Status of the present ATLAS RPC system and overview towards HL-LHC

The Resistive Plate Chamber (RPC) system covers the barrel region of the ATLAS muon spectrometer [1] in the pseudo-rapidity range |η|<1.05, with six independent detector layers exclusively providing the first level trigger signal and the track coordinate in the non-bending plane of the muon candidates. The system is designed to operate up to the nominal Large Hadron Collider luminosity ( ℒ = 1034 cm−2 s−1) which has been already exceeded thanks to the excellent performance of the collider. The experience in operating the present RPC system, up to the maximum instantaneous luminosity of 2.06 × 10 34 cm−2 s−1 reached in 2017, is reported. The performance of the system, in the severe background and pileup conditions of the last data taking period, is presented together with the improved tools implemented in order to have an effective monitoring of the detector status. The plans to successfully operate the present system during the HL-LHC phase are also introduced.

A : The Resistive Plate Chamber (RPC) system covers the barrel region of the ATLAS muon spectrometer [1] in the pseudo-rapidity range |η| < 1.05, with six independent detector layers exclusively providing the first level trigger signal and the track coordinate in the non-bending plane of the muon candidates. The system is designed to operate up to the nominal Large Hadron Collider luminosity (L = 10 34 cm −2 s −1 ) which has been already exceeded thanks to the excellent performance of the collider. The experience in operating the present RPC system, up to the maximum instantaneous luminosity of 2.06 × 10 34 cm −2 s −1 reached in 2017, is reported. The performance of the system, in the severe background and pileup conditions of the last data taking period, is presented together with the improved tools implemented in order to have an effective monitoring of the detector status. The plans to successfully operate the present system during the HL-LHC phase are also introduced.

Introduction
In 2017 the Large Hadron Collider has provided proton-proton collisions at a center of mass energy of 13 TeV with instantaneous luminosities up to 2.06 × 10 34 cm −2 s −1 , totaling an integrated luminosity of 50.4 fb −1 . This document presents the performance and the operational experience of the detector in this period of time and a brief overview of the upgrade activities for the High Luminosity phase of LHC. The document is organized as follows: after a short description of the ATLAS Resistive Plate Chambers (RPCs) in section 2, the detector status and performance with details of operational experience are presented (section 3). The improvements and upgrade activities in view of the High-Luminosity LHC phase are described in section 4, finally some concluding remarks and outlook are given in section 5.

The ATLAS Resistive Plate Chambers
RPCs [2] provide the barrel region of the ATLAS detector [3] with an independent muon trigger and a two-coordinate measurement. The chambers, arranged in three concentric double layers (middle confirm, pivot, outer confirm) are operated in a strong magnetic toroidal field and cover a surface area of about 4000 m 2 . A low p T (< 10 GeV) trigger requires a projective coincidence between hits in the pivot and the middle confirm layer while high p T triggers require an additional hit in the outer confirm layer. An ATLAS RPC chamber is made of two layers, each with two 2 mm thick bakelite laminate plates and providing two η and two φ readout strip planes (see figure 1). Made of phenolic and melaminic resins, the plates are kept apart at 2 mm by insulating spacers, enclosing a gas volume filled with a mixture of C 2 H 2 F 4 (94.7%) -C 4 H 10 (5%) -SF 6 (0.3%). The external surface of the plates is coated with a thin layer of graphite paint to allow a uniform distribution of the high voltage along the plates. The smoothness of the inner surfaces is enhanced by means of a thin layer of linseed oil. The high voltage (HV) working point is chosen to be at 9.6 kV at a temperature of 24 • C and a pressure of 970 mbar. In these conditions the RPCs work in saturated avalanche mode, inducing, for a minimum ionizing particle, a prompt charge of about 1 pC on the pick-up strips and delivering an average total charge of 30 pC in the gas. The discharge electrons drift in the gas and the signal, induced on pick-up copper strips, is read out via capacitive coupling, and detected by the front-end electronics. Read-out strips have a typical width of ∼ 30 mm and are grouped in two (η and φ) read-out panels with strips orthogonal to each other. Custom front-end electronics amplifies, discriminates and converts the detector signals to ECL standard. These signals are transmitted to the on-detector trigger electronics [4] which, by requiring appropriate coincidences in the η and φ detector layers, provide ATLAS with a Level 1 trigger decision along with the detector data for accepted events.

