Soft gluon emission at large angles
نویسنده
چکیده
I review recent results on soft gluon radiation at large angles. 1 Gribov’s legacy on soft radiation Although the major interest of Vladimir Gribov [1] in QCD was on non-perturbative aspects (notably colour confinement), his work on soft photons or soft pions emission in the pre-QCD era [2] is the basis for understanding general features of perturbative QCD dynamics. In principle soft emission is “trivial”, it reduces essentially to classical physics. That is, it depends only on external charges which are treated as classical currents (asymptotic, large-time states), thus it does not reveal the internal short distance structure of emitting systems. In spite of this “trivial” aspects, universality of soft emission allows one to compute multi-soft gluon emission distributions and thus to obtain important information on the QCD properties in general. The key point is that in computing multi-soft gluon amplitudes one deals with multi-soft scales. The softest gluon does not “see” any internal structure. However the next-to-soft gluon can “see” the softest one. In this way one can recursively reconstruct the detailed deep structure of the emitting system up to the primary hard emitting system. Therefore, while the result is “trivial” in QED since only the primary emitting charges are involved in soft photon emission, in QCD instead, due to direct gluon interaction, also the successive emitted gluons are involved and then successively resolved. In this talk I discuss various results obtained in the course of years by exploiting soft gluon emission properties obtained in this way. 2 Multi-soft gluon amplitudes I start by recalling how one obtains [3] (see also [4]) the amplitude for the emission of n soft gluons q1 · · · qn , off a colour singlet dipole of primary hard quark antiquark pb, pb as emitted for instance in ee annihilation at high c.m. energy Q (generalization to other
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