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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow Quarks and leptons arrow What is keeping the quarks together in the proton?

What is keeping the quarks together in the proton?

Apart from the electric charge, quarks also carry a so called colour charge and the interactions between the colour charges keep the quarks together. There are three different types of colour charges, usually called red, green and blue. In addition there are anti-charges in the same way as there are both positive and negative electric charges. (With this way of counting there is only one electric charge.) When the quarks interact with each other they exchange gluons that "glue" the quarks together (hence the name gluon). The force that keeps the quarks together is the same as the strong force between protons and neutrons in the atomic nucleus. The difference is that quarks interact directly with each other via the strong force by exchanging gluons, whereas protons and neutrons interact indirectly with each other by exchanging mesons, i.e., quark anti-quark states.

The quantum chromodynamic theory (QCD) describes how the coloured quarks interact with each other. The force that keeps the quarks together is so strong that they cannot exist as separate particles. Instead they only exist in colour combinations that add up to white. This means that the three-quark states are combinations of one red, one green and one blue colour charge where as the quark anti-quark states consist of one colour and its anti-colour. The reason that colour charges cannot exist on their own is connected to the fact that the gluons also carry colour. This makes the force field between a quark anti-quark pair collapse into something like a rubber band. If one tries to pull the quark and the anti-quark apart the force between them is constant and therefore more and more energy is needed. At some point, the energy stored in the field becomes so large that a new quark anti-quark pair is formed and the rubber band breaks into two. In the end a bunch of hadrons will be formed. This should be compared with the electric force which decreases quadratically with the distance between the charges.

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An illustration of how a deep inelastic scattering is viewed. A proton consisting of three quarks (white) collides with an electron (yellow). In the collision a photon (red) is exchanged and one of the quarks (blue) is scattered out of the proton. Between the scattered quark and the two quarks in the proton remnant a colour field (green) is formed. It is from the energy in this field that new quark anti-quark pairs are produced.

At DESY in Hamburg one does research, in which also researchers from the Department of Radiation Sciences in Uppsala are taking part, to learn more about the strong force and the quantum chromodynamic theory. At DESY electrons and protons are collided at very high energy in the HERA-accelerator( which has a 6.7 km circumference). When the electron interacts with the proton at these high energies the individual quarks in the proton are probed. Apart from studying the scattered electron one also tries to study the quark which is knocked out of the proton. Since the quark cannot exist as a free particle it gives rise to a shower of particles (mostly hadrons). These arise partly from the scattered quark, but also from gluons that are radiated in the collision. This is due to the acceleration of the scattered quark and is similar to the way photons are radiated by an ordinary antenna.


Photograph of the DESY area with the accelerators marked as red lines.

Apart from the u (up), d (down) and s (strange) quarks there are also three other quarks. The charm quark c was discovered in 1974 (later B. Richter and S. Ting shared the Nobel prize for this discovery) and the bottom or beauty quark b was discovered in 1977. The sixth quark t (top) was discovered at the Fermilab Tevatron in 1995. In summary, this means that there are six different types of quarks. Of these, only u and d quarks exist in ordinary matter (protons and neutrons). The other quarks can be produced in high energy particle collisions but they quickly decay. The heaviest quark, t, is two hundred times as heavy as the proton and therefore one needed the high energy of the Tevatron to see it.

Flavor

Mass(GeV/c2)

Elect. Charge

u up 0.005 +2/3
d down 0.01 -1/3
c charm 1.5 +2/3
s strange 0.2 -1/3
t top 180 +2/3
b bottom 4.7 -1/3