Dissertationen zum Thema „Physics, Astrophysics|Physics, General|Physics, Elementary Particles and High Energy“

The Large Underground Xenon (LUX) experiment has recently placed the most stringent limit for the spin-independent WIMP-nucleon scattering cross-section. The WIMP search limit was aided by an internal tritium source resulting in an unprecedented calibration and understanding of the electronic recoil background. Here we discuss corrections to the signals in LUX, the energy scale calibration and present the methodology for extracting fundamental properties of electron recoils in liquid xenon. The tritium calibration is used to measure the ionization and scintillation yield of xenon down to 1 keV, the results is compared to other experiments. Recombination probability and its fluctuation is measured from 1 to 1000 keV, using betas from tritium and Compton scatters from an external ¹³⁷Cs source. Finally, the tritium source is described and the most recent results for ER discrimination in LUX is presented.

It is the goal of this dissertation to demonstrate that beyond the standard model, certain theories exist which solve conflicts between observation and theory -- conflicts such as massive neutrinos, dark matter, unstable Higgs vacuum, and recent Planck observations of excess relativistic degrees of freedom in the early universe. Theories explored include a D-brane inspired construct of U(3) × Sp(1) × U(1) × U(1) extension of the standard model, in which we demonstrate several possible observables that may be detected at the LHC, and an ability to stabilize the Higgs mechanism. The extended model can also explain recent Planck data which, when added to HST data gives an excess of relativistic degrees of freedom of Δ N = 0.574 ± 0.25 above the standard result. Also explored is a possible non-thermal dark matter model for explanation of this result. Recent observations of Fermi bubble results indicate a signal of a 50 GeV dark matter particle annihilating into b b-bar, with a thermally averaged annihilation cross section corresponding to <σ v> = 8 × 10

(-27) cm

3/s, spurs interestin a Higgs portal model suggested by Steven Weinberg. Other implications of this model are also explored such as its ability to explain dark matter direct detection results along with LHC Higgs data, and Planck data. Particle physics is complimented by possible stochastic gravitational wave searches for which a model of second order global phase transitions is explored. These transitions generate gravitational wave spectra with amplitudes of order Ω(gw) h

2 = 10

(-24) - 10

(-15). Furthermore, techniques into such calculationsare investigated in hopes to improve the stability required in such lattice simulations.

Quantum chromodynamics (QCD) is the theory that concerns how the strong force interacts with subatomic particles. Topological configurations that interpolate between vacuum states have been shown to play an important role in the quark-gluon plasma (QGP), believed to be created in heavy ion collisions. The possible existence of [special characters omitted]-odd domains in the QGP combined with an external magnetic field that is produced in mid-central collisions may be the ingredients necessary for the so-called chiral magnetic effect (CME). The CME is the consequence of topological features called sphalerons (that are created in the hot QCD matter) in the presence of a strong magnetic field, and it induces a separation of negatively and positively charged particles along the direction of the field. This separation varies its orientation from event to event, resulting in the expectation value of any [special characters omitted]-odd observable to vanish, making it necessary to measure the variation in fluctuations. Any indication of a real charge dipole moment could be evidence for local parity violation (LPV), which would have profound implications on our understanding of the natural world.

In this dissertation, charge dependent azimuthal correlations are used to measure the charge separation fluctuations in gold ion collisions at STAR. There are three primary analyses: measuring charge distributions as a function of beam energy, by selecting specific hadron species to filter background effects, and for uranium ion collisions. The beam energy analysis shows that a small charge separation shrinks with diminishing beam energy, eventually vanishing at the lowest energies. The kaon-pion correlations are performed to eliminate specific background effects unrelated to the CME, and behave consistently with results using all types of hadrons. The uranium analysis attempts to distinguish how much of the azimuthal correlations are influenced by elliptic anisotropy, suggesting the signal is coming from a mixture of CME and strong interaction backgrounds. From the evidence gathered from these analyses, we conclude that there are signs of small charge separations congruous to predictions from the CME, however, much of the signal is obscured by other strong interaction backgrounds. The effective contribution strengths are calculated and suggestions for improvements are made in the conclusion.

