Luminosity Large Hadron Collider Research Articles

Large Area Picosecond Photodetector for the Upgrade II of the LHCb RICH

The Large Area Picosecond Photodetector (LAPPD) is a new generation microchannel plate (MCP) photodetector that is currently capturing the attention of detector physicists thanks to its excellent timing resolution, high gain and low dark count rate. It consists of a photocathode that responds to a single photon by producing a photoelectron that enters a pair of microchannel plates, where it is multiplied to a million electrons in a fast-rising pulse that is easily detected. The speed of this process provides the time resolution essential for high intensity environments such as the high luminosity Large Hadron Collider (LHC) at the CERN laboratory. The LAPPD has a large sensitive area, 200 × 200 mm 2, which reduces the number of sensors that are needed. The increased particle collision rate in the future LHC-HL phase represents a great challenge for the Ring Imaging Cherenkov (RICH) photodetectors in the LHCb experiment. The LHCb collaboration is working for the Upgrade II that will take place during the Long Shutdown 4 of the LHC (>2032). The LAPPD produced and commercialised by INCOM (US) represents one of the possible alternatives for the future RICH photodetectors, thanks to its good time resolution, low dark noise rate and high gain. The LHCb Edinburgh group is involved in the Upgrade II R&D programme, testing and characterising an LAPPD in the laboratory of the University. The LAPPD technology, the current Edinburgh setup and performance from first tests of the photodetector will be presented.

Silicon detectors beyond LHC — RD50 status report

The last decade showed the leading role of the Large Hadron Collider (LHC) experiments in particle physics. To fully exploit its physics potential, the significant increase of LHC luminosity is planned. At the High luminosity Phase-II Upgrade (HL-LHC), foreseen for 2027, a peak instantaneous luminosity of 5 × 1034 cm−2, with an integrated luminosity of 3000 fb−1 is expected. The experiments will be subjected to radiation levels up to 2 × 1016 neq/cm2 at the innermost layers of the detectors. Since more than a decade the RD50 collaboration has been conducting a significant R&D program across experimental boundaries to create silicon sensors with adequate radiation tolerance for HL-LHC trackers. HV-CMOS sensors, 3D detectors, and low gain avalanche detectors (LGADs) are important areas of detector research and development. We will discuss the current state of research and development in numerous silicon detector domains, with a focus on 3D and LGAD detectors. We will also discuss the alternatives for detector selection experiments outside of the LHC, using the FCC as an example.

Influence of the high granularity calorimeter stainless steel absorbers onto the Compact Muon Solenoid inner magnetic field

The Compact Muon Solenoid (CMS) detector is a general-purpose experimental setup at the Large Hadron Collider (LHC) at CERN to investigate the production of new particles in the proton-proton collisions at a centre of mass energy 13 TeV. The third run of the data taken is started in April 2022 and will continue till the end of 2025. Then, during a long shutdown time, the existing CMS hadron endcap calorimeter will be replaced with a new high granularity calorimeter (HGCal) designed for the higher LHC luminosity. The HGCal contains the stainless-steel absorber plates with a relative permeability limited by a value of 1.05 from estimation of the electromagnetic forces acting on this slightly magnetic material. To exclude the surprises with possible perturbation of the inner magnetic flux density in the region of the charged particle tracking system, an influence of this additional material onto the quality of the magnetic field inside the inner tracker volume is investigated at this limited value of the permeability of stainless steel. The three-dimensional model of the CMS magnet is used for this purpose. The method of the magnetic field double integrals characterizing the charged particle momentum resolution the first time is applied to the CMS detector and the first time is described in the journal publication. The results obtained with this method are presented in detail and discussed.

