used to validate the barrel sampling delay settings derived from test beam data, and to derive settings to compensate for the $\eta$ dependence of timing in the endcap and outer calorimeters. Data from LHC beam operations were recorded for the first time on September 10, 2008. In preparation for the arrival of LHC beam at CMS, the $\eta$-dependent sampling delays measured from test beam were loaded into the barrel front-end electronics. These delay values had been calculated to synchronize channels for data from collisions. Delay values for endcap and outer sectors were set to zero, since no timing measurements for these sectors were available at the time.

The LHC beam delivered to CMS consisted of a single 450 proton bunch circulating in the ring, first in one direction and then in the other. The LHC beam was also steered into collimators located 150 m upstream in either direction of the detector interaction point. Such events are referred to as “beam splash” events. Because of the large amount of material in the collimator and along the path of the secondaries, only muons were able to penetrate the entire CMS detector and form the signal in the calorimeter system. The millions of 