Impact Parameter Resolution Research Articles

First results of the upgraded ALICE Inner Tracking System in LHC Run 3

Major upgrades of the ALICE experiment at CERN were completed during the LHC Long Shutdown 2 (2019–2021). The ALICE detector is currently taking data and has been doing so from the start of the third period of operation of the LHC (Run 3) on July 5th, 2022. One key part of these upgrades is the new Inner Tracking System (ITS2), a full silicon-pixel vertexing and tracking detector constructed entirely with CMOS monolithic active pixel sensors (ALPIDE). The ITS2 consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering 10 m2 and containing 12.5 billion pixels with a pixel pitch of 27 μ m × 29 μm. It offers a significant improvement in impact-parameter resolution and tracking efficiency, thanks to the increased granularity, the very low material budget (0.35% X0 /layer in the inner barrel) as well as a smaller beam pipe radius.The ITS2 was successfully installed in the ALICE experiment in May 2021, followed by a period of comprehensive on-site commissioning, before starting data taking in July 2022. In this paper, the detector construction and commissioning will be introduced briefly. The performance results from the first phase of proton-proton collisions recorded to date in LHC Run 3 will be discussed in detail, which include detector calibration, long-term evolution of the ALPIDE sensor threshold and noise, and the first measurement of the impact parameter.

Study of MAPS silicon detector prototypes for the ALICE Inner Tracking System upgrade

The ALICE experiment, which is located at the Large Hadron Collider (LHC) at CERN, has planned an upgrade of the Inner Tracking System (ITS), called ITS3, which will be installed during the LHC Long Shutdown 3, in 2026–2028. The cornerstone of the upgrade is a new 65 nm CMOS pixel chip, produced using Monolithic Active Pixel Sensor (MAPS) technology and the stitching technology to extend the chip length to 26 cm. The produced chips will be bent and they will replace the three innermost layers of the existing detector with new ultra-light, truly cylindrical layers. The ITS3 will improve tracking performance, especially at low transverse momentum, thanks to better track impact-parameter resolution, improved by a factor two with respect to the present ITS in LHC Run 3. In addition, the detector will be closer to the interaction point and will have a much lower material budget, of 0.05% X0/layer. This proceedings paper presents the final configuration and structure of the ITS3, the challenges related to its design and construction, and the results of the current R&D program on the sensor design and characterization. Finally, it also reports the results of beam tests preformed at CERN, where an analog pixel test chip (APTS Source Follower) was investigated in terms of detection efficiency.

Developments of stitched monolithic pixel sensors towards the ALICE ITS3

The ALICE collaboration is pursuing the development of a novel and considerably improved vertexing detector called ITS3, to replace the three innermost layers of the Inner Tracker System during the LHC Long Shutdown 3. The primary goals are to reduce the material budget to the unprecedented value of 0.05% X0 per layer, and to place the first layer at a radial distance of 18 mm from the interaction point. These features will boost the impact parameter resolution by a factor two over all momenta and drastically enhance the tracking efficiency at low transverse momentum.The new detector will consist of true cylindrical layers. Each half-cylinder is based on curved wafer-scale monolithic pixel sensors. The bending radii are 18, 24 and 30 mm, and the length of the sensors in the beam direction is 27 cm. The sensors will be produced using a commercial 65 nm CMOS Imaging technology and a recent technique called stitching. This allows to manufacture chips reaching the dimensions of 27 cm × 9 cm on silicon wafers of 300 mm diameter. The sensors will be thinned down to 50 µm or below. The ITS3 concept foresees cooling by air flow, ultra-light carbon foam support elements and no flexible printed circuits in the active area. This demands a power density limit of 20 mW/cm2 for the sensor, and the need to distribute supply and transfer data over the entire sensors towards circuits located at the short edges of the chip.This contribution summarises the status of the microelectronic developments and presents selected results from the characterisation of the first prototype chips. Furthermore, it describes the ongoing efforts on the design of a first wafer-scale stitched sensor prototype, the MOSS (Monolithic Stitched Sensor) chip.

