It has been suggested that a colour-entanglement effect exists in the
Drell-Yan cross section for the ‘double T-odd’ contributions at low
transverse momentum
\bm{Q_\st},
rendering the colour structure different from that predicted by the
usual factorisation formula . These T-odd contributions can come from
the Boer-Mulders or Sivers transverse momentum dependent distribution
functions. The different colour structure should be visible already at
the lowest possible order that gives a contribution to the double
Boer-Mulders (dBM) or double Sivers (dS) effect, that is at the level of
two gluon exchanges. To discriminate between the different predictions,
we compute the leading-power contribution to the
low-\bm{Q_\st}
dBM cross section at the two-gluon exchange order in the context of a
spectator model. The computation is performed using a method of regions
analysis with Collins subtraction terms implemented. The results conform
with the predictions of the factorisation formula. In the cancellation of the colour entanglement, diagrams containing the
three-gluon vertex are essential. Furthermore, the Glauber region turns
out to play an important role – in fact, it is possible to assign the
full contribution to the dBM cross section at the given order to the
region in which the two gluons have Glauber scaling. A similar
disentanglement of colour is found for the dS effect.
Double Collins effect in
e+e−→ΛΛ¯X
and
e+e−→ΛπX
processes in a diquark spectator model
