Part of the LHC's Run 2 (which just recently started) will be dedicated to proton-ion collisions, but I haven't been able to find any firm information about what energy those collisions will run at. The best information I've seen so far suggests that there are two proposals for beam energy, $4 Z$ and $7 Z\ \mathrm{GeV}$, but that's a couple years old.

A beam energy of $4Z\ \mathrm{TeV}$ corresponds to the Run 1 pPb center-of-mass energy of $5.02\ \mathrm{TeV}$ (note $\sqrt{s_{NN}} = 2\sqrt{Z/A}\times\text{beam energy}$), which suggests that the decision made in that case was to simply set the proton beam energy at the same value used for pp collisions. Using that logic, the proton beam during Run 2 pPb would be set at $6.5\ \mathrm{TeV}$, corresponding to a center-of-mass energy of $\sqrt{s_{NN}} = 8162\ \mathrm{GeV}$. But the accelerator is designed for $7\ \mathrm{TeV}$ proton beams, which would have $\sqrt{s_{NN}} = 8790\ \mathrm{TeV}$. I don't know if a final upgrade to the design energy is scheduled for before pPb collisions resume in 2017. And it's not strictly required that pPb operation uses the same beam energy as in pp.

Does anyone know the final decision on beam energy? Or a recent source stating that the decision is not final, if that is the case.

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    $\begingroup$ I imagine the PIs saying "I need more power!" and Scotty somewhere down in the bowels of the LHC yelling "I'm givin' her all she's got Captain!" $\endgroup$ – OSE Jul 22 '15 at 14:56

I have no actual information, but I believe that it's highly likely that the protons will again run at the same energy as in the pp run. By doing this, the operations team will not have to redo the various manipulations (injection, ramping, bunching...) that have been already done for the protons.

  • $\begingroup$ Thanks... this would be my guess as well, but I'm looking for a source to confirm or validate it. Or if not a source, at least some information coming from someone with insider knowledge. $\endgroup$ – David Z Jul 22 '15 at 16:37

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