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Well, that's about it. I don't find any reasonable explanation for this fact. Are they transversally expanding as the energy increases? Please, remember that strong interactions are short ranged and the geometric cross-section of the proton is about 30mb. At 10 TeV the total cross-section is about 100mb. This can be seen in:

https://arxiv.org/abs/1712.06153

(Figure 4, in page 11 and Figure 6, in page 12).

In:

https://arxiv.org/abs/1807.06471

They claim that the black disk limit has been exceeded. That means that over 50% of the $pp$ collisions are purely elastic (which can arguably be labelled as "boring") at 13 TeV and also that this percentage will rise as the C.O.M. energies get higher.

I'm also unable to find a qualitative physical explanation for this.

It makes me wonder about the upcoming upgrades at the LHC.

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  • $\begingroup$ Seems that it is what has been observed , explained in parts with the QCD picture arxiv.org/abs/1208.3456 $\endgroup$ – anna v Aug 23 '18 at 16:24
  • $\begingroup$ Well, I don't like this paper very much. There's an almost general consensus that the best fit to total $pp$ cross-section is $Log^2(s/s_0)$ dependent at high energies. The elastic to total cross-sections ratio is above 0.5 (black disk limit exceded) at 7 and 13 T eV and increases with energy. $\endgroup$ – Carlos L. Janer Aug 23 '18 at 16:58
  • $\begingroup$ I cannot think of any physical reason that could explain this behaviour even in a qualitative way. None of the points of my previous comment are correctly addressed in that paper. The best fits are obtained in the frame of Regge's theory. $\endgroup$ – Carlos L. Janer Aug 23 '18 at 17:06
  • $\begingroup$ Wouldn't this be naively expected as larger c.o.m. energy allows the protons to come closer, plainly speaking? $\endgroup$ – Helen May 28 at 11:32

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