  The center-of-mass energy of two particles can become arbitrarily large if
they collide near the event horizon of an extremal Kerr black hole, which is
called the Ba$\rm \tilde n$ados-Silk-West (BSW) effect. We consider such a
high-energy collision of two particles which started from infinity and follow
geodesics in the equatorial plane and investigate the energy extraction from
such a high-energy particle collision and the production of particles in the
equatorial plane. We analytically show that, on the one hand, if the produced
particles are as massive as the colliding particles, the energy-extraction
efficiency is bounded by $2.19$ approximately. On the other hand, if a very
massive particle is to be produced as a result of the high-energy collision,
which has negative energy and necessarily falls into the black hole, the upper
limit of the energy-extraction efficiency is increased to $(2+\sqrt{3})^2
\simeq 13.9$. Thus, higher efficiency of the energy extraction, which is
typically as large as 10, provides strong evidence for the production of a
heavy particle.
