Browsing by Subject "Baryons"

Now showing 1 - 3 of 3
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    A search for the H0 dibaryon
    (2001-08) Jensen, Paul Thomas; Hoffmann, Gerald W.
    The H Dibaryon is a theoretical six quark (uuddss) s-wave flavor-color singlet with I=0, Y=0, B=2, S=-2 and spin-parity Jπ =0+ . Proposed in 1977 by Jaffe, the H has been the subject of more than thirty experiments. Experiment 896 at the Brookhaven National Laboratory Alternating Gradient Synchrotron searched for the H with an Au beam and an Au target. The experiment utilized a Silicon Drift Detector Array in an effort to detect the H via the H p− → Σ decay channel. This search is sensitive to weakly decaying H Dibaryons with lifetimes as small as 4 cm.
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    Studies of Baryonic Physics and Triaxiality in Galaxy Clusters
    (2018-12) Lee, Brandyn Emmerson
    This dissertation concerns the impact of baryonic physics and triaxiality on galaxy clusters. The first and primary portion of this work is to investigate the impact baryonic processes have on galaxy cluster masses, profiles, and weak lensing mass measurements. To that end, we use the cosmo-OWLS suite of hydrodynamic simulations which incorporate a variety of sub-grid baryonic processes. We obtain mass estimates by fitting spherical NFW profiles to mock weak lensing data using MCMC techniques, in particular examining the difference between dark matter-only runs and those including baryons. We find no significant difference in the mass bias when including baryonic physics. The overall masses, however, are suppressed with the introduction of feedback from active galactic nuclei (AGN). For the lowest mass systems for which a reliable mass can be obtained, we find a bias of ≈ −10 percent. The magnitude of the bias tends to decrease as mass increases and is consistent with no bias for the most massive clusters. For the lowest mass clusters, the bias is particularly sensitive to the fit radii and concentration prior, making reliable mass estimates difficult. Additionally, we find that the scatter in mass estimates due to the presence of baryons is less than the scatter between different projections of individual clusters. The next portion of the work principally concerns cluster shape. We calculate the moment of inertia tensors for ≈1100 clusters in the large volume cosmo-OWLS simulations, using all the particles bound to the clusters. From this information, we derive the cluster axes and orientations in 3D. The cluster axis distributions can be used as priors when fitting triaxial models. We also find that, in keeping with previous studies, clusters tend to be prolate. We develop a new pipeline for fitting triaxial mass models to weak lensing, strong lensing, X-ray and eventually to SZ data. This pipeline is applied to synthetic data sets from cosmological simulations, and to the CLASH data set. In particular, a new 2D prior on the shape of clusters is determined by mapping the mass using weak lensing data. The projections of the 3D models must be consistent with these 2D weak lensing priors. The strong lensing signals are incorporated into the pipeline applied to real data by requiring that the projections of the 3D models have 2D cylinder masses that are consistent with the arc features in the clusters.
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    Weak Gravitational Lensing Shear and Magnification Analysis of Hydrodynamic Simulations and Application to Weak Lensing Data
    (2019-08) Choi, Miyoung
    In the main part of this dissertation we study the impact of baryonic processes and massive neutrinos on weak lensing peak statistics that can be used to constrain cosmological parame­ters. We use the BAHAMAS simulations, which self-consistently include baryonic processes and the effect of massive neutrino free-streaming on the evolution of structure formation. We construct synthetic weak lensing maps by ray-tracing through light-cones, specifically using the so-called aperture mass statistic. The peaks detected on the maps reflect the cumulative lensing signal from massive bound objects and general large scale structure of the Universe. In particular, we quantify the impact of the prescription for baryonic physics and a range of summed neutrino masses {0.06, 0.12, 0.24, 0.48} eV on the detected peaks, so that in future this uncertainty can be factored into studies using lensing peaks to constrain cosmological models. As higher neutrino mass tends to suppress the formation of massive structures in the Universe, the halo mass function and lensing peak counts are therefore modified as a function of Mν. Both baryonic physics and massive neutrinos impact on weak lensing peaks, depending on the mass range under consideration, and should be accounted for when deriv­ing cosmological parameters from weak lensing observations. For example, baryonic physics is less important for the most massive haloes. We also present a study of the cluster merger system Abell 2146. We make new aperture mass maps by applying the aperture mass statistic to Hubble Space Telescope data of the system, using different size apertures. These maps directly show the signal-to-noise with which lensing detects features in the system. We also develop a pipeline that can in general be applied to snapshots from computer simulations of cluster merger systems. This pipeline produces maps of lensing observables that would be measured at different times during the merger process. In particular the lensing distortion field or so-called shear field can be compared with the shear field that we have estimated from Hubble Space Telescope observations, in order to find the best description of the cluster and merger parameters of the system. Finally we develop a pipeline to measure the stacked weak lensing shear and magnification signatures of galaxy clusters. We apply this pipeline to analytic models of clusters, and conclude that the shear information gives better constraints on mass models, even though the number of sources from which the shear can be measured is lower than for the magnification. In future this can be applied to synthetic catalogues for example from the BAHAMAS simulation or to real data for example from LSST or Euclid.