We consider the notion of improved and perfect actions within Regge calculus. These actions are constructed in such a way that they—although being defined on a triangulation—reproduce the continuum dynamics exactly, and therefore capture the gauge symmetries of general relativity. We construct the perfect action in three dimensions with a cosmological constant, and in four dimensions for one simplex. We conclude with a discussion about Regge calculus with curved simplices, which arises naturally in this context.

Constraints on dark energy from current observational data are sensitive to how distances are measured from Type Ia supernova (SN Ia) data. We find that flux averaging of SNe Ia can be used to test the presence of unknown systematic uncertainties, and yield more robust distance measurements from SNe Ia. We have applied this approach to the $\mathrm{\text{nearby}}+\mathrm{SDSS}+\mathrm{ESSENCE}+\mathrm{SNLS}+\mathrm{HST}$ set of 288 SNe Ia, and the “Constitution” set of 397 SNe Ia. Combining the SN Ia data with cosmic microwave background anisotropy data from Wilkinson Microwave Anisotropy Probe 5 yr observations, the Sloan Digital Sky Survey baryon acoustic oscillation measurements, the data of 69 gamma-ray bursts (GRBs) , and the Hubble constant measurement from the Hubble Space Telescope project SHOES, we measure the dark energy density function $X(z)\equiv {\rho }_X(z)/{\rho }_X(0)$ as a free function of redshift (assumed to be a constant at $z>1$ or $z>1.5$). Without the flux averaging of SNe Ia, the combined data using the Constitution set of SNe Ia seem to indicate a deviation from a cosmological constant at $\sim 95\%$ confidence level at $0\lesssim z\lesssim 0.8$; they are consistent with a cosmological constant at $\sim 68\%$ confidence level when SNe Ia are flux averaged. The combined data using the $\mathrm{\text{nearby}}+\mathrm{SDSS}+\mathrm{ESSENCE}+\mathrm{SNLS}+\mathrm{HST}$ data set of SNe Ia are consistent with a cosmological constant at 68% confidence level with or without flux averaging of SNe Ia, and give dark energy constraints that are significantly more stringent than that using the Constitution set of SNe Ia. Assuming a flat Universe, dark energy is detected at $>98\%$ confidence level for $z\le 0.75$ using the combined data with 288 SNe Ia from $\mathrm{\text{nearby}}+\mathrm{SDSS}+\mathrm{ESSENCE}+\mathrm{SNLS}+\mathrm{HST}$, independent of the assumptions about $X(z\ge 1)$. We quantify dark energy constraints without assuming a flat Universe using the dark energy figure of merit for both $X(z)$ and a dark energy equation-of-state linear in the cosmic scale factor.

We determine a constraint on the growth factor by measuring the damping of the baryon acoustic oscillations in the matter power spectrum using the Sloan digital sky survey luminous red galaxy sample. We obtain an effective upper limit on ${\sigma }_8{D}_1(z=0.3)$ using the damping of the baryon acoustic oscillation signature, where ${\sigma }_8$ is the root mean square overdensity in a sphere of radius $8h^{-1}\mathrm{Mpc}$ and $D_1(z)$ is the growth factor at redshift $z$. The above result assumes that other parameters are fixed and the cosmology is taken to be a spatially flat cold dark matter universe with the cosmological constant.

The dynamical properties of a model of dark energy in which two scalar fields are coupled by a noncanonical kinetic term are studied. We show that overall the addition of the coupling has only minor effects on the dynamics of the two-field system for both potentials studied, even preserving many of the features of the assisted quintessence scenario. The coupling of the kinetic terms enlarges the regions of stability of the critical points. When the potential is of an additive form, we find the kinetic coupling has an interesting effect on the dynamics of the fields as they approach the inflationary attractor, with the result that the combined equation of state of the scalar fields can approach $-1$ during the transition from a matter dominated universe to the recent period of acceleration.

The $V$-mode polarization of the cosmic microwave background is discussed in a weakly magnetized plasma. The VV and VT angular power spectra are computed for adiabatic initial conditions of the Einstein-Boltzmann hierarchy. Depending upon the frequency channel and upon the magnetic field intensity, the VT power spectra of the circular polarization can even be 7 orders of magnitude larger than a putative $B$-mode polarization stemming from the lensing of the primary anisotropies. Specific programs aimed at the direct detection of the $V$-mode polarization of the cosmic microwave background could provide a new observational tool for the scrutiny of predecoupling physics.

We present a cosmological model with a solution that self-accelerates at late times without signs of ghost instabilities on small scales. The model is a natural extension of the Brans-Dicke (BD) theory including a nonlinear derivative interaction, which appears in a theory with the Galilean shift symmetry. The existence of the self-accelerating universe requires a negative BD parameter but, thanks to the nonlinear term, small fluctuations around the solution are stable on small scales. General relativity is recovered at early times and on small scales by this nonlinear interaction via the Vainshtein mechanism. At late time, gravity is strongly modified and the background cosmology shows a phantomlike behavior and the growth rate of structure formation is enhanced. Thus this model leaves distinct signatures in cosmological observations and it can be distinguished from standard LCDM cosmology.