Developing an accurate and robust tool for pixel module characterization

For operation at the High Luminosity Large Hadron Collider (LHC), the ATLAS experiment is building a new all-silicon inner tracker (ITk). The production and testing of thousands of silicon pixel and strip modules is required to cover the estimated 180 m2 of the total surface area. A compact, affordable and robust module characterization system is required for in-situ testing prior any test beam campaign or installation. A test bench setup based on an infrared laser was developed at IJCLab, allowing also for the use of micro-metric precision scanning with a radioactive source. A detailed schema of the setup, operating principles and testing methods are described in this paper, together with the first results obtained with a FE-I4 silicon pixel module.

On the mechanics of MQXFB—the low-beta quadrupole for the HL-LHC

The High Luminosity Large Hadron Collider (LHC) Project target is to reach an integrated luminosity of the LHC of 3000 fb−1, corresponding to a factor 10 increase in collisions with respect to the current accelerator. One of the main components is the superconducting quadrupole called MQXF. It is based on Nb3Sn technology and has a 150 mm single aperture with a field gradient of 132.6 T m−1. The MQXF magnets are currently in preseries production in a joint collaboration between CERN and the US-LHC Accelerator Upgrade Project. The first prototype magnet based on 7 m-long coils (MQXFBP1) was assembled and preloaded in 2019. The testing and disassembly was done in 2020. The coils were equipped with optical fiber Bragg grating (FBG) sensors. Some of the FBG values exhibited strain jumps during preload and their signals were lost during cool-down. The magnet did not reach the nominal current, quenching at 15.15 kA. At the time of writing three MQXFB magnets have been preloaded from which the second prototype is soon to be tested. The objective of this paper is to analyze the mechanical behavior of MQXFB magnets in the light of current knowledge, thanks to earlier short model experiments, as well as describe in detail the process of assembly and preload. We synthesize the mechanical theory of preload and present new considerations on symmetric/asymmetric bladder-key operations and their effects. An extensive comparison between long and short model magnets is presented. In our analysis and FE modeling we take into account the measured coil sizes that vary over position. We introduce a novel technique that allows preload homogenization over the length of the magnet after preload has been done. We analyze the role of the stainless steel shell and show its mechanical behavior. Finally, the relevant data at cold is presented and analyzed.

Gas mixture quality studies for the CMS RPC detectors during LHC Run 2

Resistive Plate Chamber (RPC) detectors are widely employed in the muon trigger systems of three experiments at the CERN Large Hadron Collider (LHC). They are operated with gas mixture recirculation systems to reduce operational costs and greenhouse gas emissions since their gas mixture is based on C2H2F4, which has a high global warning potential. It is well known that the C2H2F4 molecule can break down under the effect of radiation and electric field. This leads to the creation of several impurities and free fluoride ions (F-), which could accumulate under gas recirculation and potentially attach to the RPC bakelite electrode inner surface. For these reasons, an extensive gas analysis campaign has been performed during LHC Run 2 for the CMS RPC system to verify gas mixture quality and possible accumulation of impurities.During the entire LHC Run 2 period, a gas chromatograph has been used to analyze the RPC gas mixture at different points of the gas system: quality of pure C2H2F4, fresh gas from the mixer, gas at the output of the detectors, and after the purifier module. Several impurities have been found and identified. Few impurities are already present in the C2H2F4 supply, as residuals of the industrial production. Nevertheless, it was found that some of these impurities, as well as others, are created inside the detector gas gap due to the fragmentation of the C2H2F4 molecule under the effects of electric field and radiation.During the 2018 LHC Run several gas analysis points were added to measure the fluoride ions production in different sectors of the RPC detector system. Indeed the products of the C2H2F4 fragmentation do not always recombine and F- species can stay free in the gas mixture. F- analyses were performed on the gas at the output of the detectors both for barrel and endcap regions, where radiation levels and operational conditions were different. In this way, it was possible to correlate the F- production with integrated charge, and gas mixture flow rate. In parallel, the RPC currents have been constantly monitored to look for possible correlations.Fluoride measurements have also been performed at GIF++ to better understand the correlation between F- production, radiation levels and gas volume exchanges.A comprehensive overview of the results obtained from the different types of gas analyses and possible correlation with RPC currents and LHC luminosity will be presented.

