Softness and Amplitudes' Positivity for Spinning Particles

We derive positivity bounds for scattering amplitudes of particles with arbitrary spin using unitarity, analyticity and crossing symmetry. The bounds imply the positivity of certain low-energy coefficients of the effective action that controls the dynamics of the light degrees of freedom. We show that low-energy amplitudes strictly softer than $O(p^4)$ do not admit unitary ultraviolet completions unless the theory is free. This enforces a bound on the energy growth of scattering amplitudes in the region of validity of the effective theory. We discuss explicit examples including the Goldstino from spontaneous supersymmetry breaking, and the theory of a spin-1/2 fermion with a shift symmetry.Comment: 28 pages + refs and 3 appendices, 2 figures; v2: extra refs, improved discussions, typos fixed, accepted on JHE

The surface brightness profile of the remote cluster NGC 2419

It is well known that the bright and remote Galactic globular cluster NGC2419 has a very peculiar structure. In particular its half-light radius is significantly larger than that of ordinary globular clusters of similar luminosity, being as large as that of the brightest nuclei of dwarf elliptical galaxies. In this context it is particularly worth to check the reliability of the existing surface brightness profiles for this cluster and of the available estimates of its structural parameters. Combining different datasets I derive the surface brightness profile going from the cluster center out to ~ 480 arcsec, i.e. ~25 core radii. (Abridged). The newly obtained surface brightness profile is in excellent agreement with that provided by Trager, King & Djorgovski for r>= 4 arcsec; it is best fitted by a King model having r_c=0.32 arcmin, mu_V(0)=19.55 and C=1.35. Also new independent estimates of the total integrated V magnitude (V_t=10.47 +/- 0.07) and of the half-light radius (r_h=0.96 arcmin +/- 0.2 arcmin) have been obtained. (Abridged). The structure of NGC2419 is now reliably constrained by (at least) two fully independent observational profiles that are in good agreement one with the other. Also the overall agreement between structural parameters independently obtained by different authors is quite satisfying.Comment: Research Note, accepted for publication by A&A. 6 pages with 4 figures + 3 pages of Online Material (table

Measurement of the position resolution of the Gas Pixel Detector

The Gas Pixel Detector was designed and built as a focal plane instrument for X-ray polarimetry of celestial sources, the last unexplored subtopics of X-ray astronomy. It promises to perform detailed and sensitive measurements resolving extended sources and detecting polarization in faint sources in crowded fields at the focus of telescopes of good angular resolution. Its polarimetric and spectral capability were already studied in earlier works. Here we investigate for the first time, with both laboratory measurements and Monte Carlo simulations, its imaging properties to confirm its unique capability to carry out imaging spectral-polarimetry in future X-ray missions.Comment: Submitted to Nuclear Instruments and Methods in Physics Research Section A; 15 figures, 3 table

Max-Min characterization of the mountain pass energy level for a class of variational problems

We provide a max-min characterization of the mountain pass energy level for a family of variational problems. As a consequence we deduce the mountain pass structure of solutions to suitable PDEs, whose existence follows from classical minimization argument

Quantum Wire Network with Magnetic Flux

The charge transport and the noise of a quantum wire network, made of three semi-infinite external leads attached to a ring crossed by a magnetic flux, are investigated. The system is driven away from equilibrium by connecting the external leads to heat reservoirs with different temperatures and/or chemical potentials. The properties of the exact scattering matrix of this configuration as a function of the momentum, the magnetic flux and the transmission along the ring are explored. We derive the conductance and the noise, describing in detail the role of the magnetic flux. In the case of weak coupling between the ring and the reservoirs, a resonant tunneling effect is observed. We also discover that a non-zero magnetic flux has a strong impact on the usual Johnson-Nyquist law for the pure thermal noise at small temperatures.Comment: LaTex, 6 pages, 6 figures, improved discussion of the impact of the magnetic flux on the pure thermal nois