CERN starts first lead collisions in upgraded Large Hadron Collider
arstechnica.com
arstechnica.com
Seems that since the collisions are more energetic the chance of new discoveries would be increased, no?
Edit: one of the comments on the article mention " A much more useful measure of the energy is the center of momentum energy per nucleon (either a proton or neutron). This is the energy that would be available between a single scattering of two nucleons. We only accelerate charged particles, so when we collide Pb 208 which contains 82 protons at the equivalent of proton-proton collisions at 13 TeV, have 13 TeV * 82 / 208 = 5.125 TeV/nucleon" so it seems the energy per particle is smaller
It's therefore much better to probe for unknown phenomena using a hadron collider (e.g., a proton-proton or ion-ion) since with a single beam energy you can potentially produce interactions at a wide range of center-of-mass energy. However, if you know the center-of-mass energy of the processe(s) you need to study, a lepton (e.g. electron-electron) collider is more suitable since you can precisely tune the center-of-mass energy of the interactions.
It's also possible to collide electrons with protons. This was done for example at the HERA collider at DESY in Hamburg in the 90's/00's to study the inner structure of the proton.
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.