kappa(T) and threshold resummations

We demonstrate that both the k(T) and threshold resummations can be perform ed in the Collins-Soper resummation formalism by evaluating soft gluon emis sions with infrared cutoffs for the longitudinal and transverse loop moment a, respectively. The reason the k(T) resummation for a parton distribution function leads to suppression in the large b region, b being the conjugate variable of parton transverse momentum k(T), and the threshold resummmation leads to enhancement in the large N limit, N being the moment of a distrib ution function, is a consequence of opposite directions of double-logarithm evolutions. The k(T) and threshold resummations for an energetic final-sta te jet give suppression. The switch of the threshold resummation from enhan cement to Suppression is attributed to a nonvanishing jet invariant mass. I n the same framework we derive a unification of the k(T) and threshold resu mmations for a parton distribution function by requiring infrared cutoffs f or both longitudinal and transverse loop momenta. This unified resummation exhibits suppression at large b, similar to the k(T) resummation, and exhib its enhancement at small b, similar to-the threshold resummation.