text: The most relevant point for our discussion is that this 
  model contains the physics which we identiﬁed above as 
  a source of a potential sign change of ∆WK . Namely, 
  at strong coupling the fermionic self-energy in the NS 
  is large because there exists strong scattering between 
  low-energy fermions mediated by low-energy collective 
  bosons. In the SCS, the density of low-energy fermions 
  drops and a continuum collective excitations becomes 
  gaped. Both eﬀects reduce fermionic damping and lead 
  to the increase of WK in a SCS. If this increase exceeds a 
  conventional loss of WK due to a gap opening, the total 
  ∆WK may become positive. 
  
  The CB model has been applied numerous times to the 
  cuprates, most often under the assumption that near- 
  critical collective excitations are spin ﬂuctuations with 
  momenta near Q = (π, π). This version of a CB bo- 
  son is commonly known as a spin-fermion model. This 
  model yields dx2 
  y2 superconductivity and explains in a 
  quantitative way a number of measured electronic fea- 
  tures of the cuprates, in particular the near-absence of 
  the quasiparticle peak in the NS of optimally doped and 
  underdoped cuprates39 and the peak-dip-hump structure 
  in the ARPES proﬁle in the SCS31,32,46,47. In our analy- 
  sis we assume that a CB is a spin ﬂuctuation. 