Pre-Load Studies on a 2-m Long Nb3Sn 11 T Model Magnet for the High Luminosity Upgrade of the LHC

The Large Hadron Collider luminosity upgrade requires the replacement of standard 8.3 T Nb-Ti main dipole magnets with pairs of shorter 11 T Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn dipole magnets. After the manufacturing and testing of several 2 m-long 11 T models, the full-length 11 T magnet series production is now ongoing. As a result of a degraded quench performance in several short models, attributed to excessive stress in the coils' midplanes, an upper hard limit for the coils' pre-stress was set to avoid any conductor degradation. With the aim of further investigating the influence of a lower pre-stress, an additional double-aperture short model was manufactured. The first aperture was assembled with the standard pre-stress target, in line with the series production. The second aperture was assembled with half of the standard pre-stress. Both apertures were then assembled into a 2-in-1 structure, but connected individually in order to power them independently to allow for comparison studies. Contrary to previously built 11 T magnets, the conductor type is also different as Powder-in-tube (PIT) cable was used, instead of standard Restacked-Rod-Process (RRP) cable. Additionally, the quench heaters for magnet protection were not impregnated as previously done, but placed on the outer radial surface of the coils, in agreement with the final decision for the 11 T series production. This paper describes the manufacturing and testing campaign results of this last 11 T short model.

Self-consistent PIC simulations of ultimate space charge compensation with electron lenses

Further progress of fundamental particle physics requires high intensity and high brightness of accelerated proton and ion beams. This goal is essential for the FAIR hadron beams at GSI, for the neutrino production at the facilities such as Fermilab and JPARC, and for the Large Hadron Collider luminosity at CERN. One of the most formidable obstacles toward that goal is the beam's own space charge, whose forces cause beam emittance growth, losses and lifetime degradation. Typically, such effects become intolerable when the space charge tune-shift parameter Δ QSC exceeds ∼ - (0.25–0.5). To reduce these detrimental effects, it was suggested to use electron lenses to compensate the space charge forces. This paper reports on detailed particle-in-cell space charge and electron lens compensation simulations for extremely intense proton bunches whose space charge tune-shift parameter exceeds -1.0. Different scenarios were evaluated based on reduction in the emittance growth and particle loss at a 4σ aperture. We investigate phenomena and issues related to the focusing lattice errors, importance of the transverse and longitudinal matching of the electron beam profiles to the proton ones, and vary the strength and the number of the electron lenses distributed around a circular machine to optimize the reduction of harmful space charge effects.

Studies of the gas volume current density in the ATLAS Resistive Plate Chamber detector during the 2018 data-taking at the LHC

The ATLAS Resistive Plate Chamber (RPC) detector is a tracking detector, used to primarily trigger muons in the ATLAS barrel region (which corresponds to a pseudorapidity range of |η|<1.05) at the 40 MHz collision rate, and to provide η and ϕ coordinates. The RPC system consists of about 3700 gas volumes covering a sensitive surface of about 4000 m2. It is arranged in three concentric doublet layers at radius 7 m, 8 m and 10 m, and operating in an approximately 0.5–1 T toroidal magnetic field. RPCs provide up to 6 space points along the muon track with a space-time resolution of about 1 cm2 × 1 ns. This document studies systematically the gas volume current as a function of the electric field applied across the gas volume and of temperature both with and without the Large Hadron Collider (LHC) beam induced background and up to an instantaneous luminosity Linst = 2×1034 cm−2 s−1 (twice larger than the design LHC luminosity). These measurements have been used to study the RPC working conditions and to extrapolate the detector response to the High-Luminosity LHC regime with Linst = 7.5×1034 cm−2 s−1.

