Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

CHAPTER 2: The CMS experiment at the LHC 272.2.4 Triggering and Data AcquisitionIn one hand, the LHC, while working at the design luminosity of L = 10 34 , collidesbunch of protons every 25 ns resulting in a bunch crossing rate of 40 MHz as mentionedbefore. This corresponds to obtain 10 9 proton-proton interactions per second when themachine is operating at the center of mass energy of 14 TeV. In the other hand, thecurrent technology is limited to process only about 100 interactions per second, hence10 7 times less than what is produced. Since most of the occured interactions are softcollisions of protons, then one does not lose processes of physics interest by using anoptimal rejection procedure. The process of online event selection is handled with theuse of the trigger system [54, 55], which is performed in two stages. A first stage,which is called first level trigger (Level-1), is aimed to reduce the rate of the datataking down to 100 kHz. Because the decision made by the Level-1 trigger should beas fast as possible, the basic information from calorimeters and the muon system areused to decide whether an event has to be kept or not. A second stage, which is calledHigh-Level Trigger (HLT), is designed for a further reduction of the rate down to 100Hz. The interactions selected by the Level-1 trigger are then fed to the HLT for additionalinvestigation and making a more robust decision. The HLT software containsmore sophisticated algorithms which are very close to the ones applied in the offlineanalysis. The decision which is made at each stage of the trigger to keep or rejecta specific interaction is based on the reconstruction and identification of the physicsobjects at the final state of that particular interaction. Because processes of physicsinterest, such as production and decay of rare particles like Higgs bosons, contain highenergetic objects, such as electrons, photons, muons and jets, various trigger streamsare defined, each of which requires some configurable physics conditions. An interactionwhich fires at least one of the trigger streams is archived permanently and keptfor the offline analysis.The data which pass the HLT filters are stored in computing centers distributed allaround the world. According to the CMS computing model [56], the storage resourcesare connected and managed through the Worldwide LHC Computing Grid(WLCG) [57]. The WLCG project makes it possible for the users to access the informationand data which have been distributed over the storage resources with the useof the Grid network. Computing centers are grouped in three categories referred toas Tier-0, Tier-1 and Tier-2, each of which provides some particular tasks. The rawdata which are provided by the CMS detector are first moved to the Tier-0, which islocated at CERN, in order to run the initial reconstruction algorithms and processingof the data. The reconstructed data are then transferred to the computer resources ofthe Tier-1 centers. Also the simulated proton collisions, which are produced at Tier-2 computing resources, are kept in Tier-1 storage services. In case it is needed, there-reconstruction of the data is also performed at Tier-1 centers. The several Tiersare kept in a hierarchial level. The collected data by the CMS experiment are thenstored in the form of ROOT files [58] and subsequently analyzed with some dedicatedsoftware, called CMSSW. The CMS computing model provides various data formats tofacilitate the data transfer and processing. The data which come out of the detector,containing the electronic signals and hits, is called RAW data. They are huge andcomprising the low level objects which are not suitable for the physics analaysis. After