The advent of the ep collider HERA at the DESY research center has opened a wide kinematical window for the study of the Deep Inelastic Scattering (DIS) processes e+p→ e+X. Recent measurements of the structure function F2 of the proton have confirmed the fact that the Bjorken scaling which arises from the naive Quark-Parton Model is broken in the limit where the scaling variable, x-Bjorken, is small. It also revealed that F2 grows exponentially with x-Bjorken, at low x. The standard picture of parton evolution, based on the DGLAP equations, reproduces this steep rise if it is used with an appropriate set of F2 parametrizations. However these parametrizations include "ad-hoc" assumptions on the non-perturbative behaviour of the structure functions and depend on a large set of phenomenological parameters. / The steep rise can also be reproduced by using a different perturbative treatment for the parton evolution. This treatment, based on the BFKL evolution equations, differs from the standard DGLAP picture by the lack of ordering in the transverse energy of the emitted partons. Unlike DGLAP, the BFKL picture reproduces the steep rise of F2 without assuming any particular shape of the structure function in the non-perturbative domain. / In order to gain an insight on parton dynamics in DIS at low x and discriminate between these two pictures, jet observables are used as they are expected to be closely related to the hard scattering and depend only slightly on the hadronization effects. Two jet observables are studied here in more details: the azimuthal correlation between the two leading-order jets and the cross section of the forward jet production. The measured cross sections are corrected for detector effects and compared to several DIS Monte Carlo models and next-to-leading order simulations over a wide kinematic range. The results are evaluated in the light of the BFKL and the DGLAP pictures. The experimental results are compared to other models of parton evolution as well, like the Colour Dipole Model (CDM), the Linked Dipole Chain (LDC) and the resolved photon model in DIS.

We study the effect of the thermal background on the mechanism of electroweak baryogenesis. Firstly, we model the effect of collisions by solving the Schrodinger equation in presence of a potential representing the bubble of phase transition of the Higgs field and a series of delta functions simulating the collisions amongst the particles. We validate this technique in chapter 2 by comparing it to the results obtained by application of the Born approximation. Later, we apply this "delta functions recurency technique" to the case of relativistic particles by solving the Dirac equation in the precense of similar potentials. In the second part of this thesis, we attempt to model the interaction by a totally distinct formalism that incorporates the effect of the exchange of momentum during a collision between the particles of the thermal background. To this end, we begin by describing a preexisting method for predicting the evolution of the density matrix of a system subjected to an interaction. Then we expend it in order to model the effect of the thermal background on baryogenesis. Lastly, we apply the "density matrix formalism" to the case of relativistic particles.

We derive fully covariant expressions for all two-point scattering amplitudes of two massless closed strings from a Dirichlet p-brane. This construction relies on the observation that there is a simple relation between these D-brane amplitudes in type II superstring theory and four-point scattering amplitudes for type I open superstrings. From the two-point amplitudes, we derive the long range background fields for the D-branes, and verify that as expected they correspond to those of extremally charged p-brane solutions of the low energy effective action.

A model is presented which consists of a dynamical scheme for reproducing the mass hierarchies observed in the quark sector. This is accomplished through new physics beyond the Standard Model, and does not rely upon the Higgs mechanism. By introducing a heavy fermion which couples to the quarks via the emission and reabsorption of a new boson, initially massless quarks are given their masses in a purely radiative fashion. The differences between up and down sector masses appear naturally as a result of generating the down mass matrix from corrections to the up mass matrix: accomplished via an anomalous flavour-changing coupling to the weak sector. Multipole type form-factors are inserted at each coupling, and introduce parameters which are varied to produce numerical fits to the quark masses, mixing angles and CP-violation parameter. Several extensions to the model are proposed and discussed.

