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Subjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

Grazing exclusion is one of the most efficient approaches to restore degraded grassland but may negatively affects the recovery of species diversity. Changes in plant species diversity should be a consequence of the ecological assembly process. Local community assembly is influenced by environmental filtering, biotic interactions, and dispersal. However, how these factors potentially contribute to changes to species diversity is poorly understood, especially in harsh environments. In this study, two management sites within a Stipa breviflora desert steppe community (typical natural steppe) were selected in northern China. In one of the two management sites, grazing has been excluded since 2010 and in the other with open grazing by sheep. In August 2016, three plots were established and 100 sampling units were created within each plot in a 5 m×5 m area at the two management sites. To assess the effects of grazing exclusion on S. breviflora steppe, we analyzed the vegetation biomass, species diversity, soil organic carbon, and soil particle size distribution using paired T-tests. In addition, variation partitioning was applied to determine the relative importance of environmental filtering and dispersal limitation. Null mode analysis was used to quantify the influence of biotic interactions in conjunction with EcoSim niche overlap and co-occurrence values. Our results demonstrated that (1) species diversity significantly decreased and the main improvements in soil quality occurred in the topsoil 0–10 cm after the grazing exclusion; (2) environmental filtering was important for community assembly between grazed and fenced grassland and this appears particularly true for soil particle size distribution, which may be well correlated with soil hydrological processes; and (3) however, competitive exclusion may play a significant role within the exclusion. The multiple pathways of assembly may collectively determine negative effects on the restoration of species diversity. Therefore, designers should be aware of the risk of reducing grazing exclusion-induced species diversity and account for manipulating processes. This in turn will reduce dominant species and promote environmental heterogeneity to maximize species diversity in semi-arid regions. |

submitted time
2018-04-24
From cooperative journals:《Journal of Arid Land》
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Subjects: Geosciences >> Space Physics

With a standard deviation as large as 2 cm, the sea state bias (SSB) has become the dominant source of error in satellite altimetry. The operational SSB correction models are two-dimensional (2-D) empirical (parametric or nonparametric) models based on the altimeter-measured wind speed (U) and significant wave height (SWH). However, these 2-D SSB models cannot entirely parameterize the range bias variability. The SSB uncertainty may be lowered through improved SSB models including additional measurable or predictable correlatives. This paper presents a method to estimate the SSB from crossover differences by using a three-dimensional (3-D) nonparametric model. The model is based on U, SWH from Jason-2 altimeter ocean observations, and the mean wave period from the European Centre for Medium-Range Weather Forecasts reanalysis project ERA-Interim (The SSB model developed with the method presented in this paper is called “3-D SSB model” and the SSB estimated with the 3-D SSB model is called “3-D SSB estimate”). Simulations indicate that the wave period can greatly affect the SSB. Evaluated by the separate annual datasets from 2009 to 2011, the 3-D SSB estimates can increase the explained variance by 1.32 cm2, or 1.15-cm RMS relative to the traditional 2-D SSB estimates based on U and SWH. Spatial evaluation of improvement shows that the 3-D SSB estimates are better than the traditional 2-D SSB estimates at all latitudes. The enhancement from 2-D to 3-D SSB estimates is of great significance to improve the precision of the altimeter product.[COMP]: Please set math TYPE gin the sentence below (40) as per the authors PDF. |

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2017-03-10
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Subjects: Geosciences >> Space Physics

The sea state bias (SSB) has become the dominant source of error in satellite altimetry. The operational SSB correction models are two-dimensional (2-D) nonparametric models based on the wind speed (U) and the significant wave height (SWH) that can be directly measured by the altimeters. This paper estimates the sea state bias of HY-2A radar altimeter using a three-dimensional (3-D) nonparametric model based on SWH from HY-2A interim geophysical dataset records (IGDR), U and the mean wave period (MWP) from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis project ERA-Interim. The 3-D SSB estimates can increase the explained variance by 1.72 cm2, or 1.31 cm RMS relative to the traditional 2-D SSB estimates based on U and SWH. ?2016 IEEE. |

Higgs-pair production and decay in simplest little Higgs model

Han, Xiao-Fang; Wang, Lei; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

In the framework of the simplest little Higgs model (SLHM), we study the production of a pair of neutral CP-even Higgs bosons at the LHC. First, we examine the production rate and find that it can be significantly larger than the SM prediction. Then we investigate the decays of the Higgs-pair and find that for a low Higgs mass their dominant decay mode is hh -> eta eta eta eta (eta is a CP-odd scalar) while hh -> b (b) over bar eta eta and hh -> eta eta WW may also have sizable ratios. Finally, we comparatively study the rates of pp -> hh -> b (b) over bar tau(+)tau(-), pp -> hh -> b (b) over bar gamma gamma, and pp -> hh -> WWWW in the SLHM and the littlest Higgs models (LHT). We find that for a light Higgs, compared with the SM predictions, all the three rates can be sizably enhanced in the LHT but severely suppressed in the SLHM; while for an intermediately heavy Higgs, both the LHT and SLHM can enhance sizably the SM predictions. (C) 2009 Elsevier B.V. All rights reserved. |