CMS Phase-2 Inner Tracker Upgrade

The HL-LHC conditions of instantaneous peak luminosity up to 7.5x1034 cm-2s-1 and an integrated luminosity of the order of 300 fb-1/year is expected to result in 1 MeV neutron equivalent fluence of 2.3 x 1016 neq/cm2 and a total ionizing dose (TID) of 12 MGy (1.2 Grad) at the center of the CMS Experiment, where its innermost component, the Phase-2 Pixel Detector will be installed. This detector has to survive the above radiation dose, handle hit rates of 3.2 GHz/cm2 in the layers closest to the beam line, be able to track and separate particles in extremely dense collisions, deal with a pileup of 140-200 collisions per bunch crossing and have a high impact parameter resolution. This translates into a highly granular detector design with thinner sensors and smaller pixels, and a faster and more radiation hard electronics compared to the Phase-1 detector. This contribution reviews the Phase-2 upgrade of the (silicon-based) CMS Inner Tracker focusing on the features of the detector layout and on developments of pixelated devices.

Upgrade of the ALICE experiment beyond LHC Run 3

The ALICE Collaboration completed the upgrade of the detector and is now commissioning for the beginning of the data taking during LHC Run 3. In parallel, R&D activities and simulation studies are being performed to define the future of the experiment beyond LHC Run 3. Two detector upgrades are foreseen for the next long shutdown (LS3). The first is the replacement of the three layers of the inner tracking system closest to the beam with a novel vertex detector consisting of curved wafer-scale ultra-thin silicon sensors arranged in perfectly cylindrical layers to improve impact parameter resolution and significantly extend the physics capability for the study of the heavy-flavor production and the low-mass dielectrons. The second upgrade for the LS3 is the addition of a Forward Calorimeter detector at large rapidity consisting of a Si-W electromagnetic calorimeter with pad and pixel readout, that will equip the experiment with unique capabilities to measure small-x gluon distributions via prompt photon production. A proposal of a next-generation heavy-ion experiment for LHC Run 5 is also in preparation and will be discussed. The aim is to perform novel measurements of the electromagnetic and hadronic probes of the QGP, such as the production of multiply-charmed baryons, which have so far been inaccessible, both because of detector performance and luminosity. The concept of the new apparatus foresees an extensive usage of thin silicon sensors for tracking and a modern particle identification system, combining a silicon-based time of flight detector, a RICH detector, an electromagnetic calorimeter and a muon system.

Performance of the ALICE upgraded inner tracking system

Major upgrades of the ALICE (A Large Ion Collider Experiment) detector are underway and will be completed during the LHC long shutdown 2 in order to start operation in 2022 for LHC run 3. One key part of this upgrade is the new Inner Tracking System (ITS2), a full silicon-pixel detector constructed entirely with CMOS monolithic active pixel sensors. The upgraded ITS2 detector consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering 10 m2 and containing 12.5 billion pixels with a pixel pitch of 27 μm × 29 μm. Compared with the silicon tracking system used during the LHC run 1 and run 2, the increased granularity, the very low material budget (0.35% X 0/layer in the inner barrel) as well as a smaller beam pipe radius, will result in a significant improvement of impact-parameter resolution and tracking efficiency. The assembly of the detector and services finished in December 2019, and the detector was fully installed in the ALICE experiment in May 2021. A comprehensive commissioning phase (on the surface) was completed in December 2020 to validate the detector performance. In this paper, an overview of the design and construction, as well as the performance of the ITS2 studied from the on-surface commissioning will be discussed in detail.

