Silicon Pixel Tracker Research Articles

3D silicon pixel sensors for the CMS experiment tracker upgrade

The upgrade program of the CMS experiment (CMS Collaboration, 2024) in view of the LHC High Luminosity phase (HL–LHC) includes a replacement of the silicon pixel tracker. This is necessary to guarantee the same tracking performance of the current detector under harsher operating conditions, including higher radiation fluences and hit rates. The first layer of the central (barrel) detector will be located at radial distance of 3 cm from the interaction point where the radiation fluence after 7 years of operation will be 1.9×1016neq/cm2. Following an extensive R&D program, comprising both laboratory and test beam measurements, 3D sensors will be employed in this layer due to their higher radiation tolerance and lower power consumption after irradiation. Test beam measurements after irradiation up to 1.6×1016neq/cm2 showed a hit detection efficiency larger than 96% at normal incidence with less than 2% masked pixels, for applied bias voltages between 100 V and 130 V.

Detector challenges of the strong-field QED experiment LUXE at the European XFEL

The LUXE experiment aims at studying high-field QED in electron-laser and photon-laser interactions, with the 16.5 GeV electron beam of the European XFEL and a laser beam with power of up to 350 TW. This experiment will use one electron bunch out of 2700 bunches per train from XFEL. The experiment will measure the multiplicity spectra of electrons, positrons and photons in expected ranges of 10-3 to 109 particles per 1 Hz bunch crossing, depending on the laser power and focus. These measurements have to be performed in the presence of low-energy high radiation-background. To meet these challenges, for high-rate electron and photon fluxes, the experiment will use Cherenkov radiation detectors, scintillator screens, sapphire sensors as well as lead-glass monitors for backscattering off the beam-dump. A four layer silicon-pixel tracker and a compact electromagnetic tungsten calorimeter with GaAs sensors will be used to measure the positron spectra. The layout of the experiment and the expected performance under the harsh radiation conditions will be presented. Beam tests for the Cherenkov detector and the electromagnetic calorimeter were performed at DESY recently and results will be presented. The experiment received a stage 0 critical approvement (CD0) from the DESY management and is in the process of preparing its technical design report (TDR). It is expected to start running in 2025/6.

The Upgrade of LHCb VELO

LHCb physics achievements to date include some of the world’s most precise flavour physics measurements, including the CKM phase γ and the discovery of CP violation in the charm sector. These achievements have been possible thanks to the enormous data samples collected and the performance of the subdetectors, with the efficiency and precision of the silicon vertex detector (VELO) being a major contributor to this success. The experiment has been upgraded to run at higher luminosity, requiring 40 MHz readout. The VELO Upgrade modules are composed of hybrid pixel detectors and electronics circuits glued onto a cooling substrate, made of thin silicon plates with embedded micro-channels, allowing for the circulation of evaporative CO 2 . The detectors are located in vacuum, separated from the beam by a thin aluminium box. The upgraded VELO is composed of 52 modules placed along the beam axis divided into two retractable halves. The design, production, installation and commissioning of the VELO Upgrade system is presented together with test results. • LHCb experiment relies on the VELO detector for accurate vertex reconstruction. • The VELO Upgrade is the brand new silicon pixel tracker of LHCb. • Detector comprises 52 double sided modules with 4 pixel sensors each. • Micro-channels circulating evaporative CO 2 allows for efficient heat dissipation. • Modules Electrical and thermal performance validated in operation conditions.

Track and vertex reconstruction using moving emulsion blocks and a silicon pixel tracker for particles induced by 400 GeV/c protons on a thick target

The SHiP-charm experiment is designed to measure the charm production cross section, including cascade production, of 400 GeV/c protons hitting a thick, SHiP-like target. For the detection of production and decay of heavy charmed particles, emulsion films are employed in a multilayered moving target, forming an emulsion cloud chamber. While the emulsion films provide excellent spatial resolution they do not provide timing information, integrate all events, and quickly get saturated. For the charm measurement the emulsion target is thus moving at a constant speed during data-taking. A first optimization run at the CERN SPS has been performed in 2018, with the purpose to develop the required analysis tools and to fine-tune the detector layout. We report on the experiment design, track reconstruction in the pixel tracker and the track matching with the moving emulsion detector.

