Threshold singularities in the XXZ-spin chain

We calculate the critical exponents of the threshold singularity for the spectral density of the XXZ-spin chain at zero magnetic field for the lower threshold. We show that the corresponding phase shifts are momentum independent and coincide with predictions of the effective mobile impurity Hamiltonian approach.

Fine structure of the diffraction cone: From the ISR to the LHC

Following earlier findings, we argue that the low-[Formula: see text] structure in the elastic diffractive cone, recently reported by the TOTEM Collaboration at [Formula: see text] TeV, is a consequence of the threshold singularity required by [Formula: see text]channel unitarity, such as revealed earlier at the ISR. By using simple Regge-pole models, we analyze the available data on the [Formula: see text] elastic differential cross section in a wide range of c.m. energies, namely those from ISR to LHC8, obtaining good fits of all datasets. This study hints at the fact that the non-exponential behavior observed at LHC8 is a recurrence of the low-[Formula: see text] “break” phenomenon, observed in the seventies at ISR, being induced by the presence of a two-pion loop singularity in the Pomeron trajectory.