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Data Model

The CMS Event Data Model (EDM) is centered around the concept of an Event. Physically, an event is the result of a single readout of the detector electronics and the signals that will (in general) have been generated by particles, tracks, energy deposits, present in a number of bunch crossings.

In software terms, an Event starts as a collection of the RAW data from a detector or MC event, stored as a single entity in memory, a C++ type-safe container called edm::Event. An Event is a C++ object container for all RAW and reconstructed data related to a particular collision. During processing, data are passed from one module to the next via the Event, and are accessed only through the Event. All objects in the Event may be individually or collectively stored in ROOT files, and are thus directly browsable in ROOT.

More and detailed information can be found here.

The CMS Data Hierarchy

CMS Data is arranged into a hierarchy of data tiers. Each physics event is written into each data tier, where the tiers each contain different levels of information about the event. The different tiers each have different uses. The three main data tiers written in CMS are:

  1. RAW: full event information from the Tier-0 (i.e. from CERN), containing 'raw' detector information (detector element hits, etc)
    • RAW is not used directly for analysis
  2. RECO ("RECOnstructed data"): the output from first-pass processing by the Tier-0. This layer contains reconstructed physics objects, but it's still very detailed.
    • RECO can be used for analysis, but is too big for frequent or heavy use when CMS has collected a substantial data sample.
    • RECO Data Format Table
  3. AOD ("Analysis Object Data"): this is a "distilled" version of the RECO event information, and was used for most analyses on Run 1 data.
    • AOD provides a trade-off between event size and complexity of the available information to optimize flexibility and speed for analyses.
    • AOD Data Format Table
  4. MINIAOD: slimmer version of AOD, used for analyses on Run 2 data.
    • MINIAOD is approximately one tenth of the size of AOD.
    • The reduction is obtained defining light-weight physics-object candidate representations, increasing transverse momentum thresholds for storing physics-object candidates, and reduced numerical precision when it is not required at the analysis level.
    • MINIAOD physics objects table

The data tiers are described in more detail in a dedicated WorkBook chapter on Data Formats and Tiers.