The ‘Higgs’ amplitude mode at the two-dimensional superfluid/Mott insulator transition
Spontaneous symmetry breaking plays a key role in our understanding of nature. In relativistic quantum field theory, a broken continuous symmetry leads to the emergence of two types of fundamental excitation: massless Nambu–Goldstone modes and a massive ‘Higgs’ amplitude mode. An excitation of Higgs type is of crucial importance in the standard model of elementary particle physics, and also appears as a fundamental collective mode in quantum many-body systems. Whether such a mode exists in low-dimensional systems as a resonance-like feature, or whether it becomes overdamped through coupling to Nambu–Goldstone modes, has been a subject of debate. Here we experimentally find and study a Higgs mode in a two-dimensional neutral superfluid close to a quantum phase transition to a Mott insulating phase. We unambiguously identify the mode by observing the expected reduction in frequency of the onset of spectral response when approaching the transition point. In this regime, our system is described by an effective relativistic field theory with a two-component quantum field, which constitutes a minimal model for spontaneous breaking of a continuous symmetry. Additionally, all microscopic parameters of our system are known from first principles and the resolution of our measurement allows us to detect excited states of the many-body system at the level of individual quasiparticles. This allows for an in-depth study of Higgs excitations that also addresses the consequences of the reduced dimensionality and confinement of the system. Our work constitutes a step towards exploring emergent relativistic models with ultracold atomic gases.

title: De Sitter vacua and inflation in no-scale string models
abstract: This thesis studies the question of how de Sitter vacua and slow-roll inflation may be realized in string-motivated models. More specifically, we consider 4d N = 1 supergravity theories (without vector multiplets) with Kahler potentials which are ‘no-scale’ at leading order. Such theories frequently arise in the moduli sector of string compactifications. We discuss a condition on the scalar geometry (defined by the Kahler potential) and on the direction of supersymmetry breaking in the scalar manifold, which has to be met in order for the average of the masses of the sGoldstinos to be positive, and hence for metastable vacua to be possible. This condition also turns out to be necessary for the existence of trajectories admitting slow-roll inflation. Its implications for certain scalar manifolds which arise from Calabi-Yau string compactifications are discussed. In particular, for two-moduli models arising from compactifications of heteroticand type IIB string theory, a simple criterion on the intersection numbers needs to be satisfied for possible de Sitter phases to exist. In addition, we show that subleading corrections breaking the no-scale property may allow the condition on the scalar geometry to be fulfilled, even when it is violated at leading order. Finally, we develop a procedure to construct superpotentials for a given viable Kahler potential, such that the scalar potential has a realistic local minimum. We propose two-moduli models, with superpotentials which could arise from flux backgrounds and non-perturbative effects, which have a viable vacuum without employing subleading corrections or an uplifting sector.

title: A realistic unified gauge coupling from the micro-landscape of orbifold GUTs
abstract: We consider 5-dimensional supersymmetric field theories where supersymmetry is broken by the Scherk–Schwarz mechanism (or, equivalently, by the F-term VEV of the radion). In such models, the radion effective potential is calculable in terms of the 5d gauge coupling, the UV cutoff of the 5d field theory, and the field content. We provide simple, explicit formulae for the leading part of the two-loop effective potential. Our analysis applies in particular to 5d orbifold GUTs motivated by heterotic orbifold models. We focus on potentially realistic models of this type and make the additional assumption that the UV cutoff scale is identical with the strong-coupling scale of the 5d gauge theory. Given our stabilization mechanism, the 5d radius is now fixed in terms of the 5d gauge coupling and the field content of the model. This implies a prediction for the effective 4d gauge coupling only in terms of the field content of the model. Given the ‘micro-landscape’ provided by the different possible distributions of Standard Model fields between bulk and branes, we find a subset of models with a realistic unified gauge coupling. We also discuss two possibilities for the ‘uplifting’ of our SUSY-breaking AdS vacua: One is based on the possible presence of a weak warping, the other appeals to F-terms in an extra brane-localized SUSY-breaking sector.

title: De Sitter vacua in no-scale supergravities and Calabi-Yau string models
abstract: We perform a general analysis on the possibility of obtaining metastable vacua with spontaneously broken N = 1 supersymmetry and non-negative cosmological constant in the moduli sector of string models. More specifically, we study the condition under which the scalar partners of the Goldstino are non-tachyonic, which depends only on the Kahler potential. This condition is not only necessary but also sufficient, in the sense that all of the other scalar fields can be given arbitrarily large positive square masses if the superpotential is suitably tuned. We consider both heterotic and orientifold string compactifications in the large-volume limit and show that the no-scale property shared by these models severely restricts the allowed values for the 'sGoldstino' masses in the superpotential parameter space. We find that a positive mass term may be achieved only for certain types of compactifications and specific Goldstino directions. Additionally, we show how subleading corrections to the Kahler potential which break the no-scale property may allow to lift these masses.

