Search for single production of scalar leptoquarks in proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$

A search is presented for the production of both first- and second-generation scalar leptoquarks with a final state of either two electrons and one jet or two muons and one jet. The search is based on a data sample of proton-proton collisions at center-of-mass energy $\sqrt{s}=8\mathrm{TeV}$ recorded with the CMS detector and corresponding to an integrated luminosity of $19.6{\mathrm{fb}}^{-1}$. Upper limits are set on both the first- and second-generation leptoquark production cross sections as functions of the leptoquark mass and the leptoquark couplings to a lepton and a quark. Results are compared with theoretical predictions to obtain lower limits on the leptoquark mass. At 95% confidence level, single production of first-generation leptoquarks with a coupling and branching fraction of 1.0 is excluded for masses below 1730 GeV, and second-generation leptoquarks with a coupling and branching fraction of 1.0 is excluded for masses below 530 GeV. These are the best overall limits on the production of first-generation leptoquarks to date.

Figure 1

The $s$-channel resonant LQ production diagram.

Figure 2

The $t$-channel LQ production diagrams with nonresonant components. Diagram (a) has both resonant and non-resonant components. Diagram (b) is entirely non-resonant.

Figure 3

Cross sections for single LQ production, calculated at LO in CalcHEP and scaled by the acceptance of the requirements described in Sec. 5, as a function of the LQ mass in GeV.

Figure 4

Distributions of $M_{ee}$ (top left), $S_{\mathrm{T}}$ (top right), and $M_{ej}$ (bottom) at preselection in the $eej$ channel. “Other backgrounds” include diboson, $W+\mathrm{jets}$, and single top quark contributions. The points represent the data, and the stacked histograms show the expected background contributions. The open histogram shows the prediction for an LQ signal for $M_{\mathrm{LQ}}=1000\mathrm{GeV}$ and $\lambda =0.4$. The horizontal error bars on the data points represent the bin widths. The last bin includes overflow.

Figure 5

Distributions of $M_{\mu \mu }$ (top left), $S_{\mathrm{T}}$ (top right), and $M_{\mu j}$ (bottom) at preselection in the $\mu \mu j$ channel. “Other backgrounds” include diboson, $W+\mathrm{jets}$, single top quark, and QCD multijet contributions. The points represent the data, and the stacked histograms show the expected background contributions. The open histogram shows the prediction for an LQ signal for $M_{\mathrm{LQ}}=600\mathrm{GeV}$ and $\lambda =1.0$. The horizontal error bars on the data points represent the bin widths. The last bin includes overflow.

Figure 6

Distributions of $S_{\mathrm{T}}$ and $M_{ej}$ at final selection, in the $eej$ channel. The points represent the data, and the stacked histograms show the expected background contributions. The open histogram shows the prediction for an LQ signal for $M_{\mathrm{LQ}}=1000\mathrm{GeV}$ and $\lambda =0.4$. The horizontal error bars on the data points represent the bin width. The last bin includes overflow.

Figure 7

Distributions of $S_{\mathrm{T}}$ and $M_{\mu j}$ at final selection, in the $\mu \mu j$ channel. The points represent the data, and the stacked histograms show the expected background contributions. The open histogram shows the prediction for an LQ signal for $M_{\mathrm{LQ}}=600\mathrm{GeV}$ and $\lambda =1.0$. The horizontal error bars on the data points represent the bin width. The last bin includes overflow.

Figure 8

Expected and observed upper limits at 95% C.L. on first- and second-generation leptoquark single production resonant cross sections as a function of the leptoquark mass. First generation limits are shown in the left plot with a resonant region of $M_{\ell j}>0.66M_{\mathrm{LQ}}$, $M_{\ell \ell }>110\mathrm{GeV}$, and second generation limits are shown in the right plot with a resonant region of $M_{\ell j}>0.75M_{\mathrm{LQ}}$, $M_{\ell \ell }>110\mathrm{GeV}$. The uncertainty bands on the observed limit represent the 68% and 95% confidence intervals. The uncertainty band on the theoretical cross section includes uncertainties due to PDF variation and the $k$ factor.