A search is presented for the pair production of new heavy resonances, each decaying into a top quark (t) or antiquark and a gluon (g). The analysis uses data recorded with the CMS detector from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with one muon or electron, multiple jets, and missing transverse momentum are selected. After using a deep neural network to enrich the data sample with signal-like events, distributions in the scalar sum of the transverse momenta of all reconstructed objects are analyzed in the search for a signal. No significant deviations from the standard model prediction are found. Upper limits at 95% confidence level are set on the product of cross section and branching fraction squared for the pair production of excited top quarks in the $\mathrm{t^*}$ $\to$ tg decay channel. The upper limits range from 0.12 pb to 0.8 fb for a $\mathrm{t^*}$ with spin-1/2 and from 0.015 pb to 1.0 fb for a $\mathrm{t^*}$ with spin-3/2. These correspond to mass exclusion limits up to 1050 and 1700 GeV for spin-1/2 and spin-3/2 $\mathrm{t^*}$ particles, respectively. These are the most stringent limits to date on the existence of $\mathrm{t^*}$ $\to$ tg resonances.
Expected and observed 95% CL upper limits on the product of the $t^{*} \overline{t}^{*}$ production cross section and the branching fraction squared $BR^2(t^{*} \rightarrow tg)$ for a spin-1/2 $t^{*}$ as a function of $m_{t^{*}}$. The inner (green) and outer (yellow) bands give the central probability intervals containing 68 and 95% of the expected upper limits under the background-only hypothesis. The cross section predicted by theory, following an EFT approach, is shown in blue, assuming $BR(t^{*} \rightarrow tg)=1$.
Expected and observed 95% CL upper limits on the product of the $t^{*} \overline{t}^{*}$ production cross section and the branching fraction squared $BR^2(t^{*} \rightarrow tg)$ for a spin-3/2 $t^{*}$ as a function of $m_{t^{*}}$. The inner (green) and outer (yellow) bands give the central probability intervals containing 68 and 95% of the expected upper limits under the background-only hypothesis. The cross section predicted by theory, following an EFT approach, is shown in blue, assuming $BR(t^{*} \rightarrow tg)=1$. The results of the previous CMS analysis, using data corresponding to an integrated luminosity of 35.9 $fb^{-1}$, are shown in red.
Distributions in $S_T$ in the SR for the muon channel, after a background-only fit to the data. The signal distributions are scaled to the cross section predicted by the theory. The hatched bands show the post-fit uncertainty band, combining all sources of uncertainty. The ratio of data to the background predictions is shown in the panels below the distributions.
A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb$^{-1}$ of proton--proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at $\sqrt{s}=13$ TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions.
Background fit results for regions SR-2TeV ( sumPT > 2 TeV) and SR-3TeV ( sumPT > 3 TeV) for the electron and muons channels. The errors shown are the statistical plus systematic uncertainties. The uncertainty in the total background count includes correlations between nuisance parameters and so does not reflect a quadrature sum of the uncertainties in the individual background components.
The sumPT distribution in the W+jets control region (electron channel). Expected background yields are given along with the total background uncertainty. The ttbar, W+jets and Z+jets backgrounds are normalised by the factors 0.95, 0.81 and 1.01 as obtained from the background likelihood fit. The single-top-quark and diboson background normalisations are taken from the simulation. The multijet background is obtained using a data-driven method. Additionally, the likelihood fit may constrain nuisance parameters for certain systematic uncertainties, altering the normalisation and shape of some of the distributions.
The sumPT distribution in the W+jets control region (muon channel). Expected background yields are given along with the total background uncertainty. The ttbar, W+jets and Z+jets backgrounds are normalised by the factors 0.95, 0.81 and 1.01 as obtained from the background likelihood fit. The single-top-quark and diboson background normalisations are taken from the simulation. The multijet background is obtained using a data-driven method. Additionally, the likelihood fit may constrain nuisance parameters for certain systematic uncertainties, altering the normalisation and shape of some of the distributions.
An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in $pp$ collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider, have been considered. An integrated luminosity of 3.2 fb$^{-1}$, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.
Normalisation factors for the main backgrounds obtained from the combined fit in each of the channels. The total uncertainty is given.
Search for the first generation leptoquarks (LQs). Event yields in the Z control region (CR), ttbar CR and in the signal region (SR). Each CR is treated as one bin in the profile likelihood fit. The SR is split to 7 bins according to $m_{\text{LQ}}^{\text{min}}$ for the fit. The table below shows the total number of events in each CR. For the SR, it shows the number of events per 100 GeV as a function of $m_{\text{LQ}}^{\text{min}}$. The background expectations are scaled by a scale factor extracted from the fit. However, the uncertainties shown are the pre-fit ones. The data event yield uncertainty is statistical (gaussian). The background uncertainty consists of all the experimental and theoretical components summed in quadrature. The uncertainty of the fit-extracted background scale factor is also added in quadrature.
Search for the second generation leptoquarks (LQs). Event yields in the Z control region (CR), ttbar CR and in the signal region (SR). Each CR is treated as one bin in the profile likelihood fit. The SR is split to 7 bins according to $m_{\text{LQ}}^{\text{min}}$ for the fit. The table below shows the total number of events in each CR. For the SR, it shows the number of events per 100 GeV as a function of $m_{\text{LQ}}^{\text{min}}$. The background expectations are scaled by a scale factor extracted from the fit. However, the uncertainties shown are the pre-fit ones. The data event yield uncertainty is statistical (gaussian). The background uncertainty consists of all the experimental and theoretical components summed in quadrature. The uncertainty of the fit-extracted background scale factor is also added in quadrature.
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