# How can we measure the mass of quarks?

1. Jan 30, 2013

### ndung200790

I have not yet studied experimental physics!But I would like to know how can we measure the mass of quark,because we can not to have separate quark for color confinement.In quarkonium,by resonance we know the excited state of quarkonium,but how can we know which state they lie(e.g 2$^{3}$S).

2. Jan 30, 2013

### Simon Bridge

You have the quarks in different combinations and you can measure the mass of the combinations. We can also measure the energetics of gluons.

It's like those logic puzzles you did as a kid, where you have to figure which bag of coins has the light coin in it.

3. Jan 31, 2013

### andrien

4. Jan 31, 2013

### Bill_K

The lifetime of the top quark is extremely short, 5 x 10-25 sec. Would someone who maintains that virtual particles are not "real" and just a mathematical artifact please tell me - is the top quark real?? :uhh:

5. Jan 31, 2013

### lpetrich

The masses of the quarks can be measured in several ways.

For the up, down, and strange quarks, one can use a quirk of QCD called "chiral symmetry breaking", and measure their masses using the masses of the pions and kaons:

mpi2 ~ (mu + md)*EQCD
mK+2 ~ (mu + ms)*EQCD
mK02 ~ (md + ms)*EQCD

One has to do lattice QCD to get the results, and that has the problem that one cannot make the up and down masses too small, or else the pions' Compton wavelengths will cover the entire lattice. So one does the calculations with larger masses, and extrapolates down to observed values.

For the charm and bottom quarks, one can get approximate masses from their bound states, like the J/psi, D, upsilon, and B mesons, since those quarks are nonrelativistic in those states. To improve those estimates, one must calculate those states' binding energies with lattice QCD.

For the top quark, one must get its mass from the total energy of its decay products, since it decays before it can hadronize. andrien's paper is an example of doing that.

6. Jan 31, 2013

### kurros

You can produce top-quarks on-shell right? So no problem. I do not agree with the contention that virtual particles are not real though.

7. Feb 2, 2013

### Staff: Mentor

I write that sometimes for questions like "why does exactly that Feynman diagram happen?"
Or would you accept the idea that a bound electron is constantly exchanging particles (photons) with the nucleus? If yes, how many? And how many photons does it exchange with all other particles around it?
On the other hand, if you write that in terms of Feynman diagrams, every particle is a virtual particle. Some are just "more virtual" (more off-shell, shorter living) than others.

8. Feb 2, 2013

### Bill_K

Yes! I agree with that fully.

An apparent difference between real and virtual arises because in many cases a virtual particle must be integrated over. At that point we must face the fact that Feynman diagrams represent quantum amplitudes, and consequently one virtual particle contributes to an infinite number of mutually coherent exchange processes. But it's not the particle's fault, or the idea that virtual particles are different somehow, it's just quantum mechanics coming into play.
Yes. One photon. I know that sounds odd, but each vertex contributes a factor e, and the Coulomb interaction is e2. Two-photon exchange would be e4. That one photon spends eternity being exchanged constantly and forever.

If you Fourier transform a Coulomb field between two charged particles at rest, you find that it is spatially varying but time-independent, which indicates that their photon carries momentum but zero energy,

9. Feb 2, 2013

### Staff: Mentor

Fine structure is e4, and I think you can get an e6-expression (and all higher orders) as well, if you continue to expand the interaction.

Or constant energy?