LHCb Vertex Locator Research Articles

The LHCb VELO Upgrade II: design and development of the readout electronics

The LHCb collaboration proposes a Phase-II Upgrade of the detector, to be installed during the LHC Long Shutdown 4 (2032–2034). Operating in the HL-LHC environment poses significant challenges to the design of the upgraded detector, and in particular to its tracking system. The primary and secondary vertices reconstruction will become more difficult due to the increase, by a factor of 7.5, of the average number of interactions per bunch crossing. The performance of the VErtex LOcator (VELO), which is the tracking detector surrounding the interaction region, is essential to the success of this Phase-II Upgrade. Data rates are especially critical for the LHCb full software trigger, and with the expected higher particle flux, the VELO Upgrade-II detector will have to tolerate a dramatically increased data rate: assuming the same hybrid pixel design and detector geometry, the front-end electronics (ASICs) of the VELO Upgrade-II will have to cope with rates as high as 8 Ghits/s, with the hottest pixels reaching up to 500 khits/s. With this input rate, the data output from the VELO will exceed 30 Tbit/s, with potentially a further increase if more information is added to the read-out. This paper outlines the challenges being addressed and the solutions under investigation for reading out the VELO sub-detector.

Measurement of thermal properties of the LHCb VELO detector using track-based software alignment

The thermal properties of the LHCb Vertex Locator (VELO) are studied using the real-time detector alignment procedure. The variation of the position and orientation of the detector elements as a function of the operating temperature of the VELO is presented. This study uses a dataset collected by the LHCb experiment during a VELO temperature scan performed at the end of LHC Run 2 (October 2018). Significant shrinkage of the VELO modules is observed at the operating temperature of -30°C compared to the laboratory measurements on a single module taken at a range of temperatures from +45°C to -25°C. The thermal shrinkage expected from the extrapolation of laboratory measurements to lower temperatures, and the results of this alignment study are in good agreement.

Readout Firmware of the Vertex Locator for LHCb Run 3 and Beyond

The new LHCb Vertex Locator (VELO) for LHCb, comprising a new pixel detector and readout electronics, will be installed in 2021 for data taking in Run 3 at the LHC. The electronics centers around the “VeloPix” ASIC at the front-end operating in a trigger-less readout at 40MHz. A custom serializer, called gigabit wireline transmitter (GWT), and associated custom protocol have been designed for the VeloPix. The GWT data are sent from the serializers of the VeloPix at a line rate of 5.12 Gb/s, reaching a total data rate of 2–3 Tb/s for the full VELO detector. Data are sent over 300-m optic-fiber links to the control and readout electronics cards for deserialization and processing in Intel Arria 10 FPGAs. Because of the VeloPix trigger-less design, latency variances up to 12 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> can occur between adjacent datagrams. It is therefore essential to buffer and synchronize the data in firmware prior to onward propagation or suffer a huge CPU-processing penalty. This article will describe the architecture of the readout firmware in detail with focus given to the resynchronization mechanism and techniques for cauterization. Issues found during readout commissioning, and scaling resource utilization, along with the their solutions, will be illustrated. The latest results of the firmware data-processing chain can be presented as well as the verification procedures employed in simulation. Challenges for the next generation of the detector will also be presented with ideas for a readout processing solution.

Charge collection properties of prototype sensors for the LHCb VELO upgrade

An extensive sensor testing campaign is presented, dedicated to measuring the charge collection properties of prototype candidates for the Vertex Locator (VELO) detector for the upgraded LHCb experiment. The charge collection is measured with sensors exposed to fluences of up to 8×1015 1 MeV neq cm-2, as well as with nonirradiated prototypes. The results are discussed, including the influence of different levels of irradiation and bias voltage on the charge collection properties. Charge multiplication is observed on some sensors that were nonuniformly irradiated with 24 GeV protons, to the highest fluence levels. An analysis of the charge collection near the guard ring region is also presented, revealing significant differences between the sensor prototypes. All tested sensor variants succeed in collecting the minimum required charge of 6000 electrons after the exposure to the maximum fluence.

