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\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/muon_upgrades.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:layout}Sketch of the ATLAS muon spectrometer illustrating the planned upgrades for HL-LHC. The new BMG sMDT chambers have been installed in winter 2016/2017. 
    The new BIS~78 sMDT and RPC chambers have been installed in the fall of 2020 together with new inner endcap layers. The full replacement of the BIS~1-6 MDT chambers by sMDT and RPC detectors 
    in the BIS inner barrel layer sectors are under construction for the HL-LHC upgrade in LS3.}
% add BIL (RPC) labels    
    \end{center}
\end{figure*}


\subsection{Muon Spectrometer Upgrades}
\label{subsec:muon_upgrades}

The ATLAS muon spectrometer consists of three layers of muon tracking chambers with high spatial resolution of better than 40~$\mu$m, Micromega detectors in the inner endcap wheels installed in the long shutdown
LS2 in 2021 and Monitored Drift Tube (MDT) chambers in the rest of the spectrometer, which are operated in the toroidal magnetic field of superconducting air-core coils. 
The muon detector is designed to trigger on muons with high transverse momenta $p_\mathrm{T}$ up to a pseudorapidity $|\eta|=2.4$ and to measure the muon momenta up to $|\eta|=2.7$
with 4\% resolution over a wide momentum range and with 10\% precision at $p_\mathrm{T}=1$~TeV. The tracking chambers are complemented by chambers with high time resolution of a few nanoseconds 
but moderate spatial resolution, Resistive Plate Chambers (RPC) in the middle and outer barrel layers and Thin Gap Chambers (TGC) in the inner and middle endcap wheels, for $pp$ bunch crossing identification 
and in the first-level muon trigger~\cite{atl_det_paper}. In the winter shutdown 2016-17, new small-diameter Muon Drift Tube (sMDT) chambers have been installed in barrel middle (BMG) layer inside the detector feet
increasing the detector acceptance~\cite{BMG},\cite{BMG_BIS78}, and in LS2 at the ends of the barrel inner layer (BIS~78) combined with new thin-gap RPC detectors to improve the 
first-level trigger selectivity~\cite{BIS78_1}-\cite{BIS78_4}.

The upgrade of the ATLAS muon spectrometer for HL-LHC operation in LS3 in the years 2026-28 (Phase-2 upgrade) is driven by the limited rate capability of the ATLAS muon chambers, especially of the RPC detectors in the barrel, 
and the need for higher selectivity of the first-level muon trigger. Both need to be increased by an order of magnitude in order to cope with the increases of instantanious and integrated luminosity.  
The Phase-2 upgrade program of the ATLAS muon spectrometer~\cite{MuonTDR} lead by MPP comprises the installation of new sMDT and thin-gap RPC chambers in the barrel inner layer, the use of the MDT and sMDT precision tracking chambers 
in the first-level muon trigger, and the replacement of the (s)MDT on-chamber electronics. It is illustrated in Figure~\ref{ATLAS:Muon:Upgrade:layout}.


\subsubsection{sMDT Precision Muon Chambers}
\label{subsec:sMDT_chambers}

MDT chambers with 6 or 8 layers of 30~mm diameter drift tubes are used for precision tracking in the ATLAS muon spectrometer. For this purpose, the sense wire positioning accuracy in the MDT chambers is required to be better 
than $20~\mu$m rms. The MDT chambers can only operate up to background rates of 500~Hz/cm$^{-2}$ and 300~kHz per tube. 
At background fluxes above 500~Hz\,cm$^{-2}$, the gas gain drops by more than 20\% leading to inefficiencies and rapid deterioration of the spatial resolution with increasing background flux. 
Background counting rates per tube above 300~kHz with occupancies beyond 30\% make efficient track reconstruction impossible.
At HL-LHC, the rate-capability of the MDT chambers will be exceeded in the Small Wheel and in the inner barrel layer where they will have to be replaced by chambers with about 10 times better rate capability.

The limitations are overcome by employing sMDT chambers with drift tubes of half the diameter of the ATLAS MDT tubes while keeping the operating parameters, Ar:CO$_2$ (93:7) gas mixture at 3 bar and gas gain of 20000, 
unchanged. The high reliability and mechanical precision of the drift tube chambers are maintained. 15~mm diameter sMDT drift tubes show gain drop only at 8 times higher background rates than the 30~mm diameter MDT drift tubes. 
At the same background flux, the small-diameter drift tubes have 8 times lower occupancy due to the 4 times shorter maximum drift time and the twice smaller tube cross section exposed to the radiation. Gas gain loss due to ion 
space charge is reduced by an order of magnitude compared to the 30~mm diameter MDT tubes and can be largely eliminated by adjusting the applied high-voltage at constant gain.
With careful selection of the component materials, the sMDT drift tubes have shown no sign aging up to 9~C/cm charge accumulation on the wire which goes well beyond the requirements at HL-LHC and future hadon colliders.  

The small-diameter Muon Drift Tube (sMDT) chambers have been developed at MPP since 2008~\cite{sMDT1}-\cite{sMDT4}. A sense wire positioning accuracy of better than 10~$\mu$m and about 10 times higher rate capability 
have been demonstrated in 2010 with the construction of a 16-layer prototype chamber of trapezoidal shape suitable for the highest rate region in the inner endcap layer of the ATLAS muon spectrometer at HL-LHC.
Since then, it has been shown with measurements at the Gamma Irradiation Facility GIF++ at CERN that the sMDT chambers provide sufficient performance up to $\gamma$ background counting rates of 2~MHz per tube and background fluxes 
of 35~kHz/cm$^2$ expected at future high energy hadron colliders like FCC-hh~\cite{ASD6}-\cite{FCC2}. Figures~\ref{ATLAS:Muon:Upgrade:resolution} and \ref{ATLAS:Muon:Upgrade:efficiency} show the average spatial resolution and the muon 
detection efficiency of 1.6~m long sMDT tubes as a function of the $\gamma$ background counting rate. Spatial resolutions of better than 150~$\mu$m and muon detection efficiencies of better than 80\% 
are achieved for background rates up to 2~MHz in 1.6~m long tubes, sufficient to keep up 99\% tracking efficiency and $35~\mu$m track resolution in 8-layer sMDT chamber. 
The observed degradation of both is solely due to signal pile-up effects in the readout electronics at high rates.  
The rate capability of the sMDT detectors can only be fully exploited by using improved readout electronics for high counting rates compared to the legacy MDT electronics, with faster shaping and employing baseline restoration 
techniques in order to mitigate signal pile-up effects (see Section~\ref{subsec:Upgrade_elx}).

