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\section{The CMS detector}
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\label{sec:detector}
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A detailed description of the Compact Muon Solenoid (CMS) experiment
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can be found elsewhere~\cite{CMS}. The central feature of the CMS detector
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is a 3.8~T superconducting solenoid enclosing the
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silicon pixel and strip tracker, the crystal electromagnetic
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calorimeter (ECAL), and the brass-scintillator hadronic calorimeter
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(HCAL). Muons are measured in gaseous detectors embedded in the iron
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return yoke. The ECAL has an energy resolution of better than 0.5\,\% above
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100~GeV. The HCAL, when combined with the ECAL, measures jets with a
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resolution $\Delta E/E \approx 100\,\% / \sqrt{E} \oplus 5\,\%$. The
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calorimeter cells are grouped in projective towers, of granularity
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$\Delta \eta \times \Delta \phi = 0.087\times0.087$ at central
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rapidities and $0.175\times0.175$ in the forward region.
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The muons are measured in the pseudorapidity\footnote{Pseudorapidity $\eta$ is defined as $\eta = -\ln(\tan\frac{\theta}{2})$, where
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$\theta$ is a polar angle with respect to the direction of the counterclockwise beam} window $|\eta|< 2.4$.
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Matching the muons to the tracks measured in the silicon tracker results in a transverse
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momentum resolution between 1 and 5\,\%, for $p_T$ values up to
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1~TeV/$c$. The CMS trigger system consists of two levels. The first level (L1), composed of custom
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hardware, uses information from the calorimeters and muon
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detectors to select a fraction of a per cent of most interesting events for
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further readout and analysis. The High Level Trigger
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(HLT) processor farm further decreases the event rate to
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$\sim 500$~Hz for data storage. The instantaneous luminosity of the machine is measured using information from forward hadronic calorimeters~\cite{lumi}.
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