Tracking and vertex detectors at FCC-ee

The combined vertexing and tracking performance of the innermost part of the FCC-ee experiments must deliver outstanding precision for measurement of the track momentum together with an impact parameter resolution exceeding by at least a factor five that typically achieved at LHC experiments. Furthermore, precision measurements require stability and fiducial accuracy at a level which is unprecedented in collider experiments. For the innermost vertex layers these goals translate into a target hit resolution of approximately 3 mu hbox {m} together with a material budget of around 0.2% of a radiation length per layer. Typically this performance might be provided by silicon-based tracking, together with a careful choice of a low-mass cooling technology, and a stable, low-mass mechanical structure capable of providing measurements with a low enough systematic error to match the tremendous statistics expected, particularly for the run around the Z resonance. At FCC-ee, the magnetic field will be limited to approximately 2 T, in order to contain the vertical emittance at the Z pole, and a tracking volume up to relative large radius is needed. The technological solution could be silicon- or gaseous-based tracking, in both cases with the focus on optimising the material budget, and particle identification capability would be an advantage. Depending on the global design, an additional silicon tracking layer could be added at the outer radius of the tracker to provide a final precise point contributing to the momentum or possibly time-of-flight measurement. Current developments in monolithic and hybrid silicon technology, as well as advanced gaseous tracking developments, provide an encouraging road map towards the FCC-ee detector. The current state of the art and potential extensions will be discussed and a generic call for technology which could have a significant impact on the performance of an FCC-ee tracking and vertexing detector is outlined.

Charm and multi-charm baryon measurements via strangeness tracking with the upgraded ALICE detector

We present a new method for detection of multiply charmed baryons via their decays into strange baryons, using ‘strangeness tracking’. This method makes use of the state-of-the-art upgraded silicon detectors in ALICE during Runs 3, 4 and beyond will enable the novel possibility of tracking strange hadrons directly before they decay, leading to a very significant improvement in impactparameter resolution. In this work, we will discuss how this new technique will be crucial to distinguish secondary strange baryons originating from charm decays from primary strange baryons. This is a particularly interesting possibility for the Ω− baryon coming from Ωc0 → Ω−π+ decays, since there is no other relevant feeddown source for Ω−. This, in turn, means that the main Ω− background for the Ωc measurement will point most accurately to the primary vertex, unlike pions or protons from other charm baryon decays. We will illustrate the achievable performance of strangeness tracking for the Run 3 configuration of ALICE with the upgraded Inner Tracking System, which is fully instrumented with silicon pixel detectors. Moreover, we will discuss the potential of this technique in a future experiment with an extensive silicon tracking detector with a first layer very close to the interaction point. Finally, we will also cover other potential major applications of strangeness tracking, including measurements of hypernuclei such as the Λ3H.

A High-Granularity Timing Detector for the Phase-II upgrade of the ATLAS Calorimeter system: detector concept, description and R&D and beam test results

The particle flux increase (pile-up) at the HL-LHC with luminosities of L = 7.5 × 1034 cm−2 s−1 will have a significant impact on the reconstruction of the ATLAS detector and on the performance of the trigger. The forward region and the end-cap where the internal tracker has poorer longitudinal track impact parameter resolution, and where the liquid argon calorimeter has coarser granularity, will be significantly affected. A High Granularity Time Detector (HGTD) is proposed to be installed in front of the LAr end-cap calorimeter for the mitigation of the pileup effect, as well as measurement of luminosity. It will have coverage of 2.4 to 4.0 from the pseudo-rapidity range. Two dual-sided silicon sensor layers will provide accurate timing information for minimum-ionizing particles with a resolution better than 30 ps per track (before irradiation), for assigning each particle to the correct vertex. The readout cells are about 1.3 mm × 1.3 mm in size, which leads to a high granular detector with 3 million channels. The technology of low-gain avalanche detectors (LGAD) with sufficient gain was chosen to achieve the required high signal-to-noise ratio. A dedicated ASIC is under development with some prototypes already submitted and evaluated. The requirements and general specifications of the HGTD will be maintained and discussed. R&D campaigns on the LGAD are carried out to study the sensors, the related ASICs and the radiation hardness. Both laboratory and test beam results will be presented.

