String theories with topologically complex compactification manifolds suggest the simultaneous presence of many unbroken $U(1)$’s without any light matter charged under them. The gauge bosons associated with these $U(1)$’s do not have direct observational consequences. However, in the presence of low energy supersymmetry the gauge fermions associated with these $U(1)$’s, the “photini,” mix with the bino and extend the minimal supersymmetric standard model neutralino sector. This leads to novel signatures at the LHC. The lightest ordinary supersymmetric particle (LOSP) can decay to any one of these photini. In turn, photini may transition into each other, leading to high lepton and jet multiplicities. Both the LOSP decays and interphotini transitions can lead to displaced vertices. When the interphotini decays happen outside the detector, the cascades can result in different photini escaping the detector, leading to multiple reconstructed masses for the invisible particle. If the LOSP is charged, it stops in the detector and decays out of time to photini, with the possibility that the produced final photini vary from event to event. Observation of a plenitude of photini at the LHC would be evidence that we live in a string vacuum with a topologically rich compactification manifold.

We present a simple supersymmetric model where the dominant decay mode of the lightest Higgs boson is $h\to 2\eta \to 4c$, where $\eta$ is a light pseudoscalar and $c$ is the charm quark. For such decays the Higgs mass can be smaller than 100 GeV without conflict with experiment. Together with the fact that both the Higgs and the pseudoscalar $\eta$ are pseudo-Goldstone bosons, this resolves the little hierarchy problem.

We study the phenomenology of the charged Higgs boson in the flipped two-Higgs-doublet model, in which one doublet gives mass to up-type quarks and charged leptons, and the other gives mass to down-type quarks. We present the charged Higgs branching ratios and summarize the indirect constraints. We extrapolate existing LEP searches for $H^{+}{H}^{-}$ and Tevatron searches for $t\overline{t}$ with $t\to H^{+}b$ into the flipped model and extract constraints on $M_{H^{+}}$ and the parameter $\tan \beta$. We finish by reviewing existing LHC charged Higgs searches and suggest that the LHC reach in this model could be extended for charged Higgs masses below the $tb$ threshold by considering $t\overline{t}$ with $t\to H^{+}b$ and $H^{+}\to q{\overline{q}}^{\prime }$, as has been used in Tevatron searches.

We study the static potential of a color singlet quark-antiquark pair with (fixed) distance $r$ in $D=3$ and $D=2$ space-time dimensions at weak coupling ($\alpha r\ll 1$ and $gr\ll 1$, respectively). Using the effective theory potential nonrelativistic QCD (pNRQCD) we determine the ultrasoft contributions, which cannot be computed in conventional perturbative QCD. We show in detail how the ultrasoft renormalization in pNRQCD is carried out. In three dimensions the precision of our results reaches $\mathcal{O}({\alpha }^3{r}^2)$, i.e. next-to-next-to-leading order (NNLO) in the multipole expansion, and next-to-next-to-leading logarithmic order (NNLL) in a $\alpha /\Delta V$ expansion, where $\Delta V\sim \alpha \ln (\alpha r)$. We even present results up to partly ${\mathrm{N}}^4\mathrm{LL}$ order and compare them to existing lattice data. Finally we discuss the relevance of the perturbative calculation in two dimensions, where the exact result is known.

We discuss the production of $W$ bosons in association with three jets at the LHC. We investigate how next-to-leading order QCD corrections modify basic kinematic distributions of jets and leptons. We also address the magnitude of next-to-leading QCD effects in $W+3$ jet observables, relevant for supersymmetry searches at the LHC.

At a 14 TeV proton-proton collider, the Large Hadron Collider (LHC), we show that top quark pair production is dominated at low invariant mass by the fusion of two like-helicity gluons, producing top quark pairs in the left-left or right-right helicity configurations. Whereas, at higher invariant mass the production is dominated by the fusion of unlike-helicity gluons, producing top quark pairs in the up-down or down-up off-diagonal configurations, identical to top quark pair production via quark-antiquark annihilation. We study in detail the low invariant mass region, and show that the spin correlations can be easily observed in this region by looking at the distribution of the difference in the azimuthal angles, $\Delta \varphi$, of the dileptons decay products of the top quarks in the laboratory frame. Because of the large cross section for top pair production at the LHC, even with a cut requiring that the invariant mass of the top quark pair be less than 400 GeV, the approximate yield would be ${10}^4$ dilepton $(\mathrm{e},\mu )$ events per ${\mathrm{fb}}^{-1}$ before detector efficiencies are applied. Therefore, there is ample statistics to form the $\Delta \varphi$ distribution of the dilepton events, even with the invariant mass restriction. We also discuss possibilities for observing these spin correlations in the lepton plus jets channel.

