We present a light-front calculation of the box diagram in Yukawa theory. The covariant box diagram is finite for the case of spin-$1/2$ constituents exchanging spin-0 particles. In light-front dynamics, however, individual time-ordered diagrams are divergent. We analyze the corresponding light-front singularities and show the equivalence between the light-front and covariant results by taming the singularities.

Using post-Newtonian equations of motion for fluid bodies that include radiation-reaction terms at 2.5 and 3.5 post-Newtonian (PN) order ($O[(v/c{)}^5]$ and $O[(v/c{)}^7]$ beyond Newtonian order), we derive the equations of motion for binary systems with spinning bodies, including spin-spin effects. In particular we determine the effects of radiation-reaction coupled to spin-spin effects on the two-body equations of motion, and on the evolution of the spins. We find that radiation damping causes a 3.5PN order, spin-spin induced precession of the individual spins. This contrasts with the case of spin-orbit coupling, where we earlier found no effect on the spins at 3.5PN order. Employing the equations of motion and of spin precession, we verify that the loss of total energy and total angular momentum induced by spin-spin effects precisely balances the radiative flux of those quantities calculated by Kidder et al.

We consider a black ring with a cosmological constant in the five-dimensional $\mathcal{N}=4$ de Sitter supergravity theory. Our solution preserves half of the de Sitter supersymmetries and has one rotation symmetry. Unlike the flat case, there is no angular momentum and the stability against gravitational self-attraction is balanced by the cosmological repulsion due to the cosmological constant. Our solution describes a singular black ring since, although it has horizons of topology $S^1\times S^2$, the horizons are singular. Despite the singularity, our solution displays some interesting regular physical properties: it carries a dipole charge and this charge contributes to the first law of thermodynamics; it has an entropy and mass which conform to the entropic $N$-bound proposal and the maximal mass conjecture. We conjecture that the Gregory-Laflamme instability leads to a resolution of the singularity and results in a regular black ring.

We study the background cosmology governed by the tensor-vector-scalar theory of gravity proposed by Bekenstein. We consider a broad family of functions that lead to modified gravity and calculate the evolution of the field variables both numerically and analytically. We find a range of possible behaviors, from scaling to the late time domination of either the additional gravitational degrees of freedom or the background fluid.

We investigate the clustering properties of a dynamical dark energy component. In a cosmic mix of a pressureless fluid and a light scalar field, we follow the linear evolution of spherical matter perturbations. We find that the scalar field tends to form underdensities in response to the gravitationally collapsing matter. We thoroughly investigate these voids for a variety of initial conditions, explain the physics behind their formation, and consider possible observational implications. Detection of dark energy voids will clearly rule out the cosmological constant as the main source of the present acceleration.

We calculate the flux of cosmic positrons from the dark matter annihilation in the littlest Higgs model with $T$ parity. The dark matter annihilates mainly into weak gauge bosons in the halo, and high-energy positrons are produced through leptonic and hadronic decays of the bosons. We investigate a possibility to detect the positron signal in upcoming experiments such as PAMELA and AMS-02. We found that the dark matter signal can be distinguished from the background in the PAMELA experiment when the dark matter mass is less than 120 GeV and the signal flux is enhanced due to a small scale clustering of dark matter. Furthermore, the signal from the dark matter annihilation can be detected in the AMS-02 experiment, even if such enhancement does not exist. We also discuss the invisible width of the Higgs boson in this model.

We show that under a new $U(1)$ gauge symmetry, which is nonanomalous in the presence of one “right-handed neutrino” per generation and consistent with the standard model Yukawa couplings, the most general fermion charges are determined in terms of four rational parameters. This generalization of the $B–L$ symmetry with generation-dependent lepton charges leads to neutrino masses induced by operators of high dimensionality. Neutrino masses are thus naturally small without invoking physics at energies above the TeV scale, whether neutrinos are Majorana or Dirac fermions. This “leptocratic” model predicts the existence of light quasisterile neutrinos with consequences for cosmology, and implies that collider experiments may reveal the origin of neutrino masses.

We analyze the screening of static diquark sources in 2-flavor QCD and compare results with the screening of static quark-antiquark pairs. We show that a two quark system in a fixed color representation is screened at short distances like a single quark source in the same color representation whereas at large distances the two quarks are screened independently. At high temperatures we observe that the relative strength of the interaction in diquark and quark-antiquark systems, respectively, obeys Casimir scaling. We use this result to examine the possible existence of heavy quark-quark bound states in the high temperature phase of QCD. We find support for the existence of $bb$ states up to about $2T_c$ while $cc$ states are unlikely to be formed above $T_c$.

