Searching for Dark Matter with tt‾t\overline{t} Resonance

Many models containing particles which are candidates for dark matter, assume the standard model particles and the dark matter candidates are mediated by a spin-0 particle. At the LHC, one can use these models for dark matter searches. One of the possible approaches for the search of these models is by considering the decay of the spin-0 particle to a pair of $t\bar{t}$, thus modifying the pattern of the top quark pair invariant mass spectrum. This search suggests a good sensitivity in a parameter space different than the more traditional searches. We examine this sensitivity and put limits on two benchmark models containing candidates for dark matter, using previous ATLAS results. It was found that when the mediator mass ($m_{Y_0}$) and the dark matter candidate mass ($m_{\chi}$) have values of $m_{Y_0} \sim 2 \cdot m_{\chi}$, mediator masses in the range of $[400,600]$ GeV are excluded. We compare our results to direct detection experiments and show that we gain sensitivity for new regions which are not covered by other searches

Establishing a Search for b→sℓ+ℓ−b \rightarrow s \ell^{+} \ell^{-} Anomalies at the LHC

One of the fundamental predictions of the Standard Model is Lepton Flavour Universality. Any deviation from this prediction would indicate the existence of physics beyond the Standard Model. Recent LHCb measurements present a pattern of deviations from this prediction in rare B-meson decays. While not yet statistically significant (currently $2.2-2.6 \sigma$), these measurements show an imbalance in the ratio of B-meson decays to a pair of muons in association with a Kaon and decays to a pair of electrons in association with a Kaon. If the measured deviations are indeed present in nature, new physics may mediate interactions involving a pair of same flavour leptons, a $b$- and an $s$-quark. We present the prospect for a search of new physics in this type of interactions at the LHC, in a process that involves an $s$-quark, and a final state with two leptons and a $b$-jet. The proposed search can improve the sensitivity to new physics in these processes by a factor of four compared to current searches with in the total dataset expected at the LHC

Hiding the Higgs at the LHC

We study a simple extension of the standard model where scalar singlets that mix with the Higgs doublet are added. This modification to the standard model could have a significant impact on Higgs searches at the LHC. The Higgs doublet is not a mass eigenstate and therefore the expected nice peak of the standard model Higgs disappears. We analyze this scenario finding the required properties of the singlets in order to make the Higgs "invisible" at the LHC. In some part of the parameter space even one singlet could make the discovery of the SM Higgs problematic. In other parts, the Higgs can be discovered even in the presence of many singlets.Comment: 9 pages, 1 figure. V2- References added. V3- Several examples and one fig. adde

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements