Data Acquisition & Trigger

The LHC is designed to provide a bunch crossing rate of 40 MHz. One event recorded by the CMS experiment amounts to about 1 MB of zero-suppressed data. The processing and storage of all events would largely exceed the resources provided. The available storage capabilities can store data at O(1) kHz and O(100) MB/s. Accordingly, a huge fraction of the collision events has to be rejected at an early stage. The rejection rate

necessary corresponds to a factor of about 106_{. The CMS trigger and data acquisition}

system achieves such high rejection rates based on a two-staged approach: the Level-1 (L1) trigger and the HLT.

L1 trigger

The front-end electronics situated in the detector receive signals from the various subde- tector channels. Part of this information is passed on to the L1 trigger system located in the service cavern, a second cavern next to the one accommodating the CMS detector. A schematic illustration of the L1 trigger is shown in Fig. 2.6. The L1 trigger system selects only events with simple signs of interesting physics [49]. For this purpose, simple objects, so-called trigger-primitive objects, are reconstructed mainly using calorimeter and muon system information. They are processed in several steps before the combined event information is evaluated in the global trigger (GT) and a decision is made whether to accept the event or not.

The L1 calorimeter trigger comprises two stages, a regional calorimeter trigger (RCT) and a global calorimeter trigger (GCT). The RCT receives the transverse energies and quality flags from over 8000 ECAL and HCAL towers, giving trigger coverage over |⌘| < 5. The RCT processes this information in parallel and sends as output e/ candidates and

regional ETsums based on 4 ⇥4 towers. The GCT sorts the e/ candidates further, finds

jets (classified as central, forward, and tau) using the ET sums, and calculates global

quantities such as Emiss. It sends as output four e/ candidates each of two types, iso-

lated and non-isolated, four each of central, tau, and forward jets, and several global quantities.

All three muon detector systems in CMS participate in the L1 muon trigger. The front- end trigger electronics of DT and CSC identifies track segments from the hit information registered in multiple detector planes of a single measurement station. These segments are collected and then transmitted via optical fibres to regional track finders in the elec- tronics service cavern, which then applies pattern recognition algorithms that identifies muon candidates and measure their momenta from the amount they bend in the magnetic field of the flux-return yoke of the solenoid. Information is shared between the DT track finder (DTTF) and CSC track finder (CSCTF) for efficient coverage in the region of over- lap between the two systems at |⌘| ⇡ 1. The hits from the RPCs are directly sent from the front-end electronics to pattern comparator trigger (PACT) logic boards that identify muon candidates. The three regional track finders sort the identified muon candidates

38 CHAPTER 2. THE CMS EXPERIMENT AT LHC

Figure 2.6: Overview of the CMS L1 trigger system. Data from the HF, HCAL, and ECAL are processed first regionally and then globally. Energy deposits from RPC, CSC, and DT are processed either via a pattern comparator or via a system of segment- and track-finders and sent onwards to a global muon trigger. The information from the global calorimeter and muon triggers are combined in a global trigger, which makes the final trigger decision. This decision is sent to the tracker (TRK), ECAL, HCAL or muon systems (MU) via the trigger, timing and control (TTC) system. The data acquisition system (DAQ) reads data from various subsystems for offline storage.

and transmit to the global muon trigger (GMT) up to 4 (CSCTF, DTTF) or 8 (RPC)

candidates every bunch crossing. Each candidate is assigned a pT and quality code as

well as a (⌘, ) position in the muon system. The GMT then merges muon candidates found by more than one system to eliminate a single candidate passing multiple-muon

triggers (with several options on how to select pT between the candidates). The GMT

also performs a further quality assignment so that, at the final trigger stage, candidates can be discarded if their quality is low and they are reconstructed only by one muon track finder.

The GT is the final step of the CMS L1 trigger system and implements a menu of triggers, a set of selection requirements applied to the final list of objects (i.e., electrons/photons, muons, jets, or ⌧ leptons), required by the algorithms of the HLT algorithms to meet the physics data-taking objectives. This menu includes trigger criteria ranging from simple

single-object selections with ET above a preset threshold to selections requiring coinci-

dences of several objects with topological conditions among them.

The Level-1 trigger system mainly consists of customised hardware, such as application specific integrated circuits (ASICs), in order to ensure a fast processing of the data. Nev- ertheless, also programmable hardware, like field programmable gate arrays (FPGAs), is used. Until the response of the L1 trigger is returned, the entire information of the events is stored in pipelined memory given by the buffers of the frontend electronics. The time period from sending the data to the Level-1 trigger system until the response is received adds up to about 4 µs, where about 1 µs is reserved for the decision making in the L1 trigger system. Selected events are released for further processing, while the rejected events are dropped. At this stage, the event rate is reduced to less than 100 kHz. The data passed on by the front-end electronics is further merged, before it is transferred to

2.2. THE CMS EXPERIMENT 39 the CMS computing installations on the surface. There, an event-builder network collects the data of each event and distributes them to various processing units.

HLT

The second stage of data reduction is the HLT software running on each of these pro- cessing units. It is implemented in software running on a farm of commercial computers which includes about 16 000 CPU cores, and reduces the L1 output rate to the sustainable level for storage and physics analysis of about 1 kHz. The HLT software consists of a streamlined version of the offline reconstruction algorithms; it exploits the same sophisti- cated software used for offline reconstruction and analysis, optimised in order to comply with the strict time requirements of the online selection. The software follows a strategy of rejecting events as soon as possible. This is achieved by sequentially reconstructing analysis objects in different paths that together form a modular structure. At different stages of this reconstruction procedure, events are checked for selection criteria. Colli- sion events passing this selection process are transferred to the CERN Tier-0 computing facility for further processing and storage.