Detector status and operation
RPC performance. In 2017 LHC provided proton-proton collisions at center of mass energy √ s = 13 TeV with 25 ns bunch spacing. The peak instantaneous luminosity was 2.06×10 34 cm −2 s −1 and the peak number of collisions per bunch crossing (pile-up) was about 80. The 2017 full dataset is made of a delivered integrated luminosity of 50.4 fb −1 a recorded integrated luminosity of 46.8 fb −1 out of which 43.8 fb −1 were considered good for physics analysis. The 2017 data taking has been very successful for the RPC system which has been running with a fraction of up-time and good data quality efficiency of 97.8%. The number of gas volumes either disconnected or operated not at nominal conditions has been around 6% with 239 total disconnected gaps (out of 3714), out of which 152 were disconnected on chamber, 83 on rack (partially recoverable during Technical Stops) and 4 kept at low voltage for recovery due to bad gas supply or too high currents. During 2017 three main hardware issues were responsible for the RPC down time: a power crate failure affecting one (out of 100) Caen Easy crate; a Caen Power Supply mainframe failure, stopping one RPC quadrant; some power supply glitches, due to the muon common system infrastructure, causing the main part of the RPC down time.
Detector Control System. ATLAS RPCs are monitored and operated through the Detector Control System (DCS) which is coherently integrated in the muon system so that a single shifter can operate and monitor all the muon sub-detectors. Several automatic monitoring and control tools to simplify the detector operation and optimize the data taking conditions. One important feature is the full automatic control of the HV settings, which are automatically adjusted in order to follow the different LHC beam phases from injection, stable beams, to the final dump; to compensate for the local changes of the environmental conditions (mainly local temperature and atmospheric pressure); to automatically check the individual gas-gaps currents, recalibrate at each end of fill the pedestals ad finally to provide an online measurement of the cavern background rates and an instantaneous luminosity The DCS is in charge of safely operating and monitoring the detector power system including the detector HV and LV supply. In the RPC DCS [5], a large number of settings (DAC ∼ 4000) and monitoring (ADC ∼ 6500) channels have been integrated into the system in order to optimize the detector performance and allow a fine monitoring at the level of the single individual RPC gas gap (∼ 3600). The remaining ADC channels are used to monitor with high granularity the current draw of the front end electronics and RPC gas and environmental sensors (temperature, atmospheric pressure, relative humidity and gas flow). The ability to control by tuning thresholds, and monitoring the current of each RPC gap has shown to be very powerful for the detector operation both for tracing problems and fine tune the detector. This is particularly important as the RPC performance and aging is strongly related to the environmental parameters, namely the temperature (T), the atmospheric pressure (P), and the relative humidity. The gas gain, the noise rate and the dark current of the chamber depend on these parameters following the formula: V appl = V eff · (T 0 /T) · (P/P 0 ) where V appl is the applied voltage, T, P are the environment measurements and T 0 , P 0 , V eff are the reference environmental values and HV settings.
Technical issues. During the years of operation, some gas volume inlets have developed fractures or complete breakups. This issue is currently affecting about 6% of gas gaps and is intensely investigated, as it is responsible for most of the RPCs inefficiency. As most of the volumes can be operated normally, a repair campaign is ongoing. The type of intervention depends on the crack features: if the inlet is completely broken a substitution procedure must be done. On the other hand, if the inlet has developed only a partial fracture, it is possible to intervene by spraying glue on the break, building a sealing membrane (see figure 2). In the past years many tests both on the procedure for the glue spraying and on the choice of the particular glue to be used have undergone, so that now the procedure is very solid and the results very reliable. In order to provide online monitoring of the   figure 4) and are monitored with automatic alarms. It is thus possible to promptly identify the single chamber leaks at a very early stage, allowing a rapid intervention at a time when the reparation is easier. A good news for the year 2018 comes from the fact that, taking into account the experience from previous years when a large increase of the gas leaks after the system restart (after the winter shutdown) was observed, the decision not to stop the gas system was taken. At the 2018 operation restart no increase in the gas leak was observed (see figure 5), allowing for the possibility to start the repair campaign from a better portion than last years.  . Averaged RPC hit rates in BM chambers measured in Run1 and extrapolated (including a safety factor of two) to a luminosity of 7.5 × 10 34 cm −2 s −1 . The plot shows rates in Hz / cm 2 as a function of the φ sector (vertical axis) and the station number along Z (horizontal axis). The rates have been derived form detector currents by applying a conversion factor of 30 pC / hit [8].

Overview towards HL-LHC
ATLAS RPCs are certified for 10 years of operation at a luminosity of 10 34 cm −2 s −1 with a rate of 100 Hz cm −2 for a total integrated charge of 0.3 C cm −2 . The counting rates at the High Luminosity phase of LHC will exceed this limit, for example the rate at |η| = 1 will be about 300 Hz cm −2 (see figure 6). Moreover the extent of the running time from 10 years to 30 years will bring the integrated charge to 0.48 C cm −2 at |η| = 1 for an integrated luminosity of 4000 fb −1 . As a consequence the charge per count of 30 pC / hit must be reduced in order to compensate the increase in the rate (see [8]). This would nonetheless lower the single-gap efficiency of 15% on average and of 35% at large η. The BIS 7/8 upgrade project and the BI upgrade project are presented in [6,7], so the focus here is on the BO refurbishment project. The basic idea is that the reduction of trigger efficiency at |η| > 0.8 can be recovered by refurbishing a limited number of BO chambers by replacing the front-end electronics and the readout strip panels, as shown in figure 7. This can be done during the long shutdown of 2019-2020.

Conclusions
The ATLAS RPCs have very successfully operated during 2017, maintaining high efficiency and high data quality over the whole period. A few technical issues have been promptly tackled and absorbed by the system redundancy without considerable effects on the data taking as the RPCs are the only system with trigger capability in the barrel region of the muon spectrometer. The consolidation and upgrade works will ensure the ATLAS RPCs will be able to exploit at best the luminosity delivered by the Large Hadron Collider in the next years.