# What are the main differences between $p p$ and $p \bar p$ colliders

I know that it is somehow related to the parton distribution functions, allowing specific reactions with gluons instead of quarks and anti-quarks, but I would really appreciate more detailed answers !

Thanks

-

The difference in scattering cross sections is more evident the lower the energy of collisions. Fig 41.11. At the energies of TeV the probability of new physics observations is the same for both choices of collisions.

The reason is that at low energies the fact that the proton has three quarks and the anti proton three anti quarks predominates. Quark antiquark scattering at low energies has much higher cross section than quark quark due to the extra possibility of annihilation of the quarks. At low energy the gluon "sea" plays a small part. The higher the energy of interactions the higher the number of energetic gluons that scatter and finally at TEV energies that is what predominates and the two cross sections converge. Thus for physics it makes no difference whether one uses as targets protons or antiproitons, as far as discovery potential goes.

There may be some technical advantage in the construction, in that in principle the antiproton-proton beams can circulate in the same magnetic configuration as mirror images and make the magnet construction circuits simpler. I guess that the need for high luminosity made LHC a proton proton collider, since it is more difficult to store antiprotons. I would have to research this guess.

-
But using proton proton isn't a way to reduce $q \bar q$ reaction and favour g g ? –  gdz Apr 22 '11 at 14:35
My remark is related to higgs production by gluon fusion –  gdz Apr 22 '11 at 14:47
Within each, nucleon and antinucleon, the distributions of the gluon "sea" are the same, that is why at high energies it makes no difference which one uses, since then the percentage of energy carried by the original quarks of the incoming quarks/antiquarks is small in the interactions of interest ( high transverse momentum, i.e. deep inelastic) and the sea gluons predominate. –  anna v Apr 23 '11 at 4:30
I would add to @anna's answer that $p\bar{p}$ collider such as the Tevatron is CP symmetric. This was one of the arguments for continuing the Tevatron Experiments. To quote from the proposal