We have studied the effects of the standard-model extension (SME) on hydrogen as a realization of new physics effects that incorporate Lorentz and $CPT$ violation. Specifically, we calculated the SME-induced energy level shifts at order ${\alpha }^2$ times the SME parameters. We obtained contributions at this order both from the nonrelativistic effective Hamiltonian for motion of a spin-$1/2$ particle in the presence of SME interactions and also from SME corrections to the propagator for exchange photons. We applied our result to the $2S-1S$ transition in hydrogen, which has been measured with extremely high precision. The results obtained in this work give the leading SME corrections for this transition.

We consider the origins of the gluon chain model. The model serves as a realization of the dynamics of the chromoelectric flux between static quark-antiquark sources. The derivation is based on the large-$N_C$ limit of the Coulomb gauge Hamiltonian in the presence of a background field introduced to model magnetic charge condensation inducing electric confinement.

We apply a perturbative approach to evaluate the Casimir energy for a massless real scalar field in $3+1$ dimensions, subject to Dirichlet boundary conditions on two surfaces. One of the surfaces is assumed to be flat, while the other corresponds to a small deformation, described by a single function $\eta$, of a flat mirror. The perturbative expansion is carried out up to the fourth order in the deformation $\eta$, and the results are applied to the calculation of the Casimir energy for corrugated mirrors in front of a plane. We also reconsider the proximity force approximation within the context of this expansion.

We show that the Gordon metric belongs to a larger class of geometries, which are responsible to describe the paths of accelerated bodies in moving dielectrics as geodesics in a metric ${\widehat{q}}_{\mu \nu }$ different from the background one. This map depends only on the background metric and on the motion of the bodies under consideration. As a consequence, this method describes a more general property that concerns the elimination of any kind of force acting on bodies by a suitable change of the substratum metric.

The spin foam formalism provides transition amplitudes for loop quantum gravity. Important aspects of the dynamics are understood, but many pressing questions remain. In this paper we address some of them using a twistorial description, which sheds new light on both classical and quantum aspects of the theory. At the classical level, we clarify the covariant properties of the discrete geometries involved, and the role of the simplicity constraints in leading to SU(2) Ashtekar-Barbero variables. We identify areas and Lorentzian dihedral angles in twistor space, and show that they form a canonical pair. The primary simplicity constraints are solved by simple twistors, parametrized by SU(2) spinors and the dihedral angles. We construct a SU(2) holonomy and prove it to correspond to the (lattice version of the) Ashtekar-Barbero connection. We argue that the role of secondary constraints is to provide a nontrivial embedding of the cotangent bundle of SU(2) in the space of simple twistors. At the quantum level, a Schrödinger representation leads to a spinorial version of simple projected spin networks, where the argument of the wave functions is a spinor instead of a group element. We rewrite the Liouville measure on the cotangent bundle of SL(2,C) as an integral in twistor space. Using these tools, we show that the Engle-Pereira-Rovelli-Livine transition amplitudes can be derived from a path integral in twistor space. We construct a curvature tensor, show that it carries torsion off shell, and that its Riemann part is of Petrov type D. Finally, we make contact between the semiclassical asymptotic behavior of the model and our construction, clarifying the relation of the Regge geometries with the original phase space.

Numerous claims in the literature suggest that gravity induced on a higher codimensional surface violates unitarity in the weak coupling regime. However, it remained unclear, why a conserved source localized on this surface and giving rise to an induced gravity term at low energies would absorb and emit the associated ghost, given a consistent source-free theory. In this article it is shown that the appearance of the induced Einstein-Hilbert term does not threaten the unitarity of the theory. It is shown that the would-be ghost highlighted in previous works is nondynamical and therefore not associated with a state in the Hilbert space. The physics arguments behind this statement are presented in a semicovariant language, but the detailed proof is given using Dirac’s constraint analysis. The Hamiltonian on the constraint surface of the linearized theory is derived and turns out to be manifestly positive definite. As a result of these investigations, brane-induced gravity goes without a ghost, opening an exciting window of opportunity for consistent deformations of gravity at the largest observable distances.

