We study the gauge covariance of the massive fermion propagator in three as well as four-dimensional quantum electrodynamics (QED). Starting from its value at the lowest order in perturbation theory, we evaluate a nonperturbative expression for it by means of its Landau-Khalatnikov-Fradkin (LKF) transformation. We compare the perturbative expansion of our findings with the known one-loop results and observe perfect agreement up to a gauge parameter independent term, a difference permitted by the structure of the LKF transformations.

We investigate radiation-reaction effects for a charged scalar particle accelerated by an external potential realized as a space-dependent mass term in quantum electrodynamics. In particular, we calculate the position shift of the final-state wave packet of the charged particle due to radiation at lowest order in the fine structure constant $\alpha$ and in the small $ħ$ approximation. We show that it disagrees with the result obtained using the Lorentz-Dirac formula for the radiation-reaction force, and that it agrees with the classical theory if one assumes that the particle loses its energy to radiation at each moment of time according to the Larmor formula in the static frame of the potential. However, the discrepancy is much smaller than the Compton wavelength of the particle. We also point out that the electromagnetic correction to the potential has no classical limit.

We investigate particle velocity in $\kappa$-Minkowski space-time, which is one of the realizations of noncommutative space-time. We emphasize that arrival time analyses by high-energy $\gamma$ rays or neutrinos, which have been considered as powerful tools to restrict the violation of Lorentz invariance, are not effective in detecting space-time noncommutativity. In contrast with these examples, we point out the possibility that low-energy massive particles play an important role in detecting it.

Gravitational waves generated by the final merger of compact binary systems depend on the structure of the binary’s members. If the binary contains neutron stars, measuring such waves can teach us about the properties of matter at extreme densities. Unfortunately, these waves are typically at high frequency where the sensitivity of broadband detectors is not good. Learning about dense matter from these waves will require networks of broadband detectors combined with narrowband detectors that have good sensitivity at high frequencies. This paper presents an algorithm by which a network can be “tuned,” in accordance with the best available information, in order to most effectively measure merger waves. The algorithm is presented in the context of a toy model that captures the qualitative features of narrowband detectors and of certain binary neutron star merger wave models. By using what is learned from a sequence of merger measurements, the network can be gradually tuned in order to accurately measure the waves. The number of measurements needed to reach this stage depends upon the waves’ signal strength, the number of narrowband detectors available for the measurement, and the detailed characteristics of the waves that carry the merger information. Future studies will go beyond this toy model, encompassing a more realistic description of both the detectors and the gravitational waves.

We address the issue of the reconstruction of the scalar potential of a two-Higgs-doublet model having in mind that of the MSSM. We first consider the general $\mathrm{CP}$ conserving dim-4 effective potential. To fully reconstruct this potential, we show that even if all the Higgs boson masses and their couplings to the standard model particles are measured, one needs not only to measure certain trilinear Higgs self-couplings but some of the quartic couplings as well. We also advocate expressing the Higgs self-couplings in the mass basis. We show explicitly that in the so-called decoupling limit the most easily accessible Higgs self-couplings are given in terms of the Higgs boson mass while all other dependences on the parameters of the general effective potential are screened. This also helps to easily explain how, in the MSSM, the largest radiative corrections which affect these self-couplings are reabsorbed by using the corrected Higgs boson mass. We also extend our analysis to higher order operators in the effective Higgs potential. While the above screening properties do not hold, we argue that these effects must be small and may not be measured considering the foreseen poor experimental precision in the extraction of the SUSY Higgs self-couplings.

Supersymmetry with bilinear R-parity violation provides a predictive framework for neutrino masses and mixings in agreement with current neutrino oscillation data. The model leads to striking signals at future colliders through the R-parity violating decays of the lightest supersymmetric particle (LSP). Here we study charged scalar lepton decays and demonstrate that if the scalar tau is the LSP (i) it will decay within the detector, despite the smallness of the neutrino masses, (ii) the relative ratio of branching ratios $\mathrm{Br}({\tilde{\tau }}_1\to e\sum {\nu }_i)/\mathrm{Br}({\tilde{\tau }}_1\to \mu \sum {\nu }_i)$ is predicted from the measured solar neutrino angle, and (iii) scalar muon and scalar electron decays will allow us to test the consistency of the model. Thus, bilinear R-parity breaking SUSY will be testable at future colliders also in the case where the LSP is not the neutralino.

