<p> A search for Higgs boson pair production in the four b-jet final state is carried out with up to 36.1/fb of LHC proton--proton collision data collected at &radic;<i>s</i> = 13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. This thesis presents a search in events with four individually resolved b-tagged jets. Higgs bosons produced with large momenta are reconstructed as single large radius jets with substructure. The analysis of this topology is presented in CERN-THESIS-2018-118. The two analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to four b-quarks are set in each model. The combined result searches for resonance masses in the range 260&ndash;3000 GeV. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.</p><p>

No description available.

<p>Astrophysical jets are collimated streams of magnetized plasma launched from compact objects, such as neutron stars or black holes. These jets, powered by the accretion of surrounding gas onto the compact object, can accelerate particles to extreme energies and produce powerful radiation.</p> <p><br></p> <p>In this report, I investigate energy dissipation and particle acceleration in two key regions in jets: (i) external shocks which form where jets interact with ambient gas and (ii) internally in the jet where particles are likely to be energized through the process of magnetic reconnection.</p> <p><br></p> <p>First, I explore inverse Compton scatterings of electrons accelerated at the external shock as a candidate for the high energy emissions from gamma-ray burst afterglows. I consider two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton) and photon fields external to the shock (external Compton) from the star-forming region in the host galaxy. I develop an analytical model to predict the high-energy spectra from these blasts and reproduce the observed spectra and lightcurves of GRB~190114C. The model implies that inverse Compton can dominate the sub-TeV/TeV emission in this event.</p> <p><br></p> <p>Second, I study the particle acceleration mechanism of magnetic reconnection internally in astrophysical jets. I employ particle-in-cell (PIC) simulations of 3D relativistic magnetic reconnection. My analysis reveals a novel acceleration mechanism that only operates in 3D that results in faster particle acceleration. Unlike in 2D simulations where particles are trapped in the reconnected plasma and stop being accelerated, a fraction of particles in 3D can escape from this region (along the third direction) and be further accelerated. The escaped particles are characterized by a harder energy spectrum with a higher cutoff compared to those found in previous studies. Based on the PIC simulation findings, I build an analytical model for the particle kinetics, which divides particles into two groups --- one undergoing fast energization in the reconnection upstream region and the other residing in the reconnected plasma without energy change. The model predicts a power-law spectra for both groups of particles. PIC simulations reveal a universal magnetization-independent spectra with $dN/d\gamma\propto \gamma^{-2}$ for the overall particle population. The results demonstrate that relativistic reconnection in jets may be a promising mechanism for generating Ultra-High-energy Cosmic Rays. </p>

Using dimuon events produced in proton-antiproton collisions at √s = 1.8 TeV at the D⊘ detector, the integrated inclusive b-quark production cross section has been extracted. The inclusive b-quark production cross section for Pᵇ(T) > 9 GeV/c is 2.61 ± 0.07(Stat) ± 1.13(Syst) nb. The cross section, dσ/dΔφ(μμ) , where Δφ(μμ), is the azimuthal angle difference between the dimuons, has also been measured. Both measurements are in good agreement with next-to-leading order QCD predictions.

<p> In this thesis, we develop manifestly covariant methods for 4 dimensional, <i> N</i> = 1 superconformal field theories. First, we generalize the embedding formalism in conformal field theories (CFTs) to <i>N</i> = 1 superconformal field theories (SCFTs). As applications we construct manifestly superconformally covariant expressions for land 3-point correlation functions involving the supercurrent multiplet or the global symmetry current superfield. Next, we combine this superembedding formalism with the shadow formalism in CFTs into a new method for computing superconformal blocks appearing in 4-point functions of SCFTs. This new method, called the supershadow formalism, expresses a superconformal block as manifestly covariant integrations over a product of 3-point functions. The supershadow formalism is much more efficient computationally than the brute force methods used previously in the literature. We obtain the superconformal blocks appearing in the 4-point functions of general scalar operator and then specialize to the 4-point functions involving chiral and global symmetry current multiplets. The results in the chiral case can be further generalized to <i> N</i> = 2 SCFTs. Finally, we present a systematic algorithm to extract the correlation functions of conformal primary component operators from the superfield correlation functions. We implemented this algorithm in <i> Mathematica</i> and applied it to the superfield 2-point function between general operators, from which we obtain all the component 2-point functions and all possible shortening conditions for a <i>N</i> = 1 superconformal multiplet. We also discuss a few potential directions for future researches. </p>

No description available.

