There is no tree-level flavor changing neutral current (FCNC) in the standard model (SM) which contains only one Higgs doublet. If more Higgs doublets are introduced for various reasons, the tree-level FCNC would be inevitable, except that extra symmetry was imposed. Therefore, FCNC processes are an excellent probe for physics beyond the SM (BSM). In this paper, we study the lepton flavor violated decay processes $h\to \mu \tau$ and $\tau \to \mu \gamma$ induced by the Higgs boson-$\mu \text{−}\tau$ vertex. For $\tau \to \mu \gamma$, its branching ratio is also related to the $ht\overline{t}$, $h{\tau }^{+}{\tau }^{-}$, and $h{W}^{+}{W}^{-}$ vertices. We categorize the BSM into two scenarios for the Higgs boson coupling strengths near or away from the SM. For the latter scenario, we take the spontaneously broken two Higgs doublet model (the Lee model) as an example. We consider the constraints by recent data from the LHC and B factories, and we find that the measurements give weak constraints. At LHC run II, $h\to \mu \tau$ will be confirmed or will have a stricter limit set on its branching ratio. Accordingly, $\mathrm{Br}(\tau \to \mu \gamma )\lesssim \mathcal{O}({10}^{-10}-{10}^{-8})$ for general chosen parameters. For the positive case, $\tau \to \mu \gamma$ can be discovered with $\mathcal{O}({10}^{10})$ $\tau$ pair samples at the SuperB factory, the Super $\tau$-charm factory, and the new Z factory. The future measurements for $\mathrm{Br}(h\to \mu \tau )$ and $\mathrm{Br}(\tau \to \mu \gamma )$ will be used to distinguish these two scenarios or set strict constraints on the correlations among different Higgs couplings.

We study a system of nanowires, i.e., the theory of $1+1$ dimensional massless fermions interacting with $3+1$ dimensional U(1) gauge fields. When allowing for nonzero chemical potentials, this system has a complex action problem in the conventional formulation. We show that the partition sum can be mapped to a dual representation where the fermions correspond to dimers and oriented loops on two-dimensional planes embedded in 4 dimensions. The dual degrees of freedom for the gauge fields are surfaces that either are closed or bounded by the fermion loops. In terms of the dual variables the complex action problem is overcome and Monte Carlo simulations are possible for arbitrary chemical potentials.

We study the chiral behavior of the electromagnetic (EM) form factors of pions and kaons in three-flavor lattice QCD. In order to make a direct comparison of the lattice data with chiral perturbation theory (ChPT), we employ the overlap quark action that has exact chiral symmetry. Gauge ensembles are generated at a lattice spacing of 0.11 fm with four pion masses ranging between $M_{\pi }\simeq 290\mathrm{MeV}$ and 540 MeV and with a strange quark mass $m_s$ close to its physical value. We utilize the all-to-all quark propagator technique to calculate the EM form factors with high precision. Their dependence on $m_s$ and on the momentum transfer is studied by using the reweighting technique and the twisted boundary conditions for the quark fields, respectively. A detailed comparison with SU(2) and SU(3) ChPT reveals that the next-to-next-to-leading order terms in the chiral expansion are important to describe the chiral behavior of the form factors in the pion mass range studied in this work. We estimate the relevant low-energy constants and the charge radii, and find reasonable agreement with phenomenological and experimental results.

Using the vector meson dominance ansatz for the ${\mathrm{\Lambda }}_c\to Vp\to l^{+}{l}^{-}p$ decay, we analyzed the distributions of differential decay widths in the $R$-parity-violating supersymmetric model and the extra heavy up-vector-like quark model. It is shown that the new physics contribution to this decay is very small.

We investigate the impact of unquenching effects on QCD Green’s functions, in the form of quark-loop contributions to both the gluon propagator and three-gluon vertex, in a three-loop inspired truncation of the three-particle irreducible (3PI) effective action. The fully coupled system of Dyson-Schwinger equations for the quark-gluon, ghost-gluon and three-gluon vertices, together with the quark propagator, are solved self-consistently; our only input are the ghost and gluon propagators themselves that are constrained by calculations within lattice QCD. We find that the two different unquenching effects have roughly equal, but opposite, impact on the quark-gluon vertex and quark propagator, with an overall negative impact on the latter. By taking further derivatives of the 3PI effective action, we construct the corresponding quark-antiquark kernel of the Bethe-Salpeter equation for mesons. The leading component is gluon exchange between two fully dressed quark-gluon vertices, thus introducing for the first time an obvious scalar-scalar component to the binding. We gain access to time-like properties of bound states by analytically continuing the coupled system of Dyson-Schwinger equations to the complex plane. We observe that the vector axial-vector splitting is in accord with experiment and that the lightest quark-antiquark scalar meson is above 1 GeV in mass.

The latest data released by the BABAR Collaboration on azimuthal correlations measured for pion-kaon and kaon-kaon pairs produced in $e^{+}{e}^{-}$ annihilations allow, for the first time, a direct extraction of the kaon Collins functions. These functions are then used to compute the kaon Collins asymmetries in semi-inclusive deep inelastic scattering processes, which result in good agreement with the measurements performed by the HERMES and COMPASS collaborations.

