I derive an upper bound on the electron neutrino component of the diffuse supernova neutrino flux from the constraint on the antineutrino component at SuperKamiokande. The connection between antineutrino and neutrino channels is due to the similarity of the muon and tau neutrino and antineutrino fluxes produced in a supernova, and to the conversion of these species into electron neutrinos and antineutrinos inside the star. The limit on the electron neutrino flux is $5.5{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ above 19.3 MeV of neutrino energy, and is stronger than the direct limit from Mont Blanc by 3 orders of magnitude. It represents the minimal sensitivity required at future direct searches, and is intriguingly close to the reach of the Sudbury Neutrino Observatory (SNO) and of the ICARUS experiment. The electron neutrino flux will have a lower bound if the electron antineutrino flux is measured. Indicatively, the first can be smaller than the second at most by a factor of 2–3 depending on the details of the neutrino spectra at production.

We give a joint description of weak radiative (WR) and nonleptonic (NL) hyperon decays (HD) in broken SU(3). The two groups of decays are linked via $\mathrm{SU}(2{)}_W$ spin symmetry and vector-meson dominance (VMD). We use experimental information on the parity-conserving NLHD amplitudes to fix the corresponding WRHD amplitudes. With the latter known, the data on the WRHD branching ratios and asymmetries permit us to determine the parity-violating WRHD amplitudes in terms of two parameters corresponding to the two-quark and single-quark transitions. We obtain a good description of the data, and, in particular, a large ${\Sigma }^{+}\to p\gamma$ asymmetry. Then, using the $\mathrm{SU}(2{)}_W+\mathrm{VMD}$ route we determine the non-soft-meson correction term in the parity-violating NLHD amplitudes. The latter is shown to subtract a substantial amount from the current-algebra commutator thus leading towards the resolution of the $S:P$ discrepancy in NLHD.

In some models with an extra $U(1)$ gauge boson $Z^{\prime }$, the gauge couplings of the $Z^{\prime }$ to different generations of fermions may not be universal. Flavor mixing in general can be induced at the tree level in the up-type and/or down-type quark sector after diagonalizing their mass matrices. In this work, we concentrate on the flavor mixing in the up-type quark sector. We deduce a constraint from $D^0-{\overline{D}}^0$ mixing. We study in detail single top-quark production via flavor-changing $Z^{\prime }$ exchange at the Large Hadron Collider (LHC) and the International Linear Collider (ILC). We found that for a typical value of $M_{Z^{\prime }}=1\mathrm{TeV}$, the production cross section at the LHC can be of the order of 1 fb. However, the background from the standard model single top-quark production makes it difficult to detect the flavor-changing $Z^{\prime }$ signal unless a decent charm tagging method is implemented. On the other hand, at the ILC, the production cross section at the resonance energy of $\sqrt{s}\approx M_{Z^{\prime }}$ can reach a size of more than 100 fb. Even away from the resonance, the cross section at ILC is shown to be larger than the threshold of observability of 0.01 fb.

We calculate the response of the microscopic Dirac spectrum to an imaginary isospin chemical potential for QCD with two dynamical flavors in the chiral limit. This extends our previous calculation from the quenched to the unquenched theory. The resulting spectral correlation function in the $ϵ$-regime provides here, too, a new and efficient way to measure $F_{\pi }$ on the lattice. We test the method in a hybrid Monte Carlo simulation of the theory with two staggered quarks.

The introduction of an interaction for dark energy to the standard cosmology offers a potential solution to the cosmic coincidence problem. We examine the conditions on the dark energy density that must be satisfied for this scenario to be realized. Under some general conditions we find a stable attractor for the evolution of the Universe in the future. Holographic conjectures for the dark energy offer some specific examples of models with the desired properties.

As a candidate for the dark energy, the hessence model has been recently introduced. We discuss the critical points of this model in an almost general case, that is for arbitrary hessence potential and almost arbitrary hessence-background matter interaction. It is shown that, in all models, there always exist some stable late-time attractors. It is shown that our general results coincide with those solutions obtained earlier for special cases, but some of them are new. These new solutions have two unique characteristics. First, the hessence field has finite value in these solutions and, second, their stabilities depend on the second derivative of the hessence potential.