Upgrading the Inner Tracking System and the Time Projection Chamber of ALICE

The ALICE experiment at CERN is undergoing a major upgrade during the Long Shutdown 2 (LS2) of the LHC during 2019-2020. The key elements regarding the central barrel are the installation of a new Inner Tracking System (ITS) and the upgrade of the large Time Projection Chamber (TPC). The TPC is currently being upgraded with a new readout system, including new GEM-based Readout Chambers and new front-end electronics enabling us to operate it in continuous mode. The new ITS based on CMOS Monolithic Active Pixel Sensors will significantly improve the impact parameter resolution and the tracking efficiency, especially for particles with low transverse momenta, as well as the readout-rate capability. In the following, the upgrades and their assembly and commissioning status will be outlined.Furthermore, an outlook will be given on plans for the upgrade of the ITS foreseen for the third long shutdown of the LHC in 2025-2026. For this upgrade, the three innermost layers could be replaced by cylindrical detector layers, made of curved wafer-scale-sized CMOS sensors, to lower even further the material budget and to improve the pointing resolution.

Multiparameter seismic elastic full-waveform inversion with combined geophone and shaped fiber-optic cable data

Distributed acoustic sensing (DAS) is a rapidly growing technology for seismic acquisition, with the potential to sample rarely available wavefield components in reservoir settings. How to best use the information that DAS supplies to estimate reservoir properties is an open question. Full-waveform inversion (FWI) of DAS data, alone or in combination with geophone data, is a natural possibility to pursue. A mixed formulation must accommodate particle velocity data and 1C measurements of strain or strain rate in the direction tangent to a fiber-optic cable, which itself may take on some characteristic shape. Expecting that these amplitude and directionality properties of DAS data will impact parameter resolution in FWI, especially when incorporating finite gauge lengths, we have developed two appraisal methods. The first is an analytic description of the relationship between the spatial period and the elastic-wave sensitivity within a helical-wound fiber (which builds on a symmetry class of fibers insensitive to shear strains). The second is an extension of scattering radiation pattern analysis to DAS sensors of arbitrary geometry. We then numerically analyze the FWI response. Using 2D simulations and several simple models including the Marmousi2, we analyze the effect that shaping of the DAS fiber has on parameter estimations, by comparing inversion results derived from straight and various coiled fibers in a horizontal well. Fiber geometry is observed to have important implications for the accuracy and fidelity of DAS-FWI parameter estimates. It is also clear that the complementary features of DAS and standard geophone data impact FWI. Simultaneous inversions of surface geophone and DAS data from horizontal wells convincingly outperform inversions from either data set alone.

Heavy-flavour Measurements in Pb–Pb Collisions with the Upgraded ALICE Inner Tracking System

During the second LHC long shutdown (LS2) the Inner Tracking System (ITS) of ALICE (A Large Ion Collider Experiment) will be replaced by seven layers of CMOS Monolithic Active Pixel Sensors (MAPS). The latest innovations in silicon imaging technology allow for the construction of large, ultra-thin silicon wafers which can further improve the capabilities of the ALICE tracker. The research and development studies towards the construction of a novel vertex detector have started. The detector installation has been proposed for the third LHC long shutdown (LS3) during which the three innermost layers shall be replaced by three cylindrical layers of large curved CMOS wafers. This upgrade (ITS3) will further improve the impact parameter resolution and the tracking efficiency of low momentum particles. The innermost layer will be positioned closer to the interaction point and the material budget will be reduced down to 0.05$\%X_0$ per layer. Monte Carlo simulations of a simplified ITS3 geometry within the ITS2 design indicate an improvement in the impact parameter resolution and the tracking efficiency, which are of crucial importance for measurements of heavy-flavour hadrons. This contribution shows the improved performance for the example of the $\Lambda_{\mathrm{b}}$, for which the significance of its measurement is extracted based on these MC simulations. A significant improvement by almost a factor of three in the low momentum region compared to the ITS2 is observed.