We present an $A4$-based model where neutrino masses arise from a combination of seesaw mechanisms. The model is motivated by several small mixing and mass parameters indicated by the data. These are ${\theta }_{13}$, the solar mass splitting, and the small deviation of ${\theta }_{23}$ from maximal mixing ($=\pi /4$). We take the above as indications that at some level the small quantities are well approximated by zero. In particular, the mixing angles to zeroth order should be either 0 or $\pi /4$. Accordingly, in this model the type-II seesaw dominates and generates the larger atmospheric mass splitting and sets ${\theta }_{23}=\pi /4$. The other mixing angles are vanishing as is the solar splitting. We show how the $A4$ assignment for the lepton doublets leads to this form. We also specify the $A4$ properties of the right-handed neutrinos which result in a smaller type-I seesaw contribution that acts as a perturbation and shifts the angles ${\theta }_{12}$ and ${\theta }_{13}$ into the correct range and the desired value of $\mathrm{\Delta }{m}_{\text{solar}}^2$ is produced. The $A4$ symmetry results in relationships between these quantities as well as with a small deviation of ${\theta }_{23}$ from $\pi /4$. If the right-handed neutrino mass matrix $M_R$ is chosen real then there is no leptonic $CP$ violation and only normal ordering is admissible. If $M_R$ is complex then inverted ordering is also allowed with the proviso that the $CP$ phase $\delta$ is large, i.e., $\sim \pi /2$ or $-\pi /2$. The preliminary results from $\mathrm{NO}\nu \mathrm{A}$ favoring normal ordering and $\delta$ near $-\pi /2$ imply quasidegenerate neutrino masses in this model.

At zero temperature, the Lorentz invariance is strictly preserved in three-dimensional quantum electrodynamics. This property ensures that the velocity of massless fermions is not renormalized by the gauge interaction. At finite temperature, however, the Lorentz invariance is explicitly broken by the thermal fluctuation. The longitudinal component of gauge interaction becomes short ranged due to thermal screening, whereas the transverse component remains long ranged because of local gauge invariance. The transverse gauge interaction leads to singular corrections to the fermion self energy and thus results in an unusual renormalization of the fermion velocity. We calculate the renormalized fermion velocity $v^R(p_0,\mathbf{p},T)$ by employing a renormalization group analysis, and discuss the influence of the anomalous dimension ${\eta }_n$ on the fermion specific heat.

We present a gauged twistor model of a free massive spinning particle in four-dimensional Minkowski space. This model is governed by an action, referred to here as the gauged generalized Shirafuji (GGS) action, that consists of twistor variables, auxiliary variables, and $U(1)$ and $SU(2)$ gauge fields on the one-dimensional parameter space of a particle’s worldline. The GGS action remains invariant under reparametrization and the local $U(1)$ and $SU(2)$ transformations of the relevant variables, although the $SU(2)$ symmetry is nonlinearly realized. We consider the canonical Hamiltonian formalism based on the GGS action in the unitary gauge by following Dirac’s recipe for constrained Hamiltonian systems. It is shown that just sufficient constraints for the twistor variables are consistently derived by virtue of the gauge symmetries of the GGS action. In the subsequent quantization procedure, these constraints turn into simultaneous differential equations for a twistor function. We perform the Penrose transform of this twistor function to define a massive spinor field of arbitrary rank, demonstrating that the spinor field satisfies generalized Dirac-Fierz-Pauli equations with $SU(2)$ indices. We also investigate the rank-one spinor fields in detail to clarify the physical meanings of the $U(1)$ and $SU(2)$ symmetries.

We study four-dimensional quantum gravity using nonperturbative renormalization group methods. We solve the corresponding equations for the fully momentum-dependent propagator, Newtons coupling and the cosmological constant. For the first time, we obtain a global phase diagram where the non-Gaussian ultraviolet fixed point of asymptotic safety is connected via smooth trajectories to a classical infrared fixed point. The theory is therefore ultraviolet complete and deforms smoothly into classical gravity as the infrared limit is approached.

We construct a class of Einstein-vector theories where the vector field couples bilinearly to the curvature polynomials of arbitrary order in such a way that only the Riemann tensor rather than its derivative enters the equations of motion. The theories can thus be ghost free. The $U(1)$ gauge symmetry may emerge in the vacuum and also in some weak-field limit. We focus on the two-derivative theory and study a variety of applications. We find that in this theory, the energy-momentum tensor of dark matter provides a position-dependent gauge-violating term to the Maxwell field. We also use the vector as an inflaton and construct cosmological solutions. We find that the expansion can accelerate without a bare cosmological constant, indicating a new candidate for dark energy. Furthermore, we obtain exact solutions of de Sitter bounce, generated by the vector which behaves like a Maxwell field at later times. We also obtain a few new exact black holes that are asymptotic to flat and Lifshitz spacetimes. In addition, we construct exact wormholes and Randall-Sundrum II domain walls.

