Soft and ultrasoft gluons in QCD

  • Thread starter petergreat
  • Start date
  • #1
267
3

Main Question or Discussion Point

I'm reading about factorization in QCD. The terms "soft gluons" and "ultrasoft gluons" are frequently referred to. I know how they are defined in terms of power counting, but don't understand their physical significance. In which QCD processes are soft gluons important, and in which are ultrasoft gluons significant?
 

Answers and Replies

  • #2
blechman
Science Advisor
779
8
It depends a great deal on what it is you are looking at, and to some extent it is just semantics. But as a rule, if [itex]\lambda[/itex] is your (IR) power counting parameter, then in terms of light cone momenta (and setting the hard scale to 1):

[tex](p^+,p^-,p_\perp)\sim (\lambda,\lambda,\lambda)\Rightarrow{\rm soft}[/tex]
[tex](p^+,p^-,p_\perp)\sim (\lambda^2,\lambda^2,\lambda^2)\Rightarrow{\rm ultrasoft}[/tex]

So that the "offshellness" of a soft mode is [itex]\lambda^2[/itex] while the usoft mode is [itex]\lambda^4[/itex].

When dealing with fully inclusive processes this distinction is irrelevant. The trouble comes in when you have exclusive or semi-inclusive processes. Then you have to distinguish soft from usoft.

See, for instance, Bauer, Pirjol, Stewart, hep-ph/0109045
 

Related Threads for: Soft and ultrasoft gluons in QCD

  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
0
Views
2K
Replies
22
Views
2K
Replies
3
Views
6K
Replies
9
Views
1K
  • Last Post
Replies
4
Views
7K
  • Last Post
2
Replies
35
Views
6K
Replies
1
Views
744
Top