High Luminosity LHC Operation Research Articles

Longevity study of a triple-GEM chamber for the HL-LHC upgrade

As part of the high-luminosity LHC (HL-LHC) upgrade, the CMS muon spectrometer will extend its coverage in the very forward region with triple-GEM detectors. These detectors will cope with a harsh environment with expected background fluxes up to 150 kHz/cm (Sauli, 2016) [2]. At the end of HL-LHC operation, the total accumulated charge at the hottest point is expected to reach 7.9 C/cm (Sauli, 2016) [2]. This accumulated charge is unprecedented for a GEM detector.A full-size triple-GEM chamber was built using the design planned for the most forward CMS muon station using GEM foils produced at CERN. While operated with a gas mixture of Ar/CO2 (70/30)%, the chamber was irradiated with photons from a silver-target X-ray source over 1.5 years to reach an accumulated charge of 8 C/cm2. Before and after irradiation the chamber was validated using the standard quality control procedures; gas gain and energy resolution were measured. During irradiation the gain was monitored weekly. The irradiation was interrupted only for the certification measurements. A non-irradiated sector was monitored for comparison. Temperature and pressure in the lab were continuously measured to derive correction factors to calibrate the gas gain. No degradation in gas gain was observed when reaching 8 C/cm2. Irradiated and non-irradiated chamber components were inspected for potential impacts from irradiation.

Long-term irradiation study of sMDT drift tubes with an integrated accumulated charge of 60 C per wire using beta-electrons from a 90Sr source

Two ATLAS sMDT drift tubes have been irradiated for almost 1 year using a 90Sr beta-decay source. An integrated charge of 62C has been accumulated on each of both anode wires over an anode-wire region of about 7.5 cm. Taking into account the intensity distribution of the irradiation corresponds to a maximum accumulated line charge density of about 14C/cm. At the innermost position of the ATLAS forward muon spectrometer 10C/cm are expected for 10 years of high-luminosity LHC operation and for this detector type at gas gain 20000. To investigate potential outgassing, the endplug region of the drift-tubes, where no gas amplification occurs, was irradiated additionally using about half the beta-electrons emitted from the source. The other beta-electrons were irradiating an active part of the gas volume for monitoring purpose. During four months the endplugs were irradiated by 5C/cm equivalent. All observed anode currents were very stable over the whole period of irradiation and thus no sign of deterioration in the performance of both drift tubes was observed. This indicates that no ageing effects occurred and that no performance loss due to outgassing of any plastic surfaces has been observed. All components that have potential contact to the detector gas Ar:CO2 with a mixture of 93:7 (percent volume) have been carefully and properly chosen. The required cleanliness of all tube- and gas components has been achieved during construction and operation of these drift tubes.

A muon tracking algorithm for the Level 1 trigger in the CMS barrel muon chambers during HL-LHC

This contribution presents results on the Analytical Method (AM) algorithm for trigger primitive (TP) generation in the CMS Drift Tube (DT) chambers during the High Luminosity LHC operation (HL-LHC or LHC Phase II). The algorithm has been developed and validated both in software with an emulation approach, and through hardware implementation tests. The obtained performance on Phase II simulated data shows timing and position resolutions close to the ultimate performance of the DT chambers, with resilience to potential ageing situations. The firmware version has been implemented in the so-called AB7 (TwinMux), spare μTCA boards from the present DT system which host Xilinx Virtex 7 FPGAs, and included in a prototype chain of the HL-LHC electronics operated with real DT chambers during cosmic data taking. Agreement between the software emulation and the firmware implementation has been verified using different data samples, including a sample of real muons collected during 2016 data taking.

Optimal injection voltage in the LHC

Finding the optimal rf injection voltage in the LHC is a trade-off between minimising capture and flat-bottom losses, which call for an increased voltage, and minimising rf power consumption as well as improving beam stability, both of which call for a reduced voltage. From the beam stability point of view, earlier particle-tracking simulations showed that the decrease of the injection voltage from 6MV to 4MV is beneficial. This reduction was performed in the LHC Run 2 over a three-week period in steps of 0.5MV. The impact of different voltages and injection energy errors on the evolution of beam parameters such as bunch length and beam losses are analysed in this paper. Operation at 4MV, maintained in the machine after the reduction period, is studied in more detail. The implications for the future High-Luminosity LHC operation with high-intensity beams are also discussed.