Microfabricated silicon substrates for pixel detectors assembly and thermal management a.k.a. Silicon Microchannel Cooling Plates

At CERN, the Detector Technologies (DT) group of the Experimental Physics (EP) department is actively investigating a number of innovative solutions for heat management and detector module assembly in HEP experiments. Among these, recent research carried out at EP-DT has focused on the development of microfluidic devices to cool silicon pixel detectors. In this respect, continuous advances in microengineering have opened the door to smaller and more efficient cooling devices capable of handling increasing power densities with a minimum mass penalty. In 2014, the NA62 experiment has pioneered the use of a silicon microfluidic system with single-phase liquid C6F14 for the thermal management of its GigaTracKer (GTK) pixel detectors. Following the NA62 success, LHCb became the first LHC experiment to adopt a similar solution. The future upgrade of the LHCb Vertex Locator (VELO) will combine multiple silicon plates with embedded microchannels with an evaporative CO2 system to cool fifty-two pixel detector modules dissipating a total of about 2 kW. This paper will present the implementation of this novel approach for the construction and thermal management of the NA62 GTK and LHCb VELO pixel detectors. Future developments such as 3D-printed microfluidics and microchannels embedded on monolithic pixel detectors will also be discussed.

Deep Learning Approach to Track Reconstruction in the upgraded VELO

The LHCb experiment will undergo a major upgrade for LHC Run III, scheduled to start taking data in 2021. In order to exploit the full physics potential of the upgraded detector, LHCb will employ a high level event filter entirely implemented in software. The event filter has to operate in real time at the 40 MHz LHC bunch crossing rate. The LHCb collaboration is currently exploring a variety of new new computing paradigms in order to cope with the challenges posed by the high data taking rates. One contribution to this effort is the application of Machine Learning (ML) techniques to particle track reconstruction. We explore an ML approach to the track reconstruction in the upgraded LHCb Vertex Locator (VELO). The reconstruction algorithm is evaluated with fully simulated Minimum-Bias data that reflects the input to the real time event filter. We have achieved a reasonable track reconstruction efficiency and low ghost rate with our first approach and are currently exploring several methods to further quality of the track reconstruction.

Development of the LHCb VELO Detector Modules into a Standalone, Non-Invasive Online Beam Monitor for Medical Accelerators

Knowledge of the beam properties in proton therapy through beam monitoring is essential, ensuring an effective dose delivery to the patient. In clinical practice, currently used interceptive ionisation chambers require daily calibration and suffer from a slow response time. A new non-invasive method for dose online monitoring is under development based on the silicon multi-strip sensor LHCb VELO (VErtex LOcator), originally used for the LHCb experiment at CERN. The proposed method relies on proton beam halo measurements. Several changes in the system setup were necessary to operate the VELO module as a standalone system outside of the LHC environment and are described in this paper. A new cooling, venting and positioning system was designed. Several hardware and software changes realised a synchronised readout with a locally constructed Faraday Cup and the RF frequency of a medical cyclotron with quasi-online monitoring. The adapted VELO module will be integrated at the 60 MeV proton therapy beamline at the Clatterbridge Cancer Centre (CCC), UK and the capability as a beam monitor will be assessed by measuring the beam current and by monitoring the beam profile along the beamline in spring 2019.

Mapping the material in the LHCb vertex locator using secondary hadronic interactions

Precise knowledge of the location of the material in the LHCb vertex locator (VELO) is essential to reducing background in searches for long-lived exotic particles, and in identifying jets that originate from beauty and charm quarks. Secondary interactions of hadrons produced in beam-gas collisions are used to map the location of material in the VELO. Using this material map, along with properties of a reconstructed secondary vertex and its constituent tracks, a p-value can be assigned to the hypothesis that the secondary vertex originates from a material interaction. A validation of this procedure is presented using photon conversions to dimuons.

Automatised data quality monitoring of the LHCb Vertex Locator

The LHCb Vertex Locator (VELO) is a silicon strip semiconductor detector operating at just 8mm distance to the LHC beams. Its 172,000 strips are read at a frequency of 1.1 MHz and processed by off-detector FPGAs followed by a PC cluster that reduces the event rate to about 10 kHz. During the second run of the LHC, which lasts from 2015 until 2018, the detector performance will undergo continued change due to radiation damage effects. This necessitates a detailed monitoring of the data quality to avoid adverse effects on the physics analysis performance. The VELO monitoring infrastructure has been re-designed compared to the first run of the LHC when it was based on manual checks. The new system is based around an automatic analysis framework, which monitors the performance of new data as well as long-term trends and using dedicated algorithms flags issues whenever they arise. The new analysis framework then analyses the plots that are produced by these algorithms. One of its tasks is to perform custom comparisons between the newly processed data and that from reference runs. The most-likely scenario in which this analysis would identify an issue is the parameters of the readout electronics no longer being optimal and requiring retuning. The data of the monitoring plots can be reduced further, e.g. by evaluating averages, and these quantities are input to long-term trending. This is used to detect slow variation of quantities, which are not detectable by the comparison of two nearby runs. Such gradual change is what is expected due to radiation damage effects. It is essential to detect these changes early such that measures can be taken, e.g. adjustments of the operating voltage, to prevent any impact on the quality of high-level quantities and thus on physics analyses. The plots as well as the analysis results and trends are made available through graphical user interfaces (GUIs). These GUIs are dynamically configured by a single configuration that determines the choice and arrangement of plots and trends and ensures a common look and feel.

