 GeV and $\beta =0.075$. The signal is multiplied by a factor of ten in the left plot. In the case of the $eejj$ analysis, less than one signal event is expected to pass the selection. The horizontal lines on the data points show the variable bin width.

Figure 10

Frame on top (bottom): the expected and observed upper limits at 95% CL on the LQ pair production cross section times ${\beta }^2$ ($2\beta (1-\beta )$) as a function of the first generation LQ 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 left shaded regions are excluded by Ref. [77] and the middle shaded regions are excluded by Ref. [24]. The right shaded region is excluded by the analysis presented in this paper. The ${\sigma }_{\text{theory}}$ curves and their bands represent, respectively, the theoretical scalar LQ pair production cross section and the uncertainties due to the choice of PDF and renormalization/factorization scales.

Figure 11

Frame on top (bottom): the expected and observed upper limits at 95% CL on the LQ pair production cross section times ${\beta }^2$ ($2\beta (1-\beta )$) as a function of the second generation LQ 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 left shaded regions are excluded by Ref. [78] and the middle shaded regions are excluded by Ref. [24]. The right shaded region is excluded by the analysis presented in this paper. The ${\sigma }_{\text{theory}}$ curves and their bands represent, respectively, the theoretical scalar LQ pair production cross section and the uncertainties due to the choice of PDF and renormalization/factorization scales.

Figure 12

The expected and observed exclusion limits at 95% CL on the first (left) and second (right) generation scalar LQ hypothesis in the $\beta$ versus LQ mass plane using the central value of signal cross section for the individual $\ell \ell jj$ and $\ell \nu jj$ channels and their combination. The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals. Solid lines represent the observed limits in each channel, and dashed lines represent the expected limits.

Figure 13

Frame on top (bottom): the expected and observed upper limits at 95% CL on the vector leptoquark pair production cross section times ${\beta }^2$ ($2\beta (1-\beta )$) as a function of the first generation vector leptoquark mass, obtained with the $eejj$ ($e\nu jj$) analysis for the four coupling scenarios (MC, YM, MM, and AM). The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals using the MC scenario. Because of the kinematic similarity between the MC scenario and the other coupling scenarios, cross section limits are found to be the same within the uncertainties.

Figure 14

Frame on top (bottom): the expected and observed upper limits at 95% CL on the vector leptoquark pair production cross section times ${\beta }^2$ ($2\beta (1-\beta )$) as a function of the second generation vector leptoquark mass, obtained with the $\mu \mu jj$ ($\mu \nu jj$) analysis for the four coupling scenarios (MC, YM, MM, and AM). The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals using the MC scenario. Because of the kinematic similarity between the MC scenario and the other coupling scenarios, cross section limits are found to be the same within the uncertainties.

Figure 15

Frame on top (bottom): the expected and observed upper limits at 95% CL on the top squark pair-production cross section for a Higgsino-mediated RPV SUSY model in the $eejj$ $(\mu \mu jj)+4\mathrm{b}$ quark final state as a function of the top squark mass, obtained with the $eejj$ ($\mu \mu jj$) analysis. The expected limits and uncertainty bands represent the median expected limits and the 68% and 95% confidence intervals. The ${\sigma }_{\text{theory}}$ curves and their bands represent, respectively, the theoretical top squark pair production cross section and the uncertainties due to the choice of PDF and renormalization/factorization scales.