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_resolution.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:resolution}
    Average spatial resolution of BIS~1-6 sMDT drift tubes of 1.6~m length as a function of the $\gamma$ background counting rate with the new Phase-2 ASD readout chip.
    The red line is a fit to the data. The expected maximum background rates at HL-LHC and FCC-hh are indicated.}
\end{center}
\end{figure}

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_efficiency.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:efficiency}
    Muon detection efficiency (probability of finding a hit within $3\sigma$ of the extrapolated muon track) of sMDT drift tubes 
    as a function of the $\gamma$ background counting rate. 
    The red line is a fit to the data. The expected maximum background rates at HL-LHC and FCC-hh are indicated.}
\end{center}
\end{figure}

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_endplug.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:endplug}
    Design of the endplugs of the sMDT drift tubes. The radial sense wire position is defined with respect to the external reference surface on the 
    central brass insert inside the plastic insulator with micron precision.}
\end{center}
\end{figure}

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_elx.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:MDT_elx}
    Stacked 24-channel readout electronics boards of the sMDT chambers.  
    The white plastic casings between the first two PCBs connecting to the drift tube signal caps and ground pins (so called hedgehog board)
    electrically insulate the high-voltage decoupling capacitors of each channel from each other. The board carrying the three ASD front-end ASICs 
    is shielded from the hedgehog board by the front cover of the Faraday cage. The outermost board carries the TDC ASIC and signal cable connector.
    The two active readout boards finally are shielded inside their individual aluminium housings.} 
\end{center}
\end{figure}


\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_tubes_in_comb.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:combs}
    Layer-by-layer assembly of sMDT chambers using stacked precision combs of few micrometer precision on a flat granite table.}
\end{center}
\end{figure}


\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_CMM_measurement.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:sMDT_CMM}
    Measurement of the individual sense wire positions at the drift tube endplugs in an assembled sMDT chamber using an automated coordinate measuring machine.}
\end{center}
\end{figure}




\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_RPC_assembly_1.jpg}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_RPC_assembly_2.jpg}
    \caption{\label{ATLAS:Muon:Upgrade:BIS78_RPC_triplet}
    Assembly of the RPC gas gaps with readout planes in the support frame (left). 
    The RPC triplets inside the frame are pressed together by pre-bent aluminum plates (right).}
\end{center}
\end{figure}


\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_integration.jpg}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_commissioning_1.jpg}  
    \caption{\label{ATLAS:Muon:Upgrade:BIS78_integration}
    Integration of a BIS~7 RPC on an BIS~78 sMDT chamber at CERN (left).
    Commissioning of combined BIS~78 sMDT and RPC chambers at CERN prior to installation (right).}
\end{center}
\end{figure}


\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/BIS78_installation_1.jpg}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_installation_2.jpg}
    \includegraphics[width=0.49]{ATLAS/Upgrade/figures/BIS78_installation_3.jpg}  
    \caption{\label{ATLAS:Muon:Upgrade:BIS78_installation}BIS~78 muon chamber installation in fall 2020:
    Lowering of the chamber into the ATLAS cavern between barrel muon spectrometer and endcap wheel (top). 
    Transfer of the chamber from the installation frame to the rails mounted on the barrel toroid magnet coils (bottom left).
    Chamber in final position in the small gap between toroid coil and barrel hadronic calorimeter (bottom right).}
\end{center}
\end{figure}




\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/BIS1_sMDT_RPC_integration.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:BIS1_sMDT_RPC_integration}
    Integrated design of a BIS~1-6 smDT and RPC chamber package with cable routing.}
\end{center}
\end{figure}

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_elx.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:sMDT_elx}
    24-channel signal distribution board (``hedgehog board'') connected to a quadruple sMDT chamber layer.      
    The high-voltage decoupling capacitors are enclosed in plastic shields between two PCB boards to ensure high-voltage stability. 
    The hedgehog board carries stacked mezzanine cards, the first with three 8-channels amplifier-shaper-discriminator ASICs and 
    and the second with the TDC ASIC and the signal cable connector (see Section~\ref{subsec:Upgrade_elx}).} 
\end{center}
\end{figure}


\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_drifttube_production.jpg}
    \caption{\label{ATLAS:Muon:Upgrade:sMDT_drifttube_production}
    Assembly of the sMDT drift tubes using semi-automated stations in a temperature-controlled class 1000 clean room.}
\end{center}
\end{figure}


\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS1_glueing.jpg}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS1_platforms.pdf}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS1_gassystem.jpg}    
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS1_electronics.jpg}
    \caption{\label{ATLAS:Muon:Upgrade:assembly}Photographs illustrating the sMDT chamber assembly sequence. 
    Top left: The drift tubes inserted into the precision combs at both ends layer-by-layer are glued together on a granite table in a temperature and humidity controlled class 10000 clean room using an automated glue dispenser. 
    Top right: After all tube layers have been assembled, platforms for the external optical alignment system are glued onto the chamber with $20~\mu$m accuracy relative to the sense wire grid. 
    The platform positions are then measured with $10~\mu$m precision using an electro-mechanical feeler arm. 
    Bottom left: Gas distribution system with injection molded plastic gas connectors between the endplugs of the drift tubes, sealed with gold-plated signal caps, and the gas distribution bars. 
    The ground foil serving as back plane of the Faraday cage and connecting all tube walls is also visible.    
    Bottom right: Signal distribution boards and readout mezzanine cards mounted on a BIS~1 sMDT chamber in the cosmic ray teststand.}    
\end{center}
\end{figure*}

