  Reconnection is a fundamental event in many areas of science, from the
interaction of vortices in classical and quantum fluids, and magnetic flux
tubes in magnetohydrodynamics and plasma physics, to the recombination in
polymer physics and DNA biology. By using fundamental results in topological
fluid mechanics, the helicity of a flux tube can be calculated in terms of
writhe and twist contributions. Here we show that the writhe is conserved under
anti-parallel reconnection. Hence, for a pair of interacting flux tubes of
equal flux, if the twist of the reconnected tube is the sum of the original
twists of the interacting tubes, then helicity is conserved during
reconnection. Thus, any deviation from helicity conservation is entirely due to
the intrinsic twist inserted or deleted locally at the reconnection site. This
result has important implications for helicity and energy considerations in
various physical contexts.