We discuss the constraints on the time-varying equation of state for dark energy and the curvature of the universe using observations of type Ia supernovae from Riess et al. and the most recent Supernova Legacy Survey (SNLS), the baryon acoustic oscillation peak detected in the SDSS luminous red galaxy survey and cosmic microwave background. Because of the degeneracy among the parameters which describe the time dependence of the equation of state and the curvature of the universe, the constraints on them can be weakened when we try to constrain them simultaneously, in particular, when we use a single observational data. However, we show that we can obtain relatively severe constraints when we use all data sets from observations above even if we consider the time-varying equation of state and do not assume a flat universe. We also found that the combined data set favors a flat universe even if we consider the time variation of the dark energy equation of state.

We study the polarization of the anti-Lambda particle in polarized high energy $pp$ collisions at large transverse momenta. The anti-Lambda polarization is found to be sensitive to the polarization of the antistrange sea of the nucleon. We make predictions using different parametrizations of the polarized quark distribution functions. The results show that the measurement of longitudinal anti-Lambda polarization can distinguish different parametrizations, and that similar measurements in the transversely polarized case can give some insights into the transversity distribution of the antistrange sea of nucleon.

We simulate lattice QCD with 2 flavours of massless quarks on lattices of temporal extent $N_t=8$, to study the finite temperature transition from hadronic matter to a quark-gluon plasma. A modified action which incorporates an irrelevant chiral 4-fermion interaction is used, which allows simulations at zero quark mass. We obtain excellent fits of the chiral condensates to the magnetizations of a 3-dimensional $O(2)$ spin model on lattices small enough to model the finite size effects. This gives predictions for correlation lengths and chiral susceptibilities from the corresponding spin-model quantities. These are in good agreement with our measurements over the relevant range of parameters. Binder cumulants are measured, but the errors are too large to draw definite conclusions. From the properties of the $O(2)$ spin model on the relatively small lattices with which we fit our data, we can see why earlier attempts to fit staggered lattice data to leading-order infinite-volume scaling functions, as well as finite size scaling studies, failed and led to erroneous conclusions.

We study charmonium production at high-energy colliders (Tevatron, HERA, and LEP2) in the framework of the $k_T$-factorization approach and the factorization formalism of nonrelativistic quantum chromodynamics at leading order in the strong-coupling constant ${\alpha }_s$ and the relative velocity $v$. The transverse-momentum distributions of direct and prompt $J/\psi$-meson production measured at the Fermilab Tevatron are fitted to obtain the nonperturbative long-distance matrix elements for different choices of unintegrated gluon distribution functions in the proton. Using the matrix elements thus obtained, we predict charmonium production rates in $\gamma \gamma$, $\gamma p$, and deep-inelastic $ep$ collisions including the contributions from both direct and resolved photons. The results are compared with the known ones obtained in the conventional parton model and with recent experimental data from HERA and LEP2.

In the framework of perturbative QCD and nonrelativistic bound state formalism, we calculate the production of $J/\psi$ mesons associated with open-charm particles at HERA and Tevatron conditions. We show that the contribution from $J/\psi +c+\overline{c}$ channel becomes comparable with that of $J/\psi$ sole production at high $J/\psi$ transverse momenta (approximately, at $p_{\psi ,T}>10\mathrm{GeV}$). We investigate the $J/\psi$ polarization effects and kinematic correlations between $J/\psi$ mesons and the accompanying open-charmed mesons. We show that the correlations are sensitive to the initial $k_t$ of the colliding partons, and so, can be considered as an additional test discriminating the collinear parton model and the $k_t$-factorization approach.

We study the range of the elements of the neutrino mass matrix $m_{\nu }$ in the charged lepton basis. Neutrinoless double beta decay is sensitive to the $ee$ element (the effective mass) of $m_{\nu }$. We then analyze the phenomenological implications of single texture zeros. In particular, interesting predictions for the effective mass can be obtained, in the sense that typically only little cancellation due to the Majorana phases is expected. Some cases imply constraints on the atmospheric neutrino mixing angle.

