  The Chern-Simons bosonization with U(1)xSU(2) gauge field is applied to 2-D
t-J model in the limit t >> J, to study the normal state properties of
underdoped cuprate superconductors. We prove the existence of an upper bound on
the partition function for holons in a spinon background, and we find the
optimal spinon configuration saturating the upper bound on average--a
coexisting flux phase and s+id-like RVB state. After neglecting the feedback of
holon fluctuations on the U(1) field B and spinon fluctuations on the SU(2)
field V, the holon field is a fermion and the spinon field is a hard--core
boson. We show that the B field produces a \pi flux phase for holons,
converting them into Dirac--like fermions, while the V field, taking into
account the feedback of holons produces a gap for spinons vanishing in zero
doping limit. The nonlinear sigma-model with a mass term describes the
crossover from short-ranged antiferromagnetic (AF) state in doped samples to
long range AF order in reference compounds. Moreover, we derive a low--energy
effective action in terms of spinons, holons and a self-generated U(1) gauge
field. The gauge fluctuations are not confining due to coupling to holons, but
yield an attractive interaction between spinons and holons leading to a bound
state with electron quantum numbers. The renormalisation effects due to gauge
fluctuations give rise to non--Fermi liquid behaviour for the composite
electron.This formalism provides a new interpretation of the spin gap in
underdoped superconductors (due to short-ranged AF order) and predicts the
minimal gap for the physical electron is proportional to the square root of the
doping concentration.
