May 14 – 16, 2014
University of Pennsylvania
US/Eastern timezone

Towards a Level-1 tracking trigger for the ATLAS experiment at the High Luminosity LHC

May 14, 2014, 10:30 AM
25' presentation + 5' for discussion

25' presentation + 5' for discussion

Oral presentation


Dr Tim Martin (University of Warwick)


The future plans for the LHC accelerator allow, through a schedule of phased upgrades, an increase in the average instantaneous luminosity by a factor 5 with respect to the original design luminosity. The ATLAS experiment at the LHC will be able to maximise the physics potential from this higher luminosity only if the detector, trigger and DAQ infrastructure are adapted to handle the sustained increase in particle production rates. In this paper the changes expected to be required to the ATLAS detectors and trigger system to fulfill the demands of such a high luminosity scenario are described. The increased number of interactions per bunch crossing will result in higher occupancy in the detectors and increased rates at each level of the trigger system. The trigger selection will improve the selectivity, partly from increased granularity for the sub detectors and the consequent higher resolutions achievable. One of the largest challenges will be the provision of tracking information at the first trigger level, which should allow a large increase in the rejection power at this stage of the selection and yet still allow the full physics potential of the experiment to be fulfilled. In particular, reconstructing objects at the electroweak scale will still require that the thresholds on the transverse momenta of particles be kept as low as possible. Tracking provides essential information of this kind for individual particles so it is desirable to attempt to reconstruct tracks as early in the trigger chain as possible. The ability to apply fast processing that can take account of the properties of the tracks that are being reconstructed will enhance the rejection, while retaining high efficiency for events with desired signatures, such as high momentum leptons or multiple jets. Studies to understand the feasibility of such a system have begun, and proceed in two directions: a fast readout for high granularity silicon detectors, and a fast pattern recognition algorithm to be applied just after the Front-End readout for specific sub detectors. Both existing, and novel technologies can offer solutions. The aim of these studies is to determine the parameter space to which this system must be adapted. The status of ongoing tests on specific hardware components crucial for this system, both to increase the ATLAS physics potential and fully satisfy the trigger requirements at very high luminosities are discussed.


A luminosity upgrade for the LHC[1] is foreseen for 2024 in order
to allow increased phase space coverage both for searches and precision
Higgs physics. For this high luminosity LHC (HL-LHC), instantaneous
luminosities around 7$\times10^{34}$cm$^{-2}$s$^{-1}$ are expected,
delivering around 300fb$^{-1}$ per year. At these high luminosities,
mean numbers of separate pp interactions in the range of 160-200 are
expected for every bunch crossing. Event rates for single lepton
candidates passing the Level 1 (L1) triggers in the ATLAS[2][3] detector
using the current trigger strategy are expected to be of the order of
200-400 kHz for each lepton species individually, with jet and other physics
signatures providing some significant rate in addition. The current
ATLAS trigger strategy uses only coarse granularity Calorimeter and
Muon Spectrometer information at Level 1, with full granularity data
including tracking information from the Inner Detector used in the
High Level Trigger (HLT). Processing this rate of output from L1 is
not possible in the HLT.

For the HL-LHC upgrade ATLAS will be equipped with an all-new Silicon
Inner Tracker (ITK). Studies have been performed to evaluate the
performance benefits of reconstructing tracks in the ITK at Level 1,
and matching them to either calorimeter clusters or muon
candidates. To enable the processing of the high rates from the
HL-LHC, ATLAS has therefore proposed a Level 1 Track Trigger to work
within a scenario where the current Level 1 processing is split into
consecutive harware stages, denoted Level 0 (L0) and Level 1.
Limitations from the front-end pipelines for no-longer accessible
sub-detector components mean that any upgraded ATLAS trigger system will
have only 20$\mu$s from the time of the bunch crossing until the L1
decision must arrive back at the detector front ends. The maximum L1
output rate that can be supported by these sub-detectors is 200 kHz.
To operate within these constraints, in the split L0-L1 scenario the
detector front end electronics for some sub-detectors will make use of
a double buffer. In the L0 system the Calorimeter and Muon systems
will process events using course granularity data to identify regions
of interest (RoI) as in the current Level 1 trigger. The L0 decision
will be distributed back to the detector front ends at the rate of 500
kHz to arrive 6$\mu$s after each bunch crossing. On a L0 accept the data
from the primary buffers will be read into a secondary buffer where it
will remain until the L1 decision arrives. Within the remaining
14$\mu$s both the transfer of the ITK data to the L1 track processor
and the track reconstruction must be performed.

The readout latency is critical: if the data can be read out within
6$\mu$s then 8$\mu$s will remain for the tracking algorithm. Upon
receipt of a Regional Readout Request (R3), identifying a particular RoI,
each detector element within the RoI will read out a sub-sample of the
data from the secondary buffer to the L1 processing systems.
For each L0 accept it
is expected that the average number, and size of all the RoIs in each
event will comprise only about 10% of the total data volume within the
event. In this way, only 10% of the detector need be read out, such
that the total event rate to be read out to the L1 system will be
50kHz only 10% of the 500 kHz L0 accept rate. The readout of the ITK
strip tracker is arranged by module: each module reading out the
strips using several daisy-chained identical front end ASIC (ABC130),
which use 130 nm CMOS technology. Studies using a discrete event
simulation of the complete readout chain from the distribution of the
R3 and L1 accept requests, to readout through the Hybrid Chip
Controller (HCC) to the off-detector electronics have been performed
using simulated ITK hit occupancies expected from 200 pileup interactions
per bunch crossing. These studies suggest that the R3-95% latency --
the time taken to read out all packets from a hybrid for 95% of all R3 --
for all modules in the inner most barrel layer of the ITK strip
tracker, is safely within 6$\mu$s for the standard 200 kHz L1A rate. To
help in this, the readout chip can contains logic to prioritise the
read out of R3 data over that of data from a L1 accept. An additional
FIFO can be added to buffer the input on the HCC from each daisy
chain. The latencies both with and without R3 data prioritisation and
different FIFO depths on the HCC were measured for different regions
of the tracker detector.

The tracking algorithm must complete execution in the remaining 8$\mu$s
in this scenario. This is feasible with current technologies which make
use of CAM (Content-addressable memory) to perform fast pattern-matching
in the first stage of the selection, and fast fitting procedures in a
second step, after the combinatorics have been reduced. The
pattern-matching algorithm performance will move in a large parameter
space, based on the number of coincidence layers of silicon, the
system segmentation and the pattern resolution, with constraints
provided by both hardware limitations, such as the input/output bandwidths
and size of the pre-registered pattern content per processor, and physics
requirements, such as the minimum track momentum to be selected. Studies
are ongoing using detailed simulations of the ITK geometry and detector
response in high pile-up environments in order to understand the connections
between these parameters.

In this paper the results of studies that will drive the design of the
L1 track system of ATLAS, together with the results of intermediate stage
tests on prototype hardware that will extend the potential of this system
are discussed.

L. Evans and P. Bryant, LHC Machine, Tech. Rep. JINST 3 (2008) S08001, CERN, Geneva, 2008.

[2] The ATLAS Collaboration; The ATLAS Experiment at the CERN Large Hadron Collided, JINST 3 (2008) no. 08, S08003, .

The ATLAS Collaboration, Performance of the ATLAS Trigger System in 2010, Eur. Phys. J. C72 (2012) 1849.

Primary author

Dr Tim Martin (University of Warwick)

Presentation materials