We study the radiative pion decay of ${\pi }^{+}\to e^{+}{\nu }_e\gamma$ in the light-front quark model. We also summarize the result in the chiral perturbation theory. The vector and axial-vector hadronic form factors ($F_{V,A}$) for the $\pi \to \gamma$ transition are evaluated in the whole allowed momentum transfer. In terms of these momentum dependent form factors, we calculate the decay branching ratio and compare our results with the experimental data and other theoretical predictions in the literature. We also constrain the possible size of the tensor interaction in the light-front quark model.

We develop a second order formalism for massive spin $1/2$ fermions based on the projection over Poincaré invariant subspaces in the $(\frac{1}{2},0)\oplus (0,\frac{1}{2})$ representation of the homogeneous Lorentz group. Using the $U(1{)}_{\mathrm{em}}$ gauge principle we obtain a second order description for the electromagnetic interactions of a spin $1/2$ fermion with two free parameters, the gyromagnetic factor $g$ and a parameter $\xi$ related to odd-parity Lorentz structures. We calculate Compton scattering in this formalism. In the particular case $g=2$, $\xi =0$, and for states with well-defined parity, we recover Dirac results. In general, we find the correct classical limit and a finite value $r_c^2$ for the forward differential cross section, independent of the photon energy and of the value of the parameters $g$ and $\xi$. The differential cross section vanishes at high energies for all $g$, $\xi$ except in the forward direction. The total cross section at high energies vanishes only for $g=2$, $\xi =0$. We argue that this formalism is more convenient than Dirac theory in the description of low energy electromagnetic properties of baryons and illustrate the point with the proton case.

Using $347.5{\mathrm{fb}}^{-1}$ of data recorded by the BABAR detector at the PEP-II electron-positron collider, $244\times {10}^3$ signal events for the $D^{+}\to K^{-}{\pi }^{+}{e}^{+}{\nu }_e$ decay channel are analyzed. This decay mode is dominated by the ${\overline{K}}^{*}(892{)}^0$ contribution. We determine the ${\overline{K}}^{*}(892{)}^0$ parameters: $m_{K^{*}(892{)}^0}=(895.4\pm 0.2\pm 0.2)\mathrm{MeV}/c^2$, ${\Gamma }_{K^{*}(892{)}^0}^0=(46.5\pm 0.3\pm 0.2)\mathrm{MeV}/c^2$, and the Blatt-Weisskopf parameter $r_{\mathrm{BW}}=2.1\pm 0.5\pm 0.5(\mathrm{GeV}/c{)}^{-1}$, where the first uncertainty comes from statistics and the second from systematic uncertainties. We also measure the parameters defining the corresponding hadronic form factors at $q^2=0$ ($r_V=\frac{V(0)}{A_1(0)}=1.463\pm 0.017\pm 0.031$, $r_2=\frac{A_2(0)}{A_1(0)}=0.801\pm 0.020\pm 0.020$) and the value of the axial-vector pole mass parametrizing the $q^2$ variation of $A_1$ and $A_2$: $m_A=(2.63\pm 0.10\pm 0.13)\mathrm{GeV}/c^2$. The $S$-wave fraction is equal to $(5.79\pm 0.16\pm 0.15)\%$. Other signal components correspond to fractions below 1%. Using the $D^{+}\to K^{-}{\pi }^{+}{\pi }^{+}$ channel as a normalization, we measure the $D^{+}$ semileptonic branching fraction: $\mathcal{B}(D^{+}\to K^{-}{\pi }^{+}{e}^{+}{\nu }_e)=(4.00\pm 0.03\pm 0.04\pm 0.09)\times {10}^{-2}$, where the third uncertainty comes from external inputs. We then obtain the value of the hadronic form factor $A_1$ at $q^2=0$: $A_1(0)=0.6200\pm 0.0056\pm 0.0065\pm 0.0071$. Fixing the $P$-wave parameters, we measure the phase of the $S$ wave for several values of the $K\pi$ mass. These results confirm those obtained with $K\pi$ production at small momentum transfer in fixed target experiments.

We present a measurement of the charmless semileptonic decay $B^0\to {\pi }^{-}{\ell }^{+}\nu$ using a data sample containing $657\times {10}^6$ $B\overline{B}$ events collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider operating near the ${\rm Y}(4S)$ resonance. We determine the total branching fraction of the decay, $\mathcal{B}(B^0\to {\pi }^{-}{\ell }^{+}\nu )=(1.49\pm 0.04(\mathrm{stat})\pm 0.07(\mathrm{syst}))\times {10}^{-4}$. We also report a new precise measurement of the differential decay rate and extract the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ using model-independent and model-dependent approaches. From a simultaneous fit to the measured differential decay rate and lattice QCD results, we obtain $|V_{ub}|=(3.43\pm 0.33)\times {10}^{-3}$, where the error includes both experimental and theoretical uncertainties.