Subjects: Physics >> The Physics of Elementary Particles and Fields

The left-right twin Higgs model predicts a light stable scalar (S) over cap, which is a candidate for WIMP dark matter. We study its scattering on nucleon and find that the cross section is below the CDMS II upper bound but can reach the SuperCDMS sensitivity. Then we study the Higgs phenomenology by paying special attention to the decay h -> (S) over cap(S) over cap which is strongly correlated with the dark matter scattering on nucleon. We find that such an invisible decay can be sizable, which can severely suppress the conventional decay modes like h -> VV(V = W, Z) and h -> b (b) over bar. On the other hand, compared to the SM prediction, the rates of Higgs boson productions at the LHC via gluon-gluon fusion, weak boson fusion or in association with top quark pairs are all reduced significantly, e. g., the gluon-gluon fusion channel can be suppressed by about 30%. |

Subjects: Physics >> The Physics of Elementary Particles and Fields

We study the process gamma gamma -> h -> b (b) over bar at ILC as a probe of different little Higgs models, including the simplest little Higgs model (SLH), the littlest Higgs model (LH), and two types of littlest Higgs models with T-parity (LHT-I, LHT-II). Compared with the Standard Model (SM) prediction, the production rate is found to be sizably altered in these little Higgs models and, more interestingly, different models give different predictions. We find that the production rate can be possibly enhanced only in the LHT-II for some part of the parameter space, while in all other cases the rate is suppressed. The suppression can be 10% in the LH and as much as 60% in both the SLH and the LHT-I/LHT-II. The severe suppression in the SLH happens for a large tan beta and a small m(h), in which the new decay mode h -> eta eta (eta is a light pseudo-scalar) is dominant; while for the LHT-I/LHT-II the large suppression occurs when integral and m(h) are both small so that the new decay mode h -> A(H)A(H) is dominant. Therefore, the precision measurement of such a production process at the ILC will allow for a test of these models and even distinguish between different scenarios. |

Higgs boson decays and production via gluon fusion at LHC in littlest Higgs models with T parity

Wang, Lei; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

We study the Higgs boson decays and production via gluon fusion at the LHC as a probe of two typical littlest Higgs models which introduce a top quark partner with different (even and odd) T parity to cancel the Higgs mass quadratic divergence contributed by the top quark. For each model, we consider two different choices for the down-type quark Yukawa couplings. We first examine the branching ratios of the Higgs boson decays and then study the production via gluon fusion followed by the decay into two photons or two weak gauge bosons. We find that the predictions can be quite different for different models or different choices of down-type quark Yukawa couplings, and all these predictions can sizably deviate from the standard model predictions. So the Higgs boson processes at the LHC can be a sensitive probe for these littlest Higgs models. |

Higgs boson productions at the CERN LHC as a probe of different littlest Higgs models with T parity

Wang, Lei; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

Higgs boson productions at the LHC will serve as a sensitive probe of various little Higgs models. In this work we comparatively study two littlest Higgs models with different T-parity constructions through examining their effects in three production processes of the Higgs boson at the LHC, namely, the productions of a single Higgs, a Higgs-pair, as well as a Higgs boson associated with a pair of top and antitop quarks. The two models are characterized by predicting a top partner with, respectively, even and odd T-parity, which cancels the Higgs mass quadratic divergence caused by the top quark. We find that both models can alter the standard model cross sections sizably and their corrections also differ significantly. Therefore, the Higgs boson productions at the LHC might shed some light on these two models or even distinguish them. |

LHC diphoton Higgs signal predicted by little Higgs models

Wang, Lei; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

Little Higgs theory naturally predicts a light Higgs boson whose most important discovery channel at the LHC is the diphoton signal pp -> h -> gamma gamma. In this work, we perform a comparative study for this signal in some typical little Higgs models, namely, the littlest Higgs model, two littlest Higgs models with T-parity (named LHT-I and LHT-II), and the simplest little Higgs models. We find that compared with the standard model prediction, the diphoton signal rate is always suppressed and the suppression extent can be quite different for different models. The suppression is mild (<= 10%) in the littlest Higgs model but can be quite severe (similar or equal to 90%) in other three models. This means that discovering the light Higgs boson predicted by the little Higgs theory through the diphoton channel at the LHC will be more difficult than discovering the standard model Higgs boson. |

Top quark forward-backward asymmetry and charge asymmetry in the left-right twin Higgs model

Wang, Lei; Wu, Lei; Yang, Jin MinSubjects: Physics >> The Physics of Elementary Particles and Fields

In order to explain the Tevatron anomaly of the top quark forward-backward asymmetry A(FB)(t) in the left-right twin Higgs model, we choose to give up the lightest neutral particle of (h) over cap field as a stable dark matter candidate. Then a new Yukawa interaction for (h) over cap is allowed, which can be free from the constraint of same-sign top pair production and contribute sizably to A(FB)(t). Considering the constraints from the production rates of the top pair (t (t) over bar), the top decay rates, and t (t) over bar invariant mass distribution, we find that this model with such new Yukawa interaction can explain A(FB)(t) measured at the Tevatron while satisfying the charge asymmetry A(C)(t) measured at the LHC. Moreover, this model predicts a strong correlation between A(C)(t) at the LHC and A(FB)(t) at the Tevatron, i.e., A(C)(t) increases as A(FB)(t) increases. |