14,777 research outputs found
Generalized L\"uscher Formula in Multi-channel Baryon-Meson Scattering
L\"uscher's formula relates the elastic scattering phase shifts to the
two-particle energy levels in a finite cubic box. The original formula was
obtained for elastic scattering of two massive spinless particles in the center
of mass frame. In this paper, we consider the case for the scattering of a spin
1/2 particle with a spinless particle in multi-channel scattering. A
generalized relation between the energy of two particle system and the
scattering matrix elements is established. We first obtain this relation using
quantum-mechanics in both center-of-mass frame and in a general moving frame.
The result is then generalized to quantum field theory using methods outlined
in Ref. \cite{Hansen:2012tf}. We verify that the results obtained using both
methods are equivalent up to terms that are exponentially suppressed in the box
size.Comment: One reference adde
Anisotropic but nodeless superconducting gap in the presence of spin density wave in iron-pnictide superconductor NaFe1-xCoxAs
The coexisting regime of spin density wave (SDW) and superconductivity in the
iron pnictides represents a novel ground state. We have performed high
resolution angle-resolved photoemission measurements on NaFe1-xCoxAs (x =
0.0175) in this regime and revealed its distinctive electronic structure, which
provides some microscopic understandings of its behavior. The SDW signature and
the superconducting gap are observed on the same bands, illustrating the
intrinsic nature of the coexistence. However, because the SDW and
superconductivity are manifested in different parts of the band structure,
their competition is non-exclusive. Particularly, we found that the gap
distribution is anisotropic and nodeless, in contrast to the isotropic
superconducting gap observed in an SDW-free NaFe1-xCoxAs (x=0.045), which puts
strong constraints on theory.Comment: 5 pages, 4 figures + supplementary informatio
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