ALICE@Point 2

 

The smooth function of the ALICE detector depends on a number of systems and tools that have been developed to ensure the detector's maximum performance during data taking. This applies to all the subsystems, including Trigger, Trigger Detectors, Tracking Detectors, DAQ, HLT, DCS, ESC, and the permanent data storage.

 

The collection and recording of information from all ALICE subdetectors is performed by the ALICE Data Acquisition System. ALICE had to design a data acquisition system that operates efficiently in two widely different running modes: the very frequent but small events with few produced particles, encountered in the proton-proton mode, and the relatively rare, but extremely large events with tens of thousands of particles, produced in heavy-ion operation.

The data flow from the detector electronics to the data storage at the CERN Computing Centre is organized as a sequential data-driven pipeline. Upon reception of a sequence of trigger signals requesting the data collection, the detector elements generate data that are then transferred to computers via optical links. A computing farm is used to check, label, format, and record the data.

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The High Level Trigger (HLT) combines and processes the full information from all major detectors of ALICE in a large computer cluster. Its task is to select the relevant part of the huge amount of incoming data and reduce the data volume by well over one order of magnitude to fit the available storage bandwidth, while preserving the physics information that is of interest.

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The Detector Control System (DCS) is a complex hardware and software system organized in a hierarchical way. It provides supervision and control of all subsystems such as Low Voltage (LV), High Voltage (HV), Frontend and Readout Electronics (FERO), Cooling (COO), Gas etc. In addition, the DAQ, Trigger and Offline systems, as well as the LHC Machine are linked-up with DCS.

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As millions of collisions per second take place at the LHC, ALICE uses a sophisticated system to rapidly decide, based on a number of criteria, which events are interesting, only a small fraction of which is finally recorded. Due to limitations on data storage capacity and rates, a series of "trigger" stages is employed. Triggers usually make heavy use of a parallelized design, exploiting the symmetry of the detector, with each detector having its own trigger system;  the same operation may be performed at the same time on different parts of the detector.

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The Data Quality Monitoring (DQM) system is an important aspect of every High-Energy Physics experiment, especially in the era of the LHC, when detectors are extremely sophisticated devices. To use the data taking time and precious bandwidth in an optimal way, one needs online feedback on the quality of the data being recorded. The DQM software provides this feedback and helps shifters and experts identify potential issues early. The DQM involves the online gathering of data, their analysis by a user-defined algorithm and the storage and visualization of the produced monitoring information.

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The Experiment Control System (ECS) is the top level of control of the ALICE experiment. It provides a framework in which the operator can have a unified view of all the ‘online systems’ and perform operations on the experiment seen as a set of detectors.

ALICE has adopted a hierarchical architecture, in which the ECS is a layer sitting above the ‘online systems’ (DCS, DAQ, CTP, HLT) that still preserve their autonomy to operate independently. The interface between the ECS and the ‘online systems’ apply a powerful paradigm based on inter-communicating objects. The behavioural aspects of the ECS are described using a finite-state machine model.

The ECS provides all the features necessary to split the experiment into partitions, containing one or more detectors, which can be operated independently and concurrently.

 

The LHC Interface project (LHC_IF) ensures the online data exchange between the Large Hadron Collider (LHC) and ALICE and the safe operation of the complex experiment-collider.

The LHC_IF subscribes to the beam conditions published by LHC via a dedicated network communication protocol and monitors the quality of the particle collisions in ALICE online, providing fast feedback to the control team of the LHC. Moreover, if harmful conditions for the experiment are detected, then the LHC_IF can dump the beams or inhibit the beam injection via dedicated control lines. Finally, it provides basic analysis tools to study the beam and experiment conditions offline, which is useful for physics data analysis in ALICE.

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