Numerical optimization for Artificial Retina Algorithm

High-energy physics experiments rely on reconstruction of the trajectories of particles produced at the interaction point. This is a challenging task, especially in the high track multiplicity environment generated by p-p collisions at the LHC energies. A typical event includes hundreds of signal examples (interesting decays) and a significant amount of noise (uninteresting examples).This work describes a modification of the Artificial Retina algorithm for fast track finding: numerical optimization methods were adopted for fast local track search. This approach allows for considerable reduction of the total computational time per event. Test results on simplified simulated model of LHCb VELO (VErtex LOcator) detector are presented. Also this approach is well-suited for implementation of paralleled computations as GPGPU which look very attractive in the context of upcoming detector upgrades.

The VeloPix ASIC

VeloPix, a 130 nm CMOS technology chip with data driven and zero suppressed readout, will be used as a readout chip for the hybrid pixel system of the LHCb Vertex Locator (VELO) upgrade. The upgrade, scheduled for LHC Run-3, will enable the experiment to be read out at 40 MHz in trigger-less mode, with event selection being performed in the CPU farm. The highest occupancy ASICs will experience rates of more than 900 Mhits/s, and the closest pixels are 5.1 mm from the LHC beams. This paper will present the VeloPix ASIC along with the first test results without a sensor.

Imaging the LHC beams with silicon and scintillating fibre vertex detectors

The LHCb Vertex Locator (VELO) is used to reconstruct beam–gas interaction vertices which allows one to obtain precise profiles of the LHC beams. In LHCb, this information is combined with the profile of the reconstructed beam–beam collisions and with the LHC beam currents to perform precise measurements of the luminosity. This beam–gas imaging (BGI) method also allows one to study the transverse beam shapes, beam positions and angles in real time. Therefore, a demonstrator beam–gas vertex detector (BGV) based on scintillating fibre modules has been built and installed in LHC Ring 2 at point 4.

Developments towards the LHCb VELO upgrade

The Vertex Locator (VELO) is a silicon strip detector surrounding the interaction region of the LHCb experiment. The upgrade of the VELO is planned to be installed in 2019–2020, and the current detector will be replaced by a hybrid pixel system equipped with electronics capable of reading out at a rate of 40MHz. The new detector is designed to withstand the radiation dose expected at an integrated luminosity of 50fb−1. The detector will be composed of silicon pixel sensors, read out by the VeloPix ASIC that is being developed based on the TimePix/MediPix family. The prototype sensors for the VELO upgrade are being irradiated in five different facilities and the post-irradiation performance is being measured with testbeams, and in the lab. These proceedings present the VELO upgrade and briefly discuss the results of the sensor testing campaign.

Study of the Radiation Induced Effects in the LHCb Vertex Locator

LHCb is a dedicated heavy-flavour physics experiment at the Large Hadron Collider at CERN. The VErtex LOcator (VELO) is a critical part of a LHCb tracking system, enabling the full topological reconstruction of beauty and charm mesons’ decays and providing essential input for the High Level Trigger (HLT) system used by the experiment to select events. The VELO comprises 42 modules made of two n-on-n 300 μm thick halfdisc silicon sensors with Rand Φ-measuring micro-strips, arranged in two retractable halves, operating only about 8 mm from the proton beams. In these paper, selected aspects of the VELO performance during the Run 1 data-taking period is shortly summarised along with the radiation damage studies. The track finding efficiency is typically greater than 98%. An impact parameter resolution of less than 35 μm is achieved for particles with transverse momentum greater than 1 GeV/c. An overview of all important performance parameters will be given. The VELO sensors have received a large and non-uniform radiation dose of up to 1.2 × 10 1 MeV neutron equivalent cm−2. Silicon-type inversion has been observed in regions close to the interaction point.

