  We investigate the effects of secondary (knockout) protons, which constitute
about 20% of the observed protons at STAR, on the higher order cumulants of
proton and baryon numbers measured by event-by-event analyses in relativistic
heavy ion collisions. We argue that the contribution of this background effect
on the cumulants is expressed by a simple formula, and that hence their effects
can be removed in the experimental analysis. It is discussed that this
background effect has non-negligible contribution to recently observed proton
number cumulants at STAR, especially the third-order one, and that the removal
of this effect is crucial to investigate the thermodynamical properties of the
primordial hot medium appropriately.
