The rare decay K(L) → e⁺e⁻μ⁺μ⁻ offers the best opportunity to study the dynamics behind the KL γ*γ* vertex. This information is needed to subtract the long-distance contribution to the decay K(L) → μ⁺μ⁻, allowing for a calculation of rho, the real part of the CKM matrix element Vtd. Analysis of data from the 1997 and 1999 runs of the KTeV experiment has isolated a sample of 132 K(L) → e⁺e⁻μ⁺μ⁻ events, with an estimated background of 0.8 events. Based on this data, the branching ratio for this mode was calculated to be B(K(L) → e⁺e⁻μ⁺μ⁻) = (2.63 ± 0.23(stat) ± 0.13(syst)) x 10⁻⁹, where a radiative cutoff has been imposed at x(e⁺e⁻μ⁺μ⁻) > 0.95. The D'Ambrosio, Isidori, and Portoles K(L) γ*γ* linear form factor parameter alpha was measured to be -1.52 ± 0.38, consistent with previous experimental results. Little sensitivity was found to the quadratic parameter beta, resulting in a measured value of +12.3 ± 77.1. The asymmetry in the angular distribution of the K(L) → e⁺e⁻μ⁺μ⁻ decay products was measured to be A < 25.5% with 90% confidence, constraining the size of any CP-violating contribution to the K(L) gamma*gamma* interaction. Finally, a 90% C.L. upper limit of 4.12 x 10⁻¹¹ was placed on the branching ratio of the lepton flavor-violating mode K(L) → e⁺/⁻ e⁺/⁻μ⁺/⁻μ⁺/⁻, a full factor of 3 improvement over the current limit.

We have measured the tt̄ production cross section at √s = 1.96 TeV using data collected by the D∅ experiment at Fermilab. The integrated luminosity of the data set is 140 pb⁻¹ and a total of four candidate events are seen, with an expected background of 2.61 events. The measured cross section of σ(tt̄) = 11.1⁺²²·¹₋₉.₃ (stat.) ⁺⁴·³₋₄.₅ (sys.) pb is in agreement with a NNLO calculation of 6.77 pb.

QCD higher order corrections (HOC) to three processes are considered: (i) Direct photon production in longitudinally polarized hadron-hadron collisions with numerical applications to proton-proton collisions; (ii) lepton-pair production in transversely polarized hadron-hadron collisions with numerical applications to proton-proton collisions; (iii) heavy quark pair production by polarized and unpolarized photons. The HOC to all three processes are found to be significant. Processes (i) and (ii) are shown to be sensitive probes of the proton's polarized gluon distribution and the transversity distributions, respectively. The asymmetries are found to exhibit perturbative stability. Process (iii) is considered as a background to $ gamma gamma rightarrow H sp* rightarrow b =b$ (standard model). As well, top-quark production not too far above threshold is considered.

This thesis presents an investigation of semileptonic B meson decays with a narrow P-wave charm meson in the final state. The data sample consists of 3.29 x 106 BB&macr; events collected with the CLEO II detector at the Cornell Electron-positron Storage Ring. The P-wave charm mesons are reconstructed in the chain of decays: D0J&rarr;D*+p- ,D*+&rarr;D0p+,D 0&rarr;K-p+ or D0&rarr;K-p+p 0 . Study of the decay B-&rarr;D*+p0&ell; -n&ell;&d1; reveals useful information about the deficit observed in inclusive charm semileptonic B decays and the effective couplings of the W boson to heavy quark mesons. The results obtained for the exclusive semileptonic product branching fractions are BB-&rarr;D0 1&ell;-n&ell; &d1;B D01&rarr;D*+p - = (0.373 +/- 0.085 +/- 0.052 +/- 0.024)% and BB-&rarr;D*0 2&ell;-n&ell;&d1; BD*0 2&rarr;D*+p- < 0.16% (90% C.L.). The assumption BD01&rarr;D* +p- = 67% and BD*02&rarr;D *+p- = 20% implies BB-&rarr;D01 &ell;-n&ell;&d1; (0.56 0.13 +/- 0.08 +/- 0.04)% and BB-&rarr;D*0 2&ell;n&ell;&d1; < 0.8% (90% C.L.). These results indicate that at least 18% of the total B semileptonic rate is still unaccounted for by the observed exclusive decays, B&d1;&rarr;D0&ell;- n&d1;&ell;, B&d1;&rarr;D*&ell;n &d1;&ell;,B&d1; &rarr;D1&ell;n&d1; &ell;, and B&d1;&rarr;D*2 &ell;n&d1;&ell; . Furthermore, the first measurement of the q 2 spectrum for B-&rarr;D01&ell;n &d1;&ell; is presented. The present analysis also suggests that the &Lambda; QCD/mQ corrections beyond the HQS prescriptions might be significant in the theoretical treatment of the dynamics of B semileptonic decays to excited charm mesons.