Trigger primitive generation algorithm in the CMS barrel muon chambers during HL-LHC

This contribution presents an update on the Analytical Method (AM) algorithm for trigger primitive (TP) generation in the CMS Drift Tube (DT) chambers during the High Luminosity LHC operation (HL-LHC or LHC phase 2). The algorithm has been developed and validated both in software with an emulation approach, and through hardware implementation tests. The algorithm is mainly divided into the following steps: a grouping (pattern recognition) step that finds the path of a given muon, a fitting step to extract the track parameters (position and bending angle), and a correlation step that matches the information from the different super-layers and with signal from the resistive plate chambers. Agreement between the software emulation and the firmware implementation has been verified using different data samples, including a sample of real muons collected during 2016 data taking. In this contribution, an update of the grouping step using a pseudo-Bayes classifier will be discussed.

Beam test results of silicon sensor module prototypes for the Phase-2 Upgrade of the CMS Outer Tracker

The start of the High-Luminosity LHC (HL-LHC) in 2027 requires upgrades to the Compact Muon Solenoid (CMS) experiment. In the scope of the upgrade program the complete silicon tracking detector will be replaced. The new CMS Tracker will be equipped with silicon pixel detectors in the inner layers closest to the interaction point and silicon strip detectors in the outer layers. The new CMS Outer Tracker will consist of two different kinds of detector modules called PS and 2S modules. Each module will be made of two parallel silicon sensors (a macro-pixel sensor and a strip sensor for the PS modules and two strip sensors for the 2S modules). Combining the hit information of both sensor layers, it is possible to estimate the transverse momentum of particles in the magnetic field of 3.8 T at the full bunch-crossing rate of 40 MHz directly on the module. This information will be used as an input for the first trigger stage of CMS. It is necessary to validate the Outer Tracker module functionality before installing the modules in the CMS experiment. Besides laboratory-based tests several 2S module prototypes have been studied at test beam facilities at CERN, DESY and FNAL. This article concentrates on the beam tests at DESY during which the functionality of the module concept was investigated using the full final readout chain for the first time. Additionally the performance of a 2S module assembled with irradiated sensors was studied. By choosing an irradiation fluence expected for 2S modules at the end of HL-LHC operation, it was possible to investigate the particle detection efficiency and study the trigger capabilities of the module at the beginning and end of the runtime of the CMS experiment.

The ATLAS strip detector system for the High-Luminosity LHC

The ATLAS experiment at the Large Hadron Collider is currently preparing for a significant upgrade of the Inner Detector (ID) for the High-Luminosity LHC operation, scheduled to start in 2027. A planned integrated luminosity of 4000 fb−1 implies integrated hadron fluences over 2×1016 neq/cm2, requiring a complete replacement of the existing ID. An all-silicon Inner Tracker is under development with a pixel detector surrounded by a strip detector. The strip detector consists of four barrel layers in the centre, while the forward regions are made of six disks at each end, with silicon-strip modules as basic units. With the production of modules scheduled to begin in 2020, a thorough understanding of the current prototype modules is critical. Electrical characterization of the module aims to assess the front-end performance in terms of input noise and noise occupancy. Results of input noise from modules built with different front-end designs are presented. Tracking resolutions and detection efficiency are evaluated in beam tests. Several module prototypes have been built and tested in 2018 and 2019. In this report, the beam test results from different module types are presented. Included are results from the first double-sided end-cap prototype module. The results focus on hit detection efficiencies and spatial resolution of the modules.

Single Event Upset rates in the CBC in CMS

The CMS Binary Chip (CBC) is a front-end ASIC to be used by the CMS tracker following its upgrade for High Luminosity LHC operation. It will instrument special silicon microstrip detectors to identify high transverse momentum particles in real time so tracking data can be used in the L1 trigger. The CBC should be robust against Single Event Upsets (SEUs). SEU rates have been measured in a series of tests in a 62 MeV proton beam. Each version of the chip has increased the digital circuitry, and hence the SEU susceptibility, and has also been subject to design improvements which affect SEU tolerance. The relevant design features are explained and SEU measurements reported. The expected SEU rates at the HL-LHC are estimated.

