Search for Pair Production of Third-Generation Leptoquarks and Top Squarks in $pp$ Collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$

Results are presented from a search for the pair production of third-generation scalar and vector leptoquarks, as well as for top squarks in $R$-parity-violating supersymmetric models. In either scenario, the new, heavy particle decays into a $\tau$ lepton and a $b$ quark. The search is based on a data sample of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$, which is collected by the CMS detector at the LHC and corresponds to an integrated luminosity of $4.8{\mathrm{fb}}^{-1}$. The number of observed events is found to be in agreement with the standard model prediction, and exclusion limits on mass parameters are obtained at the 95% confidence level. Vector leptoquarks with masses below 760 GeV are excluded and, if the branching fraction of the scalar leptoquark decay to a $\tau$ lepton and a $b$ quark is assumed to be unity, third-generation scalar leptoquarks with masses below 525 GeV are ruled out. Top squarks with masses below 453 GeV are excluded for a typical benchmark scenario, and limits on the coupling between the top squark, $\tau$ lepton, and $b$ quark, ${\lambda }_{333}^{\prime }$ are obtained. These results are the most stringent for these scenarios to date.

Figure 1

The measured $S_{\mathrm{T}}$ distribution (points) compared to the stacked distribution of the SM backgrounds (shaded region) and a simulated $M_{\mathrm{LQ}}=450\mathrm{GeV}$ LQ signal (solid line) after the final selection.

Figure 2

Top: the expected and observed upper limit at 95% CL on the LQ (${\tilde{t}}_1$, VLQ) pair production cross section times $\mathcal{B}(\mathrm{LQ}/{\tilde{t}}_1/\mathrm{VLQ}\to \tau b)$ as a function of the LQ (${\tilde{t}}_1$, VLQ) mass. The $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties on the expected limit are also shown as green (inner) and yellow (outer) bands around the expected limit. The blue (solid) curve, magenta (dashed) curve, and red (dotted) curve and the matching shaded bands represent the theoretical LQ, ${\tilde{t}}_1$, and VLQ pair production cross section and the uncertainties due to the choice of PDF and renormalization and factorization scales, respectively. Bottom: the expected and observed 95% CL limit on the RPV coupling ${\lambda }_{333}^{\prime }$ for $M_2=250\mathrm{GeV}$ and $M_2=1\mathrm{TeV}$.