The ATLAS Level 2 Trigger

The size of the Region of Interest (RoI).

Motivation.

The aim here is to optimize the size of the RoI. If we manage to reduce the size of the RoI then we will have less Space Points to take into account thus less combinatories and finally we can make the algorithm faster. In all these steps, we must ensure that we don't loose our efficiency.

How the RoI is constructed.

The RoI is constructed after the the LVL1 processes. At LVL1 the information comes from the Calorimeters and from the Muon Spectrometer and defines an active region in (&eta, &phi) space. The polar angle is again given with respect to Z=0.

Then the RoI is constructed as the geometrical object with dimensions:

  • Z=0 ± 168 cm (~ 3&sigma of the beam spread)
  • &eta(LVL1) ± 0.1
  • &phi(LVL1) ± 0.1 rad

IDScan: A LVL2 Trigger algorithm.

The aim of the IDScan is to find the particles' tracks inside the Inner Detector.Based on the RoI information coming from LVL1, IDScan uses the Pixel and SCT Space Points inside that RoI.

IDScan proceeds in the following steps:

  1. Import the SpacePointCollection
  2. Find the z position of the primary vertex
  3. Make internal space points - Remove noise
  4. Find and fit tracks
  5. Store tracks in the Transient Data Store

Methodology.

The study should have the next steps:
  1. Access the online information from the EM Calorimeter. Get the parameters for each sampling.
  2. Calculate the z vertex using the shower position of the 1st and 2nd sampling.
  3. Construct the new RoI.
  4. From the SpacePointCollection get the Space Points inside the new RoI (NewSpacePoints).
  5. Feed IDScan with the NewSpacePoints.
  6. Get the new track parameters. Compare - Correct.

An event using the ATLANTIS event display. The Space Points inside the RoI are with white dots. The inner most come from the SCT detectors (8 layers arranged in 4 strips), while the outer most come from the Pixel detectors (3 layers). The track found by IDScan is given in orange and the other tracks in red:

New RoI Construction - Formalism.

Results.