Theories postulating extra spatial dimensions into which the gravitational field can propagate provide interesting extensions to the Standard Model addressing the hierarchy problem. These frameworks predict TeV-scale gravity signatures, such as black hole or string ball production, that could be observed at the Large Hadron Collider. Said black holes decay into a high multiplicity of particles with typical energies ranging in the few 100 GeV. The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 13.0 fb−1 of proton-proton collision data recorded by the ATLAS detector during 2012 at [special characters omitted] = 8 TeV. No excess beyond Standard Model expectations is observed, and upper limits on the cross sections for non-Standard Model production of these final states are set.

Hadronic collisions at the LHC at CERN probe particle interactions at the highest energy scale of any experiment to date. We present a research program measuring Rjet = [special characters omitted] as a function of a number of hadronic variables. The measurements are performed with the ATLAS detector at the LHC, using the 2011 data set, consisting of 4.64 fb-1 of pp collisions at a center of mass energy of 7 TeV. This measurement is a robust way to test the Standard Model and the modeling of perturbative QCD, and is sensitive to a wide variety of possible new physics in events with high jet ET, including some variations of Supersymmetry. By taking the ratio of W/Z production, a large number of systematic uncertainties cancel, including those associated with luminosity, jet energy scale and resolution, and many theoretical uncertainties. The measurement of Rjet is performed as a function of the pT and rapidity of the 1st-4th leading jet, ST, HT, and a number of dijet variables, including invariant mass and angular separations. The measurements are compared with NLO theoretical predictions from BLACKH AT+SHERPA, as well as using leading order simulations from ALPGEN and SHERPA. Over most of the kinematic phase-space, there is good agreement between the data and theoretical predictions. There is a significant deviation for exactly one selected jet above 30 GeV, where LACKHAT+SHERPA over-estimates the ratio Rjet by 12%.

The purpose of this thesis is the study of rare decays with the BABAR detector. In the first part we present the study of the leptonic decays B0 → e +e-, B0 → μ +μ- and B0 → e±μ∓, based on a dataset corresponding to (383.6 ± 4.2) × 106 BB¯ pairs. We do not find evidence of any of the three decay modes, and obtain upper limit on the branching fractions, at 90% confidence level, of [special characters omitted](B0 → e +e-) < 11.3 × 10-8 , [special characters omitted](B0 → μ+μ -) < 5.2 × 10-8, and [special characters omitted](B0 → e ±μ∓) < 9.2 × 10-8. In the second part we present the measurement of CP asymmetry in rare inclusive B → Xsγ decays on the recoil of fully-reconstructed hadronic B decays, using a dataset corresponding to (465.0 ± 5.1) × 106 BB¯ pairs. We measure a value of ACP = -0.12 ± 0.11 ± 0.03.

The non-leptonic decays of heavy mesons are considered. A novel resummation technique to study the factorization approximation based on the Operator Product Expansion is introduced and employed to explore leading non-factorizable corrections to B-meson decay. Soft Final State Interactions are studied in the heavy quark limit and found to be dominated by the multiparticle intermediate states. It is shown that soft Final State Interactions do not disappear in the large $m\sb{B}$ limit thus bringing non-perturbative uncertainty to the calculations and extractions of the CKM matrix elements. Phenomenological applications of these findings are also considered.

We explore some of the phenomenological consequences of dynamical electro-weak-symmetry breaking, where fermionic bound-states, instead of elementary scalar fields, induce the breaking. In particular, we consider processes generated by chiral anomalies associated with global-symmetry currents in anomaly-free gauge theories, as experimental studies of such processes have the potential for revealing some of the details of the mechanism responsible for the symmetry breaking. Two investigations are conducted employing simple technicolor models. The first one deals with the way that the equivalence theorem, which relates observable longitudinal gauge-bosons to the corresponding unphysical Goldstone-bosons, is satisfied in cases where the latter bosons have anomaly-generated couplings. In the second investigation, we study the production and detection of the techni-$\eta\sp\prime$, which decays via the anomaly to two photons much as the ordinary $\eta\sp\prime$ does, at a high-energy photon collider.