I develop a renormalization-group blocking framework for lattice QCD with staggered fermions. Under plausible, and testable assumptions, I then argue that the fourth-root recipe used in numerical simulations is valid in the continuum limit. The taste-symmetry violating terms, which give rise to nonlocal effects in the fourth-root theory when the lattice spacing is nonzero, vanish in the continuum limit. A key role is played by reweighted theories that are local and renormalizable on the one hand, and that approximate the fourth-root theory better and better as the continuum limit is approached on the other hand.

The QCD mechanisms underlying the exclusive strong decays and hadronic production amplitudes of charmonium remain poorly understood, despite decades of study and an increasingly detailed body of experimental information. One set of hadronic channels of special interest are those that include baryon-antibaryon states. These are being investigated experimentally at BES and CLEO-c in terms of their baryon resonance content, and are also of interest for the future PANDA experiment, in which charmonium and charmonium hybrids will be produced in $p\overline{p}$ annihilation in association with light mesons. In this paper we develop a simple initial-state light meson emission model of the near-threshold associated charmonium production processes $p\overline{p}\to {\pi }^0\Psi$, and evaluate the differential and total cross sections for these reactions in this model. (Here we consider the states $\Psi ={\eta }_c$, $J/\psi$, ${\psi }^{\prime }$, ${\chi }_0$ and ${\chi }_1$.) The predicted near-threshold cross section for $p\overline{p}\to {\pi }^{0}J/\psi$ is found to be numerically similar to two previous theoretical estimates, and is roughly comparable to the (sparse) existing data for this process. The theoretical charmonium angular distributions predicted by this model are far from isotropic, which may be of interest for PANDA detector design studies.

In this work we use the heavy-quark-light-diquark picture to study the semileptonic decay ${\Lambda }_b\to {\Lambda }_c+l+{\overline{\nu }}_l$ in the so-called hybrid scheme. Namely, we apply the heavy quark effective theory (HQET) for larger $q^2$ (corresponding to small recoil), which is the invariant mass square of $l+\overline{\nu }$, whereas the perturbative QCD approach for smaller $q^2$ to calculate the form factors. The turning point where we require the form factors derived in the two approaches to be connected, is chosen near ${\rho }_{\mathrm{cut}}=1.1$. It is noted that the kinematic parameter $\rho$ which is usually adopted in the perturbative QCD approach, is in fact exactly the same as the recoil factor $\omega =v·v^{\prime }$ used in HQET where $v$, $v^{\prime }$ are the four velocities of ${\Lambda }_b$ and ${\Lambda }_c$ respectively. We find that the final result is not very sensitive to the choice, so that it is relatively reliable. Moreover, we apply a proper numerical program within a small range around ${\rho }_{\mathrm{cut}}$ to make the connection sufficiently smooth and we parametrize the form factor by fitting the curve gained in the hybrid scheme. The expression and involved parameters can be compared with the ones gained by fitting the experimental data. In this scheme the end-point singularities do not appear at all. The calculated value is satisfactorily consistent with the data which is recently measured by the DELPHI collaboration within 2 standard deviations.

We present a measurement of the fractions $F_0$ and $F_{+}$ of longitudinally polarized and right-handed $W$ bosons in top-quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately $318{\mathrm{pb}}^{-1}$. We select $t\overline{t}$ candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle ${\theta }^{*}$, which is defined as the angle between the momentum of the charged lepton in the $W$ boson rest frame and the $W$ momentum in the top-quark rest frame. The $\cos {\theta }^{*}$ distribution in the data is determined by full kinematic reconstruction of the $t\overline{t}$ candidates. We find $F_0={0.85}_{-0.22}^{+0.15}(\mathrm{stat})\pm 0.06(\mathrm{syst})$ and $F_{+}={0.05}_{-0.05}^{+0.11}(\mathrm{stat})\pm 0.03(\mathrm{syst})$, which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed $W$ bosons of $F_{+}<0.26$ at the 95% confidence level.

We derive a spin chain Hamiltonian from a fast spinning string in the marginally deformed $\mathrm{Ad}{\mathrm{S}}_3\times S^3$. This corresponds to a closed trajectory swept out by the $SU(2)$ or $SL(2)$ spin vector on the surface of one-parameter deformed two-sphere or hyperboloid in the background of an anisotropic magnetic field interaction. In the limit of small deformation, a class of general Landau-Lifshitz equation with a nontrivial anisotropic matrix can be derived.

A common feature of all quantum gravity (QG) phenomenology approaches is to consider a modification of the mass-shell condition of the relativistic particle to take into account quantum gravitational effects. The framework for such approaches is therefore usually set up in the cotangent bundle (phase space). However it was recently proposed that this phenomenology could be associated with an energy dependent geometry that has been coined “rainbow metric”. We show here that the latter actually corresponds to a Finsler geometry, the natural generalization of Riemannian geometry. We provide in this way a new and rigorous framework to study the geometrical structure possibly arising in the semiclassical regime of QG. We further investigate the symmetries in this new context and discuss their role in alternative scenarios like Lorentz violation in emergent spacetimes or deformed special relativity-like models.

