To start, we consider a simple continuous-time CAPM model in which the market
price Mt and an asset price St evolve as follows:
dMt
Mt
= μdt + σmdWt , (1)
dSt
St
= β
dMt
Mt
+ σdZt , (2)
for constant positive volatilities σm and σ, and a slope β. This model is consistent
with CAPM in that that the return of the asset dSt
St
is a linear function of the return of
the market dMt
Mt
through the β coefficient and a Brownian-driven noise process. In this
model, under the physical probability measure P, we assume independence between
the standard Brownian motions driving the market and asset price processes:
d W, Zt = 0. (3)