What is Nc in the R Ratio formula? re CDF detector measurements

[rwooduk]

Ive attached the R ratio formula to this post, please could someone who is familiar with it tell me what Nc is?

Does it relate at all to centre of mass energy?

Thanks in advance for any help.

source:

http://books.google.co.uk/books?id=...v=onepage&q="r ratio" "color charges"&f=false

page 68.

FORMULA:

 

[mfb]

N_c is the number of colors in the strong interaction, as the sum runs over all quark flavors and colors. You can use this ratio to determine the number as 3.

This ratio does not depend directly on the center of mass energy - the energy just determines which particles are available (here: no top-quarks at LEP as the energy was not sufficient, that's why you have the prefactors of 3 (down, strange, bottom) and 2 (up, charm) in the equation).

 

[rwooduk]

ahh i see, thanks for explaining this, very helpful!

edit is it possible to relate R to centre of mass energy?

 

[Vanadium 50]

The equation has nothing to do with CDF. Are you sure you have this right?

 

[rwooduk]

The equation has nothing to do with CDF. Are you sure you have this right?

to be honest I'm not sure at all, I'm trying to make sense of lecture notes i took over the semester, a plot was given of CDF detector measurements for a p + p -> muon + muon + various hadrons

it shows how often the events were found (y-axis) with a certain invariant mass of two muons (x-axis).

the lecturer then goes on to talk about the R ratio, so perhaps these two are not related?

ive taken some notes on the R ratio, then he asks what R is at a centre of mass energy of 4Gev, but fails to give any formula?

[rant]having real trouble understanding particle physics, the course has been lectured rather than taught and there is vast amounts of information to assimilate the majority of which doesn't help in numerical questions, never met a subject like it![/rant]

thanks again

 

[mfb]

That R comes from LEP, where they measured it with a good precision. I don't see any relation to CDF here.
At 4 GeV, another quark (in addition to top) is too heavy to get produced, this changes the ratio.