121496385
Meson exchange currents in neutron-proton bremsstrahlung
Background: The meson exchange current (MEC) contribution is important in the neutron-proton bremsstrahlung process (np gamma) when the two nucleon-scattering angles are small. However, our understanding of such effects is limited, and the reason why meson exchange current effects dominate the np gamma cross section has not been thoroughly investigated. Purpose: The primary focus of this investigation is to understand the origin of the MEC contribution, to identify the leading MEC amplitudes, and to comprehend why these MEC amplitudes dominate the np gamma cross sections. Method: We used a new method that combines the one-boson-exchange (OBE) approach with the soft-photon approach to define 10 different np gamma amplitudes. These amplitudes are used to calculate np gamma cross sections at 225 MeV for nucleon laboratory scattering angles lying between 12(degrees) and 43(degrees). The results of these calculations are then compared to investigate the meson exchange current effect in np gamma. Results: (i) The OBE amplitude M(np gamma,mu)(PS) and the two-u-two-t special (TuTts) soft-photon amplitude M(np gamma,mu)(TuTts) predict quantitatively similar np gamma cross sections. (ii) The MEC effect is found to be significant when the two nucleon-scattering angles are far from the elastic limit (45(degrees)), but the effect is insignificant when the nucleon angles approach the elastic limit. (iii) The origin of the MEC effect and the leading MEC amplitudes have been identified in this investigation. Furthermore, the reason is now clear why the leading MEC amplitudes dominate the np gamma cross section when the nucleon-scattering angles are small. (iv) The contribution from the anomalous magnetic moments of the proton and the neutron is confirmed to be negligibly small. (v) In general, the theoretical cross sections using the amplitude M(np gamma,mu)(PS), or the amplitude M(np gamma,mu)(TuTts), are consistent with the triple differential cross sections recently measured at the Los Alamos National Laboratory. However, there exists an unexplained discrepancy between theory and experiment in some cases. Conclusions: The findings of this investigation have enhanced our understanding of the meson exchange current effect in np gamma. The comparative amplitude method introduced can be used for other bremsstrahlung investigations.
115635245
Topological sigma models and generalized geometries
We study the relation between topological sigma models and generalized geometries. The existence conditions for the most general type of topological sigma models obtained from twisting the N=(2,2) supersymmetric sigma model are investigated, and are found to be related to twisted generalized Calabi-Yau structures. The properties of these topological sigma models are analyzed in detail. The observables are shown to be described by the cohomology of a Lie algebroid, which is intrinsically associated with the twisted generalized Calabi-Yau structure. The Frobenius structure on the space of states and the effects of instantons are analyzed. We also study D-branes in these topological sigma models, and demonstrate that they also admit descriptions in terms of generalized geometries.
119448092
Electroweak bremsstrahlung from neutron-neutron scattering
Background: Nucleon-nucleon (NN) bremsstrahlung processes NN{gamma} (nn{gamma}, np{gamma}, and pp{gamma}) have been extensively investigated. Neutrino-pair bremsstrahlung processes from nucleon-nucleon scattering NN{nu}{nu} (nn{nu}{nu}, np{nu}{nu}, and pp{nu}{nu}) have recently attracted attention in studies of neutrino emission in neutron stars. The calculated NN{nu}{nu} cross sections (or emissivities) are found to be sensitive to the two-nucleon dynamical model used in the calculations. Purpose and Method: A realistic one-boson-exchange (ROBE) model for NN interactions is used to construct the electroweak bremsstrahlung amplitudes using the well-known nucleon electromagnetic and weak interaction vertices. The constructed nn{gamma} and nn{nu}{nu} amplitudes are investigated by applying them to calculate nn{gamma} and nn{nu}{nu} cross sections, respectively. Results: (i) The 190-MeV ROBE nn{gamma} cross sections agree well with those calculated using the TuTts amplitude, but they are in disagreement with those calculated using the Low amplitude. (ii) The calculated nn{nu}{nu} cross sections using the ROBE amplitude at the neutrino-pair energy {omega} = 1 MeV are in quantitative agreement with those calculated by Timmermans et al.[Phys. Rev. C 65, 064007 (2002)], who used the leading-order term of the soft neutrino-pair bremsstrahlung amplitude. Conclusions: The nn{gamma} amplitude in the ROBE approach, which obeys the soft-photon theorem, has a predictive power similar to that ofmore » the TuTts amplitude. The nn{nu}{nu} amplitude in the ROBE approach, which is consistent with the soft neutrino-pair bremsstrahlung theorem, has a predictive power similar to that of the soft neutrino-pair bremsstrahlung amplitude of Timmermans et al. in the low neutrino-pair energy region.« less
9262146
Open-string BRST cohomology for generalized complex branes
It has been shown recently that the geometry of D-branes in general topologically twisted (2,2) sigma-models can be described in the language of generalized complex (GC) structures. On general grounds, such D-branes (called GC branes) must form a category. We compute the BRST cohomology of open strings with both ends on the same GC brane. In mathematical terms, we determine spaces of endomorphisms in the category of GC branes. We find that the BRST cohomology can be expressed as the cohomology of a Lie algebroid canonically associated to any GC brane. In the special case of B-branes, this leads to an apparently new way to compute Ext groups of holomorphic line bundles supported on complex submanifolds: while the usual method leads to a spectral sequence converging to the Ext, our approach expresses the Ext group as the cohomology of a certain differential acting on the space of smooth sections of a graded vector bundle on the submanifold. In the case of coisotropic A-branes, our computation confirms a proposal of Orlov and one of the authors (A.K.).
