Yesterday I posed a question - Are the first collisions recorded by the LHC running at 13 TeV the highest-energy ever produced by mankind with subatomic particles ? It was a tricky one, as usual, meant to think about the matter.

I received several tentative answer in the comments thread, and thus answered there. I paste the text here as it is of some interest to some of you and I wish it does not go overlooked.

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Dear all, 
thank you for your thoughts. As it often happens, the global common knowledge of a pool of readers exceeds the wisdom of the writer of this column... As many have figured out, the question is ill-posed, and as such cannot be answered. What is "highest-energy" ? What is "man-made" ? And what are "subatomic particles" ? None of these is clear. 

Take the first. We can define the energy as the energy carried by the projectiles in the center of mass, or the energy released by the collision. We could even become more detailed by considering the variables with which deep inelastic scattering is described, like y and Q^2, or using Mandelstam variables. 

 Or take the second. Man-made in what sense ? If a cosmic ray did interact with a proton or electron in some past experiment, it might well have created a large energy deposit in the detector and be recorded (a calculation of how likely this is would be too long here - maybe it is the subject of another post). 

Or the third. Subatomic = smaller than an atom ? Or between constituents of atoms ? A nucleus is certainly a constituent, and thus one would call it a subatomic particle. But many would object to that view (I wouldn't). 

In the end, it is a matter of what is commonly understood within a given work environment. Most HEP physicists would consider the very low-Q^2 first collisions that the LHC has produced yesterday for sure not the highest energy collisions ever produced by mankind: the energy available for the creation of new massive states was ridiculously smaller than the highest values ever achieved (which are in the 4 TeV range) in the 2011-2012 run. 

The reason is that the more energetic collisions are the rarest, and thus one cannot just accelerate one pair of protons to a gazillion GeV and be sure that the energy release from their interaction will be arbitrarily high. To be quantitative about this point, the total cross section between protons at 13 TeV is in the 100 millibarn range, but a collision that releases 4 TeV of energy is in the femtobarn range, so 10^11 times less frequent. If you record one collision at 13 TeV, the chance that it is the highest ever achieved, having collected many 4 TeV collisions in the previous run, is in the one-in-a-trillion range. 

If you ask me, I am not very excited, as my heart stays closer to answer #4, for the reasons stated above. Sure, we collided lead nuclei at very high energy, but those collisions hardly made W or Z bosons - getting to 4 TeV is a far cry from that! So, I look forward to the time when the LHC will start for real - producing high luminosity collision. When the acquired data will surpass one inverse femtobarn of collisions - making it possible that a few _really_ high-energy collisions have taken place at energies larger than the other time - will the game really get interesting... So stay tuned!