LHCb Vertex Locator: Performance and radiation damage in LHC Run 1 and preparation for Run 2

LHCb is a dedicated experiment to study New Physics in the decays of heavy hadrons at the Large Hadron Collider (LHC) at CERN. Heavy hadrons are identified through their flight distance in the Vertex Locator (VELO). The VELO comprises 42 modules made of two n+-on-n 300μm thick half-disc silicon sensors with R- and Φ-measuring micro-strips. In order to allow retracting the detector, the VELO is installed as two movable halves containing 21 modules each. The detectors are operated in a secondary vacuum and are cooled by a bi-phase CO2 cooling system. During data taking in LHC Run 1 the LHCb VELO has operated with an extremely high efficiency and excellent performance. The track finding efficiency is typically greater than 98%. An impact parameter resolution of less than 35μm is achieved for particles with transverse momentum greater than 1GeV/c. An overview of all important performance parameters will be given. The VELO sensors have received a large and non-uniform radiation dose of up to 1.2×10141MeV neutron equivalent cm−2 during the first LHC run. Silicon type-inversion has been observed in regions close to the interaction point. The preparations for LHC Run 2 are well under way and the VELO has already recorded tracks from injection line tests. The current status and plans for new operational procedures addressing the non-uniform radiation damage are shortly discussed.

Non-linear centre of gravity on LHCb Vertex Locator test beam data

Cluster position reconstruction for the LHCb Vertex Locator test beam software currently uses a linear centre of gravity algorithm. To investigate possible improvements of this approach, a study was performed that made use of a non-linear centre-of-gravity algorithm. All of the sensors in this study were Timepix chips with a 300 μ m layer of p-on-n silicon. The position resolutions obtained with the linear centre-of-gravity ranged between 5.5 μ m and 6.1 μ m. Applying the tuned non-linear algorithm reduces these by at least 0.6 μ m.

The upgrade of the LHCb Vertex Locator

The LHCb experiment is set for a significant upgrade, which will be ready for Run 3 of the LHC in 2020. This upgrade will allow LHCb to run at a significantly higher instantaneous luminosity and collect an integrated luminosity of 50fb−1 by the end of Run 4. In this process the Vertex Locator (VELO) detector will be upgraded to a pixel-based silicon detector. The upgraded VELO will improve upon the current detector by being closer to the beams and having lower material modules with microchannel cooling and a thinner RF-foil. Simulations have shown that it will maintain its excellent performance, even after the radiation damage caused by collecting an integrated luminosity of 50fb−1.

The LHCb Vertex Locator — performance and radiation damage

LHCb is a dedicated flavour physics experiment at the Large Hadron Collider at CERN. The Vertex Locator (VELO) is an important part of a LHCb tracking system, enabling precision measurement of beauty and charm mesons' flight distance. The VELO consist of a set of silicon micro-strip detectors, arranged in two retractable halves, operating only 7 mm from the interaction region. In these proceedings the VELO performance during the Run 1 is summarised and radiation damage studies are presented.

Radiation damage in the LHCb vertex locator

The LHCb Vertex Locator (VELO) is a silicon strip detector designed toreconstruct charged particle trajectories and vertices produced at the LHCbinteraction region. During the first two years of data collection, the 84 VELOsensors have been exposed to a range of fluences up to a maximum value ofapproximately 45 × 1012 1 MeV neutron equivalent(1 MeV neq). At the operational sensor temperature of approximately−7 °C, the average rate of sensor current increase is18 μA per fb−1, in excellent agreement withpredictions. The silicon effective bandgap has been determined using currentversus temperature scan data after irradiation, with an average value ofEg = 1.16±0.03±0.04 eV obtained. The first observation ofn+-on-n sensor type inversion at the LHC has been made, occurring at a fluenceof around 15 × 1012 of 1 MeV neq. The only n+-on-psensors in use at the LHC have also been studied. With an initial fluence ofapproximately 3 × 1012 1 MeV neq, a decrease in theEffective Depletion Voltage (EDV) of around 25 V is observed. Following this initialdecrease, the EDV increases at a comparable rate to the typeinverted n+-on-n type sensors, with rates of (1.43±0.16) × 10−12 V/ 1 MeV neq and (1.35±0.25) × 10−12 V/ 1 MeV neq measured for n+-on-p andn+-on-n type sensors, respectively. A reduction in the charge collectionefficiency due to an unexpected effect involving the second metal layer readoutlines is observed.

Performance, radiation damage effects and upgrade of the LHCB vertex locator

LHCb is a dedicated experiment to study new physics in the decays of heavy hadrons at the Large Hadron Collider (LHC) at CERN. Heavy hadrons are identified through their flight distance in the Vertex Locator (VELO), the retractable silicon-strip vertex detector surrounding the LHCb interaction point at only 8mm from the beam during normal LHC operation. The VELO consists of a series of silicon micro-strip detectors and operates in an extreme and highly non-uniform radiation environment. The performance of the LHCb VELO during the first three years of LHC physics running and the primary results from radiation damage studies are presented. Plans for an upgraded detector by 2018 with a 40MHz readout are also presented.