Just a quick post today, to show the invariant mass distirbution of pairs of opposite-sign muons collected by the ATLAS and CMS experiment. Such plots are incredibly rich in information, as they contain the signal of ten different resonant states, and allow one to figure out the masses and production rates that these particles have, as well as the resolution of the detectors. I have shown an early such plot by CMS some time ago, but now the graph has been updated to contain all the relevant data collected in 2010 by the detector.

What is more, I could find the corresponding picture by ATLAS: the comparison shows an additional interesting datum -the relative collection efficiency of the two apparata. Now, to be fair, I believe the figures cannot be directly compared, at least as far as the low-mass region is concerned: the ATLAS figure only includes data collected by their high-Pt muon trigger, while I believe that the CMS figure includes also lower-Pt muon triggers. In other words, the CMS figure is expected to contain more low-mass resonances than the ATLAS one. Still, the comparison is quite interesting.

Here is the CMS figure: entries per bin are normalized to counts per 1 GeV (the same as is done for the ATLAS plot following below).


And here is the ATLAS figure. As you immediately notice, the number of Z events is quite similar in the two graphs, but the lower resonances are much more numerous in the CMS plot. For instance, the upper plot has a peak of the particle labeled "J/Psi" (the famous charmonium state, discovered in 1974) which gets to 5 million entries per GeV, while in ATLAS they get only 100,000 per GeV. And the lower-energy resonances are much less distinguishable below -the particle labeled "eta" ( ) does not even appear. As I said, it is due to the trigger. But still, it is not by chance that the "M" in CMS stands for "Muon"! 1-0 for CMS as far as dimuon resonances go!



I think these graphs are terrific in their capability to convey a particle physicists' awe at the subatomic world. These particles are not just coming out of our imagination -they are as real as bread and butter!