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Complete one-loop results for the decay widths of neutral Higgs bosons ( h(_a)) into lighter neutral Higgs bosons ( h(_b), h(_c). ) arc presented for the MSSM with complex parameters. The results are obtained in the Feynman-diagrammatic approach, taking into account the full dependence on the spectrum of supersymmetric particles and all complex phases of the supersymmetric parameters. The genuine triple-Higgs vertex contributions are supplemented with two-loop propagator-type corrections, yielding the currently most precise prediction for this class of processes. The genuine vertex corrections turn out to be very important, yielding a large increase of the decay width compared to a prediction based on the tree-level vertex. One-loop propagator-type mixing between neutral Higgs bosons and Goldstone and Z bosons is also consistently taken into account. Complete one-loop results for the decay of a neutral Higgs boson into fermions are also presented, which include the full dependence on complex phases. The new results are used to analyse the impact of the experimental limits from the LEP Higgs searches on the parameter space with a very light MSSM Higgs boson. It is found that a significant part of the parameter space of the CPX benchmark scenario exists where channels involving the decay h(_2)→ h(_1) h(_1) have the highest search sensitivity, and the existence of an unexcluded region with M(_h1) ~ 45 GeV is confirmed. The public code Higgs Bounds is also presented, which can be used in conjunction with models with an arbitrary number of neutral Higgs bosons to determine whether parameter points have been excluded at the 95% CL by the LEP and Tevatron Higgs searches.

Correlations in pairs of neutral strange particles have been studied with the ZEDS detector at HERA, using 320 pb-1 of integrated luminosity from the HERA II period of running (2004-2007). The distributions in Q = JM;air - 4M~o and r.",p = JΔn2 + Δǿ2 were found for pairs of ΛΛ¯, ΛΛ+Λ¯Λ¯, ΛKs+ΛKs and KsKs particles as well as for the ratios ΛΛ/ΛΛ¯, ΛΛ/ KsKs and ΛΛ/ KsKs. The distributions were compared to a mixture of ARIADNE deep inelastic scattering (DIS) Monte Carlo and PYTHIA photoproduction MC and found to be in agreement. The ratios ΛΛ/ KsKs and ΛΛ/ KsKs were compared to the measured baryon-to-meson ratio for single particles. The ΛΛ/ KsKs ratio was found to be compatible with the product of single ratios which would apply if the particles were produced independently. The ΛΛ/ KsKs ratio was found to be larger than the value predicted from the single ratios. The Q and rήǿ distributions were also obtained for DIS and PHP enriched samples of ΛKs and KsKs pairs. The DIS distributions were found to agree with ARIADNE MC. The Q distributions were found to disagree with the shape of PYTHIA MC distributions, however the rήǿ distributions agreed with the MC. The rήǿ distributions were compared to the distance in 'r/<P space between pairs of jets. The ΛΛ rήǿ distribution was consistent with the A and A being produced in the same jet, whereas the ΛΛ, ΛKs and KsKs distributions were found to peak at rήǿ distances compatible with these VO particles being preferentially produced in separate jets. In addition the fraction of ΛΛ¯ and ΛΛ pairs in a S = 1 spin state (έ( Q)) was obtained. Both the distri- butions were compatible with a statistical S = 1 fraction, έ = 0.75. The ΛΛ έ(Q) distribution was not seen to decrease at small values of Q, as was suggested by measurements at LEP. The distribution for ΛΛ pairs was fitted to the Goldhaber parameterization to find the hadron emitter dimension of the pairs. The error on the fitted measurement was too large for conclusions to be drawn due to a mismatch between the shape of the parameterization and the observed distribu- tion. The έ( Q) distribution was also obtained for the ratio ΛΛ/ΛΛ. The ratio was similarly found to have a spin fraction consistent with the statistical έ = 0.75 value.