The redshift dependence of the cosmic microwave background temperature is one of the key cosmological observables. In the standard cosmological model, one has $T(z)=T_0(1+z)$, where $T_0$ is the present-day temperature. Deviations from this behavior would imply the presence of new physics. Here we discuss how the combination of all currently available direct and indirect measurements of $T(z)$ constrains the common phenomenological parametrization $T(z)=T_0(1+z{)}^{1-\beta }$ and obtain the first subpercent constraint on the temperature growth index $1-\beta$. Specifically, we find $\beta =(7.6\pm 8.0)\times {10}^{-3}$ at the 68.3% confidence level.

We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10–500 s in a frequency band of 40–1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between $3.4\times 10^{-5}$ and $9.4\times 10^{-4}{\mathrm{Mpc}}^{-3}{\mathrm{yr}}^{-1}$ at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves.

We propose a realistic model with Majorana neutrinos in the framework of unifying the three generations of fermions by point interactions in an extra dimension. This model can simultaneously explain the origin of fermion generations, fermion masses and mixing, and the smallness of the masses of Majorana neutrinos. We show that there are two mechanisms working together to suppress the neutrino masses significantly, so we do not have to introduce a very large extra-dimension cutoff scale. One is the type-I seesaw mechanism and the other is the overlap integration of localized lepton wave functions. A singlet scalar with an exponential-like vacuum expectation value plays a central role in these two mechanisms. For consistency in this model we introduce a $U(1{)}^{\prime }$ gauge symmetry, which will be broken by the singlet scalar. Parameters of our model can fit the masses and flavor mixing data well. These parameters can also predict all $CP$ violating phases including the Majorana ones and accidentally rescue the proton from decay.

We estimate constraints on the existence of a heavy, mostly sterile neutrino with mass between 10 eV and 1 TeV. We improve upon previous analyses by performing a global combination and expanding the experimental inputs to simultaneously include tests for lepton universality, lepton-flavor-violating processes, electroweak precision data, dipole moments, and neutrinoless double beta decay. Assuming the heavy neutrino and its decay products are invisible to detection, we further include, in a self-consistent manner, constraints from direct kinematic searches, the kinematics of muon decay, cosmology, and neutrino oscillations, in order to estimate constraints on the values of $|U_{e4}{|}^2$, $|U_{\mu 4}{|}^2$, and $|U_{\tau 4}{|}^2$.

In this paper we apply a supersymmetric generalization of the method of derivative expansion to compute the induced non-Abelian Chern-Simons term in $\mathcal{N}=1$, $d=3$ superspace, for an arbitrary gauge group.

There exists a growing evidence for the anomalous transparency of the Universe for energetic gamma rays. Popular explanations include conversion of photons into hypothetical axionlike particles (ALPs) and back in astrophysical magnetic fields. Two distinctive scenarios of this conversion have been put forward: either it happens in the (host galaxy of the) gamma-ray source and in the Milky Way, or the photon-ALP oscillations take place in the intergalactic magnetic fields all along the way between the source and the observer. Here we point out that, given recent astrophysical constraints on ALPs and on intergalactic magnetic fields, these two mechanisms imply very different ALP masses and couplings. Therefore, confirmation of the anomalies and identification of one of the scenarios would mean cornering of ALP parameters to a particular narrow region. We discuss approaches to distinguish between the two mechanisms and present some preliminary indications in favor of the galactic scenario.

The theory of very special relativity (VSR) proposed by Cohen and Glashow contains an intrinsic preferred direction. Starting from the irreducible unitary representation of the inhomogeneous VSR group $ISIM(2)$, we present a rigorous construction of quantum field theory with a preferred direction. We find that although the particles and their quantum fields between the VSR and Lorentz sectors are physically different, they share many similarities. The massive spin-half and spin-one vector fields are local and satisfy the Dirac and Proca equations, respectively. This result can be generalized to higher-spin field theories. By studying the Yukawa and standard gauge interactions, we obtain a qualitative understanding on the effects of the preferred direction. Its effect is manifest for polarized processes but are otherwise absent.

Two new families of exact solutions to the Einstein equations for a conformastatic spacetime with axial symmetry are presented which describe thin disks of dust immersed in a spheroidal halo. The solutions are obtained by expressing the metric function in terms of an auxiliary function which satisfies the Laplace equation, a characteristic property of the conformastatic spacetimes. The first family of solutions is obtained from the displacement, cut, and reflection method, which introduces a discontinuity in the first $z$ derivate of the metric tensor across the plane of the disk. The second family of solutions is obtained by using the oblate spheroidal coordinates because they adapt to the shape of the source and introduce naturally a cutting radius for the disk. The energy densities of the disk and the halo are positive everywhere and well behaved, and their energy-momentum tensor agrees with all the energy conditions. Some particular solutions for the energy density of the disk and the halo are presented, and the rotational curves are obtained by solving the geodesic equation for a particle that moves in circular orbits in the plane of the disk.