Electroweakino constraints from LHC data

We investigate the sensitivity of existing LHC searches to the charginos and neutralinos of the MSSM when all the other superpartners are decoupled. In this limit, the underlying parameter space reduces to a simple four-dimensional set $\{M_1,\,M_2,\,\mu,\,\tan\beta\}$. We examine the constraints placed on this parameter space by a broad range of LHC searches taking into account the full set of relevant production and decay channels. We find that the exclusions implied by these searches exceed existing limits from LEP only for smaller values of the Bino mass $M_1 \lesssim 150$ GeV. Our results have implications for MSSM dark matter and electroweak baryogenesis.Comment: 30 pages, 15 figure

Extending LHC Coverage to Light Pseudoscalar Mediators and Coy Dark Sectors

Many dark matter models involving weakly interacting massive particles (WIMPs) feature new, relatively light pseudoscalars that mediate dark matter pair annihilation into Standard Model fermions. In particular, simple models of this type can explain the gamma ray excess originating in the Galactic Center as observed by the Fermi Large Area Telescope. In many cases the pseudoscalar's branching ratio into WIMPs is suppressed, making these states challenging to detect at colliders through standard dark matter searches. Here, we study the prospects for observing these light mediator states at the LHC without exploiting missing energy techniques. While existing searches effectively probe pseudoscalars with masses between 5 - 14 GeV and above 90 GeV, the LHC reach can be extended to cover much of the interesting parameter space in the intermediate 20 - 80 GeV mass range in which the mediator can have appreciable Yukawa-like couplings to Standard Model fermions but would have escaped detection by LEP and other experiments. Models explaining the Galactic Center excess via a light pseudoscalar mediator can give rise to a promising signal in this regime through the associated production of the mediator with bottom quarks while satisfying all other existing constraints. We perform an analysis of the backgrounds and trigger efficiencies, detailing the cuts that can be used to extract the signal. A significant portion of the otherwise unconstrained parameter space of these models can be conclusively tested at the 13 TeV LHC with 100 fb$^{-1}$, and we encourage the ATLAS and CMS collaborations to extend their existing searches to this mass range.Comment: 27 pages + 3 appendices, 20 figures, 7 table

Using Final State Pseudorapidities to Improve s-channel Resonance Observables at the LHC

We study the use of final state particle pseudorapidity for measurements of s-channel resonances at the LHC. Distinguishing the spin of an s-channel resonance can, in principle, be accomplished using angular distributions in the centre-of-mass frame, possibly using a centre-edge asymmetry measurement, A_CE. In addition, forward-backward asymmetry measurements, A_FB, can be used to distinguish between models of extra neutral gauge bosons. In this note we show how these measurements can be improved by using simple methods based on the pseudorapidity of the final state particles and present the expected results for A_FB and A_CE for several representative models.Comment: 6 pages, 4 figures, 1 table; typos fixed, improved visibility of figures for greyscale printin

Unravelling an Extra Neutral Gauge Boson at the LHC using Third Generation Fermions

We study the potential to use measurements of extra neutral gauge bosons (Z') properties in pp collisions at the Large Hadron Collider to unravel the underlying physics. We focus on the usefulness of third generation final states (tau, b, t) in distinguishing between models with non-universal Z'-fermion couplings. We present an update of discovery limits of Z's including the 2010-2011 LHC run and include models with non-universal couplings. We show how ratios of sigma(pp -> Z' -> ttbar), sigma(pp -> Z' -> bbbar), and sigma(pp -> Z' -> tau^+tau^-) to sigma(pp -> Z' -> mu^+mu^-) can be used to distinguish between models and measure parameters of the models. Of specific interest are models with preferential couplings, such as models with generation dependent couplings. We also find that forward-backward asymmetry measurements with third generation fermions in the final state could provide important input to understanding the nature of the Z'. Understanding detector resolution and efficiencies will be crucial for extracting results

Identification of Extra Neutral Gauge Bosons at the LHC Using b- and t-Quarks

New Neutral Gauge Bosons, Z' 's, are predicted by many models of physics beyond the Standard Electroweak Theory. It is possible that a Z' would be discovered early in the Large Hadron Collider program. The next step would be to measure its properties to identify the underlying theory that gave rise to the Z'. Heavy quarks have the unique property that they can be identified in the final states. In this letter we demonstrate that measuring Z' decays to b- and t-quark final states can act as an effective means of discriminating between models with extra gauge bosons.Comment: 5 pages, 4 figures, references added and minor clarification

A framework for managing airport grasslands and birds amidst conflicting priorities