We present results for $f_B,$ $f_{B_s},$ $f_D,$ $f_{D_s}$ and their ratios in the presence of two flavors of light sea quarks ${(N}_f=2).\mathrm{}$ We use Wilson light valence quarks and Wilson and static heavy valence quarks; the sea quarks are simulated with staggered fermions. Additional quenched simulations with nonperturbatively improved clover fermions allow us to improve our control of the continuum extrapolation. For our central values the masses of the sea quarks are not extrapolated to the physical u, d masses; that is, the central values are “partially quenched.” A calculation using “fat-link clover” valence fermions is also discussed but is not included in our final results. We find, for example, $f_B{=190\mathrm{}\left(7\right)(}_{-17}^{+24\mathrm{}}{\left)\right(}_{-2}^{+11\mathrm{}}{\left)\right(}_{-0}^{+8\mathrm{}})\mathrm{MeV},$ $f_{B_s}{/f}_B{=1.16\mathrm{}\left(1\right)\mathrm{}\left(2\right)\mathrm{}\left(2\right)(}_{-0}^{+4\mathrm{}}),$ $f_{D_s}{=241\mathrm{}\left(5\right)(}_{-26}^{+27\mathrm{}}{\left)\right(}_{-4}^{+9\mathrm{}}{\left)\right(}_{-0}^{+5\mathrm{}})\mathrm{MeV},$ and $f_B{/f}_{D_s}{=0.79\mathrm{}\left(2\right)(}_{-4}^{+5\mathrm{}}{\left)\right(3\left)\right(}_{-0}^{+5\mathrm{}}),$ where in each case the first error is statistical and the remaining three are systematic: the error within the partially quenched $N_f=2\mathrm{}$ approximation, the error due to the missing strange sea quark and to partial quenching, and an estimate of the effects of chiral logarithms at small quark mass. The last error, though quite significant in decay constant ratios, appears to be smaller than has been recently suggested by Kronfeld and Ryan, and Yamada. We emphasize, however, that as in other lattice computations to date, the lattice $u,d$ quark masses are not very light and chiral log effects may not be fully under control.

We adopt an effective-Lagrangian approach to compute the new-physics contributions to T-violating triple-product correlations in charmless ${\Lambda }_b$ decays. We use factorization and work to leading order in the heavy-quark expansion. We find that the standard-model (SM) predictions for such correlations can be significantly modified. For example, triple products which are expected to vanish in the SM can be enormous $(\sim 50\%)$ in the presence of new physics. By measuring triple products in a variety of ${\Lambda }_b$ decays, one can diagnose which new-physics operators are or are not present. Our general results can be applied to any specific model of new physics by simply calculating which operators appear in that model.

We describe two-flavor QCD lattice data for the pressure at a nonzero temperature and vanishing chemical potential within a quasiparticle model. Relying only on thermodynamic consistency, the model is extended to nonzero chemical potential. The results agree with lattice calculations in the region of a small chemical potential.

The evolution equation for active and sterile neutrinos propagating in a general anisotropic or polarized background environment is found and solved for a special case when heavy neutrinos do not decouple, resulting in nonunitary mixing among light neutrino states. Then new $\mathrm{CP}$ violating neutrino oscillation effects appear. In contrast with the standard unitary neutrino oscillations these effects can be visible even for two flavor neutrino transitions and even if one of the elements of the neutrino mixing matrix is equal to zero. They do not necessarily vanish with $\delta m^2\to 0$ and they are different for various pairs of flavor neutrino transitions $({\nu }_e\to {\nu }_{\mu }),$ $({\nu }_{\mu }\to {\nu }_{\tau }),$ $({\nu }_{\tau }\to {\nu }_e).\mathrm{}$ Neutrino oscillations in vacuum and Earth’s matter are calculated for some fixed baseline experiments and a comparison between unitary and nonunitary oscillations are presented. It is shown, taking into account the present experimental constraints, that heavy neutrino states can affect $\mathrm{CP}$ and T asymmetries. This is especially true in the case of ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations.

