underdoped cuprates39 and the peak-dip-hump structure
in the ARPES proﬁle in the SCS 31,32,46,47. In our analy-
sis we assume that a CB is a spin ﬂuctuation.
The results for the conductivity within a spin-fermion
model depend in quantitative (but not qualitative) way
on the assumption for the momentum dispersion of a col-
lective boson. This momentum dependence comes from