117248378
Accidental Supersymmetry and the Naturalness of Codimension-2 Branes
This thesis addresses two separate naturalness issues which generically come to bear on physical theories with large extra dimensions, and so a gravity scale much lower than the Planck scale. The first is related to the observed stability of the proton, wherein we determine the relevant constraints on an additional gauge boson which conserves baryon number. Although several such proposals have been previously considered, our analysis is distinctive in its interest in lighter gauge boson masses (which naturally arise in such models), and in its focus on the dependence of constraints due to kinetic mixing effects. The second is related to the main purpose of large extra dimensions—namely, to address the smallness of the observed vacuum energy—wherein we compute the leading-order quantum corrections to the four-dimensional (4D) vacuum energy resulting from loops of extradimensional fields. We compute the contributions from bulk scalars (spin 0), fermions (spin 12 ), and gauge fields (spin 1) in a flux-stabilized, spheroidal extra-dimensional geometry whose rugbyball shape is due to two codimension-2 branes—one at each pole. (We also obtain the corresponding beta functions for both bulk and brane operators.) These results are then combined to obtain the net contribution from various multiplets in the context of a particular supersymmetric extra-dimensional model that has been shown to give a vanishing result for the 4D vacuum energy at the classical level. Surprisingly, we find that supersymmetry can be preserved dynamically at one loop in the case of identical branes, without arranging any particular relationship between the brane parameters. Perturbing away from the case of identical branes is shown to give a positive 1-loop contribution to the 4D vacuum energy whose size is set by the radius of the extra dimensions.
119159946
Inflating with Large Effective Fields
We re-examine large scalar fields within effective field theory, in particular focussing on the issues raised by their use in inflationary models (as suggested by BICEP2 to obtain primordial tensor modes). We argue that when the large-field and low-energy regimes coincide the scalar dynamics is most effectively described in terms of an asymptotic large-field expansion whose form can be dictated by approximate symmetries, which also help control the size of quantum corrections. We discuss several possible symmetries that can achieve this, including pseudo-Goldstone inflatons characterized by a coset G/H (based on abelian and non-abelian, compact and non-compact symmetries), as well as symmetries that are intrinsically higher dimensional. Besides the usual trigonometric potentials of Natural Inflation we also find in this way simple large-field power laws (like V ∝ φ{sup 2}) and exponential potentials, V(φ) = ∑{sub k}V{sub x}e{sup −kφ/M}. Both of these can describe the data well and give slow-roll inflation for large fields without the need for a precise balancing of terms in the potential. The exponential potentials achieve large r through the limit |η| || e and so predict r ≅ (8/3)(1-n{sub s}); consequently n{sub s} ≅ 0.96 gives r ≅ 0.11 but not much larger (and so could be ruled out as measurements on r and n{sub s} improve).more » We examine the naturalness issues for these models and give simple examples where symmetries protect these forms, using both pseudo-Goldstone inflatons (with non-abelian non-compact shift symmetries following familiar techniques from chiral perturbation theory) and extra-dimensional models.« less
53692432
Distributed SUSY breaking: dark energy, Newton’s law and the LHC
A bstractWe identify the underlying symmetry mechanism that suppresses the low-energy effective 4D cosmological constant within some 6D supergravity models, generically leading to results suppressed by powers of the KK scale, mK K2, relative to the much larger size, m4, associated with mass-m particles localized in these models on codimension-2 branes. These models are examples for which the local conditions for unbroken supersymmetry can be satisfied locally everywhere within the extra dimensions, but are obstructed only by global conditions like flux quantization or by the mutual inconsistency of the boundary conditions required at the various branes. Consequently quantities (like vacuum energies) forbidden by supersymmetry cannot become nonzero until wavelengths of order the KK scale are integrated out, since only such long wavelength modes can see the entire space and so ‘know’ that supersymmetry has broken. We verify these arguments by extending earlier rugby-ball calculations of one-loop vacuum energies within these models to more general pairs of branes within two warped extra dimensions. For the Standard Model confined to one of two otherwise identical branes, the predicted effective 4D vacuum energy density is of order ρvac ⋍ C(mMg/4πMp)4 = C(5.6 × 10−5 eV)4, where Mg ≳ 10 TeV (corresponding to extra-dimensional size r ≲ 1 μm) and Mp = 2.44 × 1018 GeV are the 6D and 4D rationalized Planck scales, and m is the heaviest brane-localized particle. (For numerical purposes we take m to be the top-quark mass and take Mg as small as possible, consistent with energy-loss bounds from supernovae.) C is a constant depending on the details of the bulk spectrum, which could easily be of order 500 for each of hundreds of fields in the bulk. The value C ∼ 6 × 106 would give the observed Dark Energy density.
