Institut de recherche sur les lois fondamentales de l'Univers

The LHC provides collisions to ATLAS at a bunch-crossing rate of 26 millions per second. Since not all collisions are interesting, and because the volume of data would be too large to record all of them, only a small fraction is selected by the ATLAS trigger system. The first level of trigger (Level-1, hardware-based) uses coarse information about the event and makes the decision to keep or reject it in less than 2.5 microseconds, reducing the rate from 26 MHz to 100 kHz . The second level (High-Level Trigger, software-based) does a full reconstruction of the events and reduces the rate by another order of magnitude. The trigger system uses various information from the collision, in particular the presence in the event of rare particles such as muons.

The NSW project was motivated by the need to reduce the high background observed in the muon endcap Level-1 trigger during the LHC Run 1 (2010-2013) in order to be able to cope with the luminosity expected at the High-Luminosity LHC. A muon coming from a collision at the center of the ATLAS detector goes through, in that order: an NSW, an endcap toroid magnet, a first Big Wheel (Middle), then a second Big Wheel (Outer). Originally only the Big Wheels Middle were equipped with a trigger system. The main purpose of the NSW is to add another layer of triggering in front of the endcap toroid magnet to veto particles not coming from the interaction point. The design chosen was a combination of two technologies both capable of triggering (requiring a quick response and a precise time measurement) and precise tracking reconstruction. The first one uses small-strip Thin Gap Chambers (sTGC, emphasis on triggering) with pads (rectangles about 8 cm wide in the bending plane), 3.2 mm-pitch strips, and wires orthogonal to the strips and pads. The second one uses Micromegas (Micro-Mesh Gaseous Structure) chambers with resistive strips of less than half a millimeter of pitch (emphasis on precision tracking). With 8 layers of measurement each, the two technologies offer a good redundancy. As part of the ATLAS phase-1 upgrade programme the wheels were installed in 2021, at the end of the LHC Long Shutdown 2 (LS2, 2019-2021), just before the start of the LHC Run 3 (2022-2026).

The muon endcap trigger puts NSW information in coincidence with the Big Wheels Middle trigger system. The NSW trigger is being implemented in several steps. As a first step, in 2023 several sectors were activated using the sTGC pad trigger only, which does not require the use of the dedicated NSW trigger processor. On May 8^th 2024, NSW information was included in up to 85 % of the trigger sectors, leading to a Level-1 trigger rate reduction from 25 kHz to 13 kHz. On May 21^st 2024, information from Micromegas for those same sectors was added for the very first time. Finally, on May 28^th 2024, thanks to the addition of Micromegas information, all remaining sectors (some suffering from low sTGC efficiency) could be included, leading to a total reduction of the main Level-1 muon trigger rate of 14 kHz, while retaining 96% of the interesting muons. The total ATLAS trigger rate is limited to 100 kHz, so a 14 kHz reduction is significant, and allows to take data at higher luminosity. Currently ATLAS is able to take data with an instantaneous luminosity of 2.2 10³⁴ cm^-2s^-1 at 13.6 TeV, which produces 65 pile-up events per bunch-crossing. The addition of Micromegas information is especially interesting in sectors suffering from low sTGC efficiency: not only does it increase the trigger efficiency, but it also allows to activate sectors that would not be efficient enough otherwise, thus contributing to the background reduction as well.

In Run 4 (expected to start in 2030), the luminosity will be up to three times higher than today, putting even more pressure on the trigger. The next step for the NSW will be to use, in addition to the sTGC pads and the Micromegas strips, the sTGC strips, providing a more precise reconstruction than the sTGC pads and additional redundancy, thus reducing further the background.

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