Search for Quantum Black Hole Production in High-Invariant-Mass $\mathbf{\text{Lepton}}+\mathbf{\text{Jet}}$ Final States Using $pp$ Collisions at $\sqrt{s}=8\mathrm{TeV}$ and the ATLAS Detector

This Letter presents a search for quantum black-hole production using $20.3{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector in $pp$ collisions at the LHC at $\sqrt{s}=8\mathrm{TeV}$. The quantum black holes are assumed to decay into a final state characterized by a lepton (electron or muon) and a jet. In either channel, no event with a lepton-jet invariant mass of 3.5 TeV or more is observed, consistent with the expected background. Limits are set on the product of cross sections and branching fractions for the $\text{lepton}+\text{jet}$ final states of quantum black holes produced in a search region for invariant masses above 1 TeV. The combined 95% confidence level upper limit on this product for quantum black holes with threshold mass above 3.5 TeV is 0.18 fb. This limit constrains the threshold quantum black-hole mass to be above 5.3 TeV in the model considered.

Figure 1

Distribution of the invariant mass of the lepton and highest-$p_{\mathrm{T}}$ jet in (a) the $\text{electron}+\text{jet}$ channel and (b) the $\text{muon}+\text{jet}$ channel, for data (points with error bars) and for SM backgrounds (solid histograms). Overlaid are two examples of QBH signals. The sum of the uncertainties due to the finite MC sample size and from various sources of systematic uncertainty is shown by the hatched area. To extract the upper limit on the $\text{lepton}+\text{jet}$ cross section, a fit to the invariant-mass distribution is performed, replacing the uncertainties due to MC sample size by the statistical uncertainties on the fit parameters.

Figure 2

The combined 95% C.L. upper limits on $\mathrm{\Sigma }{\sigma }_{\text{qq}}\times {\mathrm{BF}}_{\text{qq}}$ for QBHs decaying to a lepton and jet, as a function of $M_{\text{th}}$, assuming $M_{\mathrm{D}}=M_{\text{th}}$ and $n=6$ ADD extra dimensions. The limits take into account statistical and systematic uncertainties. Points along the solid black line indicate the mass of the signal where the limit is computed. Also shown are the $\pm 1\sigma$ and $\pm 2\sigma$ bands, indicating the underlying distribution of possible limit outcomes under the background-only hypothesis. The predicted cross section for QBHs is shown as the solid curve.