Strip sensor performance in prototype modules built for ATLAS ITk

ATLAS experiment is preparing an upgrade of its detector for High-Luminosity LHC (HL-LHC) operation. The upgrade involves installation of the new all-silicon Inner Tracker (ITk). In the context of the ITk preparations, more than 80 strip modules were built with prototype barrel sensors. They were tested with electrical readout on a per-channel basis. In general, an excellent performance was observed, consistent with previous ASIC-level and sensor-level tests. However, the lessons learned included two phenomena important for the future phases of the project. First was the need to store and test the modules in a dry environment due to humidity sensitivity of the sensors. The second was an observation of high noise regions for 2 modules.The high noise regions were tested further in several ways, including monitoring the performance as a function of time and bias voltage. Additionally, direct sensor-level tests were performed on the affected channels. The inter-strip resistance and bias resistance tests showed low values, indicating a temporary loss of the inter-strip isolation. A subsequent recovery of the noise performance was observed. We present the test details, an analysis of how the inter-strip isolation affects the module noise, and the relationship with sensor-level quality control tests.

Design and performance of a TDC ASIC for the upgrade of the ATLAS Monitored Drift Tube detector

We present the prototype of a time-to-digital (TDC) ASIC for the upgrade of the ATLAS Monitored Drift Tube (MDT) detector for high-luminosity LHC operation. This ASIC is based on a previously submitted demonstrator ASIC designed for timing performance evaluation, and includes all features necessary for the various operation modes, as well as the migration to the TSMC 130 nm CMOS technology. We present the TDC design with the emphasis on added features and performance optimization. Tests of the timing performance demonstrate that this ASIC meets the design specifications. The TDC has a bin size of about 780 ps, and a timing bin variations within 40 ps for all 24 channels with leading and trailing edge digitization, while the power consumption has been limited to 250 mW, corresponding to a consumption of about 5.2 mW per edge measurement.

Conception and design of a cooling system for the LHC injection kicker magnets

The CERN Large Hadron Collider is equipped with two fast single-turn injection kicker systems that deflect the incoming particle beam onto the accelerator’s orbit. The high intensity LHC beam, circulating for many hours, can cause considerable heating of the injection kicker magnets. Numerical models have been developed and validated to study the beam induced heating and thermal behaviour of these magnets. The models are used to predict the power deposition and temperatures for various operation scenarios. According to predictions, heating issues are expected for High Luminosity LHC operation with high intensity beams unless appropriate measures are taken. Cooling of the magnet yokes is complicated as the magnets are in vacuum and are pulsed at high voltage. A description of the evolution of the studies is presented, culminating with the conception and design of a cooling system which is thoroughly described, from the conception process to the final proposed solution.

Observation of proton-tagged, central (semi)exclusive production of high-mass lepton pairs in pp collisions at 13 TeV with the CMS-TOTEM precision proton spectrometer

The process pp → pℓ+ℓ−p(*), with ℓ+ℓ− a muon or an electron pair produced at midrapidity with mass larger than 110 GeV, has been observed for the first time at the LHC in pp collisions at sqrt{s}=13 TeV. One of the two scattered protons is measured in the CMS-TOTEM precision proton spectrometer (CT-PPS), which operated for the first time in 2016. The second proton either remains intact or is excited and then dissociates into a low-mass state p*, which is undetected. The measurement is based on an integrated luminosity of 9.4 fb−1 collected during standard, high-luminosity LHC operation. A total of 12 μ+μ− and 8 e+e− pairs with m(ℓ+ℓ−) > 110 GeV, and matching forward proton kinematics, are observed, with expected backgrounds of 1.49 ± 0.07 (stat) ± 0.53 (syst) and 2.36 ± 0.09 (stat) ± 0.47 (syst), respectively. This corresponds to an excess of more than five standard deviations over the expected background. The present result constitutes the first observation of proton-tagged γγ collisions at the electroweak scale. This measurement also demonstrates that CT-PPS performs according to the design specifications.

