Higher Point Spin Field Correlators in D=4 Superstring Theory

Calculational tools are provided allowing to determine general tree-level scattering amplitudes for processes involving bosons and fermions in heterotic and superstring theories in four space-time dimensions. We compute higher-point superstring correlators involving massless four-dimensional fermionic and spin fields. In D=4 these correlators boil down to a product of two pure spin field correlators of left- and right-handed spin fields. This observation greatly simplifies the computation of such correlators. The latter are basic ingredients to compute multi-fermion superstring amplitudes in D=4. Their underlying fermionic structure and the fermionic couplings in the effective action are determined by these correlators.Comment: 61 pages, LaTeX; v2: enlarged introduction; final version to appear in NP B; v3: little typos remove

Integration studies of a positive neutral beam injector system into the design of a volumetric neutron source

A feasibility study regarding a volumetric neutron source (VNS) is presently conducted in the EUROfusion Consortium. The VNS uses Positive Neutral Beam Injection (P-NBI) for plasma heating, current drive and particularly to drive beam-target fusion, aiming for a high neutron production ( 0.5 MW/m2 neutron wall load in the equatorial plane). P-NBI is a reliable auxiliary heating system, widely employed in plasma devices such as ASDEX Upgrade (AUG), W7-X and JT60-SA. The paper describes the integration of the P-NBI system into the design of the VNS. This includes the neutral beam (NB) duct from the torus vacuum vessel to the NBI box with the gate valves, the space requirements of the NB injectors in the building as well as the connections for cooling water, cryo-supplies, electrical high voltage and radio frequency (RF) cable connections. Since the VNS is a nuclear machine with lifetime doses significantly exceeding those of ITER, the requirements for remote maintenance (RM) are very different from any existing NBI design. The RM concept and the RM sequences will be discussed. Neutronics studies for the NB duct were performed and design iterations undertaken to assure that neutron heating and lifetime fluences remain below limits at the superconducting toroidal and poloidal field coils adjacent to the NB duct. The work considers the return of experience from AUG, ITER and some other fusion experimental machines in operation, under construction or in conceptual design such as DEMO

On RR couplings on D-branes at order O(α′2)O(\alpha'^2)

Recently, it has been found that there are couplings of the RR field strength $F^{(p)}$ and the B-field strength $H$ on the world volume of D$_p$-branes at order ${\cal O}(\alpha'^2)$. These couplings which have both world-volume and transverse indices, are invariant under the linear T-duality transformations. Consistency with the nonlinear T-duality indicates that the RR field strength $F^{(p)}$ in these couplings should be replaced by ${\cal F}^{(p)}=d{\cal C}^{(p-1)}$ where ${\cal C}=e^{B}C$. This replacement, however, reproduces some non-gauge invariant terms. On the other hand, the nonlinear terms are invariant under the linear T-duality transformations at the level of two B-fields. This allows one to remove some of the nonlinear terms in ${\cal F}^{(p)}$. We fix this by comparing the nonlinear couplings with the S-matrix element of one RR and two NSNS vertex operators. Our results indicate that in the expansion of ${\cal F}^{(p)}$ one should keep only the B-field gauge invariant terms, e.g. $B\wedge dC^{(p-3)}$ where both indices of B-field lie along the brane. Moreover, in this case one should replace $B$ with $B+2\pi\alpha'f$ to have the $B$-field gauge invariance.Comment: 23 pages, Latex file, 1 figure; v2:typos corrected, to appear in JHE

Two-loop scattering amplitudes from ambitwistor strings: from genus two to the nodal Riemann sphere

We derive from ambitwistor strings new formulae for two-loop scattering amplitudes in supergravity and super-Yang-Mills theory, with any number of particles. We start by constructing a formula for the type II ambitwistor string amplitudes on a genus-two Riemann surface, and then study the localisation of the moduli space integration on a degenerate limit, where the genus-two surface turns into a Riemann sphere with two nodes. This leads to scattering amplitudes in supergravity, expressed in the formalism of the two-loop scattering equations. For super-Yang-Mills theory, we import `half' of the supergravity result, and determine the colour dependence by considering a current algebra on the nodal Riemann sphere, thereby completely specifying the two-loop analogue of the Parke-Taylor factor, including non-planar contributions. We also present in appendices explicit expressions for the Szego kernels and the partition functions for even spin structures, up to the relevant orders in the degeneration parameters, which may be useful for related investigations in conventional superstring theory.Comment: 66 pages plus appendices, 14 figures. v2: small changes, published version. v3: typos fixed in appendix