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_production_test.jpg}
    \caption{\label{ATLAS:Muon:Upgrade:BIS_sMDT_production_test}
    BIS sMDT chambers in their transport and storage frames in various stages of completion of electronics and Faraday cage mounting and cable routing. 
    The cosmic ray teststand is visible in the background with chambers waiting to get moved in.} 
\end{center}
\end{figure}


\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_progress_MPP.jpg}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_progress_Michigan.jpg}
    \caption{\label{ATLAS:Muon:Upgrade:BIS_sMDT_progress}
    BIS~1-6 sMDT chamber construction progress at the two production sites MPP (left) and Michigan (right) since the start in January 2020.}    
\end{center}
\end{figure*}


\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_CMM_rsigma.pdf}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_CMM_ypitch.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:BIS_sMDT_CMM_precision}
    Chamber geometry information from the measurements of the individual sense wire positions in the assembled BIS~1-6 sMDT chambers at MPP:
    Sense wire positioning accuracy (left) and average layer distance, the y-pitch (right) over the chamber production.}   
\end{center}
\end{figure*}

\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_noiserates.pdf}
    \includegraphics[width=0.49\linewidth]{ATLAS/Upgrade/figures/BIS_sMDT_resolution.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:BIS_sMDT_noise_resolution}
    Chamber validation in the cosmic ray testand: Noise rates (left) and average drift tube spatial resolution (right) for the different BIS~1-6 sMDT chambers produced.  
\end{figure*}


The ATLAS experiment has adopted the sMDT chamber technology for the upgrade of the muon spectrometer for HL-LHC because
the sMDT chambers have proven to be robust, cost effective tracking detectors with high precision and spatial resolution over large areas rate and rate capability and lifetime far beyond the requirements for HL-LHC, 
sufficient also for future high-energy and high-luminosity hadron colliders. The drift tubes and the chamber assembly procedure and tooling as well as the gas distribution system have been optimized for mass production 
and allow for two times higher wire positioning accuracy at twice higher construction speed than for the legacy MDT chambers.   

The high mechanical precision is achieved by relating the sense wire positions within the tubes to external reference surfaces of the endplugs (see Figure~\ref{ATLAS:Muon:Upgrade:endplug}) and 
inserting these precision surfaces in precise combs when assembling the tubes in a chamber (see Figure~\ref{ATLAS:Muon:Upgrade:combs}) before they are glued together.
The external reference surfaces of the endplugs are accessible after the chamber assembly and can be used to measure the chamber geometry with an automated coordinate measuring machine or manual feeler gauges 
with few micron individual wire position measurement accuracy (cf. Figure~\ref{ATLAS:Muon:Upgrade:sMDT_CMM}).


\subsubsection{BIS~78 Chamber Commissioning and Installation}
\label{subsec:BIS_installation}


The construction of 16 BIS-78 chambers at the ends of the innermost layer of the barrel muon spectrometer (BIS) in the years from 2017-2019
can be considered as a pilot project for the replacement of the complete layer during the phase-2 upgrade (see Figure~\ref{ATLAS:Muon:Upgrade:layout}).
The existing BIS~7 and BIS~8 MDT chambers are replaced by combined BIS~78 sMDT chambers as precision tracking detectors making space for additional 
BIS~7 and BIS~8 RPC chambers to enhance the first-level muon trigger selectivity in the transition region between barrel and endcap muon spectrometer.  
The 16 sMDT chambers are all different and have complicated geometries to fit into the overlap regions
between barrel and the new endcap inner wheels installed in LS2 as well. The BIS~78 sMDT chambers contain 648 or 744 drift tubes of different lengths 
in two quadruple layers separated by an only 30~mm high space frame. The optical chamber planarity and global position monitoring sensors are mounted on the outer tube layer opposite to the 
RPC chamber. 

The RPC chambers consist of a triplet of novel thin-gap RPC detectors, each with two independent strip readout planes for $\eta$ and $\phi$ 
coordinate measurement, mounted inside aluminum support frames of only 50~mm available height (see Section~\label{subsec:RPC}). 
The two chamber types are independently supported on the ATLAS rail system onto which they are inserted as one package during installation.

The design and construction of the sMDT chambers and of the
RPC mechanics as well as their combined integration in the ATLAS muon spectrometer have been carried out by MPP.

The RPC triplets were installed in their support frames (see Figure~\ref{ATLAS:Muon:Upgrade:BIS78_RPC_triplet}), combined with the sMDT chambers on independent rail supports 
and commissioned together (see Figure~\ref{ATLAS:Muon:Upgrade:BIS78_integration)} in the BB5 hall at CERN during 2020~\cite{BIS78_commissioning}. 
The integrated chambers have been successfully installed in the fourth quarter of 2020 
before the installation of the new inner endcap wheels and are now operational in ATLAS in Run-3~\cite{BIS78_installation} (see Figure~\ref{ATLAS:Muon:Upgrade:BIS78_installation}).



\subsubsection{BIS~1-6 sMDT Chamber Design and Production}
\label{subsec:BIS_sMDT_prod}

The BIS~1-6 sMDT chambers consist of two quadruple layers of drift tubes separated by an only 30~mm high spacer which incorporates an optical planarity monitoring system. 
They will be combined with thin-gap RPC triplets in support frames like the BIS~78 chambers (see Section~\ref{subsec:RPC}).
They all have rectangular shape with equal tube length of 1.6~m and two different widths (BIS~1 and BIS~2-6). However, there is a large variety of global alignment sensor platform 
positions on the outer tube layer opposite to the RPC chambers. The mechanical design of the combined sMDT and RPC chambers and their integration into the muon spectrometer environment 
has been carried out at MPP (see Figure~\ref{ATLAS:Muon:Upgrade:BIS1_sMDT_RPC_integration}).

The design of the gas distribution system 
together with high-voltage stable, low-noise electronics connections to the densely packed tube array required great care (see Figure~\ref{ATLAS:Muon:Upgrade:assembly}). 
For the signal and high-voltage distribution boards and the active readout (mezzanine) cards, a dedicated stacked layout has been developed 
(see Figure~\ref{ATLAS:Muon:sMDT_elx} and Section~\ref{subsec:Upgrade_elx}). The aluminum tube walls are chromatized on the in- and outside in order to ensure high cleanliness,
which is essential for the lifetime of the detectors, and good electrical ground contacts, respectively.
The tube walls are connected through ground screws to the back plane of the chromatized Faraday cage and to the ground planes of the high-voltage and readout distribution boards. 
The mezzanine card ground planes are connected to the Faraday cage covers by springs. 