Detector challenges of the strong-field QED experiment LUXE at the European XFEL

The LUXE experiment aims at studying high-field QED in electron–laser and photon–laser interactions, with the 16.5 GeV electron beam of the European XFEL and a laser beam with power of up to 350 TW. The experiment will measure the spectra of electrons, positrons and photons in the expected range of 10−3 to 109 per 1 Hz bunch crossing, depending on the laser power and focus. These measurements have to be performed in the presence of low-energy high radiation-background. To meet these challenges, for high-rate electron and photon fluxes, the experiment will use Cherenkov radiation detectors, scintillator screens, sapphire sensors as well as lead-glass monitors for backscattering off the beam-dump. A four-layer silicon-pixel tracker and a compact electromagnetic tungsten calorimeter with GaAs sensors will be used to measure the positron spectra. The layout of the experiment and the expected performance under the harsh radiation conditions, together with the test of the Cherenkov detector and the electromagnetic (EM) calorimeter performed recently at DESY, are presented. The experiment received a stage 0 critical approval (CD0) from the DESY management and is in the process of preparing its technical design report (TDR). It is expected to start running in 2025/2026.

Track reconstruction and matching between emulsion and silicon pixel detectors for the SHiP-charm experiment

In July 2018 an optimization run for the proposed charm cross section measurement for SHiP was performed at the CERN SPS. A heavy, moving target instrumented with nuclear emulsion films followed by a silicon pixel tracker was installed in front of the Goliath magnet at the H4 proton beam-line. Behind the magnet, scintillating-fibre, drift-tube and RPC detectors were placed. The purpose of this run was to validate the measurement's feasibility, to develop the required analysis tools and fine-tune the detector layout. In this paper, we present the track reconstruction in the pixel tracker and the track matching with the moving emulsion detector. The pixel detector performed as expected and it is shown that, after proper alignment, a vertex matching rate of 87% is achieved.

Studies of high-field QED with the LUXE experiment at the European XFEL

The LUXE experiment aims at studying high-field QED in electron-laser and photon-laser interactions, with the 16.5 GeV electron beam of the European XFEL and a laser beam with power of up to 350 TW. The experiment will measure the spectra of electrons and photons in non-linear Compton scattering where production rates in excess of 109 are expected per 1 Hz bunch crossing. At the same time positrons from pair creation in either the two-step trident process or the Breit-Wheeler process will be measured, where the expected rates range from 10−3 to 104 per bunch crossing, depending on the laser power and focus. These measurements have to be performed in the presence of low-energy high radiation-background. To meet these challenges, for high-rate electron and photon fluxes, the experiment will use Cherenkov radiation detectors, scintillator screens, sapphire sensors as well as lead-glass monitors for back-scattering off the beam-dump. A four-layer silicon-pixel tracker and a compact sampling electromagnetic calorimeter will be used to measure the positron spectra. The layout of the experiment and the expected performance under the harsh radiation conditions will be presented.

Convolutional Neural Network for Track Seed Filtering at the CMS High-Level Trigger

