  By taking the virtual inter-band transitions along with the intra-band ones
into full account, here we first propose an effective band mass theorem that is
suitable for a wide-class of single-particle Hamiltonians exhibiting multiple
energy bands. Then, for the special case of two-band systems, we show that the
inter-band contribution to the effective band mass of a particle at a given
quantum state is directly controlled by the quantum metric of the corresponding
state. As an illustration, we consider a spin-orbit coupled spin-$1/2$ particle
and calculate its effective band mass at the band minimum of the lower helicity
band. Independent of the coupling strength, we find that the bare mass $m_0$ of
the particle jumps to $2m_0$ for the Rashba and to $3m_0$ for the Weyl
coupling. This geometric mass enhancement is a non-perturbative effect,
uncovering the mystery behind the effective mass of the two-body bound states
in the non-interacting limit. As a further illustration, we show that a
massless Dirac particle acquires a linearly dispersing band mass (equivalent to
the effective cyclotron one up to a prefactor) with its momentum through the
same mechanism.
