Relation between momentum and mass of quarks

1. Jan 1, 2014

Hluf

We say on-shell and off-shell mass of quarks. 1) What is the difference on-shell and off-shell mass of quarks. 2) At lab. center of mass frame for lepton particles p2= -m2. Can we apply this equation for quarks.
Thank you!

2. Jan 1, 2014

tiny-tim

Hi Hluf!
a quark always has on-shell mass (usually just called "mass")

a quark never has off-shell mass

off-shell mass is a mathematical trick which helps in the calculations for Feynman diagrams
(p is the four-momentum)

yes this applies to everything (and in any frame): leptons hadrons and photons

you can regard it as the definition of m (the mass)

3. Jan 1, 2014

Bill_K

If it works once, it's a trick. If it works twice, it's a method!

4. Jan 1, 2014

tiny-tim

But if you don't know how it works, it's magic!

5. Jan 1, 2014

Meir Achuz

If you don't know how it works, go to school to learn how.
Quarks can appear as intermediate particles in Feynman diagrams, where they are
'off-shell'.

6. Jan 1, 2014

Staff: Mentor

Well, there is no center of mass frame for (single) photons.

7. Jan 1, 2014

kurros

Some people would argue the complete opposite. The longer time a particle lives for, the more probably that its mass would be measured to be almost exactly the on-shell mass, but really the whole universe is quantum mechanical and every process a "little bit" virtual, so everything is always off-shell, the exact on-shell mass occupying measure zero of the probability distribution :p.

8. Jan 1, 2014