title: Constructing de Sitter vacua in no-scale string models without uplifting
abstract: We develop a method for constructing metastable de Sitter vacua in N = 1 supergravity models describing the no-scale volume moduli sector of Calabi-Yau string compactifications. We consider both heterotic and orientifold models. Our main guideline is the necessary condition for the existence of metastable vacua coming from the Goldstino multiplet, which constrains the allowed scalar geometries and supersymmetry-breaking directions. In the simplest non-trivial case where the volume is controlled by two moduli, this condition simplifies and turns out to be fully characterised by the intersection numbers of the Calabi-Yau manifold. We analyse this case in detail and show that once the metastability condition is satisfied it is possible to reconstruct in a systematic way the local form of the superpotential that is needed to stabilise all the fields. We apply then this procedure to construct some examples of models where the superpotential takes a realistic form allowed by flux backgrounds and gaugino condensation effects, for which a viable vacuum arises without the need of invoking corrections to the Kahler potential breaking the no-scale property or uplifting terms. We finally discuss the prospects of constructing potentially realistic models along these lines.

title: Hybrid anomaly-gravity mediation with flavor
abstract: We study models with contributions of similar size to the soft masses from anomaly-and gravity mediation, thereby curing the tachyonic slepton problem of anomaly mediation. A possible origin of this hybrid setup in a 5-dimensional brane world is briefly discussed. The absence of excessive flavor violation is explained by alignment. The gravitino can be heavy enough so that the gravitino problem of supersymmetric theories with leptogenesis is avoided. The model has a characteristic signature: It predicts the distinctive gaugino mass pattern of anomaly mediation and, at the same time, (1) slepton mass splittings.

title: Orbital dependent nucleonic pairing in the lightest known isotopes of tin.
abstract: By studying the (109)Xe→(105)Te→(101)Sn superallowed α-decay chain, we observe low-lying states in (101)Sn, the one-neutron system outside doubly magic (100)Sn. We find that the spins of the ground state (J=7/2) and first excited state (J=5/2) in (101)Sn are reversed with respect to the traditional level ordering postulated for (103)Sn and the heavier tin isotopes. Through simple arguments and state-of-the-art shell-model calculations we explain this unexpected switch in terms of a transition from the single-particle regime to the collective mode in which orbital-dependent pairing correlations dominate.

title: Measurement of g factors of excited states in radioactive beams by the transient field technique : 132Te
abstract: The g factor of the 2 1 + state in 132 52 Te, E ( 2 1 + ) = 0.9739 MeV , τ = 2.6 ps , was measured by the transient field technique applied to a radioactive beam. The development of an experimental approach necessary for work in radioactive beam environments is described. The result g = 0.28 ( 15 ) agrees with the previous measurement by the recoil-in-vacuum technique, but here the sign of the g factor is measured as well.

title: Test of calculations with single-particle density dependent pairing inTe132
abstract: R. O. Hughes, N. V. Zamfir, R. F. Casten, D. C. Radford, C. J. Barton, C. Baktash, M. A. Caprio, A. Galindo-Uribarri, C. J. Gross, P. A. Hausladen, E. A. McCutchan,   USA Department of Physics, University of Surrey, Guildford GU 2 7XH, United Kingdom Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA University of York, Heslington YO10 5DD, United Kingdom Center for Theoretical Physics, Sloane Physics Laboratory, Yale University, New Haven, , USA (Received 29 January 2004;  May 2004)

title: Lifetime Measurements of Normally Deformed and Superdeformed States in 82Sr
abstract: Lifetimes of a superdeformed band in Sr-82 were measured with the centroid shift method. The measured average quadrupole moment of this band corresponds to a quadrupole deformation of beta(2) approximate to 0.49, which is slightly smaller than both the theoretical prediction, and the measured deformation of the SD band in the neighboring isotone Zr-84. Lifetimes of high spin states of three normally deformed rotational bands in Sr-82 were also measured with the Doppler shift attenuation method technique. The quadrupole moments of these normally deformed bands show a decrease at the highest spins, supporting the predicted band terminations.

title: Dynamic polarization in the Coulomb breakup of loosely bound 17F
abstract: Angular distributions of the Coulomb breakup of radioactive 17 F were measured by impinging a 10 MeV/nucleon beam on 208 Pb and on 58 Ni to study the dynamic polarization effects. The breakup products, oxygen and a proton, were detected in coincidence. First-order perturbation theory significantly overpredicts the breakup cross section for the 208 Pb target. Dynamical calculations with a dynamic polarization as the leading order correction were performed. The calculations reproduce the data for 17 F on 58 Ni but overpredict the breakup of 17 F on 208 Pb by a factor of two at forward angles.