Decay ${\rm Y}(1S)\to \gamma f_2(1270)$ is studied by an approach in which the tensor meson, $f_2(1270)$, is strongly coupled to gluons. Besides the strong suppression of the amplitude ${\rm Y}(1S)\to \gamma gg$, $gg\to f_2$ by the mass of the $b$-quark, $d$-wave dominance in ${\rm Y}(1S)\to \gamma f_2(1270)$ is revealed from this approach, which provides a large enhancement. The combination of these two factors leads to larger $B({\rm Y}(1S)\to \gamma f_2(1270))$. The decay rate of ${\rm Y}(1S)\to \gamma f_2(1270)$ and the ratios of the helicity amplitudes are obtained and they are in agreement with the data.

Within the standard model with the 4th generation quarks $b^{\prime }$ and $t^{\prime }$ we have analyzed $CP$-violating flavor changing neutral current processes $t\to cX$, $b^{\prime }\to sX$, $b^{\prime }\to bX$, $t^{\prime }\to cX$, and $t^{\prime }\to tX$, with $X=H,Z,\gamma ,g$, by constructing and employing a global, unique fit for the 4th generation mass mixing matrix (CKM4) at $300\le m_{t^{\prime }}\le 700\mathrm{GeV}$. All quantities appearing in the CKM4 were subject to our fitting procedure. We have found that our fit produces the following $CP$ partial rate asymmetry dominance: $a_{CP}(b^{\prime }\to s(H,Z;\gamma ,g))\simeq (94,62;47,41)\%$, at $m_{t^{\prime }}\simeq 300,350\mathrm{GeV}$, respectively. From the experimental point of view the best decay mode, out of the above four, is certainly $b^{\prime }\to s\gamma$, due to the presence of the high energy single photon in the final state. We have also obtained relatively large asymmetry $a_{CP}(t\to cg)\simeq (8-18)\%$ for $t^{\prime }$ running in the loops. There are fair chances that the 4th generation quarks will be discovered at LHC and that some of their decay rates will be measured. If $b^{\prime }$ and $t^{\prime }$ exist at energies we assumed, with well executed tagging, large $a_{CP}$ could be found too.

We derive in a simple way the $(Z_2{)}^3$ symmetry which characterizes uniquely the phenomenologically successful tripartite form leading to the tribimaximal mixing in the neutrino mass matrix. We impose this symmetry in a setup including the charged leptons and find that it can accommodate all the possible patterns of lepton masses in the framework of type-I and type-II seesaw mechanisms. We also discuss the possibility of generating enough baryon asymmetry through lepton-lepton asymmetry.

We study effective models of chiral fields and Polyakov loop expected to describe the dynamics responsible for the phase structure of two-flavor QCD. We consider the chiral sector described either using a linear sigma model or a Nambu-Jona-Lasinio model and study how these models, on the mean-field level when coupled with the Polyakov loop, behave as a function of increasing bare quark (or pion) mass. We find qualitatively similar behaviors for the cases of the linear sigma model and the Nambu-Jona-Lasinio model and, by comparing with existing lattice data, show that one cannot conclusively decide which of the two approximate symmetries drives the phase transitions at the physical point.

We discuss consistent power counting for integrating soft and collinear degrees of freedom over arbitrary regions of phase space in the soft-collinear effective theory, and illustrate our results at one-loop with several jet algorithms: JADE, Sterman-Weinberg and $k_{\perp }$. Consistently applying soft-collinear effective theory power counting in phase space, along with nontrivial zero-bin subtractions, prevents double counting of final states. The resulting phase space integrals over soft and collinear regions are individually ultraviolet divergent, but the phase space ultraviolet divergences cancel in the sum. Whether the soft and collinear contributions are individually infrared safe depends on the jet definition. We show that while this is true at one-loop for JADE and Sterman-Weinberg, the $k_{\perp }$ algorithm does not factorize into individually infrared safe soft and collinear pieces in dimensional regularization. We point out that this statement depends on the ultraviolet regulator, and that in a cutoff scheme the soft functions are infrared safe.

We present a comprehensive phenomenological study of heavy flavor distributions and correlations in longitudinally polarized proton-proton collisions at BNL-RHIC. All results are obtained with a flexible parton-level Monte Carlo program at next-to-leading order accuracy and include the fragmentation into heavy mesons, their subsequent semileptonic decays, and experimental cuts. Next-to-leading order QCD corrections are found to be significant for both cross sections and double-spin asymmetries. The sensitivity of heavy flavor measurements at BNL-RHIC to the gluon polarization of the nucleon is assessed. Electron-muon and muon-muon correlations turn out to be the most promising observables. Theoretical uncertainties are estimated by varying renormalization and factorization scales, heavy quark masses, and fragmentation parameters.