Commissioning of the upgraded ALICE Inner Tracking System

The ALICE Inner Tracking System (ITS) detector is undergoing a major upgrade in order to cope with the increased data rates and to meet the requirements as set out by the physics goals of the experiment after the LHC Long Shutdown 2 (LS2). The new ITS will be completely made up of monolithic active pixel sensors (MAPS), called ALPIDE, based on the CMOS 180 nm process. A single sensor measures 30 mm × 15 mm and contains half a million pixels, with a pitch of 27μm×29μm, distributed over 512 rows and 1024 columns. The ALPIDE sensors, thinned to 50μm and 100μm for the Inner and Outer Barrels, respectively, are mounted on ultra-lightweight carbon composite support structures with an embedded cooling system. This results in a considerable reduction of the material budget and a significant improvement of the impact parameter resolution and tracking efficiency for low-transverse momentum particles. The construction of the ITS detector has been completed and the commissioning on surface is ongoing. The detector will be installed in ALICE in 2020. This paper will give a brief overview of the motivation for the upgrade and will present the first results of the detector performance obtained during the commissioning.

Upgrade of the ALICE inner tracking system: Construction and commissioning

ALICE (A Large Ion Collider Experiment) is the CERN LHC experiment optimized for the study of the strongly interacting matter produced in heavy-ion collisions, in particular the characterization of the quark-gluon plasma. After the successful operation of the experiment during the first two runs of the LHC, the ALICE collaboration is currently working on a major upgrade of its detectors, to be installed during the 2019–2020 Long Shutdown (LS2). The main goal is to increase the readout capabilities and allow recording of Pb–Pb minimum bias events at rates in excess of 50 kHz, which is the expected Pb–Pb interaction rate at the LHC after LS2. One key part of the upgrade is the construction of a new Inner Tracking System (called ITS2) that will significantly improve the impact parameter resolution of the tracks, tracking efficiency, and readout capacity which enables precise measurements of low momentum charged particles. The ITS2 consists of seven approximately-cylindrical detector layers of CMOS Monolithic Active Pixel Sensors (MAPS) with the sensor matrix and readout integrated on a single chip, named ALPIDE (ALice PIxel DEtector), covering an area of 10 m2 and containing about 12.5 billion pixels. All layers are mounted on ultra-lightweight carbon support structures with an embedded cooling system. This allows a reduction of the material budgets down to about 0.3% X0 for the inner layers and 1% X0 for the outer layers, with respect to the previous ITS system. After a brief overview of the upgrade motivation, details of the overall layout will be given as well as the construction and commissioning status and plans.

Reconstruction of D0 meson in d+Au collisions at = 200 GeV by the STAR experiment

Owing to their large masses, charm quarks are predominantly produced through initial hard scatterings in heavy-ion collisions. Therefore, they can serve as penetrating probes to study the intrinsic properties of the hot medium created in heavy-ion collisions. However, Cold Nuclear Matter effects can also affect the charm quark production in nuclear collisions with respect to p + p collisions. These effects can be measured in small collision systems such as d/p+Au. In these proceedings, D0 meson reconstruction in d+Au collisions at at the STAR experiment is described. Thanks to the excellent impact parameter resolution provided by the Heavy Flavor Tracker detector, D0() mesons are topologically reconstructed from their hadronic decay channel D0 () (). The Boosted Decision Trees machine learning algorithm from the TMVA package is applied in order to improve signal/background separation.

Belle II pixel detector: Performance of final DEPFET modules

A DEpleted P-channel Field Effect Transistor (DEPFET) based pixel detector was developed for the Belle II VerteX Detector (VXD). It is designed to achieve a good impact parameter resolution better than 15μm at the very high luminosity conditions of this experiment. In the first half of 2018 four final production modules have been deployed in the commissioning run of the detector and their performance is discussed.

