  The group described in this paper appeared while studying fundamental groups
of complements of branch curves. It turned out that a certain quotient of the
braid group acts on those fundamental groups and studying this action is
essential for understanding the structure of the fundamental groups. We
describe here the quotient of the Artin braid group by commutators of
transversal half-twists and we investigates its group actions. We denote the
quotient by B_n~ and refer to the groups which admit an action of B_n~ , as
B_n~-groups.
  We distinguish special elements in B_n~-groups which we call prime elements
and we give a criterion for an element to be prime. This criterion will be
applied to the study of the structure of fundamental groups of complements of
branch curves. The group B_n~ itself turns out to be an extension of a solvable
group by a symmetric group.
