7 TeV collisions: why not antiprotons?
First 7 TeV events: http://atlas.web.cern.ch/Atlas/public/EVTDISPLAY/events.html
I’ve been wondering for a while why the LHC is designed for proton-proton collisions, and not proton-antiproton like the Tevatron. The Tevatron uses the same set of magnets to circulate both beams, and the LHC needs two sets.
First of all, apparently, it’s very difficult to produce antiprotons at a high enough rate to sustain the rate of collisions needed for the LHC detectors to see rare events.
There are also very interesting considerations that make proton-antiproton collisions a better choice for the lower energy accelerator. The momentum of a moving proton is not equal to the sum of momenta of the three quarks. Some small fraction of the momentum is carried by the “strong field” – gluons and virtual quarks.
At the Tevatron, the virtual particle collisions do not contribute to interesting events (like W boson production) because they don’t have enough energy. Only the 3 valence quarks count. So if some of the valence quarks are antiquarks, the electric attraction dramatically increases the probability of collision.
And at the LHC energies the virtual quarks (and antiquarks) and even gluons carry enough momentum to be involved into production of heavy particles. That is why the 7-fold increase in energy is expected to give a 120-fold increase in rate of production of heavy particles. Introducing 3 antiquarks would not make any difference.