The Galilean conformal algebra (GCA) arises in taking the nonrelativistic limit of the symmetries of a relativistic conformal field theory in any dimensions. It is known to be infinite dimensional in all spacetime dimensions. In particular, the 2d GCA emerges out of a scaling limit of linear combinations of two copies of the Virasoro algebra. In this paper, we find metrics in dimensions greater than 2 which realize the finite 2d GCA (the global part of the infinite algebra) as their isometry by systematically looking at a construction in terms of cosets of this finite algebra. We list all possible subalgebras consistent with some physical considerations motivated by earlier work in this direction and construct all possible higher-dimensional nondegenerate metrics. We briefly study the properties of the metrics obtained. In the standard one higher-dimensional “holographic” setting, we find that the only nondegenerate metric is Minkowskian. In four and five dimensions, we find families of nontrivial metrics with a rather exotic signature. A curious feature of these metrics is that all but one of them are Ricci-scalar flat.

We describe general features of frequency-dependent charge transport near strongly interacting quantum critical points in $2+1$ dimensions. The simplest description using the AdS/CFT correspondence leads to a self-dual Einstein-Maxwell theory on ${\mathrm{AdS}}_4$, which fixes the conductivity at a frequency-independent self-dual value. We describe the general structure of higher-derivative corrections to the Einstein-Maxwell theory, and compute their implications for the frequency dependence of the quantum critical conductivity. We show that physical consistency conditions on the higher-derivative terms allow only a limited frequency dependence in the conductivity. The frequency dependence is amenable to a physical interpretation using transport of either particlelike or vortexlike excitations.

We calculate four classes of high-energy massive string scattering amplitudes of fermionic string theory at arbitrary mass levels in the Regge regime (RR). We show that all four leading order amplitudes in the RR can be expressed in terms of the Kummer function of the second kind. Based on the summation algorithm of a set of extended signed Stirling number identities, we show that all four ratios calculated previously by the method of decoupling of zero-norm states among scattering amplitudes in the Gross regime can be extracted from this Kummer function in the RR. Finally, we conjecture and give evidence that the existence of these four Gross regime ratios in the RR persists to subleading orders in the Regge expansion of all high-energy fermionic string scattering amplitudes.

We investigate the structure of two-point functions for the quantum field theory dual to an asymptotically Lorentzian Anti de Sitter (AdS) wormhole. The bulk geometry is a solution of five-dimensional second-order Einstein-Gauss-Bonnet gravity and causally connects two asymptotically AdS spacetimes. We revisit the Gubser-Klebanov-Polyakov-Witten prescription for computing two-point correlation functions for dual quantum field theories operators $\mathcal{O}$ in Lorentzian signature and we propose to express the bulk fields in terms of the independent boundary values ${\varphi }_0^{\pm }$ at each of the two asymptotic AdS regions; along the way we exhibit how the ambiguity of normalizable modes in the bulk, related to initial and final states, show up in the computations. The independent boundary values are interpreted as sources for dual operators ${\mathcal{O}}^{\pm }$ and we argue that, apart from the possibility of entanglement, there exists a coupling between the degrees of freedom living at each boundary. The ${\mathrm{AdS}}_{1+1}$ geometry is also discussed in view of its similar boundary structure. Based on the analysis, we propose a very simple geometric criterion to distinguish coupling from entanglement effects among two sets of degrees of freedom associated with each of the disconnected parts of the boundary.

The predictive soft-wall AdS/QCD model with a modified 5D metric at the infrared region is constructed to obtain a nontrivial dilaton solution for three flavor quarks $u$, $d$ and $s$. Such a model is shown to incorporate both the chiral symmetry breaking and linear confinement. After considering some high-order terms including the $\mathrm{U}(1{)}_L\times \mathrm{U}(1{)}_R$ chiral symmetry breaking term, we find that the resulting predictions for the SU(3) octet and singlet resonance states of pseudoscalar, scalar, vector and axial-vector mesons agree well with the experimentally confirmed resonance states. Contributions from the instanton effects given by the determinant term are also discussed. It is observed that the chiral symmetry breaking phenomena of $\mathrm{SU}(3{)}_L\times \mathrm{SU}(3{)}_R$ and $\mathrm{U}(1{)}_L\times \mathrm{U}(1{)}_R$ can be well described in this model, while the SU(3) flavor symmetry breaking effect due to quark mass difference in the source term is not enough to explain all of the current experimental data.

The most important criteria for a successful inflation are: explaining the observed temperature anisotropy in the cosmic microwave background radiation, and exiting inflation in a vacuum where it can excite the standard model quarks and leptons required for the success of big bang nucleosynthesis. In this paper, we provide the first ever closed-string model of inflation where the inflaton couplings to hidden sector, moduli sector, and visible sector fields can be computed, showing that inflation can lead to reheating the standard model degrees of freedom before the electro-weak scale.

