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Not native speaker, so apologies for the bad english. Just wanted to to add to untitled's comment: electron-on-proton collisions are still being done at Jefferson Lab in Newport News, VA. It just doesn't get as much press as CERN with its impressive PR machine (which I have nothing against). The motivation is the same as at HERA: understand the structure of protons, with the added possibility of polarizing the electron beam or the proton target, which allows to access components of the nucleon structure that are otherwise not accessible. As a bonus, they also have photon beams that collide with proton target exciting different interaction channels. To come back to gp's question, one can in principle collide anything with anything, but the ideal tool really depends on what one wants to study. Attaining ever higher energy is not necessarily the ultimate goal. Even though it is useful to explore new boundaries, there are also a lot of unanswered questions that can be addressed with much lower energy collisions. For example the Jefferson Lab beam maxes out at 12 GeV, orders of magnitude lower than that available at the LHC, but it's still an extremely useful machine. Also the comment quoted by gp from the article is wrong. Even though only the protons respond to the electromagnetic fields of the accelerator, the whole nucleus is accelerated. Therefore if the pairs of protons attain a center-of-mass energy of 13TeV, the same holds true for the neutrons, because (1) neutrons have very similar mass as protons (2) neutrons are dragged along for the ride, as they are bound to the protons by a force that is much much stronger than that of the electromagnetic forces involved in acceleration. (It is not true that only protons are accelerated). So the available energy per pair of colliding nucleon (whether it is proton or neutron) is still 13TeV, and the total energy potentially available in the center-of-mass system is 13 TeV * 208 = 2.704 PeV. However, not all of it necessarily released, since the number of nucleon-nucleon collisions that can take place is dependent on the area overlap between the two. |