Citizen Science Entry 2
Here's my entry for @lemouth's citizen science program, episode 2. Glad to be back :).
Before starting MadGraph, I picked apart the acronyms in the name of the package: 'MadGraph5_aMC@NLO'. Specifically, I wanted to know what the 'aMC' and 'NLO' terms meant. After some searching, here's what I concluded:
- aMC: adjoint Monte Carlo.
- NLO: Next-to-Leading-Order.
I was most interested in the 'aMC' acronym since 'aMC' appears prominently in the MadGraph5 prompt. So it's gotta be important!
Now for the meat-n-potatoes: simulating 10000 proton-proton collisions where each individual collision yields one top quark and one top antiquark. Following lemouth's lead in episode 2, here's the commands I entered.
$ ./bin/mg5_aMC
MG5_aMC>generate p p > t t~ # Task 1. Defines the collider process of interest.
MG5_aMC>display diagrams # Task 1. Writes diagrams to /tmp/ in my container.
MG5_aMC>output pp_tt # Task 2. Set output directory and build Fortran code.
MG5_aMC>launch pp_tt # Task 3. Run simulation and compute top anti-top production rates.
Note the presence of '#' at the end of the MG5 commands above. Through trial and error, I found that '#' indicates a comment in MadGraph's REPL. For example, generate p p > t t~ # Collider process is a valid command.
An aside on filesystems
Initially I ran the simulation on a shared filesystem (/host/c). That was slow. So I tried both a container-local filesystem and a filesystem mounted in RAM (ramfs). For comparison, I measured the elapsed time of the 'launch' command (minus menu interactions). Here are the results:
- Shared filesystem (/host/c/temp/pp_tt): 14m29s
- Container filesystem (./pp_tt): 6m31s
- RAM filesystem (~/ramfs/pp_tt): 5m47s
The learning: don't use a shared filesystem like I did :(. It was more than 2 times slower. And ramfs helps, but not too significantly. The minor effect of ramfs makes sense as the simulation is likely compute bound.
Since I'm running MadGraph in a container, there is no interactive display session. It's (nearly) headless. So I'm forced to copy graphical results (like images) to the host machine before viewing. That does incur some overhead.
Production Rates and Verification
A screenshot of my final output:
My cross-section is: 505.8 +- 0.8 pb, which yields the same production rate @lemouth determined. Assuming 140/fb has two significant figures.
I verified parton showering, hadronisation, and decay.
That's about it. Looking forward to the next episode and seeing you folks around!