Cosmological inflation generates a spectrum of density perturbations that can seed the cosmic structures we observe today. These perturbations are usually computed as the result of the gravitationally induced spontaneous creation of perturbations from an initial vacuum state. In this paper, we compute the perturbations arising from gravitationally induced stimulated creation when perturbations are already present in the initial state. The effect of these initial perturbations is not diluted by inflation and survives to its end, and beyond. We consider a generic statistical density operator $\rho$ describing an initial mixed state that includes probabilities for nonzero numbers of scalar perturbations to be present at early times during inflation. We analyze the primordial bispectrum for general configurations of the three different momentum vectors in its arguments. We find that the initial presence of quanta can significantly enhance non-Gaussianities in the so-called squeezed limit. Our results show that an observation of non-Gaussianities in the squeezed limit can occur for single-field inflation when the state in the very early inflationary Universe is not the vacuum, but instead contains early-time perturbations. Valuable information about the initial state can then be obtained from observations of those non-Gaussianities.

We calculate the next-to-leading-order (NLO) correction to the pion electromagnetic form factor at leading twist in the $k_T$ factorization theorem. Partons off-shell by $k_T^2$ are considered in both quark diagrams and effective diagrams for the transverse-momentum-dependent pion wave function. The light-cone singularities in the transverse-momentum-dependent pion wave function are regularized by rotating the Wilson lines away from the light cone. The soft divergences from gluon exchanges among initial- and fal-state partons cancel exactly. We derive the infrared-finite $k_T$-dependent NLO hard kernel for the pion electromagnetic form factor by taking the difference of the above two sets of diagrams. Varying the renormalization and factorization scales, we find that the NLO correction is smaller, when both the scales are set to the invariant masses of internal particles: it becomes lower than 40% of the leading-order contribution for momentum transfer squared $Q^2>7{\mathrm{GeV}}^2$. It is observed that the NLO leading-twist correction does not play an essential role in explaining the experimental data, but the leading-order higher-twist contribution does.

The existence of anisotropic stress of a purely geometrical origin seems to be a characteristic of higher order gravity models, and has been suggested as a probe to test these models observationally, for example, in weak lensing experiments. In this paper, we seek to find a class of higher order gravity models of $f(R,G)$ type that would give us a zero anisotropic stress and study the consequences for the viability of the actual model. For the special case of a de Sitter background, we identify a subclass of models with the desired property. We also find a direct link between anisotropic stress and the stability of the model as well as the presence of extra degrees of freedom, which seems to be a general feature of higher order gravity models. Particularly, setting the anisotropic stress equal to zero for a de Sitter background leads to a singularity that makes it impossible to reach the de Sitter evolution.

We study the cosmic microwave background temperature and polarization spectra sourced by multitension cosmic-superstring networks. First, we obtain solutions for the characteristic length scales and velocities associated with the evolution of a network of F-D strings, allowing for the formation of junctions between strings of different tensions. We find two distinct regimes describing the resulting scaling distributions for the relative densities of the different types of strings, depending on the magnitude of the fundamental string coupling $g_s$. In one of them, corresponding to the value of the coupling being of order unity, the network’s stress-energy power spectrum is dominated by populous light F and D strings, while the other regime, at smaller values of $g_s$, has the spectrum dominated by rare heavy D strings. These regimes are seen in the cosmic microwave background (CMB) anisotropies associated with the network. We focus on the dependence of the shape of the B-mode polarization spectrum on $g_s$ and show that measuring the peak position of the B-mode spectrum can point to a particular value of the string coupling. Finally, we assess how this result, along with pulsar bounds on the production of gravitational waves from strings, can be used to constrain a combination of $g_s$ and the fundamental string tension ${\mu }_F$. Since CMB and pulsar bounds constrain different combinations of the string tensions and densities, they result in distinct shapes of bounding contours in the $({\mu }_F,g_s)$ parameter plane, thus providing complementary constraints on the properties of cosmic superstrings.

