We consider the energy flow into gaps between hard jets. It was previously
believed that the accuracy of resummed predictions for such observables can be
improved by employing the kt​ clustering procedure to define the gap energy
in terms of a sum of energies of soft jets (rather than individual hadrons) in
the gap. This significantly reduces the sensitivity to correlated soft
large-angle radiation (non-global leading logs), numerically calculable only in
the large Nc​ limit. While this is the case, as we demonstrate here, the use
of kt​ clustering spoils the straightforward single-gluon Sudakov
exponentiation that multiplies the non-global resummation. We carry out an
O(αs2​) calculation of the leading single-logarithmic terms
and identify the piece that is omitted by straightforward exponentiation. We
compare our results with the full O(αs2​) result from the
program EVENT2 to confirm our conclusions. For e+e−→2 jets and DIS
(1+1) jets one can numerically resum these additional contributions as we show,
but for dijet photoproduction and hadron-hadron processes further studies are
needed.Comment: 11 pages, 5 figure