LHCb VELO Research Articles

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

In 2032–2034, the LHCb will be upgraded to efficiently explore the new HL-LHC conditions. The increased number of interactions per event and radiation level brings several challenges including high non-uniform radiation doses (up to 6×1016MeVneq/cm2), operation under vacuum (power dissipation below 1.5 W/cm2), high data rates (up to order of 100 Gbps/ASIC) and ASIC timing resolution (30 ps or better). This proceedings describes the requirements for two different upgrade scenarios (different pixel pitch, radiation doses and material budget while keeping the same scattering terms), application specific integrated circuit status to readout the sensor pixels (PicoPix and IGNITE), electronic integration (Anisotropic Conductive Bonding) and data transmission (FALAPHEL). In summary, the open challenges mentioned before are being addressed in the current R&D phase and the most suitable candidate solutions will be consolidated in the technical design report in the coming years.

Spatial resolution and efficiency of prototype sensors for the LHCb VELO Upgrade

A comprehensive study of the spatial resolution and detection efficiency of sensor prototypes developed for the LHCb VELO upgrade is presented. Data samples were collected at the CERN SPS H8 beam line using a hadron mixture of protons and pions with momenta of approximately 180 GeV/c. The sensor performance was characterised using both irradiated and non-irradiated sensors. Irradiated samples were subjected to a maximum fluence of 8 × 1015 1 MeV neq cm-2, of both protons and neutrons. The spatial resolution is measured comparing the detected hits to theposition as predicted by tracks reconstructed by the Timepix3 telescope. The resolution is presented for different applied bias voltages and track angles, sensor thickness and implant size.

Microchannel cooling for the LHCb VELO Upgrade I

The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R&D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm2 W−1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

Timing performance of the LHCb VELO Timepix3 Telescope

We performed a detailed study of the timing performance of the LHCb VELO Timepix3 Telescope with a 180 GeV/c mixed hadron beam at the CERN SPS. A twofold method was developed to improve the resolution of single-plane time measurements, resulting in a more precise overall track time measurement. The first step uses spatial information of reconstructed tracks in combination with the measured signal charge in the sensor to correct for a mixture of different effects: variations in charge carrier drift time; variations in signal induction, which are the result of a non-uniform weighting field in the pixels; and lastly, timewalk in the analog front-end. The second step corrects for systematic timing offsets in Timepix3 that vary from −2 to 2 ns. By applying this method, we improved the track time resolution from 438(16) ps to 276(4) ps.

Real-time cluster finding for LHCb silicon pixel VELO detector using FPGA

In the Run-3 of LHCb, the High Level Trigger will have to process events at full LHC collision rate (30 MHz). This is a very challenging goal, and delegating some low-level tasks to FPGA accelerators can be very helpful by saving precious computing time. In particular, the 2D pixel geometry of the new LHCb VELO detector makes the cluster-finding process particularly CPU-time demanding. We realized and tested a highly parallel FPGA-based clustering algorithm, capable of performing this reconstruction in real time at 30 MHz event rate using a modest amount of hardware resources, that can be a viable alternative solution.

LHCb VELO Timepix3 telescope

The LHCb VELO Timepix3 telescope is a silicon pixel tracking system constructed initially to evaluate the performance of LHCb VELO Upgrade prototypes. The telescope consists of eight hybrid pixel silicon sensor planes equipped with the Timepix3 ASIC . The planes provide excellent charge measurement, timestamping and spatial resolution and the system can function at high track rates. This paper describes the construction of the telescope and its data acquisition system and offline reconstruction software. A timing resolution of 350 ps was obtained for reconstructed tracks. A pointing resolution of better than 2\\mum was determined for the 180 GeV/c mixed hadron beam at the CERN SPS . The telescope has been shown to operate at a rate of 5 million particles s−1⋅ cm−2 without a loss in efficiency.

