Search for Pair Production of First-Generation Scalar Leptoquarks in $pp$ Collisions at $\sqrt{s}=7\mathrm{TeV}$

A search for pair production of first-generation scalar leptoquarks is performed in the final state containing two electrons and two jets using proton-proton collision data at $\sqrt{s}=7\mathrm{TeV}$. The data sample used corresponds to an integrated luminosity of $33{\mathrm{pb}}^{-1}$ collected with the CMS detector at the CERN LHC. The number of observed events is in good agreement with the predictions for the standard model background processes, and an upper limit is set on the leptoquark pair production cross section times ${\beta }^2$ as a function of the leptoquark mass, where $\beta$ is the branching fraction of the leptoquark decay to an electron and a quark. A 95% confidence level lower limit is set on the mass of a first-generation scalar leptoquark at 384 GeV for $\beta =1$, which is the most stringent direct limit to date.

Figure 1

Top: the $M_{\mathrm{ee}}$ distribution for events that have passed the preselection requirements. Bottom: the $S_T$ distribution for events that have passed the preselection requirement, except the preselection requirement on $S_T$ itself ($S_T>250\mathrm{GeV}$), and have $M_{\mathrm{ee}}>125\mathrm{GeV}$. The MC distributions for the signal ($\beta =1$) and the contributing backgrounds are shown. The $Z/{\gamma }^{*}+\mathrm{\text{jets}}$ MC has been normalized as described in the text. Other backgrounds include $W+\mathrm{\text{jets}}$, di-boson, and single top. All background histograms are cumulative.

Figure 2

Top: the expected and observed upper limit at 95% C.L. on the LQ pair production cross section times ${\beta }^2$ as a function of the LQ mass. The systematic uncertainties reported in Table 2 are included in the calculation. The shaded region is excluded by the current D0 limit for $\beta =1$. The ${\sigma }_{\mathrm{theory}}$ curve and its band represent, respectively, the theoretical LQ pair production cross section and the uncertainties due to the choice of PDF and renormalization-factorization scales [13]. Bottom: minimum $\beta$ for a 95% C.L. exclusion of the LQ hypothesis as a function of LQ mass. The observed (expected) exclusion curve is obtained using the observed (expected) upper limit and the central value of the theoretical LQ pair production cross section. The band around the observed exclusion curve is obtained by considering the observed upper limit while taking into account the uncertainties on the theoretical cross section. The shaded region is excluded by the current D0 limits, which combines results from searches in the two electron, electron-neutrino, and two neutrino channels.