RE: Simulating top quark production at CERN’s LHC: @mengene's Progress Blog 02 for #citizenscience Project

Thanks a lot for this very nice report. It is definitely addressed to everyone: other participants and me (through the few questions). Therefore, it ticks all the boxes :)

First of all, I would like to stress that I really appreciated the figures that you joined to this report. I believe that those with a screenshot of my post on one side and your terminal output on the other side could be very useful for other users, and those with a few things highlighted are very readable too. Thanks for the efforts you put in there!

In addition, it is great to see that you have a 16-cores machine. This will be really helpful for the CPU-heaviest of the next steps. This will allow you to save a lot of time.

Anyway, let me move on with the question you raised.

I noted that g g > t t~ was the only possible instance that generated 3 diagrams, i am hoping @lemouth could help explain why this particular instance yielded more than one diagram, which is quite different from the other instances. In addititon, does it mean that if the equation were to be derived from this particular instance would it take just one of the diagrams or it would factor in the other two diagrams during the equation generation process.

That’s the mysteries of quantum chromodynamics, the quantum theory describing the strong force. The strong force allows for the following interactions:

• quark-antiquark-gluon (where the quark and the antiquark must be of the same flavour: for instance up and anti-up, top and antitop, etc.);
• gluon-gluon-gluon;
• gluon-gluon-gluon-gluon.

In addition, we must keep in mind that protons contain quarks, antiquarks and gluons. The proton content defines the initial state of the different relevant subprocesses. We can have various quark-antiquark pairs, two gluons, or even one gluon and one quark or antiquark. We need to try them all and try to find a (diagrammatic) way, with the above three interactions, to get to a top-antitop final state.

By brute forcing it (note that MG5aMC is a bit more clever than this), we can find quickly that there are 4 possible quark-antiquark initial states that work, respectively with an up, down, strange, and charm flavour. In order to produce a top-antitop final state, we use twice the first of the above interactions (once with a top flavour, and once with the flavour of the initial state), and there is one possibility. This is the diagram that you have found and that is thus four times the same (once for an up-antiup initial state, once for a charm-anticharm initial state, once for a strange-antistrange initial state and once for a down-antidown initial state).

We can also produce a top-antitop pair from two gluons. Here, the above three interactions give 3 possibilities, that are those returned by MG5aMC. Either the first interaction is used twice (both with a top flavour), which gives two diagrams, or we used once the first interaction (with a top flavour) and once the second interaction, which gives the third diagram.

The equation derived from the diagrams is derived from all the diagrams. It would be (totally) incorrect if any diagram would be missing.

-3 : Being my curious self, I tried to mimic what @lemouth did in the screenshot below, and I managed to open the 'maxparticles.inc' file with a text editor. I share the content of the file in the next picture below

Try to open matrix.f. This will be much funnier ;)

Once again, thanks a lot for this very nice report and your questions and comments!

Cheers!