96 BIS~1-6 sMDT chambers have to be constructed, 50\% at MPP and 50\% at the partner institutes at the University of Michigan (UM) and Michigan State University (MSU). 
Individual chambers contain 464 or 560 drift tubes. Alltogether 23080 drift tubes have to be produced at each of the two construction sites. 
The spacer components, rail supports and the chamber storage and transport frames have been produced and delivered by the Institute for High Energy Physics in Protvino, Russia. 

The most time-consuming work is the assembly and test of the individual drift tubes which is performed with semi-automated tooling in a temperature controlled class 1000 clean room 
which avoids any manual contact with the sense wire (see Figure~\ref{ATLAS:Muon:sMDT_drifttube_production}). Each drift tube is certified for gas tightness, high-voltage stability 
and wire tension withing stringent margins before it is installed in a chamber. An overall failure rate of only 1\% has been achieved in the serial production. 

The main steps of the BIS~1-6 chamber assembly using precision jigging on a flat granite table in a temperature-controlled class 10000 clean room are shown in Figure~\ref{ATLAS:Muon:Upgrade:assembly}.
Still on the assembly table, the global alignment sensor platform positions are measured with an electro-mechanical feeler arm with $10~\mu$m accuracy.
The individual sense wire positions are measured at both ends of each chamber using an automated coordinate measuring machine in the same room (see Figure~\ref{ATLAS:Muon:Upgrade:CMM}).
The precision assembly and measurement of the drift tubes in the chamber is followed by the mounting of the gas distribution system in another temperature-controlled clean room, 
and by the installation of the readout electronics in Faraday cages and of the cables (see Figure~\ref{ATLAS:Muon:Upgrade:sMDT_production_test}). The final certification of the chambers 
with readout electronics and under high-voltage (2735~V) before shipment to CERN is performed in a cosmic ray test stand (see Figure~\ref{ATLAS:Muon:Upgrade:BIS_sMDT_production_test}).

The BIS~1-6 sMDT chamber construction is the first ATLAS phase-2 upgrade project to go into production. The chamber material supplies were established well ahead of time.
The chamber construction started in January 2020 at MPP and is proceeding at the planned rate of approximately two working weeks per chamber since then 
(see Figure~\ref{ATLAS:Muon:BIS_sMDT_progress})~\cite{Kevin2022}.
The technology was successfully transferred to the second production site at Michigan State University and the University of Michigan in 2019 and 2020 where the chamber production
started only two months later and is proceeding at a similar rate. The early completion of the sMDT chamber construction will free the necessary resources for the production of 
RPC gas gaps and support frames in industry and for the assembly of the RPC triplets in their frames starting 2023 (cf. Sections~\ref{subsec:RPC} and \ref{subsec:RPC_industrialization}).  


\subsubsection{BIS~1-6 sMDT Chamber Performance}
\label{subsec:BIS_sMDT_perf}

The individual sense wire positions are measured with the coordinate measuriung machine and the chamber geometry parameters (horizontal and vertical wire pitch and relative multi-layer position, 
gravitational sag) on the high-voltage distribution (HV) and readout (RO) ends of the chamber are determined for every chamber after tube assembly 
(see Figure~\ref{ATLAS:Muon:Upgrade:BIS_sMDT_CMM_precision}). 
The overall accuracy of sense wire positioning (residual width with respect to the nominal wire grid) is better than $7~\mu$m over the whole chamber production, including the unavoidable 
gravitational sag. The requirement is $20~\mu$m rms. The most critical geometry parameter, the wire pitch vertical to the chamber plane, is stable essentially within $\pm 5~mu$m around 
the nominal value of 13.090~mm~\cite{Kevin2022}. 

The final validation of each chambers equipped with gas distribution system, electronics boards and cables is performed in the cosmic ray teststand taking data over 24 hours. 
The gas leak rate, dark currents and drift tube detection efficiencies are safely within the requirement for all chambers. The noise rate is exceptionally low, around 10~Hz only at the nominal signal threshold of the readout electronics,
and well below the requirement of at most 1~kHz even for lower signal threshold (see Figure~\ref{ATLAS:Muon:Upgrade:BIS_sMDT_noise_resolution}). 
The improvement over time is due to improved grounding of the mezzanine boards to the Faraday cage. 
The increased signal rise time of the new ASD readout chip (see Section~\ref{subsec:Upgrade_elx}) at reduced noise rate results in excellent spatial resolution of the sMDT drift tubes below $100~\mu$m 
which is significantly better than the expectation based on the legacy MDT electronics (see Figure~\ref{ATLAS:Muon:Upgrade:BIS_sMDT_noise_resolution})~\cite{Kevin2022}.




\subsubsection{BIS~1-6 RPC Chamber Design and Construction}
\label{subsec:RPC}


\subsubsection{Industrialization of RPC Gas Gap Production}
\label{subsec:RPC_industrialization}


\subsubsection{Upgrade of the Muon Spectrometer Readout Electronics}
\label{subsec:Upgrade_elx}

The current read-out system of the MDT chambers was designed to cope with the LHC 
design luminosity of $1\cdot10^{34}$~cm$^{-2}$s$^{-1}$. Starting in 2025, the 
luminosity of the high-luminosity LHC (HL-LHC) will be increased by a factor of 5-7. 
For the electronics of the MDT detector this means a large increase of read-out 
bandwidth all along the data path from the preamplifier to final storage. 

In addition to the necessity of higher data through-put, the MDT read-out has to cope 
with the increase of the first level trigger rate (L1-rate), from presently 100 kHz to 
1 MHz, needed to improve the acceptance of the future ATLAS detector for low 
cross-section physics. For the MDT read-out, this rate increase calls for a new, more 
efficient read-out mode, where data are no longer packaged and transferred 
event-by-event, but are continuously transferred as raw data stream down to the 
rear-end processors. At this stage of the read-out chain, the raw data receive an 
event-related data structure and are assigned to definitive beam crossings.

