Soft gluons and colinear gluons

varphi42

Is there a difference between soft and colinear gluons?

Are soft gluons the ones with small energy, or with small PT with respect to the qqbar axe (in this case soft and colinear would have the same meaning)?

Einj

Usually soft gluons are those with low momentum, i.e. the less energetic ones, while collinear gluons are those produced near the beam direction, i.e. with low PT.
Obviously a gluon can be both soft and collinear. In fact, one can show that gluons are mostly emitted soft AND collinear as the probability of emitting a gluon from a quark is given by:

$$dP_{q\rightarrow qg}\propto \frac{dk}{k}\frac{dk_\perp^2}{k_\perp^2}$$

and so the probability is strongly centered around k≈0 (soft) and PT≈0 (collinear).

varphi42

Ok thank you!