A largely unconstrained set of relativity-violating effects is studied via the gravitomagnetic effect on intrinsic spins. The results of existing comagnetometer experiments are used to place constraints on two new combinations of these effects at the 10% level. We show that planned improvements in these experiments will make them competitive with the best existing sensitivities to this elusive class of relativity-violating effects. Prospects for measuring the conventional general-relativistic gravitomagnetic effect are also considered.

We present analytic expressions for the cosmic string unequal time correlator (UETC) in the context of the Unconnected Segment Model. This eliminates the need to simulate the many thousands of network realizations needed to estimate the UETC numerically. With our approach we can compute the UETC very accurately, over all scales of interest, in the order of $\sim 20–30$ seconds on a single CPU. Our formalism facilitates an efficient approach to computing cosmic microwave background anisotropies for cosmic strings. Discretizing the UETC and performing an eigendecomposition to act as sources in the CAMB cosmic microwave background code, the power spectrum can be calculated by summing over a finite number of eigenmodes. A much smaller number of eigenmodes are required compared to the conventional approach of averaging power spectra over a finite number of realizations of the string network. With the additional efficiency and performance improvements offered by the OpenMP CAMB code, the time required to compute string power spectra is significantly reduced compared to the standard serial CMBACT code. The latter takes $\sim 30$ hours on a modern single threaded CPU for 2000 network realizations. Similar percent level accuracy can be achieved with our approach on a moderately threaded CPU (eight threads) in only $\sim 15$ minutes. If accuracy is only required at the 10% level and the CPU is more highly threaded, cosmic string power spectra are now possible in $\sim 2–3$ minutes. This makes exploration of the string parameter space now possible for Markov chain Monte Carlo analysis.

We make a comprehensive investigation of the observational effect of the inflation consistency relation. We focus on the general single-field inflation model with the consistency relation $r=-8c_s{n}_t$, and investigate the observational constraints of sound speed $c_s$ by using the seven-year WMAP data, the background imaging of cosmic extragalactic polarization tensor power spectrum data, and the constraints on non-Gaussian parameters $f_{\mathrm{NL}}^{\mathrm{equil}}$ and $f_{\mathrm{NL}}^{\mathrm{orth}}$ from the five-year WMAP observations. We find that the constraints on the tensor-to-scalar ratio $r$ is much tighter if $c_s$ is small, since a large tilt $n_t$ is strongly constrained by the observations. We obtain $r<0.37$, 0.27, and 0.09 ($d{n}_s/d\ln k=0$) for $c_s=1$, 0.1, and 0.01 models at 95.4% confidence level (CL). When taking smaller values of $c_s$, the positive correlation between $r$ and $n_s$ also leads to a slightly tighter constraint on the upper bound of $n_s$, while the running of scalar spectral index $d{n}_s/d\ln k$ is generally unaffected. For the sound speed $c_s$, it is not well constrained if only the cosmic microwave background power spectrum data is used, while the constraints are obtainable by taking $f_{\mathrm{NL}}^{\mathrm{equil}}$ and $f_{\mathrm{NL}}^{\mathrm{orth}}$ priors into account. With the constraining data of $f_{\mathrm{NL}}^{\mathrm{equil}}$ and $f_{\mathrm{NL}}^{\mathrm{orth}}$, we find that, $c_s\lesssim 0.01$ region is excluded at 99.7% CL, and the $c_s=1$ case (the single-field slow-roll inflation) is slightly disfavored at 68.3% CL. In addition, the inclusion of $f_{\mathrm{NL}}^{\mathrm{equil}.}$ and $f_{\mathrm{NL}}^{\mathrm{orth}.}$ into the analysis can improve the constraints on $r$ and $n_s$. We further discuss the implications of our constraints on the test of inflation models.

