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The obvious answer is more energy per collision. This is discussed on p21 in this brochure of FAQs: CERN-Brochure-2017-002-Eng, which I found linked in this old question: Why not build a particle accelerator on ground level? What is the shallowest feasible depth to build one?.

Each proton beam flying around the LHC has a maximum design energy of 7 TeV, so when two protons collide the collision energy is 14 TeV. Lead ions have many protons, and together they give an even greater energy: the lead-ion beams have a maximum collision energy of 1150 TeV. Both collision energies have never been reached before in a lab.

But from Accelerator Science: Proton vs. Electron, collisions are not between lead ions so much as two quarks/antiquarks/gluons within two protons or neutrons. Mostly the ions pass through each other.

Is it simply because more massive ions can be accelerated to higher energies per nucleon, as implied on p19?

In a circular accelerator, such as the LHC, heavy particles such as protons (protons are about 2000 times more massive than electrons) have a much lower energy loss per turn through synchrotron radiation than light particles such as electrons. Therefore, in circular accelerators, to obtain the highest-energy collisions it is more effective to accelerate massive particles.

Or is it that you get more nucleons in the beam with ions?

Either way, why not use something heavier like uranium? And why are Future Circular Colliders at higher energies than the LHC planning to use protons?

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    $\begingroup$ The LHC uses protons not lead ions in its usual mode of operation. It can use lead ions, which it does a few weeks a year to study the quark gluon plasma created by heavy ion collisions. Are you asking why specifically lead ions for the QGP studies? $\endgroup$ Commented Jul 20, 2022 at 4:37

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Maybe this article will clear up your questions.

Collisions of lead nuclei will allow physicists to study specific phenomena such as quark-gluon plasma, a state of matter that is thought to have existed during the very first moments of the Universe,

You ask:

Either way, why not use something heavier like uranium? And why are Future Circular Colliders at higher energies than the LHC planning to use protons?

The choice of colliding particles in an accelerator depends on the objectives of the study, which is to see the interactions of elementary particles and test the mathematical model we have arrived to describe them, also looking for deviations and new effects. The simplest the particles colliding the easier the test of the fundamental interactions. That is why e+ e- colliders were built, the results of those experiments validated and extended the standard model.

is it that you get more nucleons in the beam with ions?

for the study of quark gluon plasma the objective is to have as many nucleons involved in the creation in order to reach if possible plasma energies. Lead is a stable nucleus and easy to manipulate. In this link one can see that uranium has also been used in the heavy ion experiments.

Protons are the simplest carriers of quarks and the experiments can study a lot of interactions and decays of new particles with the weak force and also , from deviations to the theory search for new effects due to weak interactions. Heavy nuclei are for studying plasma as quoted above. I am sure that once a new collider exists, there will be experiments with heavy nuclei to study further the plasma.

There are also plans for future e+e- and even thoughts of gamma gamma colliders to study simply fundamental interactions.

It all depends on the physics objectives of the study.

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  • $\begingroup$ There are heavier stable nuclei than lead. And I don't agree that radioactive debris is a concern. This is an absolutely miniscule amount of the particle being accelerated, and already the LHC needs to remain closed for two weeks after operation because of irradiated materials remaining radioactive. Adding a microscopic amount of uranium will not make a difference. Not sure what you mean by "for the mathematics of the analysis." Seems like you have a good description for why protons are used for the main physics goals of the LHC, but not really any answer to the question - why lead. $\endgroup$
    – AXensen
    Commented Jul 10, 2023 at 14:46
  • $\begingroup$ @AXensen cds.cern.ch/record/2065692/files/CERN-THESIS-2015-195.pdf , a thesis, has a chapter describing existing experiments with heavy ions, and yes uranium has been used, so you are correct that extra radiation dangers are not an problem for the experiments. I have edited $\endgroup$
    – anna v
    Commented Jul 10, 2023 at 15:04

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