The present status of theories of quantum gravity are reviewed from the low energy point of view. String theory relates classical black-hole type solutions of Einstein-like equations (e.g. axidilaton gravity) to the string vacuum. Several such solutions are proposed and their properties are investigated, including their behavior under supersymmetry transformations. A general feature of all possible quantum theories of gravitation is that they lead to a field theory description at low (as compared to the Planck mass) energies. The theoretical consistency, uniqueness and consequences of such an effective theory are investigated. I show that a power counting theorem allows for the momentum expansion that defines the effective theory even in the presence of large masses. I also show that graviton-graviton scattering is free of potential infrared and collinear divergencies that plague perturbative discussions of Yang-Mills theories.

Rare kaon decays are an important testing ground of the electroweak flavor theory. They can provide new signals of CP-violating phenomena and open a window into physics beyond the Standard Model. The interplay of long-distance QCD effects in strangeness-changing transitions can be analyzed with Chiral Perturbation Theory techniques. Some theoretical predictions obtained within this framework for radiative kaon decays are reviewed, together with the present experimental status. In particular, two rare kaon decays are analyzed: the first decay, $K\sb{L} \to \pi\sp0 e\sp+ e\sp-$, is being searched for as a signal of direct $\Delta S$ = 1 CP violation. We provide a thorough updating of the analysis of the three components of the decay: (1) direct CP violation, (2) CP violation through the mass matrix and (3) CP-conserving (two-photon) contributions. First the chiral calculation of the $K\sb{S} \to \pi\sp0 e\sp+ e\sp-$ rate, due to Ecker, Pich and de Rafael, is updated to include recent results on the nonleptonic amplitude. Then we systematically explore the uncertainties in this method. These appear to be so large that they will obscure the direct CP violation unless it is possible to measure the $K\sb{S} \to \pi\sp0 e\sp+ e\sp-$ rate. The CP-conserving amplitude remains somewhat uncertain, but present indications are such that there may be a sizable CP-violating asymmetry in the $e\sp+, e\sp-$ energies from the interference of CP-conserving and CP-violating amplitudes and this may potentially be useful in determining whether direct CP violation is present. The second decay, $K\sb{L} \to \pi\sp0\gamma e\sp+ e\sp-$, which occurs at a higher rate than the nonradiative process $K\sb{L} \to \pi\sp0 e\sp+e\sp-$ can be a background to CP violation studies using the latter reaction. It also has interest in its own right in the context of chiral perturbation theory, through its relation to the decay $K\sb{L} \to \pi\sp0\gamma\gamma$. The leading order chiral loop contribution to $K\sb{L} \to \pi\sp0\gamma e\sp+ e\sp-$, including the $(q\sb{e\sp+} + q\sb{e\sp-})\sp2/m\sbsp{\pi}{2}$ dependence, is completely calculable. We present this result and also include the higher order modifications that are required in the analysis of $K\sb{L} \to \pi\sp0\gamma\gamma$.

A description of SLAC experiment E154, a precision measurement of the neutron's longitudinal spin structure function $g\sbsp{1}{n},$ is presented. Deep inelastic electron scattering was used to measure the structure function in the kinematic range $0.014

In this thesis, we have explored the rich phenomenology of the chiral anomaly in order to further our understanding of this fascinating and unique aspect of quantum field theory. We studied a subset of anomalous processes viz., [special characters omitted], and [special characters omitted] with the explicit purpose of extending the current status of calculations in this sector. For this, we applied the N/D formalism, which exploits the unitarity and analyticity of the scattering amplitude, in order to provide a plausible extension of one-loop results into resonance dominated regions of phase space. In particular, we were able to account for vector meson exchange effects by demanding that the phase shifts of the N/D function reflect those appearing in [special characters omitted]-scattering. We also compared our results with calculations based on vector dominance models, for which purpose we adopted the hidden symmetry approach.

Nonperturbative methods in particle physics are introduced. Three nonperturbative approaches are used to extend chiral perturbation theory. Dispersive technique is used to address the asymptotic limit and structure of the pion form factor performing the matching with the constraints of perurbative QCD and chiral perurbation theory. Models of Bosonization technique are used to generate an effective action directly from QCD in order to predict the size of the low energy constants strictly from theory. A simple constituent quark model is also suggested to study the model dependence of [special characters omitted] non-leptonic hyperon decay amplitudes arising from isospin mixing. Finally, spin-dependent physics of monopoles are introduced to examine Montonen-Olive's electromagnetic duality conjecture by studying the long-range field of the different states in the N = 2 BPS monopole supermultiplet.