Speaker
Description
Summary
The design of the trigger of the ALICE experiment foresees a multilevel
architecture, with three levels of hardware implemented currently. The first
decision (L0) is taken 1.2 microsec, the L1 decision is taken 6.5 microsec and
the L2 decision is issued 88 microsec after the collision.
Taking advantage of the MRPC (Multigap Resistive Plate Chamber) detector, the Time
of Flight provides very fast signals with very low noise. The idea is to use the
information from this large area (150 m2), finely segmented detector (~160,000
channels) for a fast estimate of the event track multiplicity. Since the fast
signals from the detector are available shortly after the collisions, the TOF
trigger can comfortably contribute to the ALICE L0 trigger decision. The large
granularity of the TOF and the low noise of the MRPC allows to trigger on various
configurations:
- classification of events with very large/low track multiplicity corresponding to
the central/peripheral ion-ion collisions;
- identification of events with large and localized track multiplicity as jet
events in pp collisions;
- back to back patterns of cosmics events, which will be very useful during the
commissioning of the detector itself.
The TOF detector is made of 18 supermodules, each covering an
azimuthal angle of 20^o and a pseudorapidity of |eta|<1. Four Local Trigger Modules
(LTMs) are needed to process the data from one supermodule. Each LTM has 48 inputs.
Each input is made by the logical OR of 48 TOF pads, whose area is ~9 cm^2.
The architecture of the TOF trigger uses a first layer of 72 VME (9U) boards
(LTM). The LTMs provide an interface between the front end electronics and the
second trigger layer, called CTTM (Cosmic and Topology Trigger Module) and made of
a number of VME boards. The CTTM receives the input signals from the 72 cables
coming from the LTMs and each cable transmits 24 signals. The distance between the
72 LTMs and the CTTM is about 60 m.
Extensive R&D has been performed to safely transmit LVDS signals over this
distance.
A dedicated calibration is foreseen to allow the LTMs to compensate for the
relative time differences among the 1728 input signals. This is achieved by
using commercial delay line units, programmable through the LTM VME interface. The
aligned input signals are then processed in the LTMs using an Altera Cyclone FPGAs.
The clock used by the FPGA to sample the input signals is synchronized to the
LHC machine clock (40 MHz) to associate correctly the input data to the
corresponding bunch crossing and to provide to the CTTM the full set of information
required to assert a L0 trigger decision.
The CTTM generates the L0 and the L1 trigger decisions. Since the L0 decision must
reach the ALICE Central Trigger Processor (CTP) within 800 ns after the
interaction, a very fast data processing will be implemented. A more detailed
analysis will be possible for the L1 decision.
At present both the LTM boards and the protocol for 60 m LVDS signal
transmission have been successfully prototyped and tested, while the design of
CTTM boards is well advanced.
In this talk we will show the TOF trigger layout and selection capabilities, both
for cosmic and beam events. The results obtained with the prototype boards are
presented.
