The high luminosity (HL) LHC will pose significant detector challenges for radiation tolerance and event pile-up, especially for forward calorimetry. This will provide a benchmark for future hadron colliders. The CMS experiment has chosen a novel high granularity calorimeter (HGCAL) for the forward region as part of its planned Phase 2 upgrade for the HL-LHC. Based largely on silicon sensors, the HGCAL features unprecedented transverse and longitudinal readout segmentation which will be exploited in the upgraded Level 1 (L1) trigger system. The high channel granularity results in around one million trigger channels in total, to be compared with the 2000 trigger channels in the endcaps of the current detector. This presents a significant challenge in terms of data manipulation and processing for the trigger. The high luminosity will result in an average of 140 interactions per bunch crossing and along with it a higher rate of background in the endcap for trigger algorithms to mitigate. Three-dimensional reconstruction of the HGCAL clusters in events with high hit rates is also a more complex computational problem for the trigger than the two-dimensional reconstruction in the current CMS calorimeter trigger. The status of the trigger architecture and design, as well as the concepts for the algorithms needed in order to tackle these major issues and their impact on trigger object performance, will be presented.
With the end of the large hadron collider (LHC) Phase-1 in 2023, the accelerator will undergo
an upgrade in luminosity (high luminosity LHC or HL-LHC) [
The HGCAL [3] will include both electromagnetic (CE-E) and hadronic (CE-H) calorimeters in one. It is a sampling calorimeter whose active layers will adopt a fully silicon technology for the CE-E and a hybrid silicon-scintillator for CE-H. The silicon is needed in order to achieve high radiation tolerance (up to 10 MGy at 3 pb-1 see Fig. 1) and the scintillator to reduce costs where possible. The detector’s high granularity (cell size of 0.5 and 1.2 cm2 depending on θ position, where θ is the azimuthal angle and φ is the inclination) requirement will improve jets separation at low angles. Each detector endcap will be formed by 28 layers (CuW+Cu+Pb absorber and 25 X0 + 1.3 0) in the CE-E region and 22 layers (stainless steel absorber and 8 0) in the CE-H region (Fig. 1).
An HGCAL active layer is reported in Fig. 1. One of the main challenges, while designing the detector, has been the one-order-of-magnitude variability of bandwidth across the detector. In order to adapt to this variability, each detector element is mounted on motherboards of variable size (see Fig. 2).
The capability to trigger on complex objects in the forward region will be a key feature of the CMS detector during the HL-LHC. One compelling physics signature to study is weak vector boson fusion production of both standard model vector boson and new physics, which often produces two jets in the endcaps. One of the key points has been to reduce the bandwidth while balancing the reduction in physics performance. To achieve this goal, several key design choices have been studied and implemented into simulations: ● ● ● ● ●
Trigger data are readout with a coarser granularity: 1/4 or 1/9 of the full granularity (depending on the θ region), electromagnetic section CE-E contributes to the trigger using every other layer, time information are not transmitted to the trigger processor, trigger cells data are not sent to the backend if a programmable energy threshold is not reached, in case of buffer overflows the system is designed to drop data outside the latency window.
The trigger primitive generator will implement its algorithms over the Serenity platform [
The general overview of the HGCAL trigger primitive generator is presented in Fig. 3. It
consists of 2 stages. It is based on a time multiplex (TMUX) architecture (see Fig. 3) [
The detector upgrade will increase also the total allowed latency for the trigger path, from the current 4 s to 12.5 s. This will include the central trigger processor latency and all the hardware contributions. The total latency allocated to the HGCAL tigger primitive generator (TPG) is 5 s. This latency is the sum of the fixed contributions from upstream electronics (e.g. front-end, concentrator ASIC, SerDes and TMUX) and contribution from data processing int the trigger firmware. The fixed latency from upstream electronics amounts to 2.2 s, leaving ca. 2.8 s for TPG algorithm development.
Stage 1 receives data from the front-end electronics via low power gigabit transceiver links
(lpGBT) [
Stage 2 implements the trigger primitive generation. It is planned to mount Xilinx Virtex Ultrascale+ FPGAs (VU7P). Each FPGA is collecting data for the full depth of a 120o sector. The current time multiplex period is set to 18 bunch crossing (450 ns). This is a clear design choice in order to keep the system flexible for future updates of the firmware.
Currently, the baseline for the Stage 2 algorithm adopts an imaging algorithm for the cluster reconstruction. The algorithm is split into two logically separated steps, the first one generates seeds to be passed to the second where the actual clusters are built.
The seeds are generated using a histogram. The histogram is built using the position and energy of all the Trigger cells projected to the (r/z, φ/z) plane (where z is the distance from the interaction point along the beam axis and r the transversal distance form the ). Once all the trigger cells are collected into the histogram a Gaussian smearing function is applied over all the histogram bins. This is crucial to removing local fluctuations and identify the local maxima that are then used as seeds. An example of this procedure is illustrated in Fig. 5. allow us to replace the FPGA (on the top half of the picture). The bottom half of the picture shows an
FPGA mounted on the interposer. Right: Serenity board configuration for HGCAL Stage 2 TPG, which receives data from all the Stage 1 boards via 72 links at 16 Gpbs and transmits the primitives to the central correlator.
The second step consists in collecting the trigger cells around a seed and within a programmable radius (in the (r/z, φ/z) plane).
Finally the three-dimensional clusters are formed and relevant information are extracted (e.g. position, energy and shape).
The HGCAL and its trigger primitive generator designs are facing new challenges dictated by the unprecedented at LHC pile-up levels and radiation dose that the system must withstand. This note describes the main problems faced and proposed solutions found have been presented. A careful study of the trigger path and algorithms is underway in order to ensure the performance needed to fulfil the future challenges. An important role has been played by the implementation choice of adopting the Serenity platform, this has allowed the collaboration to tailor resource, hence costs, to the specific problem. The hardware tests and the firmware implementation has started.
The CMS Collaboration, “The CMS experiment at the CERN LHC”, JINST 3 (2008) S08004.