Starting with Run II, future development projects for the Large Hadron Collider will constantly bring nominal luminosity increase, with the ultimate goal of reaching a peak luminosity of 5·1034cm−2s−1 for ATLAS and CMS experiments planned for the High Luminosity LHC (HL-LHC) upgrade. This rise in luminosity will directly result in an increased number of simultaneous proton collisions (pileup), up to 200, that will pose new challenges for the CMS detector and, specifically, for track reconstruction in the Silicon Pixel Tracker. One of the first steps of the track finding work-flow is the creation of track seeds, i.e. compatible pairs of hits from different detector layers, that are subsequently fed to higher level pattern recognition steps. However, the set of compatible hit pairs is highly affected by combinatorial background resulting in the next steps of the tracking algorithm to process a significant fraction of fake doublets. A possible way of reducing this effect is taking into account the shape of the hit pixel cluster to check the compatibility between two hits. To each doublet is attached a collection of two images built with the ADC levels of the pixels forming the hit cluster. Thus the task of fake rejection can be seen as an image classification problem for which Convolutional Neural Networks (CNNs) have been widely proven to provide reliable results. In this work we present our studies on CNNs applications to the filtering of track pixel seeds. We will show the results obtained for simulated event reconstructed in CMS detector, focusing on the estimation of efficiency and fake rejection performances of our CNN classifier.

Timing resolution studies of the optical part of the AFP Time-of-flight detector.

We present results of the timing performance studies of the optical part and front-end electronics of the time-of-flight subdetector prototype for the ATLAS Forward Proton (AFP) detector obtained during the test campaigns at the CERN-SPS test-beam facility (120 GeV π+ particles) in July 2016 and October 2016. The time-of-flight (ToF) detector in conjunction with a 3D silicon pixel tracker will tag and measure protons originating in central exclusive interactions p + p → p + X + p, where the two outgoing protons are scattered in the very forward directions. The ToF is required to reduce so-called pileup backgrounds that arise from multiple proton interactions in the same bunch crossing at high luminosity. The background can fake the signal of interest, and the extra rejection from the ToF allows the proton tagger to operate at the high luminosity required for the measurement of the processes. The prototype detector uses fused silica bars emitting Cherenkov radiation as a relativistic particle passes through them. The emitted Cherenkov photons are detected by a multi-anode micro-channel plate photomultiplier tube (MCP-PMT) and processed by fast electronics.

Geometry optimization of a barrel silicon pixelated tracker**Supported by National Natural Science Foundation of China (U1232202)

We have studied optimization of the design of a barrel-shaped pixelated tracker for given spatial boundaries. The optimization includes choice of number of layers and layer spacing. Focusing on tracking performance only, momentum resolution is chosen as the figure of merit. The layer spacing is studied based on Gluckstern’s method and a numerical geometry scan of all possible tracker layouts. A formula to give the optimal geometry for curvature measurement is derived in the case of negligible multiple scattering to deal with trajectories of very high momentum particles. The result is validated by a numerical scan method, which could also be implemented with any track fitting algorithm involving material effects, to search for the optimal layer spacing and to determine the total number of layers for the momentum range of interest under the same magnetic field. The geometry optimization of an inner silicon pixel tracker proposed for BESIII is also studied by using a numerical scan and these results are compared with Geant4-based simulations.

Status of the Mu3e detector

Mu3e is an experiment searching for charged lepton flavour violation in the decay μ+ → e+e−e+. Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on 50 μm thin High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) in a 1 T magnetic field provides precise vertex and momentum information. A scintillating fibre detector and a scintillating tile detector provide sub-nanosecond time information. The status of the Mu3e detector is presented, summarizing the development of HV-MAPS chips, the pixel detector modules as well as the timing detectors.

Construction of the optical part of a time-of-flight detector prototype for the AFP detector.

We present the construction of the optical part of the ToF (time-of-flight) subdetector prototype for the AFP (ATLAS Forward Proton) detector. The ToF detector in conjunction with a 3D silicon pixel tracker will tag and measure protons originating in central exclusive interactions p + p → p + X + p, where the two outgoing protons are scattered in the very forward directions. The ToF is required to reduce so-called pileup backgrounds that arise from multiple proton interactions in the same bunch crossing at high luminosity. The background can fake the signal of interest, and the extra rejection from the ToF allows the proton tagger to operate at the high luminosity required for measurement of the processes. The prototype detector uses fused silica bars emitting Cherenkov radiation as a relativistic particle passes through it. The emitted Cherenkov photons are detected by a micro-channel plate multi-anode Photomultiplier Tube (MCP-PMT) and processed by fast electronics.