9377747
Stability Conditions For Topological D-branes: A Worldsheet Approach
We study conditions on the topological D-branes of types A and B obtained by requiring a proper matching of the spectral flow operators on the boundary. These conditions ensure space-time supersymmetry and stability of D-branes. In most cases, we reproduce the results of Marino-Minasian-Moore-Strominger, who studied the same problem using the supersymmetric Born-Infeld action. In some other cases, corresponding to coisotropic A-branes, our stability condition is new. Our results enable us to define an analogue of the Maslov class and grading for coisotropic A-branes. We expect that they play a role in a conjectural generalization of the Floer homology.
11337046
D-branes in Landau-Ginzburg models and algebraic geometry
We study topological D-branes of type B in N = 2 Landau-Ginzburg models, focusing on the case where all vacua have a mass gap. In general, tree-level topological string theory in the presence of topological D-branes is described mathematically in terms of a triangulated category. For example, it has been argued that B-branes for an N = 2 sigma-model with a Calabi-Yau target space are described by the derived category of coherent sheaves on this space. M. Kontsevich previously proposed a candidate category for B-branes in N = 2 Landau-Ginzburg models, and our computations confirm this proposal. We also give a heuristic physical derivation of the proposal. Assuming its validity, we can completely describe the category of B-branes in an arbitrary massive Landau-Ginzburg model in terms of modules over a Clifford algebra. Assuming in addition Homological Mirror Symmetry, our results enable one to compute the Fukaya category for a large class of Fano varieties. We also provide a (somewhat trivial) counter-example to the hypothesis that given a closed string background there is a unique set of D-branes consistent with it.
119494911
Deuteron Compton scattering in effective field theory and spin-independent nucleon polarizabilities
Abstract Deuteron Compton scattering is calculated to O ( Q 2 ) in pionless effective field theory using a dibaryon approach. The vector amplitude, which was not included in the previous pionless calculations, contributes to the cross section at O ( Q 2 ) and influences significantly the extracted values of nucleon electric polarizability at incident photon energy 49 MeV. We recommend future high precision deuteron Compton scattering experiments being performed at 25–35 MeV photon energy where the nucleon polarizability effects are appreciable and the pionless effective field theory is most reliable. For example, a measurement at 30 MeV with a 3% error will constrain the isoscalar nucleon electric polarizability α 0 with a ∼30% error.
118460727
N 3 LO Higgs boson and Drell-Yan production at threshold: The one-loop two-emission contribution
In this paper, we study phenomenologically interesting soft radiation distributions in massless QCD. Specifically, we consider the emission of two soft partons off of a pair of lightlike Wilson lines, in either the fundamental or the adjoint representation, at next-to-leading order. Our results are an essential component of the next-to-next-to-next-to-leading order threshold corrections to both Higgs boson production in the gluon fusion channel and Drell-Yan lepton production. Our calculations are consistent with the recently published results for Higgs boson production. As a nontrivial cross-check on our analysis, we rederive a recent prediction for the Drell-Yan threshold cross section using a completely different strategy. Our results are compact, valid to all orders in the dimensional regularization parameter, and expressed in terms of pure functions.
118619769
Matter Effects in Active-Sterile Solar Neutrino Oscillations
The matter effects for solar neutrino oscillations are studied in a general scheme with an arbitrary number of sterile neutrinos, without any constraint on the mixing, assuming only a realistic hierarchy of neutrino squared-mass differences in which the smallest squared-mass difference is effective in solar neutrino oscillations. The validity of the analytic results are illustrated with a numerical solution of the evolution equation in three examples of the possible mixing matrix in the simplest case of four-neutrino mixing.
118638462
Branching Fractions and CP Asymmetries of B --> K_0*(1430) rho and B --> K_0^*(1430) phi Decays in the Family Nonuniversal Z' Model
In this work, within the QCD factorization approach, we investigate the branching fractions and CP asymmetries of decays $B \to K_0^*(1430)\rho$ and $B \to K_0^*(1430)\phi$ under two different scenarios both in the standard model and the family nonuniversal $Z^\prime$ model. We find that the annihilation terms play crucial roles in these decays and lead to the main uncertainties. For decays $B^- \to \bar K_0^{*-} (1430)\rho^0 (\omega)$, the new $Z^\prime$ boson could change branching fractions remarkably. However, for other decays, its contribution might be clouded by large uncertainties from annihilations. Unfortunately, neither the standard model nor $Z^\prime$ model can reproduce all experimental data under one certain scenario. We also noted that the CP asymmetries of $B^-\to \bar K_0^{*-}(1430) \rho^0 (\omega)$ could be used to identify the$K_0^*(1430)$ meson and search for the new physics contribution.
119023320
Sum rules and spin-dependent polarizabilities of the deuteron in effective field theory
We construct sum rules for the forward vector and tensor polarizabilities for any spin-S target and apply them to the spin-1 deuteron. We calculate these polarizabilities of the deuteron to the next-to-leading order in the pionless effective field theory.