The upgrade of the ALICE Inner Tracking System

The Inner Tracking System (ITS) of the ALICE experiment will be upgraded during the second long LHC shutdown in 2019–2020. The main goal of the ALICE ITS upgrade is to enable high precision measurements of low-momentum particles (<1 GeV/c) by acquiring a large sample of events, benefiting from the increase of the LHC instantaneous luminosity of Pb-Pb collisions to ℒ = 6 ⋅ 1027 cm−2 s−1 during Run 3. Working in this direction the ITS upgrade project is focusing on the increase of the readout rate, on the improvement of the impact parameter resolution, as well as on the improvement of the tracking efficiency and the position resolution. The major setup modification is the substitution of the current ITS with seven layers of silicon pixel detectors. The ALPIDE chip, a CMOS Monolithic Active Pixel Sensor (MAPS), was developed for this purpose and offers a spatial resolution of 5 μm. The use of MAPS together with a stringent mechanical design allows for the reduction of the material budget down to 0.35% X0 for the innermost layers and 1% X0 for the outer layers. The detector design was validated during the research and development period through a variety of tests ensuring the proper operation for the full lifetime inside ALICE. The production phase is close to completion with all the new assembled components undergoing different tests that aim to characterize the modules and staves and determine their qualification level. This contribution describes the detector design, the measurements performed during the research and development phase, as well as the production status.

Performance and operation of the CMS Phase 1 pixel detector

The silicon pixel detector of the CMS experiment at the LHC provides high precision tracking in the region closest to the proton–proton collision point. It operates in a particularly harsh environment characterized by high track multiplicity and heavy irradiation which poses severe challenges to the detector design. At the beginning of 2017, the original pixel detector was replaced with an upgraded 4-layer pixel system to maintain the excellent tracking performance of CMS in view of the increasing luminosity scenario (up to 2 . 5 × 1 0 34 cm − 2 s − 1 during Run 2 and 3). The CMS Phase 1 detector features a new readout chip to reduce the dynamic inefficiency at high rate as well as an additional innermost layer at a radius of 3 cm from the interaction point to improve the impact parameter resolution. This paper reviews the main design features of the CMS Phase 1 pixel detector and discuss its performance and the challenges in operation during the first year of data-taking in 2017. Furthermore, the paper reports about the consolidation work performed during the LHC year end technical stop and the status of the Phase 1 pixel detector and re-commissioning toward operation in 2018. • The CMS Phase 1 Pixel detector delivered high quality data during 2017. • Significant improvement of hit rate tolerance and hit efficiency. • Discussion of DCDC converter failures as understanding is key for future detectors.

An extension of the Gluckstern formulae for multiple scattering: Analytic expressions for track parameter resolution using optimum weights

Momentum, track angle and impact parameter resolution are key performance parameters that tracking detectors are optimised for. This report presents analytic expressions for the resolution of these parameters for equal and equidistant tracking layers. The expressions for the contribution from position resolution are based on the Gluckstern formulae and are well established. The expressions for the contribution from multiple scattering using optimum weights are discussed in detail.

The new Inner Tracking System of the ALICE experiment

The ALICE experiment will undergo a major upgrade during the next LHC Long Shutdown scheduled in 2019–20 that will enable a detailed study of the properties of the QGP, exploiting the increased Pb-Pb luminosity expected during Run 3 and Run 4.The replacement of the existing Inner Tracking System with a completely new ultra-light, high-resolution detector is one of the cornerstones within this upgrade program. The main motivation of the ITS upgrade is to provide ALICE with an improved tracking capability and impact parameter resolution at very low transverse momentum, as well as to enable a substantial increase of the readout rate.The new ITS will consist of seven layers of innovative Monolithic Active Pixel Sensors with the innermost layer sitting at only 23 mm from the interaction point. This talk will focus on the design and the physics performance of the new ITS, as well as the technology choices adopted. The status of the project and the results from the prototypes characterization will also be presented.