Electronics for CMS Endcap Muon Level-1 Trigger System Phase-1 and HL LHC upgrades

To accommodate high-luminosity LHC operation at a 13 TeV collision energy, the CMS Endcap Muon Level-1 Trigger system had to be significantly modified. To provide robust track reconstruction, the trigger system must now import all available trigger primitives generated by the Cathode Strip Chambers and by certain other subsystems, such as Resistive Plate Chambers (RPC). In addition to massive input bandwidth, this also required significant increase in logic and memory resources. To satisfy these requirements, a new Sector Processor unit has been designed. It consists of three modules. The Core Logic module houses the large FPGA that contains the track-finding logic and multi-gigabit serial links for data exchange. The Optical module contains optical receivers and transmitters; it communicates with the Core Logic module via a custom backplane section. The Pt Lookup table (PTLUT) module contains 1 GB of low-latency memory that is used to assign the final Pt to reconstructed muon tracks. The μ TCA architecture (adopted by CMS) was used for this design. The talk presents the details of the hardware and firmware design of the production system based on Xilinx Virtex-7 FPGA family. The next round of LHC and CMS upgrades starts in 2019, followed by a major High-Luminosity (HL) LHC upgrade starting in 2024. In the course of these upgrades, new Gas Electron Multiplier (GEM) detectors and more RPC chambers will be added to the Endcap Muon system. In order to keep up with all these changes, a new Advanced Processor unit is being designed. This device will be based on Xilinx UltraScale+ FPGAs. It will be able to accommodate up to 100 serial links with bit rates of up to 25 Gb/s, and provide up to 2.5 times more logic resources than the device used currently. The amount of PTLUT memory will be significantly increased to provide more flexibility for the Pt assignment algorithm. The talk presents preliminary details of the hardware design program.

Beam test evaluation of electromagnetic calorimeter modules made from proton-damaged PbWO4 crystals

The performance of electromagnetic calorimeter modules made of proton-irradiated PbWO4 crystals has been studied in beam tests. The modules, similar to those used in the Endcaps of the CMS electromagnetic calorimeter (ECAL), were formed from 5×5 matrices of PbWO4 crystals, which had previously been exposed to 24 GeV protons up to integrated fluences between 2.1× 1013 and 1.3× 1014 cm−2. These correspond to the predicted charged-hadron fluences in the ECAL Endcaps at pseudorapidity η = 2.6 after about 500 fb−1 and 3000 fb−1 respectively, corresponding to the end of the LHC and High Luminosity LHC operation periods. The irradiated crystals have a lower light transmission for wavelengths corresponding to the scintillation light, and a correspondingly reduced light output. A comparison with four crystals irradiated in situ in CMS showed no significant rate dependence of hadron-induced damage. A degradation of the energy resolution and a non-linear response to electron showers are observed in damaged crystals. Direct measurements of the light output from the crystals show the amplitude decreasing and pulse becoming faster as the fluence increases. The latter is interpreted, through comparison with simulation, as a side-effect of the degradation in light transmission. The experimental results obtained can be used to estimate the long term performance of the CMS ECAL.

The CMS Electromagnetic Calorimeter: overview, lessons learned during Run 1 and future projections

The Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid (CMS) experiment at the LHC is a hermetic, fine grained, homogeneous calorimeter, containing 75,848 lead tungstate scintillating crystals. We highlight the key role of the ECAL in the discovery and elucidation of the Standard Model Higgs boson during LHC Run I. We discuss, with reference to specific examples from LHC Run I, the challenges of operating a crystal calorimeter at a hadron collider. Particular successes, chiefly in terms of achieving and maintaining the required detector energy resolution in the harsh radiation environment of the LHC, are described. The prospects for LHC Run II (starting in 2015) are discussed, building upon the experience gained from Run I. The high luminosity upgrade of the LHC (HL-LHC) is expected to be operational from about 2025 to 2035 and will provide instantaneous and integrated luminosities of around 5 × 1034/cm2/s and 3000/fb respectively. We outline the challenges that ECAL will face and motivate the evolution of the detector that is thought to be necessary to maintain its performance throughout LHC and High-Luminosity LHC operation.