In the rear-end processors, the accurate MDT track coordinates are used for a 
refinement of the muon track momentum, because the {{\it coarse}} momentum, as supplied 
by the RPC and TGC trigger chambers, is not precise enough to define a sufficiently 
sharp high-$p_T$ threshold, allowing too many of the abundant low-$p_T$ muons to pass 
the L1-trigger. In order to be available for a better definition of the L1-trigger 
threshold, the MDT data must arrive at the processors in USA15 inside a predefined 
latency of 10~$\mu s$. Based on present estimates, however, this value of the maximum 
latency seems not to be a hurdle for the MDT read-out.


\begin{figure*}[!htb]
\begin{center}
\includegraphics[width=0.9\textwidth]{ATLAS/Upgrade/figures/Modern_Muon_RO_system.jpg} 
\end{center}
\caption{\label{Modern_Muon_RO_system}Block diagram of the MDT trigger and read-out
electronics for HL-LHC runs.                 
All muon hits will be sent by the on-chamber electronics
(ASD, TDC and CSM) to the off-chamber electronics in the                 
ATLAS counting room. Relevant hits will
be extracted based on the time information provided by
RPC and TGC trigger chambers. Segment finder and track fitter
algorithms are applied on matched hits to determine the accurate
momentum of the incoming muon. The fitted muon
track information will be sent to the ATLAS Sector Logic and muon Central
Track (trigger) Processor (MuCTPi).}
\end{figure*}

% ---------------------------------------------------------------------

Figure~\ref{Modern_Muon_RO_system} shows the future MDT trigger and read-out chain. 
The MDT on-chamber electronics will send all muon hits via a number of intermediate 
stations to the processors in the ATLAS counting room. The signals supplied by the MDT 
tubes are processed by two ASICs, first shaped and discriminated by an ASD [6-8], 
subsequently time-digitized by a TDC. Data from the TDC are transmitted to an 
on-chamber data concentrator (Chamber Service Module, CSM) and further down-stream to a 
Data Processor, where trigger-relevant hits are extracted out of the raw data stream, 
to be used for an early $p_T$ measurement, leading to improved L1-trigger selectivity. 
Hit extraction is based on timing information from the fast trigger chambers (RPCs in 
the barrel and TGCs in the endcap) and used to define a time t0, when the triggering 
muon actually passed the chamber. The difference between the arrival time of the signal 
at the central wire and t0 equals the drift time of the ionization in the tube, 
corresponding to the distance of the track from the wire. The MDT 'precision 
coordinates', approximately matched to the track candidates obtained from the trigger 
chambers, are processed by segment finder and track fitter algorithms, leading to a precise 
value of the muon transverse momentum. All MDT data, matching the time window defined 
by the L1 trigger, are stored for transmission and final permanent storage to the FELIX 
read-out system [9].

MPP has played a leading role in the development of the 'legacy' read-out 
chain, presently used for MDT read-out.

In the new read-out scheme, designed for operation at the HL-LHC, MPP is taking a 
leading role in three crucial domains:

\begin{itemize}

\item  development of the ASICs for the input part of the read-out electronics, i.e. the 
       preamplifier and pulse shaping stage (ASD) and Time-to-Digital 
       converter (TDC), the latter in collaboration with the University of Michigan,
       (Section~\ref{subsec:MDT_frontend_elx}). 

\item  development of the readout boards ('mezzanine cards') for the MDT and sMDT chambers.
       Due to the high density of signal routing and the presence of small, unamplified 
       analog signals close to digital signals, the optimization of the board layout is 
       a particular challenge. Figure~\ref{stacked_mezzanine} shows the 
       readout board for sMDT readout, where components had to be mounted on two PCB layers
       due to space constraints.

\item  development of the MDT Data Processor, where data, relevant for the L1 
       trigger decision, are extracted out of the raw data stream and processed for
       an early estimate of the transverse track momentum $p_T$ 
       (Section~\ref{subsec:Trig_Limitation}).

\end{itemize}


% -------------------------------------------------------------------

\begin{figure}[hbt]
\begin{center}

    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/sMDT_mezz_card.pdf}                

    \caption{\label{stacked_mezzanine} Two-layers of PCB are mounted on top of each other to house the ASD chips and the TDC chip close to the tube ends of the sMDT. The white cylinders, visible  below the PCBs, contain the coupling capacitors, which provide DC-separation between  the readout electronics (at ground) from the central tube wire (at high voltage).}

\end{center}
\end{figure}

% ---------------------------------------------------------------------



\subsubsection{Evolution of the MDT front-end electronics}
\label{subsec:MDT_frontend_elx}
% ASD development for ATLAS Phase, performance and status


The first stage of the read-out electronics of the MDT chambers is a Charge Sensing 
Amplifier, followed by a pulse Shaping and a Discriminator stage (ASD). For best 
performance at HL-LHC, it had to be optimized for spatial resolution, i.e. accurate 
determination of the drift time inside the MDT tubes. Technically, this calls for fast 
signal rise time, low noise performance and uniformity of thresholds among the 8 
channels of the chip. Technology and design of the chip must guarantee immunity against 
ionizing radiation up to a Total Ionizing Dose (TID) of 200 Gray (20 krad). Another 
important requirement for the read-out architecture comes from the fact that MDT tubes 
operate at a voltage of 3.1 kV. Therefore the sensitive inputs of the amplifiers must 
be protected against spontaneous HV discharges in the tubes, which may be of several 
hundred Volts. Tests with HV pulses of up to the full operating voltage (both 
polarities) have demonstrated that a protective network of passive components together 
with the protective network internal to the ASIC provided sufficient immunity against 
HV discharges, while the corresponding attenuation of the tube signal of about 15\% was 
acceptable.

