Physics
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Item PHENOMENOLOGY OF WARPED EXTRA DIMENSIONS(2010) Zhu, Lijun; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Warped extra dimensions provide a very interesting and attractive framework that solves the hierarchy problem of the Standard Model (SM) of particle physics through the curvature along an extra dimension. In this thesis I will discuss various aspects of collider phenomenology of warped extra dimensions. First, I will discuss a class of models within this framework that are very attractive due to their naturalness, which are called warped/composite Pseudo-Goldstone Boson (PGB) Higgs models. A generic prediction of these models is the existence of extra gauge bosons (called coset gauge bosons), which give rise to distinctive signatures at the Large Hadron Collider (LHC). However, due to the large masses (beyond 3 Teraelectronvolt (TeV)) of the coset gauge bosons and their small couplings to standard model states, their discovery would be very challenging at the LHC, and an upgrade of the LHC is needed. My second topic is about the phenomenology of the Higgs boson in warped extra dimensions. In models where fermions propagate in the extra dimension, there exist heavy excitations of SM fermions, which are called the Kaluza-Klein (KK) fermions. These KK states give sizable new contributions to the production and decay channels of the Higgs boson. I will give a detailed analysis of the Higgs boson couplings to massless vector bosons (gluons and photons) in warped extra dimensions. I will show that KK fermions of all generations contribute to these couplings, leading to significant deviation from the prediction of the SM. Therefore, precision measurement of the properties of the Higgs boson can shed light on the structure of warped extra dimensions even if KK particles cannot be directly produced at the LHC due to their heavy masses.Item Transport coefficients and universality in hot strongly coupled gauge theories(2010) Cherman, Aleksey; Cohen, Thomas D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The gauge/gravity duality provides a valuable opportunity to study the behavior of relativistic fluids described by some strongly-interacting non-Abelian gauge theories. However, as yet no gravity duals are known for the field theories that are currently used to describe nature. Thus, it is particularly interesting to search for universal properties of theories with gravity duals. This dissertation discusses a broad class of theories with gravity duals, and it is shown that at high temperatures, the speed of sound squared is bounded from above by one-third of the speed of light squared. It is conjectured that this may be a universal property of theories with gravity duals. It is also shown that the temperature dependence of a number of transport coefficients takes a universal form in the high-temperature limit. In particular, in a high-temperature expansion, the power law of the leading correction away from the infinite temperature limit is universal for all of the transport coefficients, and is the same as that of the speed of sound squared.Item Flavor Physics in the Models with Warped Extra Dimension(2010) Azatov, Aleksandr; Mohapatra, Rabindra N; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I will first briefly review the Standard Model and gauge hierarchy problem. Then I will present the Randall-Sundrum model with warped extra dimension, which provides an elegant geometrical solution to the hierarchy problem. The main focus of this thesis will be an analysis of the flavor violation in the models with warped extra dimension. First I will discuss the bounds on the scale of the extra dimension arising from the low energy physics. I will show that there is a tension in the parameter space coming from different low energy observables, and I will also discuss possible ways to relax these bounds. Another interesting feature of the warped models is that they generically predict flavor violation in the Higgs sector. I will discuss low energy flavor constraints on the Higgs mediated flavor violation as well as its signatures at the collider experiments. In the last part of this thesis I will discuss the physics of radion, a scalar degree of freedom of the five dimensional gravity multiplet, and I will show why it has interactions which are generically flavor misaligned leading to the observable flavor violation. This, combined with the fact that radion is likely to be the lightest new physics degree of freedom will lead to the interesting phenomenology both from perspective of collider phenomenology and low energy observables.Item Search for Quantum Gravity with IceCube and High Energy Atmospheric Neutrinos(2010) Huelsnitz, Warren; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)IceCube is a cubic-kilometer neutrino telescope nearing completion in the South Pole Ice. Designed to detect astrophysical neutrinos from 100 GeV to about an EeV, it will contribute to the fields of high energy astrophysics, particle physics, and neutrino physics. This analysis looks at the flux of atmospheric neutrinos detected by IceCube while it operated in a partially-completed, 40-string configuration, from April 2008 to May 2009. From this data set, a sample of about 20,000 up-going atmospheric muon neutrino events with negligible background was extracted using Boosted Decision Trees. A discrete Fourier