An analytical approach for studying the cosmological scenario with a homogeneous tachyon field within the framework of loop quantum gravity is developed. Our study is based on the semiclassical regime where space time can be approximated as a continuous manifold, but matter Hamiltonian gets nonperturbative quantum corrections. A formal correspondence between classical and loop quantum cosmology is also established. The Hamilton-Jacobi method for getting exact solutions is constructed and some exact power law as well as bouncing solutions are presented.

In Phys. Rev. D 58, 014014 (1998) and 71, 094013 (2005), we determined nonperturbative $D^0$, $D^{+}$, $D^{*+}$, $D_s^{+}$, and ${\Lambda }_c^{+}$ fragmentation functions, both at leading and next-to-leading order in the $\overline{\mathrm{MS}}$ factorization scheme, by fitting $e^{+}{e}^{-}$ data taken by the OPAL Collaboration at CERN LEP1. The starting points for the evolution in the factorization scale $\mu$ were taken to be ${\mu }_0=2m_Q$, where $Q=c$, $b$. For the reader’s convenience, in this paper, we repeat this analysis for ${\mu }_0=m_Q$, where the flavor thresholds of modern sets of parton density functions are located.

Pole trajectories connecting light and heavy scalar mesons, both broad resonances and quasibound states, are computed employing a simple coupled-channel model. Instead of varying the coupling constant as in previous work, quark and meson masses are continuously changed, so as to have one scalar meson evolve smoothly into another with different flavor(s). In particular, it is shown, among several other cases, how the still controversial $K_0^{*}(800)$ turns into the established ${\chi }_{c0}$, via the disputed $D_s(2317)$. Moreover, a ${\chi }_{c0}^{\prime }(3946)$ is predicted, which may correspond to the recently observed $Y(3943)$ resonance. These results lend further support to our unified dynamical picture of all scalar mesons, as unitarized $q\overline{q}$ states with important two-meson components.

The dynamics of a particle in a gravitational quantum well is studied in the context of nonrelativistic quantum mechanics with a particular deformation of a two-dimensional Heisenberg algebra. This deformation yields a new short-distance structure characterized by a finite minimal uncertainty in position measurements, a feature it shares with noncommutative theories. We show that an analytical solution can be found in perturbation and we compare our results to those published recently, where noncommutative geometry at the quantum mechanical level was considered. We find that the perturbations of the gravitational quantum well spectrum in these two approaches have different signatures. We also compare our modified energy spectrum to the results obtained with the GRANIT experiment, where the effects of the Earth’s gravitational field on quantum states of ultracold neutrons moving above a mirror are studied. This comparison leads to an upper bound on the minimal length scale induced by the deformed algebra we use. This upper bound is weaker than the one obtained in the context of the hydrogen atom but could still be useful if the deformation parameter of the Heisenberg algebra is not a universal constant but a quantity that depends on the energetic content of the system.

We identify spurious states in meson-baryon scattering, those that appear in QCD for $N_c>3$ but decouple for $N_c=3$. The key observation is that the relevant flavor SU(3) Clebsch-Gordan coefficients contain factors of $1-3/N_c$. We show that this method works even if SU(3) is badly broken. We also observe that resonant scattering poles lying outside naive quark-model multiplets are not necessarily large $N_c$ artifacts, and can survive via configuration mixing at $N_c=3$.

The introduction of $T$ parity dramatically improves the consistency of little Higgs models with precision electroweak data, and renders the lightest $T$-odd particle (LTP) stable. In the littlest Higgs model with $T$ parity, the LTP is typically the $T$-odd heavy photon, which is weakly interacting and can play the role of dark matter. We analyze the relic abundance of the heavy photon, including its coannihilations with other $T$-odd particles, and map out the regions of the parameter space where it can account for the observed dark matter. We evaluate the prospects for direct and indirect discovery of the heavy photon dark matter. The direct detection rates are quite low and a substantial improvement in experimental sensitivity would be required for observation. A substantial flux of energetic gamma rays is produced in the annihilation of the heavy photons in the galactic halo. This flux can be observed by the GLAST telescope, and, if the distribution of dark matter in the halo is favorable, by ground-based telescope arrays such as VERITAS and HESS.