% -------------------------------------------------------------------

\begin{figure}[hbt]
\begin{center}

    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/ASD6_risetime.jpg}                

    \caption{\label{ASD6_risetime} Shape of the signal in front of the
             discriminator with a peak time of about 10 ns. The horizontal
             scale is 10\,ns/div.} 

\end{center}
\end{figure}

% ---------------------------------------------------------------------

Following these design optimizations, the final version of the ASD chip has been 
submitted in August 2017. Technology was the 130 nm process from Global Foundries, 
which, since its introduction in 2002, has reached a high level of maturity, resulting 
in small variations of process parameters, high production yield and comparatively 
moderate cost. The support given by CERN with respect to design tools and coordination 
of Multi Project Wafer submission was another reason for selecting this process.

After reception of prototype chips, early 2018, detailed tests \cite{BruceThesis} confirmed that the 
target specifications for this device had been reached. A crucial design parameter, the 
rise time of the shaped signal, being responsible for time resolution, is shown in 
Figure~\ref{ASD6_risetime}. Rise time and other performance parameters of this chip are 
significantly better than the ones in the presently used version. A detailed 
presentation of the design is given in \cite{ASD1,ASD2,ASD3,ASD4}.

Figure~\ref{ASD_photo} 
shows a photo of the device, where the 8-fold structure of the chip can be seen.


% --------------------------------------------------------------------

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=0.45\textwidth]{ATLAS/Upgrade/figures/2019_03_13_Photo_Padring.jpg}
\end{center}

\caption{Photograph of the production version of the 8-channel
ASD for MDT read-out. Dimensions are 2.32\,$\times$\,3.45\,$mm^{2}$.
Signal processing in each channel goes from left to right, containing
a differential stage of analog signal processing and discrimination
as well as an ADC of the Wilkinson type and a LWDS driver for the
output signal.
}

\label{ASD_photo}
\end{figure}

% --------------------------------------------------------------------


\subsubsection{Optimization of the sMDT Read-out electronics for Very High Rates}

The background count rates at the HL-LHC will be well below the rate capability limit of an sMDT chamber. Yet, in muon systems at future circular hadron colliders background levels of the order of the rate capability limit will be reached in the forward region. The bipolar shaping scheme of the present sMDT read-out electronics is not ideal for these operating conditions. At very high rates of the order of a MHz per tube, muon hits are often so close in time with the preceding background hits that the negative muon signals are reduced by the positive part of the preceding background hit. This signal pile-up degrades the spatial resolution of the drift tubes and, in particular, the tube efficiency.

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]
        {ATLAS/Upgrade/figures/sMDT_eff_vs_rate_BLR.png}
    \caption{\label{ATLAS:Upgrade:Muon:BLR:fig01}Muon detection efficiency as a function of the background hit rate for three different shaping schemes: the current MDT ASD chip with bipolar shaping, a discrete AS circuit with the same shaping scheme as the new ASD chip but shorter peaking time, and the discrete circuit with active baseline restoration (BLR) to minimize the impact of signal pile-up on the muon efficiency.}
\end{center}
\end{figure}

This effect can be highly reduced by adopting active baseline restoration (BLR) in the shaper of the MDT read-out electronics. This is illustrated in Figure~\ref{ATLAS:Upgrade:Muon:BLR:fig01} in which the dependence of the muon detection efficiency is shown as a function of the background rate for three different amplifier-shaper (AS) combinations: the current MDT ASD chip with bipolar shaping, a discrete AS circuit with the same shaping scheme as the new ASD chip but shorter peaking time, and the discrete circuit with active baseline restoration. The efficiency drop with increasing hit rate is largest for the current ASD chip. The efficiency drop is reduced in the discrete circuit with BLR due to the shorter peaking time. The BLR minimizes the influence of signal pile-up on the muon detection efficiency.

In parallel to the conceptual studies with discrete BLR circuits \cite{BLR_Nowak, BLR_kos}, the ATLAS muon group at MPP has started to design a new ASD chip with active baseline restoration for the use at future hadron collider experiments.


\subsubsection{Limitations of the ATLAS First-Level Muon Trigger}

At the LHC, the ATLAS experiment uses a 3-level trigger system. A high-$p_\mathrm{T}$ muon trigger built out of a coincidence of three RPCs in the barrel toroid magnet and three TGCs behind the end-cap toroid magnet is part of the first trigger level. The muon momentum is estimated from the sizes of the deviations of the trigger chamber hits from a straight line from the $pp$ interaction point. 

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]
        {ATLAS/Upgrade/figures/Muon_run_I_mu_trig_rate_vs_eta_new.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig17}First-level muon trigger rates for 10~GeV transverse momentum threshold as a function of pseudorapidity in LHC Run 1~\cite{NSWTDR}.}
\end{center}
\end{figure}

Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig17} shows the contributions to the first-level muon trigger rate corresponding to 10~GeV transverse momentum threshold as a function of pseudorapidity in LHC Run 1. The trigger rate is dominated by triggers in the end-cap regions caused by charged particles, mainly protons, entering into the  spectrometer from the radiation shielding and the materials of the endcap toroid magnet and leaving tracks in the TGCs in the middle layer behind the toroid magnet which look like high-$p_\mathrm{T}$ muons from the interaction point. These fake triggers can be rejected by using information from trigger chambers in front of the end-cap toroid magnet and the new BIS-78 RPCs in the barrel end-cap transition region in coincidence in the first-level trigger decision. After the removal of the fake triggers, the main source of muon triggers are muons with 
$p_\mathrm{T}<10$~GeV which are accepted due to the coarse spatial resolution of the trigger chambers and the, therefore, rather poor momentum resolution at the first trigger level. 

For operation at HL-LHC, the momentum resolution and selectivity of the first-level muon trigger has to be improved approximately to the performance of the current level-2 trigger by installing new trigger chambers with higher spatial resolution in the inner endcap layers (Phase 1) and by including the (s)MDT precision tracking chamber information in all barrel and endcap layers into the first-level muon trigger decision (Phase 2)~\cite{TDAQTDR}.



\subsubsection{MDT-Based Muon Trigger for the Operation at the HL-LHC 
\label{subsec:Trig_Limitation}}

Unprescaled single-muon triggers with nominal $p_\mathrm{T}$ thresholds of around 20~GeV are essential to fully exploit the HL-LHC physics potential. After elimination of the fake triggers using the NSW trigger chambers and the BIS-78 thin-gap RPCs, the single muon trigger rate at HL-LHC luminosity will be dominated by muons with transverse momenta below the nominal trigger threshold because of the moderate spatial resolution of the trigger chambers. This limitation can be overcome by using the MDT data for high-precision track reconstruction and momentum determination at the first trigger level. This becomes possible because the new (s)MDT on-chamber electronics will allow for streaming off all MDT hits to the trigger logic~\cite{TDC1, TDC2}. 