In this dissertation, I explore the relationship between solar energetic particles (SEPs) and the interplanetary magnetic field, and I use observations of SEPs to probe the region of space between the Sun and the Earth. After an introduction of major concepts in heliospheric physics, describing some of the history of energetic particles and defining the data sets used in the work, the rest of this dissertation is organized around three major concepts related to energetic particle transport: magnetic field-line length, interplanetary turbulence, and particle scattering and diffusion. In Chapter 2, I discuss how energetic particles can be used to measure the lengths of field lines and how particle scattering complicates the interpretation of these measurements. I then propose applying these measurements to a particular open problem: the origin and properties of heliospheric current sheets. In the next chapter, I move from the large to small scale and apply energetic particle measurements to important problems in interplanetary turbulence. I introduce two energetic-particle features, one of which I discovered in the course of this work, which have size scales roughly that of the correlation scale of the turbulence (the largest scale over which observations are expected to be similar). I discuss how multi-spacecraft measurements of these energetic particle features can provide a measure of the correlation scale independent of the magnetic field measurements. Finally, I consider interplanetary scattering and diffusion in detail. I describe new observations of particle diffusion in the direction perpendicular to the average magnetic field, showing that particles only scatter a few times between their injection at the Sun and observation at the Earth. I also provide numerical simulation results of diffusion parallel to the field which can be used to correct for the effects of transport on the particles. These corrections allow inferences to be made about the particle energies at injection from observations of the event-integrated fluences at 1 AU. By carefully including scattering, cooling, field line meandering and turbulence effects, solar-energetic particles become a powerful tool for studying the inner heliosphere.

No description available.

<p> A search for vector-like quarks (VLQs) decaying to a Z boson using multi-stage machine learning was compared to a search using a standard square cuts search strategy. VLQs are predicted by several new theories beyond the Standard Model. The searches used 20.3 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV collected with the ATLAS detector in 2012 at the CERN Large Hadron Collider. CLs upper limits on production cross sections of vector-like top and bottom quarks were computed for VLQs produced singly or in pairs, <i>T_single, B_single, T_pair, </i> and <i>B_pair.</i> The two stage machine learning classification search strategy did not provide any improvement over the standard square cuts strategy, but for <i>T_pair, B_pair,</i> and <i>T_single,</i> a third stage of machine learning regression was able to lower the upper limits of high signal masses by as much as 50%. Additionally, new test statistics were developed for use in the Neyman construction of confidence regions in order to address deficiencies in current frequentist methods, such as the generation of empty set confidence intervals. A new method for treating nuisance parameters was also developed that may provide better coverage properties than current methods used in particle searches. Finally, significance ratio functions were derived that allow a more nuanced interpretation of the evidence provided by measurements than is given by confidence intervals alone.</p>

A variant of the recombination model which we call the valon-parton model is applied simultaneously to a variety of meson inclusive reactions with proton, pion and kaon beams in the kinematic region of low transverse momentum and intermediate values of longitudinal momentum fractions. It is found that the valon distributions in hadrons show no evidence for SU(3) breaking. There are some indications of substantial gluon dissociation contributions which we interpreted through a maximally enhanced sea. For proton induced reactions the model predictions are in excellent agreement with the data; meson initiated reactions indicate additional contributions are coming from resonances which are produced recombinantly and then decay into the observed mesons. / Source: Dissertation Abstracts International, Volume: 42-10, Section: B, page: 4097. / Thesis (Ph.D.)--The Florida State University, 1981.