The dominant term in the damping rate for quarks and transverse gluons at high momentum (p $ gg$ gT) is calculated within the framework of perturbative QCD at finite temperature. It is shown that the damping rate, $ gamma$, takes the form $ gamma$ = cg$ sp2$T log(1/g) with c = N/4$ pi$ for transverse gluons and c = (N$ sp2$-1)/(8$ pi$N) for quarks where N is the number of colours, g is the coupling constant, and T is the temperature. The sign and the gauge invariance of $ gamma$ are easily verifiable due to the simplicity of the argument. This result agrees with the more complicated (unpublished) calculations of Pisarski et al. but disagrees with those of Lebedev and Smilga.

A particular kind of gaseous particle detector, called resistive plate counter, has been built and tested with cosmic rays. It is composed of two parallel highly resistive electrode plates between which circulates an ionizing gas, and of capacitively coupled readout pads. The tests look at the pulse shape, efficiency, charge distribution, time delay, and time resolution of the counter. It is shown that these characteristics are dependent upon the voltage applied to the electrodes, the gas mixture ratio (argon/butane/freon), and the surface irregularities of the electrodes. The counter exhibits a 98% efficiency and a time resolution of approximately 1 ns. These results are consistent with published results for this type of counter.

This thesis describes the data acquisition error monitoring system developed for the 1993-94 physics run of the collider detector at Fermilab (CDF). It presents an overview of the CDF data acquisition system indicating the role that the error monitoring system plays in the experiment. It then describes the custom software and software packages used to meet the error monitoring requirements of the CDF data acquisition system.

$B sp pm$ mesons are reconstructed through the decay chain $B sp pm to chi sb{c}K sp pm, chi sb{c} to J/ psi gamma,$ and $J/ psi to mu sp+ mu sp-.$ Using data taken at the Collider Detector at Fermilab in $ bar pp$ collisions at $ sqrt{s}$ = 1.8 TeV during the 1988/89 run, we have seen the signal of the $B sp pm$ mesons from this decay channel although the statistics still need to be improved. By Monte Carlo methods, we also conclude that it is possible to resolve the individual $ chi sb{c1}$ and $ chi sb{c2}$ states by making higher photon energy cuts.

The production of the final states K$ sp+$K$ sp-$ and $ eta sp prime to pi sp+ pi sp- gamma$ in $ gamma gamma$ collisions has been measured using the ARGUS detector. The product $ Gamma sb{ gamma gamma}( eta sp prime)$Br$( eta sp prime to rho gamma)$ has been determined. The topological cross section for the production of charged kaon pairs in two-photon collisions has been measured and the $ gamma gamma$-widths and interference parameters for the tensor mesons f$ sb2$(1270), a$ sb2$(1320) and f$ sbsp{2}{ prime}$(1525) have been extracted. The helicity structure assumed for the K$ sp+$K$ sp-$ continuum contribution has a significant effect on the result. Upper limits have been obtained for the $ gamma gamma$-widths of the glueball candidate states f$ sb2$(1720) and X(2230).

The perturbative regime of Quantum Chromodynamics (QCD) is considered and certain aspects related with higher order corrections (HOC) are studied. Certain large correction terms in the perturbative expansion are determined, in particular for large transverse momentum $(p sb{T})$ direct photon production. The origin of these terms is specified and simple forms, called K-factors, are provided (soft gluon approach). / Furthermore, for processes initiated by 2 $ to$ 2 particle subprocesses the structure of the complete HOC is analyzed. It is shown that when structure functions and/or fragmentation functions are involved, there is a gauge invariant part that dominates HOC over a sizable kinematic range. Simple and general expressions are derived allowing an easy calculation of this part. Also, it is shown that, under certain approximations, this part reduces to the form of the simple K-factors. / Other aspects of HOC, in particular the dependence on the choice of scales, are considered. Using complete HOC, detailed analysis of recent and old data on large-$p sb{T}$ direct photon production is carried out. The dependence of the form of the gluon distribution on the choice of the scales (physical versus optimal scales) is discussed and it is concluded that appreciable ambiguity in this distribution still remains.