The Tag
 
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The ATLAS TAG is a small ntuple like object that contains one entry per event for every DATA event (and possibly for MC as well).

Use Cases for the TAG

There are two use cases I hear talked about the most often:

  1. Preselect Data so as to access the Data Management Facilities less frequently. The TAG is compact, and very fast to run over, and, from a disk I/O perspective, very cheap. You design a set of cuts based on very loose object requirements, or a series of triggers, extract the run/event number list from the TAG, and then feed that to a real AOD job that actually extracts the data for your analysis. 
  2. Do a quick/rough analysis by looking at the physics reconstructed objects in the event. Make cuts, plots, etc.

Contents Of the TAG

At varous meetings (Computer Model Meetings, mostly) I've been given to understand that the TAG may (or may not) contain the following:

  • The HLTs that fired. This could be a complete bit mask or something similar (hopefully not requiring you to cross index this with a conditions database).
  • High Level Objects. For example, 4 vectors for electrons and jets and muons and missing ET

What Does DZERO Do?

Questions for TAG usage at ATLAS

Let me first stress there is an implied assumption in my questions below: I don't consider the TAG very useful from the data selection point-of-view if you can't significantly reduce the number of files you need to access (i.e. the # of events/file on average is much less than one).

  1. Do the TAG Contents make sense?
    1. If the TAG contains only trigger information will it be possible to make an analysis selection that both makes sense and also selects a small enough amount of data to be useful.
    2. The TAG needs info form
  2. Who is actually going to skim the data? Will it be individuals? Physics Groups? Or an experiment wide task run by some special streaming group?
    1. Individuals. Do we really have the resources to do that if people make a loose selection (1%, 2%, 10% of the data??). We will have to have something like the TAG in this case.
    2. Groups. The TAG may be useful in this