  Experimental investigations of the phase diagram of strongly interacting
matter involve collisions of heavy ions at ultrarelativistic velocities. The
medium created in such a collision is often of dimensions a few fermi, in
particular in the Beam Energy Scan experiments. An understanding of the effect
of the finite volume and the boundary is important for connecting the
experimental results to the phase diagram.
  Using the Nambu Jona-Lasinio model, an effective theory for the chiral
transition of quantum chromodynamics (QCD), we have studied the effect of the
finite volume of the fireball on the transition line at finite temperature and
density using the MIT boundary condition to mimic the condition that the system
is deconfined inside. The shift of the transition temperature for finite volume
and finite volume effect on number density and its susceptibilities are
studied. The volume effects should be considered when looking for signatures of
the phase diagram in experiments.