Beauty quarks are pair-produced by strong interactions in multi-TeV proton-proton (pp) collisions at the CERN Large Hadron Collider (LHC). Such interactions allow for a test of perturbative Quantum Chromodynamics (QCD) in a new energy regime. The primary beauty-antibeauty quark b b pair production mechanisms in perturbative QCD are referred to as flavor creation, flavor excitation, and gluon splitting. These three mechanisms produce bb pairs with characteristic kinematic behavior, which contribute differently to the shape of the differential b b production cross section with respect to the difference in the azimuthal angle Δ&phis; and the combined separation variable Δ R = [special characters omitted] between the beauty and antibeauty quarks (b and b, respectively); with Δη being the change in the pseudorapidity η = — ln (tan (&thetas;/2)), &thetas; being the polar angle. These Δ&phis; and Δ^R variables are collectively referred to as angular correlation variables and hence forth referred to as Δ A. By measuring the shape and absolute normalization of the differential production cross section distributions with respect to ΔA a test of the predictions of perturbative QCD can be performed.

This dissertation describes a measurement of the differential production cross sections with respect to the ΔA between two hadronic jets arising from the hadronization and decay of b or b (referred to as b hence forth) produced in pp collisions at the LHC observed with the Compact Muon Solenoid (CMS) detector. Hadronic jets are identified as originating from b quarks, i.e. b-tagged, based on the presence of high impact parameter tracks with respect to the primary pp interaction point in events in which a muon is also produced. The study presented in this dissertation corresponds to an integrated luminosity of 3 pb ^-1 collected in 2010 by the CMS experiment at a center-of-mass energy of 7 TeV.

The visible kinematic phase-space of the differential production cross sections probed in this study is given by the requirement of two b-tagged hadronic jets with [special characters omitted] > 30 GeV and ∥η^jet∥ < 2.4, with an angular separation of ΔR > 0.6 between them, one of these jets has a muon within its constituents with [special characters omitted] > 8 GeV and ∥η^μ∥ < 2.1. The results obtained in data are compared with predictions based on perturbative QCD calculations given by CASCADE, MADGRAPH/MADEVENT, and PYTHIA Monte Carlo event generators. The predictions of perturbative QCD are found to be in agreement the measured differential cross sections within uncertainties.

Coherent electron cooling (CeC) offers a potential new method of cooling hadron beams in colliders such as the Relativistic Heavy Ion Collider (RHIC) or the future electron ion collider eRHIC. A 22 MeV linear accelerator is currently being built as part of a proof of principle experiment for CeC at Brookhaven National Laboratory (BNL). In this thesis we present a simulation of electron beam dynamics including space charge in the 22 MeV CeC proof of principle experiment using the program ASTRA (A Space charge TRacking Algorithm).

The electric (G_E) and magnetic ( G_M) form factors of the proton are fundamental observables which characterize its charge and magnetization distributions. There are two methods to measure the proton form factors: the Rosenbluth separation method and the polarization transfer technique. However, the ratio of the electric and magnetic form factors measured by those methods significantly disagree at momentum transfer Q² > 1 GeV². The most likely explanation of this discrepancy is the inclusion of two-photon exchange (TPE) amplitude contributions to the elastic electron-proton cross section which significantly changes the extraction of G_E from the Rosenbluth separation measurement. The Jefferson Lab CLAS TPE experiment determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections. The primary electron beam was used to create an intense bremsstrahlung photon beam. Some of the photons were then converted to a mixed e⁺/ e^- beam which then interacted with a liquid hydrogen target. The e⁺p and e^-p events were detected by the CLAS (CEBAF Large Acceptance Spectrometer). The elastic cross section ratios ((σ( e⁺p)/(σ(e ^-p)) were measured over a wide range of virtual photon polarization ϵ and Q². The cross section ratios displayed a strong ϵ dependence at Q² = 1.45 GeV². There is no significant Q² dependence observed at ϵ = 0.45. The results are consistent with a recent measurement at the VEPP-3 lepton storage ring in Novosibirsk and with the hadronic calculation by Blunders, Melnitchouk and Tjon. The hadronic calculation resolves the disagreement between the Rosenbluth separation and polarization transfer extractions of G_E/G_M at Q² up to 2 – 3 GeV². Applying the GLAS TPE correction to the Rosenbluth cross section measurements significantly decreases the extracted value of G_E and brings it into good agreement with the polarization transfer measurement at Q²∼1.75 GeV². Thus, these measurements appear to resolve the proton electric form factor discrepancy for Q² < 2 GeV².