The MuPix system-on-chip for the Mu3e experiment

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ+→e+e−e+. Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on 50μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is ⩾99% in a high rate test beam.

The ultra-lightweight support structure and gaseous helium cooling for the Mu3e silicon pixel tracker

The Mu3e experiment searches for charged lepton flavor violation in the rare decay μ→eee. In order to reach a sensitivity of better than 10−16, more than 109 muon decays per second have to be observed over a running time of one year. Precise determination of particle momentum, vertex position and time are necessary for background suppression. These requirements can be met by combining an ultra-lightweight tracker based on High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) with a timing system which consists of a scintillating fiber detector and a tile hodoscope. As the momentum of particles from muon decay at rest is below 53 MeV/c, the silicon pixel tracker resolution is dominated by multiple Coulomb scattering. This leads to extreme requirements for the material budget of the tracking detector of below 0.1% of a radiation length per layer. Even though the target power consumption of the HV-MAPS detector is as low as 150 mW/cm2, the detector cooling must be very efficient and at the same time avoid adding material inside the active tracking volume.

Heavily irradiated N-in-p thin planar pixel sensors with and without active edges

We present the results of the characterization of silicon pixel modules employing n-in-pplanar sensors with an active thickness of 150 μm, produced at MPP/HLL, and 100–200 μm thin active edge sensor devices, produced at VTT in Finland.These thin sensors are designed as candidates for the ATLAS pixel detector upgrade to be operated at the HL-LHC, as they ensure radiation hardness at high fluences. They are interconnected to theATLAS FE-I3 and FE-I4 read-out chips.Moreover, the n-in-p technology only requires a single side processing and thereby it is a cost-effectivealternative to the n-in-n pixel technology presently employed in the LHC experiments.High precision beam test measurements of the hit efficiency have been performed on thesedevices both at the CERN SpS and at DESY, Hamburg. We studied the behavior of thesesensors at different bias voltages and different beam incident angles up to the maximum oneexpected for the new Insertable B-Layer of ATLAS and for HL-LHC detectors.Results obtained with 150 μm thin sensors, assembled with the new ATLAS FE-I4 chip andirradiated up to a fluence of 4 × 1015 neq/cm2, show that they are excellent candidates for largerradii of the silicon pixel tracker in the upgrade of the ATLAS detector at HL-LHC. In addition, the active edge technologyof the VTT devices maximizes the active area of the sensorand reduces the material budget to suit the requirements for the innermost layers.The edge pixel performance of VTT modules has been investigated at beam test experiments andthe analysis after irradiation up to a fluence of 5 × 1015 neq/cm2 has been performed using radioactive sources in the laboratory.

Studies of azimuthal dihadron correlations in ultra-central PbPb collisions at $ \sqrt{{{s_{NN }}}} $ =2.76 TeV

Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at $ \sqrt{{{s_{NN }}}} $ = 2.76TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 μb − 1. The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v 2 to v 6, are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T-averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-by-event fluctuating initial conditions is also presented.

Radiation hardness studies of cooling fluids epoxies and capacitors for CMS pixel system

Radiation hardness studies of C 6F 14 Fluorinert (FC72) fluid, some epoxies and tantalum capacitors were studied for the Compact Muon Solenoidal (CMS) experiment at the Large Hadron Collider (LHC) of CERN. For a successful operation of Forward Silicon Pixel Tracker of the CMS, the cooling fluid, epoxies to be used and the filter capacitors of bias voltage supplies need to be radiation hard. It was also necessary to find out whether the cooling fluid would dissociate under the expected fluence and damage beryllium cooling channels of the Forward Silicon Pixel Discs. This paper is a report of our findings from the performed tests.

Silicon pixel detectors for tracking in NA57*1

In this paper, after recalling the main features of the NA57 experiment, we describe its silicon pixel tracker.

Silicon pixel detectors for tracking in NA57

In this paper, after recalling the main features of the NA57 experiment, we describe its silicon pixel tracker.