Using a data sample of integrated luminosity &int; Ldt = 28.9 +/- 1.2 pb-1 of proton-antiproton collisions at a center-of-mass energy s = 1.8 TeV collected with the CDF detector at the Fermilab Tevatron collider, we searched for "penguin" radiative decays of B0d and B0s mesons which involve the flavor-changing neutral-current transition of a b quark into an s quark with the emission of a photon, b&rarr;sg . Specifically, we searched for the decays B0d&rarr;K*0g, K*0&rarr;K+p- and B0s&rarr;fg, f&rarr;K+K- , as well as for the charge conjugate chains. / In order to collect such decays, we designed a specialized trigger which required information on all the decay products of the B meson decay chain, the first such trigger in a hadron collider environment. This "penguin" trigger collected data during the last quarter of the 1994--1996 data taking period. After all selection criteria, we are left with one candidate B0d&rarr;K*0g decay and no B0s&rarr;fg candidates in the entire data sample. We then proceed to set upper limits on the branching fractions of the penguin channels. / We exploit the topological similarity between the B&d1;&rarr;e-D0 X,D0&rarr;K-p + and the penguin decays, by forming ratios of branching fractions between the penguin and the B&d1;&rarr;e-D0 X channels. Uncertainties associated with the B meson production cross section, common efficiency corrections and other systematic effects are minimal in the ratio of branching fractions. The uncertainty on the B&d1;&rarr;e-D0 X yield is the biggest contribution to the total uncertainty on the penguin branching fraction. We assume equal production rates for B+u and B0d mesons, while the probability of producing B0s mesons relative to B0d mesons, fs/fd, is taken to be 1/3. The inferred upper limits on the ratios of branching fractions are B&parl0;B0 s&rarr;fg&parr0;B&parl0; B&d1;&rarr;e-D0 X&parr0;<3.5x10-3 at90&percnt; C.L. B&parl0;B 0d&rarr;K*0g&parr0; B&parl0;B&d1;&rarr;e- D0X&parr0;<1.9x10-3 at90&percnt;C.L. / Relative branching fraction measurements were combined with the branching fraction measurement of the B&d1;&rarr;e-D0 X,D0&rarr;K-p + decay chain, B&parl0;B&d1;&rarr;e- D0X&parr0;xB&parl0;D0&rarr; K-p+&parr0; = (294 +/- 40) x 10-5, to extract the following absolute branching fraction limits B&parl0;B0s&rarr; fg&parr0;<2.8x10-4 at90&percnt;C.L. B&parl0;B0d &rarr;K*0g&parr0;<1.5x10-4 at90&percnt; C.L. / The upper limit for the B0d&rarr;K*0g decay is consistent with the branching fraction measurement reported by the CLEO collaboration, B&parl0;B0d&rarr;K*0 g&parr0; = (4.0 +/- 1.9) x 10-5, while the upper limit for the as yet unobserved B0s&rarr;fg decay is the most constraining one set to date.

The construction of four-dimensional string models via nonabelian twist is discussed in an operator formalism. Features of Hilbert space related to nonabelian twists are studied from the group theoretical point of view. This enables global anomalies to be removed if one insists the vacuum states to be a representation of the nonabelian group. We present a systematic procedure for the identification of the final gauge group, whose rank is generically reduced in a nonabelian twist. This general method of model-building is applied to obtain all minimal-rank strings resulting from twists by finite nonabelian subgroups of SU(2). Their partition functions, vacuum representations, gauge groups, and the elimination of global anomalies are considered individually for each case.