We perform an update of the next-to-leading order calculation of the rate for Higgs boson production in association with two jets. Our new calculation incorporates the full analytic result for the one-loop virtual amplitude. This new theoretical information allows us to construct a code including the decay of the Higgs boson without incurring a prohibitive penalty in computer running time. Results are presented for the Tevatron, where implications for the Higgs search are sketched, and also for a range of scenarios at the LHC.

Instanton effects on the Euclidean correlation function of two Wilson loops at an angle $\theta$, relevant to soft high-energy dipole-dipole scattering, are calculated in the instanton liquid model and compared with the existing lattice data. Moreover, the instanton-induced dipole-dipole potential is obtained from the same correlation function at $\theta =0$, and compared with preliminary lattice data.

In this work we study the two-body hadronic charmed meson decays, including both the $PP$ and $VP$ modes. The latest experimental data are first analyzed in the diagrammatic approach. The magnitudes and strong phases of the flavor amplitudes are extracted from the Cabibbo-favored decay modes using ${\chi }^2$ minimization. The best-fitted values are then used to predict the branching fractions of the singly Cabibbo-suppressed and doubly Cabibbo-suppressed decay modes in the flavor SU(3) symmetry limit. We observe significant SU(3) breaking effects in some of the singly Cabibbo-suppressed channels. In the case of $VP$ modes, we point out that the $A_P$ and $A_V$ amplitudes cannot be completely determined based on currently available data. We conjecture that the quoted experimental results for both $D_s^{+}\to {\overline{K}}^0{K}^{*+}$ and $D_s^{+}\to {\rho }^{+}{\eta }^{\prime }$ are overestimated. We compare the sizes of color-allowed and color-suppressed tree amplitudes extracted from the diagrammatical approach with the effective parameters $a_1$ and $a_2$ defined in the factorization approach. The ratio $|a_2/a_1|$ is more or less universal among the $D\to \overline{K}\pi$, ${\overline{K}}^{*}\pi$, and $\overline{K}\rho$ modes. This feature allows us to discriminate between different solutions of topological amplitudes. For the long-standing puzzle about the ratio $\Gamma (D^0\to K^{+}{K}^{-})/\Gamma (D^0\to {\pi }^{+}{\pi }^{-})$, we argue that, in addition to the SU(3) breaking effect in the spectator amplitudes, the long-distance resonant contribution through the nearby resonance $f_0(1710)$ can naturally explain why $D^0$ decays more copiously to $K^{+}{K}^{-}$ than ${\pi }^{+}{\pi }^{-}$ through the $W$-exchange topology. This has to do with the dominance of the scalar glueball content of $f_0(1710)$ and the chiral-suppression effect in the decay of a scalar glueball into two pseudoscalar mesons. The same final-state interaction also explains the occurrence of $D^0\to K^0{\overline{K}}^0$ and its vanishing amplitude when SU(3) flavor symmetry is exact. Owing to the $G$-parity selection rule, $D_s^{+}\to {\pi }^{+}\omega$ does not receive contributions from the short-distance $W$-annihilation and resonant final-state interactions, but it can proceed through the weak decays $D_s^{+}\to {\rho }^{+}{\eta }^{{(}^{\prime })}$ followed by the final-state rescattering of ${\rho }^{+}{\eta }^{{(}^{\prime })}$ into ${\pi }^{+}\omega$ through quark exchange.

Looking for an observable manifestation of the so-called unnaturalness of scalar fields, we introduce a seemingly new set of differential equations for connected Green functions. These equations describe the momentum dependence of the Green functions and are close relatives to the previously known renormalization group equations. Applying the new equations to the theory of scalar field with ${\varphi }^4$ interaction, we identify a relation between the four-point Green function and the propagator that expresses the unnaturalness of the scalar field. Possible manifestations of the unnaturalness at low momenta are briefly discussed.

We explore calculable models with low-energy supersymmetry where the flavor hierarchy is generated by quark and lepton compositeness, and where the composites emerge from the same sector that dynamically breaks supersymmetry. The observed pattern of standard model fermion masses and mixings is obtained by identifying the various generations with composites of different dimension in the ultraviolet. These “single-sector” supersymmetry-breaking models give rise to various spectra of soft masses which are, in many cases, quite distinct from what is commonly found in models of gauge or gravity mediation. In typical models which satisfy all flavor-changing neutral current constraints, both the first- and second-generation sparticles have masses of order 20 TeV, while the stop mass is a few TeV. In other cases, all sparticles obtain masses of order a few TeV predominantly from gauge mediation, even though the first two generations are composite.