The path integral, which generates in-in correlation functions of a scalar field in a cosmological spacetime, is shown to admit nontrivial classical solutions as stationary phases. Although the solutions exist for the Lorentzian signature, their contribution to the path integral is reminiscent of that of the instantons in Euclidean field theories. When the scalar potential has more than one locally stable vacua, the correlation functions receive contributions from all of them via these instanton-like configurations, which is similar to tunneling. We present some explicit solutions for toy models and discuss possible implications of our results.

It was recently demonstrated that, when coupled to $\mathcal{N}=1$ supergravity, the Dirac-Born-Infeld (DBI) action constructed from a single chiral superfield has the property that when the higher-derivative terms become important, the potential becomes negative. Thus, DBI inflation cannot occur in its most interesting, relativistic regime. In this paper, it is shown how to overcome this problem by coupling the model to one or more additional chiral supermultiplets. In this way, one obtains effective single real scalar field DBI models with arbitrary positive potentials, as well as multiple real scalar field DBI inflation models with hybrid potentials.

We find ourselves in an extended era of entropy production. Unlike most other observations, the arrow of time is usually regarded as a constraint on initial conditions. I argue, however, that it primarily constrains the vacuum structure of the theory. I exhibit simple scalar field potentials in which low-entropy initial conditions are not necessary, or not sufficient, for an arrow of time to arise. In particular, in some models an arrow of time obtains even if the initial vacuum has arbitrarily large entropy. In such examples, the dynamical resolution of the arrow of time problem arises from properties of the vacuum structure similar to those that allow the string landscape to solve the cosmological constant problem without producing an empty universe, particularly its high dimensionality and the large difference in vacuum energy between neighboring vacua.

Current evidence for dark matter in the Universe does not exclude heavy composite nuclear-density objects consisting of bound quarks or antiquarks over a significant range of masses. Here we analyze one such proposed scenario, which hypothesizes antiquark nuggets with a range of $B\sim {10}^{24–30}$ with specific predictions for spectral emissivity via interactions with normal matter. We find that, if these objects make up the majority of the dark matter density in the solar neighborhood, their radiation efficiency in solids is marginally constrained, due to limits from the total geothermal energy budget of the Earth. At allowed radiation efficiencies, the number density of such objects can be constrained to be well below dark matter densities by existing radio data over a mass range currently not restricted by other methods.

We construct a minimal calculable model of a light dilaton based on the scenario where only top and Higgs sectors are involved in a quasiconformal dynamics. The model consistently accommodates the electroweak precision tests even when the Higgs boson is very heavy, thereby allowing one to consider the possibility that the particle at around 125 GeV, discovered at the LHC experiments, is identified as the light dilaton rather than the Higgs boson. We find that the current LHC data allow distinct parameter regions where the observed particle is either mostly the Higgs boson or the dilaton.

We construct a minimal model inspired by the general class of pyramid schemes [T. Banks and J.-F. Fortin, J. High Energy Phys. 07 (2009) 046], which is consistent with both supersymmetry breaking and electroweak symmetry breaking. In order to do computations, we make unjustified approximations to the low energy Kähler potential. The phenomenological viability of the resultant mass spectrum is then examined and compared with current collider limits. We show that for certain regimes of parameters, the model, and thus generically the pyramid scheme, can accommodate the current collider mass constraints on physics beyond the standard model with a tree-level light Higgs mass near 125 GeV. However, in this regime the model exhibits a little hierarchy problem, and one must permit fine-tunings that are of order 5%.

Supersymmetry broken geometrically in extra dimensions naturally leads to a nearly degenerate spectrum for superparticles, ameliorating the bounds from the current searches at the LHC. We present a minimal such model with a single extra dimension, and show that it leads to viable phenomenology despite the fact that it essentially has two fewer free parameters than the conventional constrained minimal supersymmetric standard model. The theory does not suffer from the supersymmetric flavor or $CP$ problem because of the universality of geometric breaking, and it automatically yields near-maximal mixing in the scalar top sector with $|A_t|\approx 2m_{\tilde{t}}$ to boost the Higgs boson mass. Despite the rather constrained structure, the theory is less fine-tuned than many supersymmetric models.