transform method was used to constrain a directional asymmetry in right ascension. Constraints on certain interaction coefficients from the Standard Model Extension were improved by three orders of magnitude, relative to prior experiments. The event sample was also used to unfold the atmospheric neutrino spectrum at its point of origin, and seasonal and systematic variations in the atmospheric muon neutrino flux were studied. A likelihood method was developed to constrain perturbations to the energy and zenith angle dependence of the atmospheric muon neutrino flux that could be due to Lorentz-violating oscillations or decoherence of neutrino flavor. Such deviations could be a signature of quantum gravity in the neutrino sector. The impact of systematic uncertainties in the neutrino flux and in the detector response on such a likelihood analysis were examined. Systematic uncertainties that need to be reduced in order to use a two-dimensional likelihood analysis to constrain phenomenological models for Lorentz or CPT violating neutrino oscillations were identified.Item Topics in Lattice QCD and Effective Field Theory(2010) Buchoff, Michael Ireland; Bedaque, Paulo F; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Quantum Chromodynamics (QCD) is the fundamental theory that governs hadronic physics. However, due to its non-perturbative nature at low-energy/long distances, QCD calculations are difficult. The only method for performing these calculations is through lattice QCD. These computationally intensive calculations approximate continuum physics with a discretized lattice in order to extract hadronic phenomena from first principles. However, as in any approximation, there are multiple systematic errors between lattice QCD calculation and actual hardronic phenomena. Developing analytic formulae describing the systematic errors due to the discrete lattice spacings is the main focus of this work. To account for these systematic effects in terms of hadronic interactions, effective field theory proves to be useful. Effective field theory (EFT) provides a formalism for categorizing low-energy effects of a high-energy fundamental theory as long as there is a significant separation in scales. An example of this is in chiral perturbation theory (χPT ), where the low-energy effects of QCD are contained in a mesonic theory whose applicability is a result of a pion mass smaller than the chiral breaking scale. In a similar way, lattice χPT accounts for the low-energy effects of lattice QCD, where a small lattice spacing acts the same way as the quark mass. In this work, the basics of this process are outlined, and multiple original calculations are presented: effective field theory for anisotropic lattices, I=2 ππ scattering for isotropic, anisotropic, and twisted mass lattices. Additionally, a combination of effective field theory and an isospin chemical potential on the lattice is proposed to extract several computationally difficult scattering parameters. Lastly, recently proposed local, chiral lattice actions are analyzed in the framework of effective field theory, which illuminates various challenges in simulating such actions.Item A STUDY OF EFFECTIVE THEORY APPROACHES TO PROBLEMS IN NUCLEAR PHYSICS(2010) Chen, Panying; Ji, Xiangdong; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effective field theory is an efficient tool in dealing with physics involving separate scales. A typical effective field theory approach involves calculating the matching conditions for the desired parameters at some cutoff scale and obtain the renormalization group equation for the effective theory. In this thesis we study a collection of effective theories including the non-relativistic QED, the chiral perturbation theory, the soft-collinear effective theory, and their applications in nuclear physics. We study the angular momentum operator in the non-relativistic QED (NRQED). We construct its gauge-invariant decomposition into spin and orbital angular momentum operators of electrons and photons and calculate the matching conditions of operators between QED and NRQED up to one loop. We perform the matching with both dimensional regularization and UV cutoff $\Lambda$. We apply the result in the Hydrogen-like system and calculate the radiation correction of the orbital angular momentum. We study the CP-violating operators in chiral perturbation theory. We apply our general, model-independent result onto the left-right symmetric model and relate the desired operator to the standard model penguin operators through $SU(3)_L$ × $SU(3)_R$ chiral symmetry. We use the lattice result for the standard model and acquire a more strict lower bound for the mass of right-hand boson in LRSM. We study the polarized gluon distribution $\Delta g(x)$ in a longitudinally polarized proton. The first result from the MIT bag model as well as the non-relativistic quark model shows that $\Delta g(x)$ is positive at all $x$. The total gluon helicity $\Delta G$ from the bag model is about $0.3\hbar$ at the scale of 1 GeV, considerably smaller than previous theoretical expectations. We study deep-inelastic scattering factorization on a nucleon in the end-point regime $x_B \sim 1-{\cal O}( \Lambda_{\rm QCD}/Q)$ with an approach in soft-collinear effective theory. Refactorization of the scale $(1-x_B)Q^2$ in the coefficient function can be made in the SCET and remains valid in the end-point regime. On the