Dust acoustic waves in the bulk of a dust cloud in complex plasma of low-pressure gas discharge under microgravity conditions are considered. The complex plasma is assumed to conform to the ionization equation of state (IEOS) developed in our previous study. This equation implies the ionization similarity of plasmas. We find singular points of IEOS that determine the behavior of the sound velocity in different regions of the cloud. The fluid approach is utilized to deduce the wave equation that includes the neutral drag term. It is shown that the sound velocity is fully defined by the particle compressibility, which is calculated on the basis of the used IEOS. The sound velocities and damping rates calculated for different three-dimensional complex plasmas both in ac and dc discharges demonstrate a good correlation with experimental data that are within the limits of validity of the theory. The theory provides interpretation for the observed independence of the sound velocity on the coordinate and for a weak dependence on the particle diameter and gas pressure. Predictive estimates are made for the ongoing PK-4 experiment.
Abstract A large number of top quarks will be produced at the Large Hadron Collider (LHC) during the Run II period. This will allow us to measure the rare processes from the top sector in great details. We present a study of top-quark pair production in association with a bottom-quark pair ( \(t\bar tb\bar b\) ) from fast simulations for the Compact Muon Solenoid (CMS) experiment. The differential distributions of \(t\bar tb\bar b\) are compared with the top-quark pair production with two additional jets ( \(t\bar tjj\) ) and with the production in association with the Higgs ( \(t\bar tH\) ), where the Higgs decays to a bottom-quark pair. The significances of the \(t\bar tb\bar b\) process in the dileptonic and the semileptonic decay modes are calculated with the data corresponding to an integrated luminosity of 10 fb-1, which is foreseen to be collected in the early Run II period. This study will provide an important input in searching for new physics beyond the standard model, as well as in searching for the \(t\bar tH\) process where the Yukawa coupling with the top quark can be directly measured.