Graviton production due to collapsing extra dimensions is studied. The momenta lying in the extra dimensions are taken into account. A D-dimensional background is matched to an effectively four-dimensional standard radiation-dominated universe. Using observational constraints on the present gravitational wave spectrum, a bound on the maximal temperature at the beginning of the radiation era is derived. This expression depends on the number of extra dimensions, as well as on the D-dimensional Planck mass. Furthermore, it is found that the extra dimensions have to be large.

We consider the question raised by Unruh and Wald of whether mirrored boxes can self-accelerate in flat spacetime (the “self-accelerating box paradox”). From the point of view of the box, which perceives the acceleration as an impressed gravitational field, this is equivalent to asking whether the box can be supported by the buoyant force arising from its immersion in a perceived bath of thermal (Unruh) radiation. The perfect mirrors we study are of the type that rely on light internal degrees of freedom which adjust to and reflect impinging radiation. We suggest that a minimum of one internal mirror degree of freedom is required for each bulk field degree of freedom reflected. A short calculation then shows that such mirrors necessarily absorb enough heat from the thermal bath that their increased mass prevents them from floating on the thermal radiation. For this type of mirror the paradox is therefore resolved. We also observe that this failure of boxes to “float” invalidates one of the assumptions going into the Unruh-Wald analysis of entropy balances involving boxes lowered adiabatically toward black holes. Nevertheless, their broad argument can be maintained until the box reaches a new regime in which box-antibox pairs dominate over massless fields as contributions to thermal radiation.

We investigate the possibility of studying the $\overrightarrow{B}\pi$ form factor using the semi-inclusive decays $B^0\to {\pi }^{+}{+X}_q.$ In general, $\overrightarrow{B}\mathrm{PX}$ semi-inclusive decays involve several hadronic parameters. However, for $B^0\to {\pi }^{+}{X}_q$ decays, we find that in the factorization approximation, the only unknown hadronic parameters are the form factors $F_{0,1}^{\overrightarrow{B}\pi }.$ Therefore, these form factors can be studied in $B^0\to {\pi }^{+}{X}_q$ decays. Using theoretical model calculations for the form factors, the branching ratios for $B^0\to {\pi }^{+}{X}_d(\Delta S=0)$ and $B^0\to {\pi }^{+}{X}_s(\Delta S=-1),$ with $E_{\pi }>2.1\mathrm{}$ GeV, are estimated to be in the ranges of $(3.1\mathrm{}–4.9)\times {10}^{-5}\left[F_1^{\overrightarrow{B}\pi }\mathrm{}\right(0)/0.33\mathrm{}{]}^2$ and $(2.5\mathrm{}–4.2)\times {10}^{-5}\left[F_1^{\overrightarrow{B}\pi }\mathrm{}\right(0)/0.33\mathrm{}{]}^2,$ respectively, depending on the value of the $\mathrm{CP}$-violating phase $\gamma .$ The combined branching ratio for $B^0\to {\pi }^{+}{(X}_d{+X}_s)$ is about $7.4\times {10}^{-5}\left[F_1^{\overrightarrow{B}\pi }\mathrm{}\right(0)/0.33\mathrm{}{]}^2$ and is insensitive to $\gamma .$ We also discuss $\mathrm{CP}$ asymmetries in these decay modes.

In string theory, it is well known that any hard scattering amplitude inevitably suffers exponential suppression. We demonstrate that, if the string scale is $M_s<2\mathrm{}\mathrm{TeV},$ this intrinsically stringy behavior leads to a dramatic reduction in the QCD jet production rate with very high transverse momenta $p_T>~2\mathrm{TeV}$ at CERN LHC. This suppression is sufficient to be observed in the first year of low-luminosity running. Our prediction is based on the universal behavior of string theory, and therefore is qualitatively model independent. This signature is alternative and complementary to conventional ones such as Regge resonance (or string ball or black hole) production.