Energy reconstruction of the ATLAS Tile Calorimeter under high pile-up conditions using the Wiener filter

The ATLAS experiment records data from the proton-proton collisions produced by the Large Hadron Collider (LHC). The Tile Calorimeter is the hadronic sampling calorimeter of ATLAS in the region . It uses iron absorbers and scintillators as active material. Jointly with the other calorimeters it is designed for reconstruction of hadrons, jets, -lepton and missing transverse energy. It also assists in muon identification. The energy deposited by the particles in the Tile Calorimeter is read out by approximately 10,000 channels. The signal provided by the readout electronics for each channel is digitized at 40 MHz and its amplitude is estimated by an optimal filtering algorithm. The increase of LHC luminosity leads to signal pile-up that deforms the signal of interest and compromises the amplitude estimation performance. This work presents the proposed algorithm, based on the Wiener filter theory, for energy estimation in the Tile Calorimeter under high pile-up conditions during LHC Run 3. The performance of the proposed method is studied under various pile-up conditions and compared with the current optimal filtering method using proton-proton collision data.

An Investigation of Signal Kinematical Region to Study Standard Model Higgs Boson Properties in the H → WW* → ℓvℓv Decay Channel in the ATLAS Experiment at the Large Hadron Collider at 13 TeV

A signal kinematical region is analyzed by the Higgs Boson Working Group (HWW) within the framework of a study of the Standard Model Higgs boson properties in the H → WW* → evev decay channel in the ATLAS experiment on the Large Hadron Collider (LHC). The analysis is based on the full statistics of proton-proton collision data during exposures in 2015–2016 at the center-of-mass energy of 13 TeV. As compared to our previous analysis, the reconstruction software is updated and the simulation accuracy of backgrounds is improved. Events taken at low and high LHC luminosity are studied separately. In addition, the kinematics at the later stages of event selection is investigated in detail and systematic uncertainties are fully estimated. It is shown that the Monte Carlo modeling satisfactorily describes the experimental data at different selection stages. Kinematic distributions for hadron jets are studied also in the Z-boson control region, where the acquired data reach the order of ten million events. As compared to the previous investigation, the modeling accuracy of Z and background processes is noticeably improved, and events at low and high LHC luminosities are studied separately. Furthermore, all systematic uncertainties related to jets are estimated as well. As a whole, good agreement between experimental results and Monte Carlo simulation data is observed. No problems that would be caused by high luminosity are observed.

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.

Timepix3 Luminosity Determination of 13-TeV Proton–Proton Collisions at the ATLAS Experiment

Medipix and Timepix devices, installed in the ATLAS cavern, have provided valuable complementary luminosity information. Results are presented from measurements with the Timepix3 (TPX3) detectors. In contrast with previously employed frame-based data acquisition, the TPX3 detector remains active continuously, sending information on the pixel hits as they occur. The hit- and cluster-counting methods were used for the luminosity determination of the Large Hadron Collider (LHC) proton–proton collisions. The LHC luminosity versus time is determined using these two methods and fits to a simple model, which incorporates luminosity reduction from the single bunch and beam–beam interactions. The precision of the luminosity determination could be improved by counting the number of clusters, instead of just pixel hits. The internal precision and long-term stability of the TPX3 luminosity measurements are below 0.5%. TPX3, owing to its 1.56-ns time-tagging, is able to resolve the time structure of the luminosity due to the collisions of the individual proton bunches when integrated over an LHC fill.

Beam-loss damage experiment on ATLAS-like silicon strip modules using an intense proton beam

The ATLAS detector community is entering in the final stages of sensor development for the future upgrade of the Large Hadron Collider (LHC). An increase by an order of magnitude of the LHC luminosity, up to 1035 cm−2s−1, will lead to an increased radiation dose delivered to the detector, especially for the devices located in the Inner Tracker. In 2018, an experimental study of the damages induced by a beam-loss over silicon strip detectors was done in the High-Radiation to Materials facility at CERN. A progressive number of proton bunches was extracted from the CERN Super Proton Synchrotron, and focused directly over the strip sensor, in order to emulate the large amount of charge typically generated in a beam-loss failure. This work presents the results obtained during this experiment, showing that Punch-Through Protection effectively protects the AC-coupling capacitors of the silicon sensors from large charge accumulations, but still this scenario could induce damages on the read-out electronics.