Evolution of the response of the CMS ECAL and possible design options for electromagnetic calorimetry at the HL-LHC

The performance of the CMS electromagnetic calorimeter (ECAL) has been continuously monitored at the LHC. The evolution of this performance is a critical issue for the future. Work has started to assess the need for possible changes to the detector to ensure adequate performance for High-Luminosity LHC (HL-LHC) operation, planned for 2022 and beyond. Results from CMS running, beam tests and laboratory measurements on proton-irradiated crystals are combined to predict the performance of the current detector at the HL-LHC. This is achieved using MC simulations of the CMS detector, where the ECAL response has been tuned to account for the ageing of the detector components. In addition, various R&D studies are presented in case modification or replacement of the ECAL Endcaps is needed for the HL-LHC period.

Recent results of the ATLAS upgrade Planar Pixel Sensors R&D project

To extend the physics reach of the LHC, upgrades to the accelerator are planned which will increase the integrated annual luminosity by a factor of 5–10. This will increase the occupancy and the radiation damage of the inner trackers. To cope with the elevated occupancy, the ATLAS experiment plans to introduce an all silicon inner tracker for High Luminosity LHC (HL-LHC) operation. With silicon, the occupancy can be adjusted by using the appropriate pitch for the pixels/micro-strips. Constraints due to high radiation damage mean that only sensors with electrode configuration designed to read out the electron signal (n-in-p and n-in-n) are considered. To investigate the suitability of planar pixel sensors (PPS) for the ATLAS tracker upgrade, a dedicated R&D project was established, with 17 institutes and more than 80 scientists. The main focuses of research are the performance of planar pixel sensors after the high fluences expected during HL-LHC operation, the optimisation of the detector and module production technologies for cost reduction to enable the instrumentation of large volumes and the reduction of the inactive areas needed for electrical insulation of the sensitive region from the cut edge of the sensors. An overview of recent accomplishments of the PPS (Planar Pixel Sensors) R&D project is given. The performance in terms of charge collection and tracking efficiency, evaluated with radioactive sources in the laboratory and from beam tests, is presented. Sensors with different thicknesses (ranging from 75 to 300μm) were irradiated to several fluences up to 2×1016neqcm−2 to study the effect of varying thickness on the radiation hardness. The significant progresses made towards the reduction of the edge distance are reported.

Single event upset studies with the optical links of the ATLAS semiconductor tracker

Studies have been performed of Single Event Upsets in the ATLAS SemiConductor Tracker optical links system. The measurements were made using low energy neutrons, and pion and proton beams in the momentum range 300–465 MeV/c. The implications for the operation of the system in ATLAS during high luminosity LHC operation are discussed.

Pixel detectors at LHC

The design considerations and expected performance of pixel vertex detectors for the planned general purpose LHC experiments are presented. Special emphasis is given to the optimisation of the secondary vertex resolution that will be crucial for efficient b-jet tagging in high luminosity LHC operation and will allow precise studies of B-decays (e.g. measurement of B s-oscillation) during the first years of low luminosity running. The very demanding requirements for the pixel readout electronics in the high rate environment are described and the current development of an LHC-adequate analog block for pixel readout is shown. The performance of various data evacuation architectures for the pixel readout has been calculated with time domain simulations that extend from event generation up to the data flow in the integrated readout chips.

B-physics prospects with ATLAS and CMS at the LHC

TheLarge Hadron Collider (LHC) is in preparation at CERN, and the ATLAS (A Toroidal LHC Apparatus) and CMS (Compact Muon Solenoid) Collaborations have recently presented Technical Proposals for the construction of two general purpose experiments. Even though ATLAS and CMS are primarily conceived and optimized for high-luminosity LHC operation, namely in view of Standard Model and Minimal Supersymmetric Standard ModelHiggs searches, the first few years of low-luminosity operation, thanks to the high centre-of-mass energy of the LHC collider experiments with two 7 TeV proton beams, will also allow investigation of B-physics issues, which are influencing the designs of the detectors. Prospects for B physics will be described and compared, in particularCP violation, with time-dependent and time-integrated asymmetries for different channels and related sensitivities to theCP-violating parameters, oscillations and rare decays. Conclusions and possible improvements will be summarized.