and the $z$ axis along the counterclockwise beam direction. The azimuthal angle, $\phi$, is measured with respect to the $x$ axis in the $(x,y)$ plane, and the polar angle, $\theta$, is defined with respect to the $z$ axis. The pseudorapidity is $\eta =
-\ln [\tan(\theta / 2)]$.

For the data used in this analysis, the peak luminosity of the LHC increased from to over . For the data collected between , the increase was achieved by increasing the number of bunches colliding in the machine, keeping the average number of interactions per crossing at about 7. For the rest of the data, the increase in the instantaneous luminosity was achieved by increasing the number and density of the protons in each bunch, leading to an increase in the average number of interactions per crossing from around 7 to around 17. The presence of multiple interactions per crossing was taken into account in the CMS Monte Carlo (MC) simulation by adding a random number of minimum bias events to the hard interactions, with the multiplicity distribution matching that in data.

Trigger selection\[sec:TRIGGER2011\]
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The CMS experiment uses a two-stage trigger system, with events flowing from the L1 trigger at a rate 