Gas mixture quality monitoring for the RPC detectors at the LHC experiments

Resistive Plate Chamber (RPC) detectors are widely employed in the muon trigger systems of three experiments at the CERN Large Hadron Collider (LHC) thanks to their excellent time resolution. The LHC RPC systems are operated under gas recirculation to reduce operation cost and greenhouse gas emissions since their gas mixture is based on C2H2F4, which has a global warning potential of 1430 and it is subject to the European F-gas Regulation. Extensive gas analysis campaigns have been performed during LHC Run 2 for the CMS RPC and ALICE Muon Trigger (MTR) systems to verify the gas mixture quality and possible accumulation of impurities. A particular attention has been addressed to the ALICE MTR system, which has been operated under gas recirculation from the end of 2015. In order to validate the system ensuring good detector operation, the gas recirculation has been increased in steps: 30%, 60% and 70%. Detector currents and gas mixture quality have been closely monitored. A gas chromatograph and mass spectrometer (GC/MS) station has been installed to analyze the MTR gas mixture in different points of the gas system: fresh gas from the mixer, detectors output and output of the purifier module. Several impurities have been found and identified. Most of the impurities are created inside the detector gas gap due to the fragmentation of the C2H2F4 molecule under the effects of electric field and radiation. The GC/MS analyses have been regularly performed in 2016 and 2017. It has been demonstrated that impurities concentration (at the level of tens of ppm) increases with the increase of the gas recirculation fraction. GC/MS analyses have been also performed after the purifier module showing that some impurities are filtered while others not. In parallel, the RPC currents have been constantly monitored, and their trend showed no correlation with the gas recirculation fraction. In 2017 an Ion Selective Electrode station was installed to measure the fluoride ions (F−) concentration in the MTR RPC gas mixture. Indeed the products of the C2H2F4 fragmentation not always recombine and F^− species can stay free in the gas mixture. The analyses show that F− are present in the mixture exiting the RPCs and their concentration increases with the increase of luminosity, even if it stays at the level of ppb per day. A comprehensive overview of the results obtained from the different types of gas analyses and possible correlation with RPC currents and LHC luminosity will be presented.

Failure Assessments for MQXF Magnet Support Structure With a Graded Approach

The high luminosity large hadron collider (LHC) upgrade requires new quadrupoles, MQXF, to replace the present LHC inner triplet magnets. The MQXFA magnet is the first prototype that has a 150-mm aperture and uses Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting technology in a 4.2-m magnetic length structure. The support structure design of the MQXFA magnet is based on the bladder-and-key technology, where a relatively low pre-stress at room temperature is increased to the final preload targets during the cool-down by the differential thermal contraction of the various components. The magnet support structure components experience different load levels from pre-load to cool-down and excitation. Consequently, a few parts experience high stresses that may cause localized plastic deformations or internal fracture development. The concept presented in this paper for the failure assessment of support structures integrates nonlinear finite-element (FE) analysis with detailed sub-models and fracture mechanics into an advanced engineering tool. The nonlinear FE solutions enable estimations of the structural response to the given loads, and the advanced fracture analysis with failure assessment diagram assesses the structure safety index of results obtained from the FE model. The paper describes how the MQXFA end-shell segments are being optimized based on the failure analyses.

