# Browsing by Author "Chen, X"

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Item Open Access Dark Energy Survey Year 1 Results: Cosmological Constraints from Cluster Abundances and Weak LensingAbbott, TMC; Aguena, M; Alarcon, A; Allam, S; Allen, S; Annis, J; Avila, S; Bacon, D; Bechtol, K; Bermeo, A; Bernstein, GM; Bertin, E; Bhargava, S; Bocquet, S; Brooks, D; Brout, D; Buckley-Geer, E; Burke, DL; Carnero Rosell, A; Carrasco Kind, M; Carretero, J; Castander, FJ; Cawthon, R; Chang, C; Chen, X; Choi, A; Costanzi, M; Crocce, M; da Costa, LN; Davis, TM; De Vicente, J; DeRose, J; Desai, S; Diehl, HT; Dietrich, JP; Dodelson, S; Doel, P; Drlica-Wagner, A; Eckert, K; Eifler, TF; Elvin-Poole, J; Estrada, J; Everett, S; Evrard, AE; Farahi, A; Ferrero, I; Flaugher, B; Fosalba, P; Frieman, J; García-Bellido, J; Gatti, M; Gaztanaga, E; Gerdes, DW; Giannantonio, T; Giles, P; Grandis, S; Gruen, D; Gruendl, RA; Gschwend, J; Gutierrez, G; Hartley, WG; Hinton, SR; Hollowood, DL; Honscheid, K; Hoyle, B; Huterer, D; James, DJ; Jarvis, M; Jeltema, T; Johnson, MWG; Johnson, MD; Kent, S; Krause, E; Kron, R; Kuehn, K; Kuropatkin, N; Lahav, O; Li, TS; Lidman, C; Lima, M; Lin, H; MacCrann, N; Maia, MAG; Mantz, A; Marshall, JL; Martini, P; Mayers, J; Melchior, P; Mena-Fernández, J; Menanteau, F; Miquel, R; Mohr, JJ; Nichol, RC; Nord, B; Ogando, RLC; Palmese, A; Paz-Chinchón, F; Plazas, AA; Prat, J; Rau, MM; Romer, AK; Roodman, A; Rooney, P; Rozo, E; Rykoff, ES; Sako, M; Samuroff, S; Sánchez, C; Sanchez, E; Saro, A; Scarpine, V; Schubnell, M; Scolnic, D; Serrano, S; Sevilla-Noarbe, I; Sheldon, E; Smith, J Allyn; Smith, M; Suchyta, E; Swanson, MEC; Tarle, G; Thomas, D; To, C; Troxel, MA; Tucker, DL; Varga, TN; von der Linden, A; Walker, AR; Wechsler, RH; Weller, J; Wilkinson, RD; Wu, H; Yanny, B; Zhang, Y; Zhang, Z; Zuntz, J; Collaboration, DESWe perform a joint analysis of the counts and weak lensing signal of redMaPPer clusters selected from the Dark Energy Survey (DES) Year 1 dataset. Our analysis uses the same shear and source photometric redshifts estimates as were used in the DES combined probes analysis. Our analysis results in surprisingly low values for $S_8 =\sigma_8(\Omega_{\rm m}/0.3)^{0.5}= 0.65\pm 0.04$, driven by a low matter density parameter, $\Omega_{\rm m}=0.179^{+0.031}_{-0.038}$, with $\sigma_8-\Omega_{\rm m}$ posteriors in $2.4\sigma$ tension with the DES Y1 3x2pt results, and in $5.6\sigma$ with the Planck CMB analysis. These results include the impact of post-unblinding changes to the analysis, which did not improve the level of consistency with other data sets compared to the results obtained at the unblinding. The fact that multiple cosmological probes (supernovae, baryon acoustic oscillations, cosmic shear, galaxy clustering and CMB anisotropies), and other galaxy cluster analyses all favor significantly higher matter densities suggests the presence of systematic errors in the data or an incomplete modeling of the relevant physics. Cross checks with X-ray and microwave data, as well as independent constraints on the observable--mass relation from SZ selected clusters, suggest that the discrepancy resides in our modeling of the weak lensing signal rather than the cluster abundance. Repeating our analysis using a higher richness threshold ($\lambda \ge 30$) significantly reduces the tension with other probes, and points to one or more richness-dependent effects not captured by our model.Item Open Access Impact of gully incision on hillslope hydrology(Hydrological Processes, 2020-01-01) Chen, X; Kumar, M; deB Richter, D; Mau, Y© 2020 John Wiley & Sons Ltd The Southern U.S. Piedmont ranging from Virginia to Georgia underwent severe gully erosion over a century of farming mainly for cotton (1800s–1930s). Although tree succession blanketed much of this region by the middle 20th century, gully erosion still occurs, especially during wet seasons. While many studies on gully erosion have focused on soil loss, soil carbon exchange, and stormwater response, the impacts on soil moisture, groundwater, and transpiration remain under-studied. Using a newly developed 2D hydrologic model, this study analyzes the impacts of gully erosion on hillslope hydrologic states and fluxes. Results indicate that increases in gully