We present a next-to-leading order QCD calculation for ${\mathrm{e}}^{+}{\nu }_{\mathrm{e}}{\mu }^{+}{\nu }_{\mu }\mathrm{jj}$ production in vector-boson fusion, i.e., the scattering of two positively charged $W$ bosons at the LHC. We include the complete set of electroweak leading order diagrams for the six-particle final state and quantitatively assess the size of the $s$-channel and interference contributions in vector-boson fusion kinematics. The calculation uses the complex-mass scheme to describe the $W$-boson resonances and is implemented into a flexible Monte Carlo generator. Using a dynamical scale based on the transverse momenta of the jets, the QCD corrections stay below about 10% for all considered observables, while the residual scale dependence is at the level of 1%.

Interference phenomena observed in the $\psi (3770)$ resonance region in the reactions $e^{+}{e}^{-}\to D\overline{D}$ are analyzed. To avoid ambiguities in the determination of the $\psi (3770)$ resonance parameters, when analyzing data between the $D\overline{D}$ and $D\overline{D}\pi$ thresholds, the amplitudes satisfying the elastic unitarity requirement should be used. In the lack of information on the $P$ wave of $D\overline{D}$ elastic scattering, the $\psi (3770)$ parameters, determined by fitting the $e^{+}{e}^{-}\to D\overline{D}$ data, can essentially depend on the model used for the total contribution of the resonance and background. The selection of the models can be toughened by comparing their predictions with the relevant data on the shape of the $\psi (3770)$ peak in the non-$D\overline{D}$ channels $e^{+}{e}^{-}\to \gamma {\chi }_{c0}$, $J/\psi \eta$, $\varphi \eta$, etc.

We investigate the model dependence of meson resonance properties extracted from the Dalitz-plot analysis of the three-pions photo-production reactions on the nucleon. Within a unitary model developed in our earlier work, we generate Dalitz-plot distributions as data to perform an isobar model fit that is similar to most of the previous analyses of three-pion production reactions. It is found that the resonance positions from the two models agree well when both fit the data accurately, except for the resonance poles near branch points. The residues of the resonant amplitudes extracted from the two models and by the usual Breit-Wigner procedure agree well only for the isolated resonances with narrow widths. For overlapping resonances, most of the extracted residues could be drastically different. Our results suggest that even with high precision data, the resonance extraction should be based on models within which the amplitude parametrization is constrained by a three-body unitarity condition.

We propose the use of $N$-jettiness (${\tau }_N$), a global event shape, as a probe of nuclear dynamics in lepton-nucleus collisions. It characterizes the amount of soft radiation between the jet and nuclear beam directions. We give a new factorization formula for the 1-jettiness (${\tau }_1$) distribution for the production of a single hard jet ($J$) in lepton-nucleus collisions: $\ell +A(P)\to J(P_J)+X$. Each nuclear target gives rise to a unique pattern of radiation, affected by nuclear dynamics, that can be quantified by the ${\tau }_1$ distribution. Up to power corrections, the ${\tau }_1$ distribution allows for a direct measurement of the nuclear parton distribution functions. Additional nuclear-dependent effects will be dominated through power corrections of size $\sim Q_s^2(A)/({\tau }_1{P}_{JT})$, where $Q_s(A)$ is a dynamical nuclear scale and $P_{JT}$ is the transverse momentum of the jet. Such nuclear-dependent effects can be studied through a dedicated program to measure ${\tau }_1$ distributions for a range of nuclei and kinematics. We give numerical results for the 1-jettiness distribution for the simplest case of a proton target at next-to-leading-log accuracy.