In this article, we present our results on a global fit to precision electroweak data in a Higgs triplet model. In models with a triplet Higgs boson, a consistent renormalization scheme differs from that of the standard model and the global fit shows that a light Higgs boson with mass of 100–200 GeV is preferred. Triplet Higgs bosons arise in many extensions of the standard model, including the left-right model and the Little Higgs models. Our result demonstrates the importance of the scalar loops when there is a large mass splitting between the heavy scalars. It also indicates the significance of the global fit.

The ground and first excited states of the hybrid charmonium $\overline{c}cg$, with nonexotic quantum numbers $J^{PC}=0^{-+}$, $1^{--}$, and $1^{++}$ are investigated using quenched lattice QCD. The excited states are completely ignored in the literature. However, we observe strong gluonic radial excitations in the first excited states; We find their masses are completely different from the first excited states of the corresponding conventional charmonium. Their relevance to the recent discovery of the $Y(4260)$ state and future experimental search for other states are also discussed.

It has been shown that in the twisted mass formulation of Lattice QCD at maximal twist large cutoff effects are generated when the quark mass becomes of $\mathcal{O}(a{\Lambda }_{\mathrm{QCD}}^2)$. In general, these effects can be suppressed in two ways: either by choosing the critical quark mass in an “optimal way” or by adding the Clover term to the twisted action. We investigate the second option by performing a quenched lattice QCD simulation with twisted Clover fermions and pion masses as low as 280 MeV. We show that the Clover term is indeed efficient in reducing the large cutoff effects. In particular, the so-called bending phenomenon observed in the determination of the pion decay constant is cured in this way. In addition, by using maximally twisted Clover fermions, we provide a nonperturbative determination of the vector current renormalization constant $Z_V$ as well as of the nonperturbatively renormalized light quark masses. Finally, we calculate the connected contribution to the charged-neutral pseudoscalar meson mass splitting, finding that the introduction of the Clover term in the twisted action is also beneficial, in the quenched approximation, in reducing cutoff effects related to the isospin symmetry breaking at finite lattice spacing.

Flux-tube model predictions for hybrid-meson decays are beginning to be confronted by Lattice QCD. We compare the two approaches for the $S$-wave decay of the exotic $1^{-+}$, and find excellent agreement. Results suggest that in Strong QCD $q\overline{q}$ creation occurs with $S=1$.

We determine the real parts of electroweak matching conditions relevant for top quark pair production close to threshold in $e^{+}{e}^{-}$ annihilation at next-to-next-to-leading logarithmic (NNLL) order. Numerically the corrections are comparable to the NNLL QCD corrections.

We evaluate the two-photon width of the pseudoscalar charmonia, ${\Gamma }_{\gamma \gamma }({\eta }_c)$ and ${\Gamma }_{\gamma \gamma }({\eta }_c^{\prime })$, within a Heavy-Quark Spin-Symmetry setting and show that whereas the former width agrees with experiment, the latter is more than twice larger than the recent measurement by CLEO. When binding-energy effects are included in the ${\eta }_c^{\prime }$ case, the discrepancy is worse, pointing out at a possible anomaly in the ${\eta }_c^{\prime }$ decay.

Continuous $U(1{)}_{L_{\mu }-L_{\tau }}$ symmetry can generate quasidegenerate mass spectrum for both left handed light and right handed heavy Majorana neutrinos assuming that the symmetry preserving nonzero parameters are nearly same. There is an accidental $\mu \tau$ exchange symmetry in the light and heavy neutrino Majorana mass terms. This implies ${\theta }_{13}=0$ and ${\theta }_{23}=\frac{\pi }{4}$. In addition it generates another zero mixing angle and one zero mass difference. We restrict ourselves to type-I See-Saw mechanism for generation of light neutrino mass. We have found that under $U(1{)}_{L_{\mu }-L_{\tau }}$ symmetry cosmological lepton asymmetry vanishes. We break $U(1{)}_{L_{\mu }-L_{\tau }}$ such a way that the $\mu \tau$ exchange symmetry preserves in the neutrino sector. We have seen that light neutrino phenomenology can be explained under soft breaking of this symmetry. We have observed that softness of this symmetry breaking depends on the degeneracy of the light neutrino mass spectrum. Quasidegeneracy of right handed neutrino mass spectrum opens an option for resonant leptogenesis. The degeneracy of the right handed neutrino mass spectrum is restricted through light neutrino data. We observed that for generation of right sized baryon asymmetry common neutrino mass scale $m_0$ have to be of the order of $\sqrt{\Delta m_{\mathrm{atm}}^2}$ and corresponding right handed neutrino mass scale have to be nearly ${10}^{13}\mathrm{GeV}$. We also have discussed the effect of RG evolution on light neutrino spectrum and also on baryon asymmetry.

