Speaker
Description
Summary
The ATLAS barrel level-1 muon trigger system has to identify muon candidates crossing
the spectrometer and associate them to a specific bunch crossing, to a detector
region of ΔηXΔΦ=0.1X0.1 granularity and classify them by their Pt thresholds.
The ATLAS barrel level-1 muon trigger system has the following main requirements:
coarse measurement and discrimination of the muon transverse momentum pT; bunch
crossing identification; fast and coarse tracking to identify tracks in the precision
chambers that are related to the muon candidate; 2nd-coordinate measurement with a
required resolution of 5–10 mm.
The muon trigger system in the barrel is based on full granularity information
coming from three station of a dedicated trigger detector, Resistive Plate Chamber,
covering a region of –1<η<1. Two stations are located near the centre of the magnetic
field region, inside the air-core toroids, and provide the low-pT trigger (pT > 6
GeV), while the addition of the third station, at the outer radius of the magnet,
allows to increase the pT threshold to more than 20 GeV, thus providing the high-pT
trigger.
A trigger station is made of two detector layers, each one is composed by two RPC
detectors, read out by two orthogonal series of pick-up strips of about 3 cm pitch:
the η strips parallel to the MDT wires (z direction) provide the “bending” coordinate
of the trigger detector; the φ strips, orthogonal to the wires, provide the second
“non-bending” coordinate.
To reduce the rate of accidental triggers, due to low-energy background particles in
the ATLAS cavern, the algorithm is performed in both the η and φ projections for both
low-pT and high-pT triggers. The first stage of the trigger algorithm is performed
separately and independently for the two projections. A valid trigger is generated
only if the trigger conditions are satisfied for both projections. The trigger logic
requires three out of four layers in the middle stations for the low pT trigger and,
in addition, one of the two outer layers for the high-pT trigger. The η and φ trigger
information is combined to generate the Regions-of-Interest (RoI), identifying areas
in the apparatus in which track candidates are found with a granularity of ~0.1×0.1
in the η-φ pivot plane.
The signals from the RPC detector are amplified, discriminated and digitally shaped
on-detector. In the low-pT trigger, for each of the η and the φ projections, about
200 RPC signals of the two detector doublets, RPC1 and RPC2, are sent to a
Coincidence Matrix (CM) board, that contains a CM chip. This chip performs almost all
of the functions needed for the trigger algorithm and also for the read-out of the
strips. It aligns the timing of the input signals, performs the coincidence and
majority operations, and makes the pT cut on three different thresholds. It also
contains the level-1 latency pipeline memory and de-randomising buffer. The CM board
produces an output pattern containing the low-pT trigger results for each pair of RPC
doublets in the η or φ projection. The information of two adjacent CM boards in the η
projection, and the corresponding information of the two CM boards in the φ
projection, are combined together in the low-pT Pad Logic (Pad) board. The four
low-pT CM boards and the corresponding Pad board are mounted on top of the RPC2
detector. The low-pT Pad board generates the low-pT trigger result and the associated
RoI information. This information is transferred, synchronously at 40 MHz, to the
corresponding high-pT Pad board, that collects the overall result for low-pT and
high-pT. In the high-pT trigger, for each of the η and φ projections, the RPC signals
from the RPC3 doublet, and the corresponding pattern result of the low-pT trigger,
are sent to a CM board, very similar to the one used in the low-pT trigger. This
board contains the same coincidence-matrix chip as in the low-pT board, programmed
for the high-pT algorithm. The high-pT CM board produces an output pattern containing
the high-pT trigger results for a given RPC doublet in the η or φ projection. The
information of two adjacent CM boards in the η projection and the corresponding
information of the two CM boards in the φ projection are combined in the high-pT Pad
Logic board. The four high-pT CM boards and the corresponding Pad board are mounted
on top of the RPC3 detector. The high-pT Pad board combines the low-pT and high-pT
trigger results. The information is sent, synchronously at 40 MHz, via optical links,
to a receiver and Sector Logic (SL) board, located in the USA15 counting room. Each
SL board receives inputs from up to height Pad boards, combining and encoding the
trigger results of one of the 64 sectors into which the barrel trigger system is
subdivided. The trigger data elaborated by the Sector Logic is sent, again
synchronously at 40 MHz,, to the Muon Interface to the Central Trigger Processor
(MUCTPI), located in the same counting room. Data are read out from high-pT Pad
boards only. These data include the RPC strip pattern and some additional information
used in the LVL2 trigger. The read-out data for events accepted by the LVL1 trigger
are sent asynchronously to Read-Out Drivers (RODs) located in the USA15 underground
counting room and from here to the Read-Out Buffers (ROBs). The physical link between
on-detector and off-detector electronics is shared between trigger and readout data.
In particular the read-out data are sent when trigger results are not available for
the detector region of interest.