The concept of a Phase-2 muon trigger system which will employ new MDT-based muon trigger processors to reconstruct muon transverse momenta from straight track segments in (s)MDT chambers within less than 3~$\mu$s was pioneered by the MPP group \cite{MDTtrigger0}-\cite{MDTtrigger2}. The data processing consists of three steps:
\begin{itemize}
    \item   Selection of (s)MDT hits in the so-called ``regions of interest''   
            defined by the pre-trigger from RPC and TGC trigger chambers.
    \item   Reconstruction of straight muon track segments in each (s)MDT    
            chamber within the region of interest.
    \item   Determination of the muon $p_\mathrm{T}$ from the curvature of the muon
            trajectory determined from the straight muon track segments in two or three layers of (s)MDT chambers.
\end{itemize}

An initial hardware demonstrator for the feasibility study of such a trigger processor was developed at the MPP using a Xilinx Zynq evaluation board. With data recorded with a single sMDT chamber at the CERN Gamma Irradiation Facility it was demonstrated that a muon track in MDT chambers can be reconstructed within 3~$\mu$s using a fast tracking algorithm, seeded by the pre-trigger regions-of-interest, on a state-of-the art 1~GHz CPU interfaced to an FPGA for data management (ARM Cortex A9 processor)~\cite{MDTtrigger3}. 

The algorithms for the two most challenging steps of the data processing, the fast reconstruction of straight track segments and the muon momentum determination, have been developed at MPP. Recently, the group introduced further optimizations and provided the implementation of the corresponding VHDL-based firmware on the high-performance FPGAs using the prototyping platform shown in Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig19}. 
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]
        {ATLAS/Upgrade/figures/profpga_fm-zus__500x500_400x400.png}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig19}Prototyping system PCB with a high-performance Xilinx Zynq FPGA. The prototyping system is used for the development of the firmware for MDT-based muon trigger processors.}
\end{center}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
It could be demonstrated that the latency for the two mentioned crucial processing steps is less than 200~ns which is a reduction by about a factor of four compared to the ARM processor in the initial feasibility studies~\cite{MDTtrigger4}. The implemented algorithm is characterised by a low usage of FPGA resources which are a main driver of the final trigger processor costs.


Due to improvements of fast tracking algorithms, the design of an MDT-based muon trigger processor has evolved since the last review by the advisory committee in 2016~\cite{MDTtrigger3, MDTtrigger4}. Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig18} shows the corresponding block diagram of an ATCA blade with the MDT-based muon trigger processor for one of the 64 sectors in the muon spectrometer. Each blade consists of the so-called Service and the Command Module. The Service Module provides mainly the power and the slow control. The data processing now takes place entirely on the Command Module which continuously receives the region-of-interest information together with data from (s)MDT chambers via Gigabit optical links and processes them on two high-performance FPGAs.  
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]
        {ATLAS/Upgrade/figures/MDT_trigger_blade.png}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig18}Block diagram of an ATCA blade for an MDT-based muon trigger processor handling one of the 64 sectors of the muon spectrometer. }
\end{center}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
The MPP group is responsible for the design and the production of all 64 Command Modules. The first full-size hardware demonstrator board is now being designed and will be fabricated in 2020.


\subsubsection{sMDT and RPC chambers for Future Collider Experiments}


A new circular hadron collider, called ``FCC-hh'', with a centre-of-mass energy of 100~TeV and a peak luminosity of  $3\cdot10^{35}$~cm$^{-2}$s$^{-1}$ designed to deliver in its lifetime an integrated luminosity of 20~ab$^{-1}$ is one of the powerful future options to explore physics beyond the standard model of the strong and electroweak interaction. The present conceptual design of a detector at the FCC-hh has the structure depicted in Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig20}. \cite{FCCCDR} The inner detector and the calorimeters will be operated in a 4~T magnetic field created by a solenoid. A muon system will surround the solenoid. It will consist of four parts:
\begin{itemize}
    \item   $|\eta|<1.0$: barrel muon system;
    \item   $1.0\leq|\eta|<1.5$: outer end-cap muon system;
    \item   $1.5\leq|\eta|<2.2$: inner end-cap muon system;
    \item   $2.2\leq|\eta|<3.0$: forward muon system.
\end{itemize}
The muon system will have to provide a muon trigger. The angle between the straight-line interconnection of the entry point in the muon system with the proton-proton interaction point and the direction of flight at the entry point can be used as a stand-alone measurement of the muon momentum. An angular resolution better than 100~$\mu$rad is needed for a high momentum resolution up to $p_\mathrm{T}=1$~TeV.

\begin{figure*}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/FCC_conceptual_design.png}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig20}Schematic drawing of a quadrant of the baseline detector design for the FCC-hh \cite{FCCCDR}.}
\end{center}
\end{figure*}

These goals can be achieved by stations of thin-gap RPCs and sMDT chambers \cite{FCC1}. Like in the ATLAS experiment, the thin-gap RPCs would be used for the identification of $pp$ bunch crossing and the region in which the muons were created; the sMDT chambers would be used for the precise determination of the muon incidence angle required for the stand-alone muon momentum determination.

The muon chambers will be exposed to a large background of neutrons and $\gamma$ rays, which will lead to counting rates of up to 500~Hz\,cm$^{-2}$ in the barrel and the outer end-cap muon system and up to 10~kHz\,cm$^{-2}$ in the inner end-cap and forward muon system. sMDT chambers can be operated at counting rates of up to 30~kHz\,cm$^{-2}$ and background occupancies of up to 30\%. If one uses 2.8~m long tubes in the barrel and the outer end-cap muon system, the chambers will have a background occupancy below 4\%. In the inner end-cap and the forward regions chambers with 40~cm long tubes have to be used to keep the occupancy below 10\%. A full GEANT-4 simulation of sMDT chambers has been performed to determine the achievable angular resolution under the expected background conditions \cite{FCC2}. The simulation studies show that monolithic sMDT chambers with two quadruple tube layers separated by a 1.5~m thick spacer frame provide the required angular of better than 100~$\mu$rad.