Electromagnetic structure functions for electron-proton scattering are used as a test of the QCD improved parton model at low and moderate $Q^2$. Two parametrizations which work well in $ep$ scattering at low $Q^2$ are used to evaluate the inelastic muon neutrino-nucleon and muon antineutrino-nucleon cross sections for energies between 1–10 GeV, of interest in long baseline neutrino oscillation experiments. Cross sections are reduced when these low-$Q^2$ extrapolations are used.

The exclusive reactions $\gamma p\to {\overline{K}}^0{K}^{+}n$ and $\gamma p\to {\overline{K}}^0{K}^{0}p$ have been studied in the photon energy range 1.6–3.8 GeV, searching for evidence of the exotic baryon ${\Theta }^{+}(1540)$ in the decays ${\Theta }^{+}\to n{K}^{+}$ and ${\Theta }^{+}\to p{K}^0$. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. The integrated luminosity was about $70{\mathrm{pb}}^{-1}$. The reactions have been isolated by detecting the $K^{+}$ and proton directly, the neutral kaon via its decay to $K_S\to {\pi }^{+}{\pi }^{-}$ and the neutron or neutral kaon via the missing mass technique. The mass and width of known hyperons such as ${\Sigma }^{+}$, ${\Sigma }^{-}$ and $\Lambda (1116)$ were used as a check of the mass determination accuracy and experimental resolution. Approximately 100 000 ${\Lambda }^{*}(1520)$’s and 150 000 $\varphi$’s were observed in the ${\overline{K}}^0{K}^{+}n$ and ${\overline{K}}^0{K}^{0}p$ final state, respectively. No evidence for the ${\Theta }^{+}$ pentaquark was found in the $n{K}^{+}$ or $p{K}_S$ invariant mass spectra. Upper limits were set on the production cross section of the reaction $\gamma p\to {\overline{K}}^0{\Theta }^{+}$ as functions of center-of-mass angle, $n{K}^{+}$ and $p{K}_S$ masses. Combining the results of the two reactions, the 95% C.L. upper limit on the total cross section for a resonance peaked at 1540 MeV was found to be 0.7 nb. Within most of the available theoretical models, this corresponds to an upper limit on the ${\Theta }^{+}$ width, ${\Gamma }_{{\Theta }^{+}}$, ranging between 0.01 and 7 MeV.

We examine experimental constraints on scharm-stop flavor mixing in the minimal supersymmetric standard model, which arise from the experimental bounds on squark and Higgs boson masses, the precision measurements of $W$-boson mass and the effective weak mixing angle, as well as the experimental data on $B_s-{\overline{B}}_s$ mixing and $b\to s\gamma$. We find that the combined analysis can put rather stringent constraints on ${\tilde{c}}_L-{\tilde{t}}_L$ and ${\tilde{c}}_L-{\tilde{t}}_R$ mixings. As an illustration for the effects of such constraints, we examine various top-quark flavor-changing neutral-current processes induced by scharm-stop mixings at the LHC and find that their maximal rates are significantly lowered.

We describe a method that provides a model independent measurement of the $B_S$ fraction in ${\rm Y}(5S)$ resonance decays, $f_S$, using the relative rates of like-sign versus opposite-sign dileptons; the like-sign leptons result from $B^0$ and $B_S$ mixing. In addition, we show that determining the rates of single, double and triple $D_S^{\pm }$ mesons provides an alternative way of finding $f_S$.