$W\rightarrow\ell\nu$ events containing jets. The generation of fake MET will be measured using processes such as $\gamma$ + jet, where no MET is expected and the recoil against the photon provides good control of possible jet energy mismeasurement. The efficiency of background rejection can be further studied using dijet event samples containing a cosmic ray or beam halo muon as identified by the muon system.

Summary
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This paper has presented the technique used to determine the arrival time of energy deposits in the CMS HCAL to less than 25ns, and the use of this technique to suppress out-of-time backgrounds. The performance of the technique and of the synchronization system of the HCAL have been demonstrated using data from test beam operations as well as “beam splash” data from the LHC in September 2008. The ultimate resolution of the technique for hadron showers was measured to be 1.2 ns using test beam data, and the time spread of HCAL channels after hardware alignment in $\eta$ was measured to be $\pm$2 ns, with an expected potential offline synchronization of less than 1 ns. Using data from CRAFT and beam operations in 2008, the use of a timing filter to suppress fake 