Abstract
The ATLAS collaboration at LHC has endorsed the resistive Micromegas technology (MM), along with the small-strip Thin Gap Chambers (sTGC), for the high luminosity upgrade of the first muon station in the high-rapidity region, the so called New Small Wheel (NSW) project. The NSW requires fully efficient MM chambers, up to a particle rate of ∼ 15 kHz/cm2, with spatial resolution better than 100 μm independent of the track incidence angle and the magnetic field (B ≤ 0.3 T). Along with the precise tracking the MM should be able to provide a trigger signal, complementary to the sTGC, thus a decent timing resolution is required. Several tests have been performed on small (10 × 10 cm2) MM chambers using medium (10 GeV/c) and high (150 GeV/c) momentum hadron beams at CERN. Results on the efficiency and position resolution measured during these tests are presented demonstrating the excellent characteristics of the MM that fulfil the NSW requirements. Exploiting the ability of the MM to work as a Time Projection Chamber a novel method, called the μTPC, has been developed for the case of inclined tracks, allowing for a precise segment reconstruction using a single detection plane. A detailed description of the method along with thorough studies towards refining the method’s performance are shown. Finally, during 2014 the first MM quadruplet (MMSW) following the NSW design scheme, comprising four detection planes in a stereo readout configuration, has been realised at CERN. Test-beam results of this prototype are discussed and compared to theoretical expectations.
Highlights
The upgrade of the ATLAS [1] muon spectrometer is primarily motivated by the high background radiation expected during Run 3 (2020) and at L = 7 × 1034 cm−2s−1 in HL-LHC (2025)
The muon trigger rate will exceed the available bandwidth because of the fake endcap muon triggers (90% is coming from low energy particles, generated in the material located between the Small Wheel and the outer endcap muon station)
The performance of the μTPC track reconstruction method is of great importance for the ATLAS New Small Wheel (NSW) MM project and a lot of effort has been dedicated in optimising the performance of this technique
Summary
In order to demonstrate the excellent performance of the MM technology and optimise the design and operational parameters of the detector, several small and medium size chambers have been tested with medium and high momentum hadron beams at CERN. The charge centroid method reconstructs hits with high accuracy when the particle traverses the chamber plane perpendicularly. In this case the charge is shared among only a few strips allowing for a very precise charge interpolation. The μTPC method does not provide accurate results in this case as the pulse in each strip is the aggregation of pulses induced by more than one primary ionisation clusters generated at different heights within the drift gap. When a charged particle crosses the detector under an angle, the primary ionisation charge is distributed along several readout strips, with the signal induced in each strip is most probably coming from one primary cluster In this occasion the μTPC method provides a very accurate measurement of the particle hit position on the detector. The strip signal amplitude becomes sensitive to the primary cluster charge fluctuations and the accuracy of the charge centroid
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