other hand, the traditional refactorization approach introduces the spurious scale $(1-x_B)Q$ in various factors, which drives them nonperturbative in the region of our interest. We show how to improve the situation by introducing a rapidity cut-off scheme, and how to recover the effective theory refactorization by choosing appropriately the cut-off parameter. Through a one-loop calculation, we demonstrate explicitly that the proper soft subtractions must be made in the collinear matrix elements to avoid double counting.Item A Search for Muon Neutrinos from Gamma-Ray Bursts wih the IceCube 22-String Detector(2009) Roth, A Philip; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two searches are conducted for muon neutrinos from Gamma-Ray Bursts (GRBs) using the IceCube detector. Gamma-Ray Bursts are brief and transient emissions of keV/MeV radiation occuring with a rate of a few per day uniformly in the sky. Swift and other satellites of the Third Interplanetary Network (IPN3) detect these GRBs and send notices out via the GRB Coordinate Network (GCN). The fireball model describing the physics of GRBs predicts the emission of muon neutrinos from these bursts. IceCube is a cubic kilometer neutrino detector buried in the deep antarctic ice at the South Pole that can be used to find these prediceted but still unobserved neutrinos. It is sensitive to them by detecting Cherenkov light from secondary muons produced when the neutrinos interact in or near the instrumented volume. The construction of IceCube has been underway since the austral summer of 2004-2005 and will continue until 2011. The growing IceCube detector will soon be sensitivite to the high energy neutrino emission from GRBs that is predicted by the fireball model. A blind and triggered search of the 22-string IceCube data for this neutrino emission was conducted. The principal background to the observation of neutrinos in IceCube is muons generated in cosmic-ray air-showers in the atmosphere above the detector. Atmospheric neutrinos make up a separate irreducible background to the detection of extraterrestrial neutrinos. A binned stacked search of 41 bursts occuring in the northern hemisphere greatly reduces the muon background by looking for tracks moving up through the detector. The atmospheric neutrino background is greatly reduced by the temporal constraints of the search, making it effectively background free. 40 individual unbinned searches of bursts occuring in the southern hemisphere extend IceCube's sensitivity to the higher background regions above the horizon. No significant excesses over background expectations are found in either search. A 90% confidence upper limit on the neutrino fluence from northern hemisphere bursts is set at 6.52 x 10-3 erg cm-2 with 90% of the expected signal between 87.9 TeV and 10.4 PeV.Item Measurement of the W Boson Mass and Width Using a Novel Recoil Model(2009) Wetstein, Matthew Joseph; Eno, Sarah C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a direct measurement of the W boson mass (M_W) and decay width (&Gamma_W) in 1 fb^(-1) of W &rarr e &nu$ collider data at D0 using a novel method to model the hadronic recoil. The mass is extracted from fits to the transverse mass M_T, p_T(e), and MET distributions. The width is extracted from fits to the tail of the M_T distribution. The electron energy measurement is simulated using a parameterized model, and the recoil is modeled using a new technique by which Z recoils are chosen from a data library to match the p_T and direction of each generated W boson. We measure the the W boson mass to be M_W = 80.4035 ± 0.024 (stat) ± 0.039 (syst) from the M_T, M_W = 80.4165 ± 0.027 (stat) ± 0.038 (syst) from the p_T(e), and M_W = 80.4025 ± 0.023 (stat) ± 0.043 (syst) from the MET distributions. &Gamma_W is measured to be &Gamma_W=2.025 ± 0.038 (stat) ± 0.061 (syst)$ GeV.Item The Search for Neutralino Dark Matter with the AMANDA Neutrino Telescope(2008) Ehrlich, Ralf; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole. Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton. AMANDA - the Antarctic Muon and Neutrino Detector Array - makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected. If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation.Item Radiative B Meson Decay as a Probe of Physics Beyond the Standard Model: Time-Dependent CP Violation in B0 → KS π0 γ and the B → φ K γ Branching Fraction(2008) Tuggle, Joseph M.; Jawahery, Abolhassan; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present measurements of radiative B meson decays to the final states KS π0 γ and K φ γ based on data collected at the Υ(4S) resonance with the BaBar detector at the PEP-II e+e– collider at SLAC. In a data sample of 467 million BBbar pairs, the time-dependent CP asymmetry in B0 → KS π0 γ decays is measured in two regions of KS-π0 invariant mass. In the K* region, 0.8 < m(KS π0) < 1.0 GeV/c2, we find SK* γ = –0.03 ± 0.29 ± 0.03 and CK* γ = –0.14 ± 0.16 ± 0.03; in the range 1.1 < m(KS π0) < 1.8 GeV/c2, we find SKS π0 γ = –0.78 ± 0.59 ± 0.09 and CKS π0 γ = –0.36 ± 0.33 ± 0.04. With a sample of 228 million BBbar pairs we measure the branching fraction B(B+ → K+ φ γ) = (3.5 ± 0.6 ± 0.4) × 10–6 and set the limit B(B0 → K0 φ γ) < 2.7 × 10–6 at 90% confidence level. The direct CP asymmetry in B+ → K+ φ γ is found to be ACP = (–26 ± 14 ± 5)%. In each case the uncertainties are statistical and systematic, respectively.