Abstract
One of the major challenges for the Compact Muon Solenoid (CMS)experiment, is the task of reducing event rate from roughly 40 MHz down to a more manageable 1 kHz while keeping as many interesting physics events as possible. This is accomplished through the use of a Level-1 (L1) hardware based trigger as well as a software based High-Level Trigger (HLT). Monitoring and understanding the output rates of the L1 and HLT triggers is of key importance for determining the overall performance of the trigger system and is intimately tied to what type of data is being recorded for physics analyses. We present here a collection of tools used by CMS to monitor the L1 and HLT trigger rates. One of these tools is a script (run in the CMS control room) that gives valuable real-time feedback of trigger rates to the shift crew. Another useful tool is a plotting library, that is used for observing how trigger rates vary over a range of beam and detector conditions, in particular how the rates of individual triggers scale with event pile-up.
Highlights
The trigger system for the Compact Muon Solenoid (CMS) [1] is extremely important to data acquisition and is responsible for determining what data gets recorded for use in physics analysis
As the LHC pushes to higher beam intensities, CMS has to be ready to respond to emergencies if the trigger rates go out of expected ranges, potentially overwhelming the DAQ or
In addition to the real-time component, the software produces plots that compare the trigger rate versus μ to the expected rate for each fill. These plots have been integrated into the central CMS Web Based Monitoring service (WBM) [3]; a dedicated page is linked from each Fill Report page on WBM, containing plots with rates for that fill (Figure 3)
Summary
The trigger system for the Compact Muon Solenoid (CMS) [1] is extremely important to data acquisition and is responsible for determining what data gets recorded for use in physics analysis. Fits are made to the trigger rates in previous runs using linear and non-linear regression These fits are compared to the instantaneous trigger rate as data is being collected in order to spot (unexpected) deviations in the rates. The raw trigger rate is first corrected for deadtime, Level-1 (L1) and High Level Trigger (HLT) prescales, and number of colliding bunches in the LHC. These corrections facilitates comparisons between runs with different conditions and allows a smooth function to be fit across multiple runs.
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