A new model of physics, with a hidden conformal sector which manifests itself as an unparticle coupling to standard model particles effectively through higher dimensional operators, predicts strong collider signals due to unparticle self-interactions. We perform a complete analysis of the most spectacular of these signals at the hadron collider, $pp(\overline{p})\to \gamma \gamma \gamma \gamma$ and $\gamma \gamma gg$. These processes can go through the three-point unparticle self-interactions as well as through some $s$ and $t$ channel diagrams with one and/or two unparticle exchanges. We study the contributions of individual diagrams classified with respect to the number of unparticle exchanges and discuss their effect on the cross sections at the Tevatron and the LHC. We also restrict the Tevatron bound on the unknown coefficient of the three-point unparticle correlator. With the availability of data from the Tevatron, and the advent of the data emerging from the LHC, these interactions can provide a clear and strong indication of unparticle physics and distinguish this model from other beyond the standard model scenarios.

We calculate the energies of the $B=1$ and $B=2$ Skyrmions with a generalized mass term proposed in [V. B. Kopeliovich, B. Piette, and W. J. Zakrzewski, Phys. Rev. D 73, 014006 (2006).], allowing for (iso-)rotational deformations within the axially-symmetric ansatz. We show that this modification of the chiral symmetry breaking term is not sufficient to accommodate for the experimental value of the binding energy of the deuteron. Also, a computation of the different vibrational modes and energies reveals how the deuteron rigidity is affected as a function of the mass parameter $D$.

Flavor changing neutral currents coming from a new nonuniversal neutral gauge boson and from the nonunitary quark mixing matrix for the $SU(3{)}_c\otimes SU(3{)}_L\otimes U(1{)}_X$ model with right-handed neutrinos are studied. By imposing as experimental constraints the measured values of the $3\times 3$ quark mixing matrix, the neutral meson mixing, and bounds and measured values for direct flavor changing neutral current processes, the largest mixing of the known quarks with the exotic ones can be established, with new sources of flavor changing neutral currents being identified. Our main result is that for a $|V_{tb}|$ value smaller than 1, large rates of rare top decays such as $t\to c\gamma$, $t\to cZ$, and $t\to cg$ (where $g$ stands for the gluon field) are obtained; but if $|V_{tb}|\sim 1$ the model can survive present experimental limits only if the mass of the new neutral gauge bosons becomes larger that 10 TeV.

We review the status of $CPT$ violation in the neutrino sector. Apart from LSND, current data favors three flavors of light stable neutrinos and antineutrinos, with both halves of the spectrum having one smaller mass splitting and one larger mass splitting. Oscillation data for the smaller splitting are consistent with $CPT$. For the larger splitting, current data favor an antineutrino mass-squared splitting that is an order of magnitude larger than the corresponding neutrino splitting, with the corresponding mixing angle less than maximal. This $CPT$-violating spectrum is driven by recent results from MINOS, but is consistent with other experiments if we ignore LSND. We describe an analysis technique which, together with MINOS running optimized for muon antineutrinos, should be able to conclusively confirm the $CPT$-violating spectrum proposed here, with as little as 3 times the current data set. If confirmed, the $CPT$-violating neutrino mass-squared difference would be an order of magnitude less than the current most-stringent upper bound on $CPT$ violation for quarks and charged leptons.

We consider the endpoint of the Roberge-Weiss (RW) first order transition line present for imaginary baryon chemical potentials. We remark that it coincides with the finite size transition relevant in the context of large $N_c$ QCD and study its order in the theory with two degenerate flavors. The RW endpoint is first order in the limit of large and small quark masses, while it weakens for intermediate masses where it is likely in the Ising 3D universality class. Phenomenological implications and further speculations about the QCD phase diagram are discussed.

A measurement of the cross section for the inclusive production of isolated photons by the CDF experiment at the Fermilab Tevatron collider is presented. The measurement covers the pseudorapidity region $|{\eta }^{\gamma }|<1.0$ and the transverse energy range $E_T^{\gamma }>30\mathrm{GeV}$ and is based on $2.5{\mathrm{fb}}^{-1}$ of integrated luminosity. The sample is almost a factor of 7 larger than those used for recent published results and extends the $E_T^{\gamma }$ coverage by 100 GeV. The result agrees with next-to-leading order perturbative QCD calculations within uncertainties over the range $50<E_T^{\gamma }<400\mathrm{GeV}$, though the energy spectrum in the data shows a steeper slope at lower $E_T^{\gamma }$.

The coupling of the Higgs field through the Ricci tensor, put forward by Balakrishna and Wali, is derived using a conformal rescaling of the metric. Earlier results on “Bogomolny-type” equations in curved space, by Comtet and others, are recovered. The procedure can be generalized to any static background metric.