In this thesis, we discuss many-particle systems on general compact quantum graphs. The results cover systems of distinguishable particles as well as systems of bosons or fermions. The main focus lies on the introduction of many-particle interactions in order to establish a useful model regarding many-particle quantum chaos 811d onc-dimensional Bose-Einstein condensation (BEC). Using suitable quadratic forms, we will characterise self-adjoint realisations of the two- and many-particle Laplacian which incorporate two different types of interactions, i.e. singular interactions localised at the vertices of the graph and contact interactions which are also present along the edges. In that context, we will establish regularity results in order to characteristic the domains of the self-adjoint realisations explicitly. We will also discuss spectral properties of the constructed operators by establishing discreteness of their spectra and Weyl laws for the corresponding eigenvalue counts. Finally, based on the introduced models of interacting particles, we discuss BoseEinstein condensation on general quantum graphs. We will distinguish between systems of bosons for which BEC occurs and such for which no BEC is present at any finite temperature. As a final result, we prove that no Bose-Einstein condensation occurs (in the sense of phase transitions) in a system of bosons interacting via repulsive hard-core interactions.

The search for new physics with the ATLAS detector at the LHC requires a thorough understanding of Standard Model physics and the performance of the detector. A reliable prediction of the Standard Model backgrounds combined with precise measurements of collision events at a previously unreachable centre of mass energy {/s = 7 TeV) in ATLAS provides excellent opportunities for new physics discoveries.

Particular examples of one-loop string effects are explored in the context of brane-based realizations of the Standard Model. We firstly examine the consequences of a phenomenon known as Kinetic Mixing, which couples hidden U(1) gauge factors to visible U(l)'s. The effect is shown to occur in nonsupersymmetric string set-ups between D-branes and D-branes, where it acts either to give millicharges (of e.g. hyper charge) to would-be hidden sector fermions, or to generate an enhanced communication of supersymmetry breaking that dominates over the usual gravitational suppression. In either case, the conclusion is that the string scale in these nonsupersymmetric brane configurations has a generic upper bound of M(_s)≤ 10(^8) GeV. Turning to models based on intersecting branes, Yukawa interactions at one-loop on intersecting D6 branes are calculated. The non-renormalization theorem is demonstrated in supersymmetric configurations, and it is shown how Yukawa ß-functions may be extracted. In addition to the usual logarithmic running, power-law dependence on the infra-red cut-off (associated with Kaluza-Klein modes) is found. The results presented may be used to evaluate coupling renormalization in nonsupersymmetric cases. Much of the discussion is applicable to one-loop calculations on intersecting branes in general.

We present non-perturbative solutions for the fermion and boson propagators of QED in both three- and four-dimensions, and QCD. In doing so, we solve the coupled system of Schwinger-Dyson equations numerically in Euclidean space, investigating criticality, gauge dependence and phenomenology of the solutions. We do so by exploiting a new and novel three-point ansatz, the Kizilersü-Pennington vertex, designed to satisfy multiplicative renormalisability in unquenched QED. The efficacy of this is demonstrated numerically for QED(_4), where we find a marked improvement in the gauge-invarance of the photon wave-function. The critical coupling associated with dynamical mass generation is investigated for a variety of gauges; remarkably a lessening of this dependence is seen, despite the ansatz’s origins from a massless theory, which is improved further by constructing a hybrid system. As with many studies in the past, we apply this ansatz to the three-dimensional non-compact formulation of QED, checking gauge covariance of the propagators through a momentum-space formulation of the Landau-Khalatnikov-Pradkin transformations. The critical dependence on the number of active fermions was investigated, with the gauge dependence of the condensate unresolved. As an aside, we found numerically that LKF transforming the propagators gave rise to a constant condensate; a fact supported analytically through an explicit proof. We turn our attention towards QCD, where we explore a variety of phenomeno-logical models, including the full ghost-gluon system, in which we make comparisons between traditional vertices and the new KP-Vertex. These models are used in a determination of the physical quark condensate for massive quarks, through the exploitation of a class of non-positive definite solutions accessible for small quark masses. Finally, we examine Generalised Ward-Takahashi identities, which hold promise to further constrain the tranvserse part of the vertex. The identity is shown to hold true at one-loop through an explicit calculation, and a constraint on one of the basis coefficients is given as an example of its use.