Search for Rare Decays of D^{0} Mesons into Two Muons

A search for the very rare D^{0}→μ^{+}μ^{-} decay is performed using data collected by the LHCb experiment in proton-proton collisions at sqrt[s]=7, 8, and 13 TeV, corresponding to an integrated luminosity of 9  fb^{-1}. The search is optimized for D^{0} mesons from D^{*+}→D^{0}π^{+} decays but is also sensitive to D^{0} mesons from other sources. No evidence for an excess of events over the expected background is observed. An upper limit on the branching fraction of this decay is set at B(D^{0}→μ^{+}μ^{-})<3.1×10^{-9} at a 90% C.L. This represents the world's most stringent limit, constraining models of physics beyond the standard model.

Hadronic and Hadron-Like Physics of Dark Matter

The problems of simple elementary weakly interacting massive particles (WIMPs) appeal to extend the physical basis for nonbaryonic dark matter. Such extension involves more sophisticated dark matter candidates from physics beyond the Standard Model (BSM) of elementary particles. We discuss several models of dark matter, predicting new colored, hyper-colored or techni-colored particles and their accelerator and non-accelerator probes. The nontrivial properties of the proposed dark matter candidates can shed new light on the dark matter physics. They provide interesting solutions for the puzzles of direct and indirect dark matter search.

Hadronic Dark Matter

The possibility of hadronic Dark Matter particles is analised in the framework of the simplest extensions of the Standard Model. Hadronic particles possess strong interaction and consist of new heavy quark and light standard one. It is shown that the existence of heavy quarks does not contradict to the precision electro-weak restrictions on new physics. The neutral and charged pseudoscalar low-lying heavy states are considered as the Dark Matter particle and its mass-degenerated partner. We evaluated the values of their masses and lifetime of the charged component. The potential of low-energy interactions of these particles with nucleons is described in the framework of the exchangemeson model. Some peculiarities of the hadronic Dark Matter scenario are also discussed.

Vector-like top/bottom-quark partners and Higgs physics at the LHC

Vector-like quarks (VLQ) that are partners of the heavy top and bottom quarks are predicted in many extensions of the standard model (SM). We explore the possibility that these states could explain not only the long-standing anomaly in the forward-backward asymmetry in b-quark production at LEP, [Formula: see text], but also the more recent [Formula: see text]2[Formula: see text] deviation of the cross section for the associated Higgs production with top-quark pairs at the LHC, [Formula: see text]. Introducing three illustrative models for VLQs with different representations under the SM gauge group, we show that the two anomalies can be resolved while satisfying all other theoretical and experimental constraints. In this case, the three different models predict VLQ states in the 1-2 TeV mass range that can be soon probed at the LHC. In a second step, we discuss the sensitivity on the VLQ masses and couplings that could be obtained by means of a percent level accuracy in the measurement of ratios of partial Higgs decay widths, in particular [Formula: see text] and [Formula: see text]. We show that top and bottom VL partners with masses up to [Formula: see text]5 TeV and exotic VLQs with masses in the 10 TeV range can be probed at the high-luminosity LHC.