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

Dynamic simulation of catastrophic late Pleistocene glacial-lake drainage, Altai Mountains, central Asia

Numerical simulations of the catastrophic draining of Pleistocene glacial-lake Kuray–Chuja quantify the discharge history of the draining event in detail. The plan-view basin flows are modelled as water emptied due to the instantaneous failure of the impounding ice-dam when the lake was at maximum capacity. The Chuja Basin water exited as a jet-flow into the Kuray Basin via a narrow conjoining valley. The peak discharge from the Chuja Basin is etermined to be 1.20 × 107 m3 s?1, and the peak discharge (3.19 × 107 m3 s?1 > Q ? 2.0 × 107 m3 s?1) that flowed from the Kuray Basin at the failed impoundment is also calculated for two limiting conditions. The variations inlake volume and depth indicate complete drainage within 50 h. In both basins, fields of relict gravel bedforms reflect sediment transport due to entrained lake-bed sediments. Thus, in addition to the general overview of drainage, the detailed temporal and spatial evolutions of drainageparameters are reported, including for the locations of the bedform fields. Local flow above the bedforms is considered in relation to thresholds for sediment motion, bedform development, and orientations. Within the simple bathymetry of the Chuja Basin, the flow field was fairly uniformwith flow conducive to bedform evolution only occurring close to the exit from the basin, which accords with field evidence. In contrast, within the Kuray Basin, the flow responded sensitively to the complex bathymetry, which included rapid changes in flow direction due to interaction of the Kuray water with the jet-flow from Chuja, and as submerged ridges shoaled. Thus the Kuray flow field was complex but with time-dependent flow conditions in ccordance with bedform development. It is concluded that the location of the bedforms can be explained in terms of the flow modelling and suggestions are made as to how future drainage models might be improved

Unsteady 1D and 2D hydraulic models with ice dam break for quaternary megaflood, Altai mountains, southern Siberia

One of the largest known floods occurred during the Late Quaternary, emanating from an ice-dammed lake in Asia. Glacial lake Kuray–Chuja was formed by a 600-m-high ice dam converging in the Chuja River valley of the Altai Mountains in southern Siberia. The dam impounded up to 594 km3 of water in the Kuray and Chuja basins.At least three floods from lake Kuray–Chuja occurred, but only the largest, or the most recent, is modelled herein. The discharge, through an ice dam breach by tunnelling or over-topping, is analysed using dam breach equations including one specifically developed for ice dam failures. From these calculations it is concluded that the ice dam need not have failed when the water was at a maximum depth (i.e. 600 m deep) but, in consideration with flood routing models, it is probable that the lake emptied by over-topping under conditions of maximum water level. Although an over-topping model is favoured, a collapse of the ice dam due to initial tunnel development in the ice body cannot be precluded.The resultant flood wave ran down the Chuja River valley to the confluence with the Katun River and beyond. One-dimensional and two-dimensional unsteady and non-uniform flow modelling of the flood wave routed down the river valleys is presented that includes modelling a channel bifurcation at the confluence and backwater effects. The depth of the flood model is constrained by the altitudes of the tops of giant bars deposited by the palaeoflood, which indicate maximum flood stage.The results of the ice dam failure calculations and the flow modelling are independent of each other and are consistent, indicating in each case a flood of the order of 10 M m3 s? 1, with best-fit solutions providing estimated peak flood discharges of 9 to 11 M m3 s? 1. A breach, 1 km wide and 250 m deep, developed in the ice dam in as little as 11.6 h whereas the flood duration required to evacuate the total lake volume was around 1 day