# EW global fit,pole mass,gluon fusion

1. Jul 4, 2008

### wlln

Could you explain the meanings of EW global fit,pole mass,gluon fusion? Thank you very much!

2. Jul 5, 2008

### blechman

EW Global fit: Ignoring neutrino masses for the moment, there are 19 parameters in the Standard Model of particle physics. However, there are dozens of measured quantities. Therefore, the study of particle physics is overconstrained and we can use this over-measuring to see if we can find discrepancies. So we use the first 19 of the measurements to nail down the parameters, and then we can compare the remaining measurements to what the theory predicts.

Pole Mass: Mass is a funny thing in quantum field theory. Technically it's not well defined (it's divergent). One must specify a "renormalization scheme" by which we define the mass. "Pole Mass Scheme" is a particular (but by no means unique!) choice, where we DEFINE the mass as the value of the momentum where the field propagator (2-point Green's function) diverges (has a pole). The full inverse propagator goes like

(p^2 - m0^2 - A(p^2))

and we define the number Mpole^2 to be the value of p^2 that makes this denominator vanish. Here m0 is the "bare mass" - the parameter in the theory before renormalization.

Gluon fusion: This is just the Feynman diagram where two gluons come together and produce something. Gluons (unlike photons) can do this due to the nature of QCD - that is, photons cannot interact with each other (since they're electrically neutral) but gluons carry strong nuclear charge and can therefore interact. In fact, there IS such a thing as photon fusion, but it's VERY suppressed and therefore not worth talking about, unless you're doing quantum optics, which is another forum.

Hope this helps get you started. Also check out the standard sites like wikipedia, or even google.

3. Jul 6, 2008

### wlln

thank you very much