53667729
Accidental SUSY: enhanced bulk supersymmetry from brane back-reaction
A bstractWe compute how bulk loops renormalize both bulk and brane effective interactions for codimension-two branes in 6D gauged chiral supergravity, as functions of the brane tension and brane-localized flux. We do so by explicitly integrating out hyper- and gauge-multiplets in 6D gauged chiral supergravity compactified to 4D on a flux-stabilized 2D rugby-ball geometry, specializing the results of a companion paper, arXiv:1210.3753, to the supersymmetric case. While the brane back-reaction generically breaks supersymmetry, we show that the bulk supersymmetry can be preserved if the amount of brane- localized flux is related in a specific BPS-like way to the brane tension, and verify that the loop corrections to the brane curvature vanish in this special case. In these systems it is the brane-bulk couplings that fix the size of the extra dimensions, and we show that in some circumstances the bulk geometry dynamically adjusts to ensure the supersymmetric BPS-like condition is automatically satisfied. We investigate the robustness of this residual supersymmetry to loops of non-supersymmetric matter on the branes, and show that supersymmetry-breaking effects can enter only through effective brane-bulk interactions involving at least two derivatives. We comment on the relevance of this calculation to proposed applications of codimension-two 6D models to solutions of the hierarchy and cosmological constant problems.
118724352
Gravitational forces on a codimension-2 brane
A bstractWe compute the gravitational response of six dimensional gauged, chiral supergravity to localized stress energy on one of two space-filling branes, including the effects of compactifying the extra dimensions and brane back-reaction. We find a broad class of exact solutions, including various black-brane solutions. Several approximate solutions are also described, such as the near-horizon geometry of a small black hole which is argued to be approximately described by a 6D Schwarzschild (or Kerr) black hole, with event horizon appropriately modified to encode the brane back-reaction. The general linearized far-field solutions are found in the 4D regime very far from the source, and all integration constants are related to physical quantities describing the branes and the localized energy source. The localized source determines two of these, corresponding to the source mass and the size of the strength of a coupling to a 4D scalar mode whose mass is parametrically smaller than the KK scale. At large distances the solutions agree with those of 4D general relativity, but for an intermediate range of distances (larger than the KK scale) the solutions better fit a Brans-Dicke theory. For a realistic choice of parameters the KK scale could lie at a micron, while the crossover to Brans-Dicke behaviour could occur at around 10 microns. While allowed by present data this points to potentially measurable changes to Newton’s Law arising at distances larger than the KK scale.
118339212
Technically natural vacuum energy at the tip of a supersymmetric teardrop
A minimal supersymmetric brane-world model is presented which has: i) zero classical four-dimensional vacuum curvature, despite the large naive vacuum energy due to contributions from Standard Model particles; ii) one-(bulk)-loop quantum corrections to the vacuum energy with a size set by the radius of the extra-dimensional spheroid. These corrections are technically natural because a BPS-like relation between the brane tension and R-charge---which would have preserved (half of) the bulk supersymmetry---is violated by the requirement that the stabilizing R-symmetry gauge flux be quantized. The extra-dimensional geometry is similar to previous rugby-ball geometries, but is simpler in that there is only one brane and so fewer free parameters. Although the sign of the renormalized vacuum energy ends up being the unphysical one for this model (in the limit considered here, where the massive bulk loop is the leading contribution), it serves as an illustrative example of the relevant physics.
118294173
A measurement of the inclusive branching ratio for $b o sgamma$ with the ALEPH detector at LEP
110521797
Ganga: User-friendly Grid job submission and management tool for LHC and beyond
Ganga has been widely used for several years in ATLAS, LHCb and a handful of other communities. Ganga provides a simple yet powerful interface for submitting and managing jobs to a variety of computing backends. The tool helps users configuring applications and keeping track of their work. With the major release of version 5 in summer 2008, Ganga's main user-friendly features have been strengthened. Examples include a new configuration interface, enhanced support for job collections, bulk operations and easier access to subjobs. In addition to the traditional batch and Grid backends such as Condor, LSF, PBS, gLite/EDG a point-to-point job execution via ssh on remote machines is now supported. Ganga is used as an interactive job submission interface for end-users, and also as a job submission component for higher-level tools. For example GangaRobot is used to perform automated, end-to-end testing of distributed data analysis. Ganga comes with an extensive test suite covering more than 350 test cases. The development model involves all active developers in the release management shifts which is an important and novel approach for the distributed software collaborations. Ganga 5 is a mature, stable and widely-used tool with long-term support from the HEP community.
56334536
uBoost: a boosting method for producing uniform selection efficiencies from multivariate classifiers
The use of multivariate classifiers, especially neural networks and decision trees, has become commonplace in particle physics. Typically, a series of classifiers is trained rather than just one to enhance the performance; this is known as boosting. This paper presents a novel method of boosting that produces a uniform selection efficiency in a selected multivariate space. Such a technique is well suited for amplitude analyses or other situations where optimizing a single integrated figure of merit is not what is desired.
121580852
New results in ω photoproduction using high-statistics data obtained from CLAS at Jefferson Lab
High-statistics differential cross sections and spin density matrix elements for the reaction γ p → p ω have been measured using the CLAS at Jefferson Lab for center-of-mass energies, W , from threshold up to 2.84 GeV. An event-based partial wave analysis (PWA) has also been performed on the data. This analysis benefits from access to the world's first high precision spin density matrix element measurements, available to the event-based PWA through the decay distribution of ω → π + π − π 0 . Strong evidence has been found for nucleon resonance contributions.
18677949
Separating Signals from Non-Interfering Backgrounds using Probabilistic Event Weightings
A common situation in experimental physics is to have a signal which can not be separated from a non-interfering background through the use of any cut. In this paper, we describe a procedure for determining, on an event-by-event basis, a quality factor (Q-factor) that a given event originated from the signal distribution. This Q-factor can then be used as an event weight in subsequent analysis procedures, allowing one to more directly access the true spectrum of the signal.