We consider models that are natural extensions of those where supersymmetry is broken at low energy scales and transmitted to visible matter by gauge interactions. We investigate the situation where the quark and lepton superfields of the minimal supersymmetric standard model (MSSM) are localized to a brane in a higher dimensional space while the messenger fields and the sector that breaks supersymmetry dynamically are localized to another brane in the same space. The MSSM gauge and Higgs fields are assumed to propagate in the bulk. If some of the messenger fields and the Higgs fields have the same quantum numbers, this allows the possibility of mixing between these fields so that the physical Higgs and messenger fields are admixtures of the brane and bulk fields. This manifests itself in direct couplings of the quark and lepton fields to the physical messengers which are proportional to the MSSM Yukawa couplings and hence preserve the flavor structure of the Cabibbo-Kobayashi-Masakawa (CKM) matrix. The result is new contributions to the soft supersymmetry breaking parameters that are related to the Yukawa couplings and which therefore naturally satisfy the constraints from flavor changing neutral currents. For messenger scales greater than 1000 TeV these new contributions are parametrically of the same order of magnitude as gauge mediation. This scenario naturally avoids the cosmological problems associated with stable messengers and admits a simple and natural solution to the μ problem based on the next-to-minimal supersymmetric standard model.

When one constructs specific models with a fundamental scale as low as the TeV scale, there arise many difficulties. In this Brief Report we examine hybrid inflation due to bulk scalar fields, which has been proposed to solve the problem of fine-tuning in producing density perturbations. We find that the Kaluza-Klein modes play significant roles, which enhance the speed of the phase transition and alter the reheating process. We also argue that a lower bound must be put to the fundamental scale in order to construct successful hybrid inflation due to bulk scalar fields.

To better understand the possible breakdown of locality in quantum gravitational systems, we pursue the identity of precursors in the context of the anti-de Sitter/conformal field theory correspondence. Holography implies a breakdown of standard bulk locality which we expect to occur only at extremely high energy. We consider precursors that encode bulk information causally disconnected from the boundary and whose measurement involves nonlocal bulk processes. We construct a toy model of holography which encapsulates the expected properties of precursors and compare it with previous such discussions. If these precursors can be identified in the gauge theory, they are almost certainly Wilson loops, perhaps with decorations, but the relevant information is encoded in the high-energy sector of the theory and should not be observable by low energy measurements. This would be in accord with the locality bound, which serves as a criterion for situations where breakdown of bulk locality is expected.

We study some of the novel properties of conformal field theories (CFTs) with noncompact target spaces as applied to string theory. Standard CFT results get corrected by boundary terms in the target space in a way consistent with the expected spacetime physics. For instance, one-point functions of general operators on the sphere and boundary operators on the disk need not vanish; we show that they are instead equal to boundary terms in spacetime. By applying this result to vertex operators for spacetime gauge transformations with support at infinity, we derive formulas for conserved gauge charges in string theory. This approach provides a direct CFT definition of Arnowitt-Deser-Misner (ADM) energy-momentum in string theory.

We consider nonplanar contributions to the correlation functions of Berenstein-Maldacena-Nastase (BMN) operators in free N = 4 super Yang-Mills theory. We recalculate these nonplanar contributions from a different kind of diagram and find some exact agreement. The vertices of these diagrams are represented by free planar three-point functions; thus, our calculations provide some interesting identities for correlation functions of BMN operators in N = 4 super Yang-Mills theory. These diagrams look very much like loop diagrams in a second quantized string field theory; thus these identities could possibly be interpreted as natural consequences of the $\mathrm{pp}$-wave/conformal field theory correspondence. For convenience we will call these diagrams “string theory diagram,” although there are reasonable doubts as to whether these calculations are truly string theory calculations since it is not known how to compute general loop amplitudes in a second quantized string theory.

A free massless scalar field is coupled to homogeneous and isotropic loop quantum cosmology. The coupled model is investigated in the vicinity of the classical singularity, where discreteness is essential and where the quantum model is nonsingular, as well as in the regime of large volumes, where it displays the expected semiclassical features. The particular matter content (massless, free scalar) is chosen to illustrate how the discrete structure regulates pathological behavior caused by kinetic terms of matter Hamiltonians (which in standard quantum cosmology leads to wave functions with an infinite number of oscillations near the classical singularity). Because of this modification of the small volume behavior the dynamical initial conditions of loop quantum cosmology are seen to provide a meaningful generalization of DeWitt’s initial condition.