In this work thermal dilepton production from a hot medium created in heavy ion collision is studied. Using an effective Lagrangian technique for particle decays and annihilations, a complete method for calculation of the lepton pair production rate is developed. The importance of axial vector meson contributions to the dilepton spectrum is analyzed. Different effective Lagrangians for the a1rhopi interaction are considered, and a new form of b1o(&phis;)pi effective Lagrangian is introduced. / A systematic study of light meson contributions is performed. The most significant decay and reaction contributions are calculated and summed for low and intermediate invariant mass dileptons. The calculated dilepton rate is compared to that obtained using spectral functions extracted from data, and it is shown that the chosen set of mesonic reactions and decays accounts for all significant contributions to the thermal dilepton emission. / A hydrodynamic approach to the space-time evolution of the hot medium formed as a result of a central heavy ion collision at ultra-relativistic energies is considered. A theoretical curve of intermediate invariant mass dilepton spectrum is computed and compared to the NA50 data from central Pb(158 AGeV)+Pb collisions. Experimental acceptance cuts are accounted for. Drell-Yan processes are considered as well. We find that our thermal dileptons account for the intermediate mass excess observed by the NA50 Collaboration. We see no need to invoke charm enhancement. Predictions for the future experiments at RHIC and are made.

The production of charmonium states, $ chi sb1$ and $ chi sb2$, was measured at 300 GeV/c with $ pi sp+$, $ pi sp-$, and proton beams on a lithium target. The cross-sections were determined by analyzing the full data sample recorded, during the 1987-1988 running period, by experiment E705 at Fermilab. The $ chi$ mesons were detected through their radiative decay into J/$ psi$ $ gamma$ and the subsequent decay, $ psi to mu sp+ mu sp-$. / The measured cross-sections for the $ chi$ mesons and estimates of the cross section for direct production of J/$ psi$'s have been compared with measurements obtained in other experiments and with theoretical predictions within the framework of Quantum ChromoDynamics.

Two aspects of quantum field theory in curved spacetimes are discussed. First, the limits for applicability of the equivalence principle in the context of low energy effective field theories is considered. In particular, we find three classes of higher-derivative interactions for the gravitational and electromagnetic fields which produce dispersive photon propagation. One of these classes of interactions also produces birefringent propagation. This result is illustrated by calculating the energy-dependent contribution to the bending of light. In the second part, the divergences appearing in statistical black hole entropy are analysed. Using a Pauli-Villars regulator, it is shown that 't Hooft's approach to evaluating black hole entropy through a statistical-mechanical counting of states for a scalar field propagating outside the event horizon yields precisely the one-loop renormalization of the standard Bekenstein-Hawking formula, $S = { cal A}/(4G),$ where $ cal A$ is the black hole area. The calculation also yields a constant contribution to the black hole entropy, which may be associated with the one-loop renormalization of certain higher curvature terms in the gravitational action. The calculation of black hole entropy is done for a Schwarzschild black hole as well as for a Reissner-Nordstrom black hole.

This thesis is arranged in two distinct parts. The first part involves the construction of a novel dynamical scheme which is capable of generating the masses of the quarks and the elements of the Kobayashi-Maskawa matrix, and which does not rely upon the Higgs mechanism. The up quarks acquire their masses by coupling to a new heavy fermion via emission and reabsorption of a new scalar boson. The down quarks, on the other hand, obtain their masses via a correction to the up mass matrix as a result of an anomalous flavour changing interaction of the $W sp pm$. Extensions of the mass generation model are proposed and discussed. The second part involves the leading-log s study of hadron-hadron elastic scattering to order $ alpha sb{s} sp3$. By introducing wave functions, it is found that there are three types of form factor for the proton, and two for the pion. These form factors turn out to cancel the infrared divergence which occurs in quark-quark scattering. To this order in $ alpha sb{s}$, the amplitude of such elastic scattering is shown to have the form $ sim$(1 + log(s)($ alpha sb{o}$ + $ alpha sp prime t$ + $O(t sp2))$ for the imaginary part and a constant $ sim alpha sb{odd}$ for the real part, where s and t are the usual Mandelstam variables, and the $ alpha$'s are constants.