$SU(2{)}_L$ triplet scalars appear in models motivated for the left-right symmetry, neutrino masses, and dark matter (DM), etc. If the triplets are the main decay products of the DM particle, and carry nonzero lepton numbers, they may decay dominantly into lepton pairs, which can naturally explain the current experimental results reported by PAMELA and Fermi-LAT or ATIC. We discuss this possibility in an extended left-right symmetric model in which the decay of the DM particle is induced by tiny soft charge-conjugation ($C$) violating interactions, and calculate the spectra for cosmic-ray positrons, neutrinos, and gamma rays. We show that the DM signals in the flux of high energy neutrinos can be significantly enhanced, as the triplets couple to both charged leptons and neutrinos with the same strength. In this scenario, the predicted neutrino-induced muon flux can be several times larger than the case in which the DM particle only directly decays into charged leptons. In addition, the charged components of the triplet may give an extra contribution to the high energy gamma rays through an internal bremsstrahlung process, which depends on the mass hierarchy between the DM particle and the triplet scalars.

It has been recently suggested that the anomalous excess of low-energy electronlike events observed by the MiniBooNE experiment could be explained by the radiative decay of a heavy sterile neutrino ${\nu }_h$ of the mass around 500 MeV with a muonic mixing strength in the range $|U_{\mu h}{|}^2\simeq (1–4)\times {10}^{-3}$. If such ${\nu }_h$ exists its admixtures in the decay $D_s^{+}\to {\mu }^{+}{\nu }_{\mu }$ would result in the decay $D_s^{+}\to {\mu }^{+}{\nu }_h$ with the branching fraction $\mathrm{Br}(D_s^{+}\to {\mu }^{+}{\nu }_h)\simeq (1.2–5.5)\times {10}^{-4}$, which is in the experimentally accessible range. Interestingly, the existence of the $D_s^{+}\to {\mu }^{+}{\nu }_h$ decay at this level may also explain why the currently measured decay rate of $D_s^{+}\to {\mu }^{+}{\nu }_{\mu }$ is slightly higher than the predicted one. This enhances motivation for a search for this decay mode and makes it interesting and complementary to neutrino experiments probing sterile-active neutrino mixing. Our estimates show that the sensitive search could be performed with the analysis of existing data, e.g. from the CLEO-c or another analogous experiment. The discrepancy between the measurements and theoretical description of the decay $D_s^{+}\to {\tau }^{+}{\nu }_{\tau }$ is also briefly discussed.

The quark-lepton complementarity (QLC) is very suggestive in understanding possible relations between quark and lepton mixing matrices. We explore the QLC relations in all the possible angle-phase parametrizations and point out that they can approximately hold in five parametrizations. Furthermore, the vanishing of the smallest mixing angles in the Cabibbo-Kobayashi-Maskawa and Pontecorvo-Maki-Nakagawa-Sakata matrices can make sure that the QLC relations exactly hold in those five parametrizations. Finally, the sensitivity of the QLC relations to radiative corrections is also discussed.

We consider Deutsch’s computational model of a quantum system evolving in a spacetime containing closed timelike curves. Although it is known that this model predicts nonlinear and nonunitary evolutions of the system, we demonstrate that it also gives rise to evolutions which are a discontinuous function of the input state. These discontinuities persist for the most natural modifications of Deutsch’s approach.

There exists several modified gravity theories designed to reproduce the empirical Milgrom’s formula, modified Newtonian dynamics (MOND). Here we derive analytical results in the context of the static weak-field limit of two of them (bimetric MOND, leading for a given set of parameters to the quasilinear MOND, and Bekenstein’s Tensor-Vector-Scalar). In this limit, these theories are constructed to give the same force field for spherical symmetry, but their predictions generally differ out of it. However, for certain realizations of these theories (characterized by specific choices for their free functions), the binding potential-energy of a system is increased, compared to its Newtonian counterpart, by a constant amount independent of the shape and size of the system. In that case, the virial theorem is exactly the same in these two theories, for the whole gravity regime and even outside of spherical symmetry, although the exact force fields are different. We explicitly show this for the force field generated by the two theories inside an elliptical shell. For more general free functions, the virial theorems are, however, not identical in these two theories. We finally explore the consequences of these analytical results for the two-body force.