Detector-size upper bounds on dark hadron lifetime from cosmology

We show that in a confining hidden valley model where the lightest hidden particles are dark hadrons that have mass splittings larger than mathcal{O}(0.1) GeV, if the lightest dark hadron is either stable or decays into Standard Model (SM) hadrons/charged leptons during the big-bang nucleosynthesis (BBN), at least one of the heavier dark hadrons needs to decay into SM particles within mathcal{O}(10) nanosec. Once being produced at collider experiments, this heavier dark hadron is likely to decay within mathcal{O}(10) meter distance, which strengthens the motivation of searching for long-lived particles with sub-meter scale decay lengths at colliders. To illustrate the idea, we study the lifetime constraint in scenarios where the lightest dark particle is a pseudo-scalar meson, and dark hadrons couple to SM particles either through kinetic mixing between the SM and dark photons or by mixing between the SM and dark Higgs. We study the annihilation and decay of dark hadrons in a thermal bath and calculate upper bounds on the lightest vector meson (scalar hadron) lifetime in the kinetic mixing (Higgs portal) scenario. We discuss the application of these lifetime constraints in long-lived particle searches that use the LHCb VELO or the AT-LAS/CMS inner detectors.

Digital column readout architectures for hybrid pixel detector readout chips

In this paper, two digital column architectures suitable for sparse readout of data from a pixel matrix in trigger-less applications are presented. Each architecture reads out a pixel matrix of 256 x 256 pixels with a pixel pitch of 55 μm. The first architecture has been implemented in the Timepix3 chip, and this is presented together with initial measurements. Simulation results and measured data are compared. The second architecture has been designed for Velopix, a readout chip planned for the LHCb VELO upgrade. Unlike Timepix3, this has to be tolerant to radiation-induced single-event effects. Results from post-layout simulations are shown with the circuit architectures.

The Timepix Telescope for high performance particle tracking

The Timepix particle tracking telescope has been developed as part of the LHCb VELO Upgrade project, supported by the Medipix Collaboration and the AIDA framework. It is a primary piece of infrastructure for the VELO Upgrade project and is being used for the development of new sensors and front end technologies for several upcoming LHC trackers and vertexing systems. The telescope is designed around the dual capability of the Timepix ASICs to provide information about either the deposited charge or the timing information from tracks traversing the 14×14mm matrix of 55×55μm pixels. The rate of reconstructed tracks available is optimised by taking advantage of the shutter driver readout architecture of the Timepix chip, operated with existing readout systems. Results of tests conducted in the SPS North Area beam facility at CERN show that the telescope typically provides reconstructed track rates during the beam spills of between 3.5 and 7.5kHz, depending on beam conditions. The tracks are time stamped with 1ns resolution with an efficiency of above 98% and provide a pointing resolution at the centre of the telescope of ∼1.6μm. By dropping the time stamping requirement the rate can be increased to ∼15kHz, at the expense of a small increase in background. The telescope infrastructure provides CO2 cooling and a flexible mechanical interface to the device under test, and has been used for a wide range of measurements during the 2011–2012 data taking campaigns.

Merlin: a fast versatile readout system for Medipix3

This contribution reports on the development of a new high rate readout system for the Medipix3 hybrid pixel ASIC developed by the Detector Group at Diamond Light Source. It details the current functionality of the system and initial results from tests on Diamond's B16 beamline. The Merlin system is based on a National Instruments PXI/FlexRIO system running a Xilinx Virtex5 FPGA. It is capable of recording Medipix3 256 by 256 by 12 bit data frames at over 1 kHz in bursts of 1200 frames and running at over 100 Hz continuously to disk or over a TCP/IP link. It is compatible with the standard Medipix3 single chipboards developed at CERN and is capable of driving them over cable lengths of up to 10 m depending on the data rate required. In addition to a standalone graphical interface, a system of remote TCP/IP control and data transfer has been developed to allow easy integration with third party control systems and scripting languages. Two Merlin systems are being deployed on the B16 and I16 beamlines at Diamond and the system has been integrated with the EPICS/GDA control systems used. Results from trigger synchronisation, fast burst and high rate tests made on B16 in March are reported and demonstrate an encouraging reliability and timing accuracy. In addition to normal high resolution imaging applications of Medipix3, the results indicate the system could profitably be used in `pump and probe' style experiments, where a very accurate, high frame rate is especially beneficial. In addition to these two systems, Merlin is being used by the Detector Group to test the Excalibur 16 chip hybrid modules, and by the LHCb VELO Pixel Upgrade group in their forthcoming testbeams. Additionally the contribution looks forward to further developments and improvements in the system, including full rate quad chip readout capability, multi-FPGA support, long distance optical communication and further functionality enhancements built on the capabilities of the Medipix3 chips.

