Speaker
Dr
Patricia Conde Muíño
(LIP-Lisbon)
Description
ATLAS is one of the four major LHC experiments, designed to cover a wide range
of physics topics. In order to cope with a rate of 40MHz and 25 interactions per
bunch crossing, the ATLAS trigger system is divided in three different levels. The
first one (LVL1, hardware based) identifies signatures in 2 microseconds that are
confirmed by
the the following trigger levels (software based). The Second Level Trigger (LVL2)
only looks at a region of the space around the LVL1 signature (called Region of
Interest or ROI), confirming/rejecting the event in about 10 ms, while the Event
Filter (Third Level Trigger, EF) has potential full event access and larger processing
times, of the order of 1 s.
The jet selection starts at the LVL1 with dedicated processors that search for
high ET hadronic energy depositions. At the LVL2, the jet signatures are verified
with the execution of a dedicated, fast jet reconstruction algorithm. Given the fact
that the main jet's background are jets,the energy calibration at the LVL2 is one
of the major dificulties of this trigger, allowing to distinguish low/high energy jets.
The algorithm for the calibration has been chosen to be fast and robust, with a good
performance.
The other major dificulty is the execution time of the algorithm,dominated by
the data unpacking time due to the large sizes of the jet ROI. In order to reduce
the execution time, three possible granularities have been proposed and are being
evaluated: cell based (standard), energy sums calculated at each Fron-End Board
(FEB) and the use of the LVL1 Trigger Towers. The FEB and Trigger Tower
granularities are also being used/evaluated for the reconstruction of the missing ET
triggers at the Event Filter, given the short times available to process the full event.
In this presentation, the design and implementation of the jet trigger of ATLAS
will be discussed in detail, emphasasing the major dificulties of each selection step.
The performance of the jet algorithm, including timing, eficiencies and rates will
also be shown, with detailed comparisons of the different unpacking modes.
| Submitted on behalf of Collaboration (ex, BaBar, ATLAS) | ATLAS HLT group |
|---|
Primary author
Dr
Patricia Conde Muíño
(LIP-Lisbon)