The Zee model generates neutrino masses at the one-loop level by adding charged $SU(2{)}_L$-singlet and extra $SU(2{)}_L$-doublet scalars to the standard model of particle physics. We introduce the softly broken $A_4$ symmetry to the Zee model as the origin of the nontrivial structure of the lepton flavor mixing. This model is compatible with the tribimaximal mixing which agrees well with neutrino oscillation measurements. Then, a sum rule $m_1{e}^{i{\alpha }_{12}}+2m_2+3m_3{e}^{i{\alpha }_{32}}=0$ is obtained and it results in $\Delta m_{31}^2<0$ and $m_3\ge 1.8\times {10}^{-2}\mathrm{eV}$. The effective mass $|(M_{\nu }{)}_{ee}|$ for the neutrinoless double beta decay is predicted as $|(M_{\nu }{)}_{ee}|\ge 1.7\times {10}^{-2}\mathrm{eV}$. The characteristic particles in this model are $SU(2{)}_L$-singlet charged Higgs bosons $s_{\alpha }^{+}(\alpha =\xi ,\eta ,\zeta )$ which are made from a $\mathbf{3}$ representation of $A_4$. Contributions of $s_{\alpha }^{\pm }$ to the lepton flavor violating decays of charged leptons are almost forbidden by an approximately remaining $Z_3$ symmetry; only $\mathrm{BR}(\tau \to \overline{e}\mu \mu )$ can be sizable by the flavor changing neutral current interaction with $SU(2{)}_L$-doublet scalars. Therefore, $s_{\alpha }^{\pm }$ can easily be light enough to be discovered at the LHC with satisfying current constraints. The flavor structures of $\mathrm{BR}(s_{\alpha }^{-}\to \ell \nu )$ are also discussed.

The $\mathcal{F}$-lipped $SU(5)\times U(1{)}_X$ grand unified theory supplemented by TeV-scale vectorlike particles from $\mathcal{F}$-theory, together dubbed $\mathcal{F}\mathrm{\text{−}}SU(5)$, offers a natural multiphase unification process which suggests an elegant implementation of the no-scale supergravity boundary conditions at the unification scale $M_{\mathcal{F}}\simeq 7\times {10}^{17}\mathrm{GeV}$. Enforcing the no-scale boundary conditions, including $B_{\mu }(M_{\mathcal{F}})=0$ on the Higgs bilinear soft term, with the precision 7-year WMAP value on the dark matter relic density isolates a highly constrained “golden point” located near $M_{1/2}=455\mathrm{GeV}$ and $\tan \beta =15$ in the $\tan \beta –M_{1/2}$ plane, which simultaneously satisfies all known experiments, and moreover corresponds to an imminently observable proton decay rate. Because the universal gaugino mass is actually determined from established low energy data via renormalization group equation running, there are no surviving arbitrary scale parameters in the present model.

The LEP experiments give a lower bound on the neutralino mass of about 46 GeV which, however, relies on a supersymmetric grand unification relation. Dropping this assumption, the experimental lower bound on the neutralino mass vanishes completely. Recent analyses suggest, however, that in the minimal supersymmetric standard model (MSSM), a light neutralino dark matter candidate has a lower bound on its mass of about 7 GeV. In light of this, we investigate the mass sensitivity at the ILC for very light neutralinos. We study slepton pair production, followed by the decay of the sleptons to a lepton and the lightest neutralino. We find that the mass measurement accuracy for a few-GeV neutralino is around 2 GeV, or even less if the relevant slepton is sufficiently light. We thus conclude that the ILC can help verify or falsify the MSSM neutralino cold dark matter model even for very light neutralinos.

The decay mode $\psi (2S)\to J/\psi +\gamma \gamma$ is proposed in order to experimentally identify the effects of the coupling of charmonium states to the continuum $D\overline{D}$ states. To have a better understanding of such a two-photon decay process, in this work we restrict ourselves to investigate the contribution of the discrete part, in which the photons are mainly produced via the intermediate states ${\chi }_{cJ}(nP)$. Besides calculating the resonance contributions of ${\chi }_{cJ}(1P)(J=0,1,2)$, we also take into account the contributions of the higher excited states ${\chi }_{cJ}(2P)$ and the interference effect among the $1P$ and $2P$ states. We find that the contribution of the $2P$ states and the interference terms to the total decay width is very tiny. However, for specific regions of the Dalitz plot, off the resonance peaks, we find that these contributions are sizable and should also be accounted for. We also provide the photon spectrum and study the polarization of $J/\psi$.

By determining the quark momentum fractions of the octet baryons from $N_f=2+1$ lattice simulations, we are able to predict the degree of charge symmetry violation in the parton distribution functions of the nucleon. This is of importance, not only as a probe of our understanding of the nonperturbative structure of the proton, but also because such a violation constrains the accuracy of global fits to parton distribution functions and hence the accuracy with which, for example, cross sections at the LHC can be predicted. A violation of charge symmetry may also be critical in cases where symmetries are used to guide the search for physics beyond the standard model.

It is shown that a tensor-to-scalar ratio close to $r=0.03$, which can be observed by Planck, is realized in supersymmetric hybrid inflation models with TeV-scale soft supersymmetry breaking terms. This extends our previous analysis, which also found $r\lesssim 0.03$ but employed intermediate-scale soft terms. Other cosmological observables such as the scalar spectral index are in good agreement with the WMAP data.