Management of modern airports is a task beset by conflicting priorities. Airports are vital to the global market economy, but impose costly environmental disturbances including habitat loss, noise, reduced air quality, erosion, introduction of invasive organisms, and polluted storm-water runoff (Blackwell et al. 2009). Airport environments also attract some wildlife hazardous to aviation safety, namely species involved in wildlife-aircraft collisions or ‘strikes’ (ICAO 2001, Blackwell et al. 2009, DeVault et al. 2011). Since 1912 at least 276 human lives have been lost due to bird strikes (Thorpe 2010), and from 1990 to 2010, more than 106 000 bird strikes involving civil aircraft were reported to the US Federal Aviation Administration (FAA; http://wildlife-mitigation. tc.faa.gov/wildlife/). Dolbeer (2006) reported that for strikes resulting in substantial aircraft damage (ICAO 1989), 66% occurred below 152 m altitude and within 1.5 km of a runway for airports servicing piston-powered aircraft only, and within 3 km of a runway for airports servicing turbine-powered aircraft (FAA 2009). Consequently, aviation authorities prioritize human safety over wildlife conservation in management of airport habitats (ICAO 2001, FAA 2009). Despite these problems, airports have been proposed as candidates for biodiversity conservation (Kelly & Allan 2006, Blackwell et al. 2009). For example, Kutschbach- Brohl et al. (2010) report that airport grasslands can provide habitat for a range of arthropod communities (e.g. Lepidoptera), and suggest the possibility of conserving these communities while minimizing provision of prey resources to birds recognized as hazardous to aviation. Moreover, declines in grassland bird populations in Europe and North America due to agricultural intensification and development have focused attention on enhancing quality and quantity of remnant grasslands (Herkert 1994, Vickery et al. 2004), including airport grasslands. In North America, airport properties have been identified as key areas of remnant grasslands important to obligate grassland bird species; species that both nest and forage in grasslands (Vickery et al. 1994, Askins et al. 2007). Airport properties in the contiguous USA include \u3e 330 000 ha of grassland, mostly annually mown areas, constituting 39–50% of airport property (DeVault et al. 2012). However, there is little research specific to airport environments that considers food resources for birds (Bernhardt et al. 2010, Kutschbach-Brohl et al. 2010), how birds perceive and react to predation risk (Baker & Brooks 1981) or disturbance (Kershner & Bollinger 1996), and no adequate assessment of how grassland management might affect strike risk (Blackwell et al. 2009, Martin et al. 2011). In this context, we contend that promoting conservation of obligate grassland birds and managing to reduce bird hazards to aviation safety combines two potentially conflicting objectives in a single management framework. Ecologically based guidance to solve this potential conflict is limited, if not oversimplified. Here, we question the potential use of airports to conserve grassland birds, and assess the challenges in managing airport grasslands in light of current ecological and behavioral frameworks. We consider conditions for conservation of obligate grassland birds on airports, and evidence on the use of airports by frequently struck, grassland birds (both obligate and facultative). We also provide a framework to manage grassland birds at airports, given current information and uncertainty. Because of the availability of strike data via the FAA, our focus is on North America. However, problems associated with bird use of airport grasslands are international (ICAO 2001). Therefore, our ultimate purpose is better to inform current management, but also identify research gaps and establish specific predictions that will guide future studies on the ecological basis of use of airport grasslands by birds

An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation

[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(–1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology

The pulse of the Amazon: fluxes of dissolved organic carbon, nutrients, and ions from the world's largest river

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 35(4), (2021): e2020GB006895, https://doi.org/10.1029/2020GB006895.The Amazon River drains a diverse tropical landscape greater than 6 million km2, culminating in the world's largest export of freshwater and dissolved constituents to the ocean. Here, we present dissolved organic carbon (DOC), organic and inorganic nitrogen (DON, DIN), orthophosphate (PO43−), and major and trace ion concentrations and fluxes from the Amazon River using 26 samples collected over three annual hydrographs. Concentrations and fluxes were predominantly controlled by the annual wet season flood pulse. Average DOC, DON, DIN, and PO43− fluxes (±1 s.d.) were 25.5 (±1.0), 1.14 (±0.05), 0.82 (±0.03), and 0.063 (±0.003) Tg yr−1, respectively. Chromophoric dissolved organic matter absorption (at 350 nm) was strongly correlated with DOC concentrations, resulting in a flux of 74.8 × 106 m−2 yr−1. DOC and DON concentrations positively correlated with discharge while nitrate + nitrite concentrations negatively correlated, suggesting mobilization and dilution responses, respectively. Ammonium, PO43−, and silica concentrations displayed chemostatic responses to discharge. Major and trace ion concentrations displayed clockwise hysteresis (except for chloride, sodium, and rubidium) and exhibited either dilution or chemostatic responses. The sources of weathered cations also displayed seasonality, with the highest proportion of carbonate- and silicate-derived cations occurring during peak and baseflow, respectively. Finally, our seasonally resolved weathering model resulted in an average CO2 consumption yield of (3.55 ± 0.11) × 105 mol CO2 km−2 yr−1. These results represent an updated and temporally refined quantification of dissolved fluxes that highlight the strong seasonality of export from the world's largest river and set a robust baseline against which to gauge future change.This work was supported by a grant from the Harbourton Foundation to R. G. M. Spencer and R. M. Holmes. T. W. Drake was supported by ETH Zurich core funding to J. Six. R. G. M. Spencer was additionally supported by NSF OCE-1333157.2021-09-1

Role of microstructure and surface defects on the dissolution kinetics of CeO2, a UO2 fuel analogue.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesised CeO2 analogue for UO2 fuel. Dissolution was performed on: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects; and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90°C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce3+ influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long-term performance of spent fuel in geological disposal