We have searched for the decays $B^0\to D_s^{+}{a}_0^{-}$, $B^0\to D_s^{*+}{a}_0^{-}$, $B^0\to D_s^{+}{a}_2^{-}$ and $B^0\to D_s^{*+}{a}_2^{-}$ in a sample of about $230\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector at the PEP-II asymmetric-energy $B$ Factory at SLAC. We find no evidence for these decays and set upper limits at 90% C.L. on the branching fractions: $\mathcal{B}(B^0\to D_s^{+}{a}_0^{-})<1.9\times {10}^{-5}$, $\mathcal{B}(B^0\to D_s^{*+}{a}_0^{-})<3.6\times {10}^{-5}$, $\mathcal{B}(B^0\to D_s^{+}{a}_2^{-})<1.9\times {10}^{-4}$, and $\mathcal{B}(B^0\to D_s^{*+}{a}_2^{-})<2.0\times {10}^{-4}$.

The Newton-Hooke algebras in $d$ dimensions are constructed as contractions of dS(AdS) algebras. Nonrelativistic brane actions are WZ terms of these Newton-Hooke algebras. The NH algebras appear also as subalgebras of multitemporal relativistic conformal algebras, $SO(d+1,p+2)$. We construct generalizations of pp-wave metrics from these algebras.

We study in more detail the dynamics of chiral primaries of the $\mathrm{D}1/\mathrm{D}5$ system. From the CFT given by the $S_N$ orbifold a study of correlators resulted in an interacting (collective) theory of chiral operators. In ${\mathrm{AdS}}_3\times S^3$ SUGRA we concentrate on general $1/2$ BPS configurations described in terms of a fundamental string. We first establish a correspondence with the linearized field fluctuations and then present the nonlinear analysis. We evaluate in detail the symplectic form of the general degrees of freedom in SUGRA and confirm the appearance of chiral bosons. We then discuss the appearance of interactions and the cubic vertex, in correspondence with the $S_N$ collective field theory representation.

We investigate Big bang nucleosynthesis (BBN) in scalar-tensor theories of gravity with arbitrary matter couplings and self-interaction potentials. We first consider the case of a massless dilaton with a quadratic coupling to matter. We perform a full numerical integration of the evolution of the scalar field and compute the resulting light element abundances. We demonstrate in detail the importance of particle mass thresholds on the evolution of the scalar field in a radiation dominated universe. We also consider the simplest extension of this model including a cosmological constant in either the Jordan or Einstein frame.

It is argued that the noncommutativity approach to fully supersymmetric field theories on the lattice suffers from an inconsistency. Supersymmetric quantum mechanics is worked out in this formalism and the inconsistency is shown both in general and explicitly for that system, as well as for the Abelian super BF model.

The coupling constant of $a_0\to \rho \gamma$ and $f_0\to \rho \gamma$ decays are calculated using 3-point QCD sum rules. We estimate the coupling constant $g_{a_0\rho \gamma }$ and $g_{f_0\rho \gamma }$ which are an essential ingredient in the analysis of physical processes involving isoscalar $f_0(980)$ and isovector $a_0(980)$ mesons.

We examine the $\gamma p$ photoproduction and the hadronic $\gamma \gamma$ total cross sections by means of a QCD eikonal model with a dynamical infrared mass scale. In this model, where the dynamical gluon mass is the natural regulator for the tree level gluon-gluon scattering, the $\gamma p$ and $\gamma \gamma$ total cross sections are derived from the $pp$ and $\overline{p}p$ forward scattering amplitudes assuming vector meson dominance and the additive quark model. We show that the validity of the cross section factorization relation ${\sigma }_{pp}/{\sigma }_{\gamma p}={\sigma }_{\gamma p}/{\sigma }_{\gamma \gamma }$ is fulfilled depending on the Monte Carlo model used to unfold the hadronic $\gamma \gamma$ cross section data, and we discuss in detail the case of $\sigma (\gamma \gamma \to \mathrm{\text{hadrons}})$ data with $W_{\gamma \gamma }>10\mathrm{GeV}$ unfolded by the Monte Carlo generators PYTHIA and PHOJET. The data seems to favor a mild dependence with the energy of the probability ($P_{\mathrm{had}}$) that the photon interacts as a hadron.