Recent results at RHIC seem to confirm T.D.Lee's hypothesis that a new form of matter, the quark-gluon plasma (QGP), could be formed in heavy-ion collisions at high energies. Heavy quarks, being formed in the early stages of heavy-ion collisions, form a good probe for the properties of the QGP. The energy loss of heavy quarks as they traverse the medium is predicted to be less than that of the lighter quarks. However; previous measurements of the nuclear modification factor at RIHC indicate that the energy loss of heavy and light quarks is comparable. Thus measurements of the in-medium energy loss of heavy-quarks are of particular interest. In this thesis, a measurement of the differential production cross-section of electrons from the semi-leptonic decay of heavy-flavor quarks in [special characters omitted] = 2.76 TeV pp is presented. This provides a stringent test of perturbative QCD in a new energy regime, and forms a crucial baseline for Pb-Pb collisions where the in-medium energy loss mechanism can be studied.

Experimental high energy physics (HEP) techniques are applied to accurate simulated collider data in search for existence or exclusion of supersymmetric (SUSY) particles. Supersymmetry is a leading candidate to resolve the hierarchy problem in particle physics as well as offer a stable dark matter candidate. Techniques and practices are explored and applied to the leptonic decay process production followed by and where is the proton, is the chargino, , are neutralinos and , are the standard model W and Higgs Bosons respectively. Signal yields are in general agreement with other researchers and ranged from 0.5 to 62.6 events. Reduction in the background to signal ratio is demonstrated through isolating the SUSY process and applying theoretical knowledge of the signal and associated dominant backgrounds. Results from this study establish procedures for future work with actual data, offer a benchmark for this specific leptonic decay process and may motivate variable selection and cut criteria choices in future analysis of similar signal processes.

The main motivation of the Compact Muon Solenoid (CMS) experiment is to explore and to discover physics underlying electro-weak asymmetry breaking. Beside this, CMS detector provides an opportunity to do various experiments for detecting new physics signatures beyond the Standard Model (SM). Investigation of these signatures requires the identification and precise energy and moment measurement of electrons, muons, photons, and jets. The objective of this thesis is the calculation of the efficiencies for the measurement of W charge asymmetry in inclusive pp → W (&mgr;υ) production. The charge asymmetry is defined to be the difference between W ⁺ and W⁻ bosons, normalized to the sum. This asymmetry is sensitive to the u-quark and d-quark ratios in the proton and precise measurement of the W charge asymmetry can provides new insights to the proton structure functions. Therefore, to improve understanding of SM backgrounds in search for new physics, the moun trigger, isolation, reconstruction, identification efficiencies has been studied using partial data collected by the CMS detector during pp collisions at the LHC in 2011. The dataset corresponds to an integrated luminosity of 2.31 fb⁻¹. The efficiencies are measured as functions of the decay muon pseudo rapidity and transverse momentum based on "tag and probe" method. The efficiency measurements are compared to their estimated value from the Monte Carlo simulations so as to provide scaling factors to correct to the residual mis-modeling of the CMS muon performance. The comparison with simulations based on MC simulations opens a gate for validation of the detector simulation and optimization of selection strategies.