A Monolithic 32-Channel Front End and DSP ASIC for Gaseous Detectors

Conseil Européen pour la Recherche Nucléaire is currently undergoing a major upgrade within the A Large Ion Collider Experiment (ALICE) detectors, one of the four main experiments at the Large Hadron Collider (LHC) accelerator. The LHC luminosity will increase making the heavy ions collision rate rise from 500 Hz to 50 kHz. Both the time projection chamber (TPC) and muon chamber (MCH) detectors demand a new and faster readout electronics, which support continuous readout to cope with this higher rate. This paper presents the serialized analog-digital multipurpose application-specific integrated circuit (ASIC) (SAMPA) integrated circuit for upgrading the TPC and MCH front-end and signal processing of charge induced signals, addressing the higher data rate specification. The SAMPA comprises 32 equal channels, twice the present ALICE TPC readout circuitry capacity. Each channel is composed of a positive/negative polarity charge sensitive amplifiers, a pulse shaper, and a 10-bit analog-to-digital converter. It also integrates a full custom digital signal processor, which receives the data from 32 channels, compensate signal perturbations or distortion through several filters and transmits the processed data. Designed for Taiwan Semiconductor Manufacturing Company 130-nm CMOS process, the manufactured chip occupies an area of approximately 85 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and consumes about 8.3 mW per channel at a nominal voltage supply of 1.25 V. The SAMPA is silicon proven and the experimental test results have confirmed its functionality and suitability for the ALICE Upgrade (2019-2020).

Mechanical analysis of the Nb3Sn 11 T dipole short models for the High Luminosity Large Hadron Collider

For the Large Hadron Collider luminosity upgrade, two of the NbTi 8.3 T main bending dipoles will be replaced by two shorter Nb3Sn 11.2 T dipoles to create space for the installation of collimators in the dispersion suppression region. With the aim to verify the design features, several 2 m long 11 T models have been constructed and tested at CERN. During the fabrication and assembly of, so far, seven single and two double aperture short model magnets, several challenges were identified and tackled. These include reproducibility in coil fabrication and assembly procedure, as well as control of mechanical stresses in the conductor and surrounding structure. In order to limit coil over-compression and improve reproducibility, the cable insulation was re-optimized. In addition, a review of the collaring procedure of the 11 T magnet was launched with the goal of reducing the risk of conductor degradation due to excessive stress. In this paper, the main fabrication and assembly steps are described, including the description of the actions taken to resolve the identified weakness.

A New Interface Board for the Level-1 Muon Barrel Trigger Upgrade of the ATLAS Experiment

The Level-1 barrel trigger of the ATLAS experiment is based on the resistive plate chambers (RPCs) detectors. The on-detector trigger electronics identifies muons with specific values of transverse momentum, by using coincidences between different layers of detectors. Trigger data are then transferred from on-detector to off-detector trigger electronics boards. Data are processed by a complex system, which combines trigger data from the barrel and the end-cap regions and provide the combined muon candidate to the central trigger processor (CTP). The system has been performing well for almost a decade. However, in order to cope with continuously increasing large hadron collider luminosity and more demanding requirements on trigger efficiency and performance, various upgrades for the full-trigger system were already deployed and others are foreseen in the next years. Most of the trigger upgrades are based on the state-of-the-art technologies and allow designing more complex trigger menus, increasing processing power and data transfer bandwidth in order to send more trigger candidates, to perform topological selections, and to support new physics studies. In this paper, we describe the design of the first prototype of the barrel interface board, designed around a Xilinx field-programmable gate array, which transfers RPC trigger data to the CTP system; the board supports the optical transmission of trigger data with fixed latency and new trigger algorithms. We discuss the design strategies, the hardware implementation, and the results of the first functional and integration tests.

Study on Geant4 Simulation Toolkit Using a Low-Energy Physics Profiling System

The Geant4 simulation toolkit needs to be optimized for central processing unit (CPU) time and memory size in order to be used for a high luminosity Large Hadron Collider (LHC) experiment. We have developed a low-energy physics profiling system and studied Geant4 with respect to both software and hardware. The results of profiling with Geant4 for low-energy physics were provided to the users and developers of the Geant4 simulation toolkit. In the case of the software, we performed profiling for dependence on different Geant4 versions and provided feedback to Geant4 developers. Profiling for dependence on the number of events showed their linear dependency. In the case of the hardware, we performed profiling for machine dependency on the KISTI-4 supercomputer. This action showed that machines were homogenous. The results obtained in this study will serve as a reference for Geant4 users.