Production, measurement and simulation of a low mass flex cable for multi gigabit/s readout for the LHCb VELO upgrade

The goal of this project is to examine the feasibility ofdata transmission up to ∼ 5 Gbit/s on a short ( ∼ 60 cm) low massflex cable, for the readout of the upgraded vertex detector (VELO) of theLHCb experiment. They will be in a vacuum and very high radiationenvironment and also partly in the particle acceptance. For the full system1600 readout links will be required. A set of single-ended and differential(edge-coupled) striplines, with a variety of line parameters have beenprototyped using a material specifically tailored for this type ofapplication (Dupont Pyralux AP-plus polyimide). To reduce mass, the totalthickness of the cable is kept to 0.7 mm. We will present measurements ofthe characteristic impedance, insertion and return loss, obtained both fromtime and frequency domain, as well as a comparison with simulations andexpectations. Also the effectiveness of grounded guard traces and the use ofground via holes to reduce crosstalk will be reported. From the measurementswe were also able to extract the material properties such as the dielectricconstant and loss factor up to several GHz. The measurements were done witha Vector Network Analyzer (VNA), TDR/TDT Digital Sampling Oscilloscope,serial PRBS generator and analyzer for eye diagram and CAD tools such asAgilent ADS and ANSYS HFSS simulators.

First spatial alignment of the LHCb VELO and analysis of beam absorber collision data

A first alignment of the LHCb Vertex Locator has been obtained from beam induced tracks at the LHC. A 450 GeV/ c protons were collided on a beam absorber during the LHC synchronisation tests of the anti-clockwise beam in August and September 2008. The resulting particle tracks have been reconstructed by the Vertex Locator. This was the first full reconstruction of tracks induced by the LHC beam. The quality of the data obtained is discussed. A total of 2200 tracks were reconstructed from the full data sample, and a first spatial alignment was obtained. The detector is aligned to an accuracy of 5 μ m in the sensor plane. The results confirm that all detector modules have not been displaced from their surveyed positions by more than 10 μ m .

First LHC beam induced tracks reconstructed in the LHCb VELO

The Vertex Locator of the LHCb experiment has been used to fully reconstruct beam induced tracks at the LHC. A beam of protons was collided with a beam absorber during the LHC synchronisation test of the anti-clockwise beam on the weekend 22nd–24th August 2008. The resulting particles have been observed by the Vertex Locator. The LHCb Vertex Locator is a silicon micro-strip detector containing 21 planes of modules. Tracks were observed passing through up to 19 modules (38 silicon sensors). A total of over 700 tracks were reconstructed, and are being used to study the calibration and alignment of the detector.

The LHCb VELO and its use in the trigger

Precise vertex reconstruction is a fundamental requirement for the LHCb experiment. Displaced secondary vertices are a distinctive feature of b-hadron decays. The detector setup is presented with special emphasis on its use in the LHCb trigger system together with test results.

The signal shape from the LHCb vertex locator prototype detectors

Measurements of the SCT128A ASIC pulse shape, when reading out non-irradiated and irradiated prototype detectors for the LHCb VELO, are presented. The detectors studied were two n-on-n prototype detectors fabricated by Hamamatsu, and a p-on-n prototype detector fabricated by MICRON.

The beam test setup for LHCb VELO prototypes

Measurements of the performance of the LHCb VELO beam test telescope are discussed and compared with a simulation. The pulse shape of a fast VELO readout chip has been determined. An outlook on the 2000 beam test program for resolution and efficiency measurements on LHCb VELO prototypes is given.