A muon system consisting of a single monolithic layer as illustrated in Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig21} for the barrel muon system can therefore be used at the FCC. The sMDT chambers will have to be combined with a double layer of thin-gap RPC chambers which provide bunch crossing identification with better than 1 ns time resolution, muon trigger seeds, and coordinate measurement along the tubes.

\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/FCC_MS_conceptual_design.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig21}Conceptual design of the FCC barrel muon system equipped with a single monolithic layer of sMDT chambers enclosed by two thin-gap RPCs for triggering.}
\end{center}
\end{figure}

The stand-alone muon momentum resolution which can be obtained with the proposed muon system was studied with a GEANT-4 based simulation.  The expected stand-alone muon momentum resolution for the nominal background rates is summarized in Figure~\ref{ATLAS:Muon:Upgrade:triggerrates:Fig22} as a function of the muon transverse momentum. Up to $p_\mathrm{T}\approx1$~TeV the momentum resolution is constant and dominated by multiple scattering: 5\% resolution is achieved in the barrel region, about 10\% in the end-caps and about 20\% in the forward region. 20-40\% resolution is reached at 10~TeV. \cite{FCC2}


\begin{figure}[hbt]
\begin{center}
    \includegraphics[width=\linewidth]{ATLAS/Upgrade/figures/FCC_pt_res.pdf}
    \caption{\label{ATLAS:Muon:Upgrade:triggerrates:Fig22}Expected stand-alone muon momentum resolution for different regions of the FCC muon system.}
\end{center}
\end{figure}


\subsubsection{Electronics Development for Future Collider Experiments}


\subsubsection{Summary and Outlook} 

The MPP ATLAS group has been a leading member of the ATLAS muon collaboration since the beginning and was the proponent of the vastly successful design of the muon spectrometer 
with precision muon tracking chambers based on aluminum drift tubes in a toroidal air-core magnet. The first MDT prototype chambers were all built at MPP, 
and the institute was leading the design and construction of the chambers. The group now plays an even more decisive role for the Phase-2 upgrade of the ATLAS muon spectrometer 
for the HL-LHC which was conceived and established by the MPP group. The research and development for the upgrade has been performed at MPP, 
including the development of the new small-diameter muon drift tube (sMDT) chambers with an order of magnitude higher rate capability compared to the MDT chambers in order to cope with 
the much harsher background conditions at the HL-LHC and the concept and design of an MDT-based first-level muon track trigger system with ten times improved momentum resolution and selectivity.

Several generations of sMDT chambers have already been designed, constructed and operated in ATLAS. The latest installation of BIS~78 sMDT chambers together with new thin-gap RPCs took place 
in the fall of 2020 for operation in the just started Run-3 of the LHC. The integrated design of the sMDT and thin-gap RPC chambers for the replacement the inner barrel layer of the 
ATLAS muon spectrometer for HL-LHC has been completed at MPP. As the first ATLAS Phase-2 upgrade project, the sMDT chamber production started in January 2020 and is proceeding according to plan. 
at MPP and at the second production site in Michigan. There has been a long-standing fruitful collaboration on sMDT chamber construction with the Institute fo High Energy Physics in Protvino 
where the injection molded drift-tube endplugs and the chamber mechanics have been produced such that all parts for the Phase-2 upgrade could already be delivered.  
Half of the new sMDT chambers has already been shipped to CERN where they will be integrated with the thin-gap RPC triplets.
Furthermore, the MPP group has established the construction procedures of thin-gap RPC detectors at the institute and is in the process of 
transferring the technology to german industry for part of the ATLAS Phase-2 upgrade as well as other future large-scale applications. 
The assembly of the thin-gap RPC triplets in the support frames designed by MPP is foreseen to start at MPP in 2023 in the new building in Garching. 

The need for new front-end electronics for the (s)MDT chambers for the operation at the HL-LHC was realized early on by the MPP group. The design of the new ASD and TDC 
ASIC chips for the new readout boards was started well before the Phase-2 upgrade was approved such that he new front-end ASICs are now already available as the first 
ones for ATLAS at HL-LHC. The design of the new (s)MDT readout boards has also been completed at MPP in order to proceed to production early next year.
The MPP group is also responsible for the realization of the new MDT trigger processors both on the hardware and on the firmware side. 
The final prototype is currently under construction, in time for the planned start of the processor module production in 2024. 

Significant resources from the Technical Departments will continue to be needed until end of 2024 to supervise the RPC detector production in industry and complete the RPC triplet assembly at MPP.
Starting already in 2023, qualified personnel will also be needed at CERN for the sMDT and RPC chamber integration and commissioning to be completed before the start of LS3 in 2026 and then 
the installation and commissioning of the chambers and of the new MDT readout electronics and trigger in the ATLAS detector in 2026-2028 which will be the largest effort for the ATLAS 
upgrade in LS3 requiring MPP participation and expertise.

Based on extensive experience, the group prepares to significantly contribute to conceptual design and detector development and performance studies for future lepton and hadon collider experiments.  
The sMDT precision tracking chambers constitute also a very attractive solution for the muon detectors and hardware trigger of a future hadron collider experiment which will be exposed 
to up to 20 times higher luminosity and background rates than expected at the HL-LHC. A full design study has been performed for the FCC-hh Conceptual Design Report and proof-of-principle is planned.
in the near future. The high time resolution of the RPC detectors makes them a crucial technolgy for future collider detectors. Standardized RPC production in industry offers new opportunities 
for large scale applications in collider detectors and elsewhere. 

ASIC electronics design in modern technolgies has been establizhed at MPP by the ATLAS muon group based on the successful development of the MDT ASD and TDC chips. Further improvements of these chips
with respect to rate capability and power consumption in 65~nm CMOS technolgy for high-precision gas detectors for further upgrades of the ATLAS muon spectrometer after LS3 and for future collider experiments 
are already in progress.  