incision lead to reduction in groundwater table, root zone soil moisture, and transpiration. These reductions show seasonal variations, but the season when the reduction is maximum differs among the hydrologic variables. Spatially, the impacts are generally the greatest near the toe of the hillslope and reduce further away from it, although the reductions are sometimes non-monotonic. Overall, the impacts are larger for shallow gully depths and diminish as the incision goes deeper. Lastly, the extent of impacts on a heterogeneous hillslope is found to be very different with respect to a homogeneous surrogate made of dominant soil properties. These results show that through gully erosion, the landscape not only loses soil but also a large amount of water from the subsurface. The magnitude of water loss is, however, dependent on hydrogeologic and topographic configuration of the hillslope. The results will facilitate (a) mapping of relative susceptibility of landscapes to gullying, (b) understanding of the impacts of stream manipulations such as due to dredging on hillslope eco-hydrology, (c) prioritization of mitigation measures to prevent gullying, and (d) design of observation campaigns to assess the impacts of gullying on hydrologic response.Item Open Access Numerical method for multi-alleles genetic drift problem(SIAM Journal on Numerical Analysis, 2019-01-01) Xu, S; Chen, X; Liu, C; Yue, XGenetic drift describes random fluctuations in the number of genes variants in a population. One of the most popular models is the Wright-Fisher model. The diffusion limit of this model is a degenerate diffusion-convection equation. Due to the degeneration and convection, Dirac singularities will always develop at the boundaries as time evolves, i.e., the fixation phenomenon occurs. Theoretical analysis has proven that the weak solution of this equation, regarded as measure, conserves total probability and expectations. In the current work, we propose a scheme for 3-alleles model with absolute stability and generalize it to N-alleles case (N > 3). Our method can conserve not only total probability and expectations, but also positivity. We also prove that the discrete solution converges to a measure as the mesh size tends to zero, which is the exact measure solution of the original problem. The simulations illustrate that the probability density decays to zero first on the inner nodes, then also on the edge nodes except at the three vertex nodes, on which the density finally concentrates. The results correctly predict the fixation probability and are consistent with theoretical ones and with direct Monte Carlo simulations.Item Open Access Semiparametric Efficiency in GMM Models with Auxiliary Data(2007) Chen, X; Hong, H; Tarozzi, AWe study semiparametric efficiency bounds and efficient estimation of parameters defined through general moment restrictions with missing data. Identification relies on auxiliary data containing information about the distribution of the missing variables conditional on proxy variables that are observed in both the primary and the auxiliary database, when such distribution is common to the two data sets. The auxiliary sample can be independent of the primary sample, or can be a subset of it. For both cases, we derive bounds when the probability of missing data given the proxy variables is unknown, or known, or belongs to a correctly specified parametric family. We find that the conditional probability is not ancillary when the two samples are independent. For all cases, we discuss efficient semiparametric estimators. An estimator based on a conditional expectation projection is shown to require milder regularity conditions than one based on inverse probability weighting.Item Open Access Weak coupling of pseudoacoustic phonons and magnon dynamics in the incommensurate spin-ladder compound S r14 C u24 O41(Physical Review B - Condensed Matter and Materials Physics, 2016-10-21) Chen, X; Bansal, D; Sullivan, S; Abernathy, DL; Aczel, AA; Zhou, J; Delaire, O; Shi, L© 2016 American Physical Society.Intriguing lattice dynamics have been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr14Cu24O41, which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7-1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.