With the recent measurement of full sky cosmic microwave background (CMB) polarization from WMAP, key cosmological degeneracies have been broken, allowing tighter constraints to be placed on cosmological parameters inferred assuming a standard recombination scenario. Here we consider the effect on cosmological constraints if additional ionizing and resonance radiation sources are present at recombination. We find that the new CMB data significantly improve the constraints on the additional radiation sources, with ${log}_{10}[{ϵ}_{\alpha }]<-0.5$ and ${log}_{10}[{ϵ}_i]<-2.4$ at 95% c.l. for resonance and ionizing sources, respectively. Including the generalized recombination scenario, however, we find that the constraints on the scalar spectral index $n_s$ are weakened to $n_s=0.98\pm 0.03$, with the $n_s=1$ case now well inside the 95% c.l. The relaxation of constraints on tensor modes, scale invariance, dark energy and neutrino masses are also discussed.

We build a realistic model of curvaton cosmology, in which the energy content is described by radiation, WIMP dark matter, and a curvaton component. We calculate the curvature and isocurvature perturbations, allowing for arbitrary initial density perturbations in all fluids, following all species and their perturbations from the onset of dark matter freeze-out onto well after curvaton decay. We provide detailed numerical evaluations as well as analytical formulas which agree well with the latter. We find that substantial isocurvature perturbations, as measured relatively to the total curvature perturbation, can be produced even if the curvaton energy density is well underdominant when it decays; high precision measurements of cosmic microwave background anisotropies may thus open a window on underdominant decoupled species in the prenucleosynthesis early Universe. We also find that, in a large part of parameter space, curvaton decay produces enough dark matter particles to restore WIMP annihilations, leading to the partial erasure of any preexisting dark matter–radiation isocurvature perturbation.

At present, application of Superheated Droplet Detectors (SDDs) in WIMP dark matter searches has been limited to the spin-dependent sector, owing to the general use of fluorinated refrigerants which have high spin sensitivity. Given their recent demonstration of a significant constraint capability with relatively small exposures and the relative economy of the technique, we consider the potential impact of heavy versions of such devices on the spin-independent sector. Limits obtainable from a ${\mathrm{CF}}_3\mathrm{I}$-loaded SDD are estimated on the basis of the radiopurity levels and backgrounds already achieved by the SIMPLE and PICASSO experiments. With 34 kgd exposure, equivalent to the current CDMS, such a device may already probe to below ${10}^{-6}\mathrm{pb}$ in the spin-independent cross section.

We prove that three-dimensional Yang-Mills theories in the Landau gauge supplemented with a infrared regulating, parity preserving mass term are ultraviolet finite to all orders. We also extend this result to the Curci-Ferrari gauge.

We consider a model with anomaly-free Abelian gauge axial-vector symmetry, which is intended to mimic the standard electroweak gauge chiral $SU(2{)}_L\times U(1{)}_Y$ theory. Within this model we demonstrate: (1) Strong Yukawa interactions between massless fermion fields and a massive scalar field carrying the axial charge generate dynamically the fermion and boson proper self-energies, which are ultraviolet-finite and chirally noninvariant. (2) Solutions of the underlying Schwinger-Dyson equations found numerically exhibit a huge amplification of the fermion mass ratios as a response to mild changes of the ratios of the Yukawa couplings. (3) The “would-be” Nambu-Goldstone boson is a composite of both the fermion and scalar fields, and it gives rise to the mass of the axial-vector gauge boson. (4) Spontaneous breakdown of the gauge symmetry further manifests by mass splitting of the complex scalar and by new symmetry-breaking vertices, generated at one loop. In particular, we work out in detail the cubic vertex of the Abelian gauge boson.

We use perturbative Symanzik improvement to create a new staggered-quark action (HISQ) that has greatly reduced one-loop taste-exchange errors, no tree-level order $a^2$ errors, and no tree-level order $(am{)}^4$ errors to leading order in the quark’s velocity $v/c$. We demonstrate with simulations that the resulting action has taste-exchange interactions that are 3–4 times smaller than the widely used ASQTAD action. We show how to bound errors due to taste exchange by comparing ASQTAD and HISQ simulations, and demonstrate with simulations that such errors are likely no more than 1% when HISQ is used for light quarks at lattice spacings of $1/10\mathrm{fm}$ or less. The suppression of $(am{)}^4$ errors also makes HISQ the most accurate discretization currently available for simulating $c$ quarks. We demonstrate this in a new analysis of the $\psi -{\eta }_c$ mass splitting using the HISQ action on lattices where $a{m}_c=0.43$ and 0.66, with full-QCD gluon configurations (from MILC). We obtain a result of 111(5) MeV which compares well with the experiment. We discuss applications of this formalism to $D$ physics and present our first high-precision results for $D_s$ mesons.