The LHCb experiment
The Large Hadron Collider beauty (LHCb) experiment specialises in investigating the slight differences between matter and antimatter, by studying a type of particle called the "beauty quark", or "b quark".
Instead of surrounding the entire collision point with an enclosed detector, like ATLAS and CMS, the LHCb experiment uses a series of subdetectors to detect mainly forward particles – those thrown forwards by the collision in one direction. The first subdetector is mounted close to the collision point, with others following one after another over a length of 20 metres.
The 5600-tonne LHCb detector is 21 metres long, 10 metres high and 13 metres wide, and sits 100 metres below ground near the town of Ferney-Voltaire, France. About 1565 scientists, engineers and technicians from 20 countries make up the LHCb collaboration.
Since December 2018, CERN’s Long Shutdown 2 (LS2) has allowed the accelerator and experiment infrastructure to be upgraded. The upgrades of the LHCb experiment are summarised below.
1. VELO: New silicon pixel detector
The vertex locator (VELO) is the subdetector that measures the distance between the collision point and the point where B hadrons (composite particles containing at least one b quark or antiquark) transform into other particles.
The new VELO consists of pixel-tracking layers, which offer improved hit resolution and simpler track reconstruction. It is also closer to the beam axis: 5.1 mm as opposed to 8.4 mm. A new chip, the VELOPIX, capable of collecting signal hits from 256×256 pixels and sending data at a staggering rate of up to 20 Gb/s, was developed for this purpose.
2 & 3. RICH1 and RICH2
The two ring-imaging Cherenkov (RICH) detectors, RICH1 and RICH2, located upstream and downstream of the LHCb magnet 1 and 10 m away from the collision point, provide excellent particle identification over a wide momentum range. Both have been refurbished to cope with the more challenging data-taking conditions of LHC Run 3.
The photon detection system has been redesigned. Two types of 64-channel multi-anode photomultiplier tubes have been selected to detect single photons, while providing excellent spatial resolution and low background noise.
The optical system of the RICH1 detector has all been upgraded. To reduce the number of photons in the hottest region, its optics have been redesigned to spread the Cherenkov rings over a larger surface.
4 & 5. Trackers: UT and SciFi
Originally, the main tracking system was reconstructing the path of charged particles in four tracking stations: one between RICH1 and the LHCb dipole magnet, and three between the magnet and RICH2.
Now, a new upstream tracker (UT) with innovative silicon-microstrip sensors has been installed in place of the station before the magnet.
It is composed of four planes of silicon-microstrip detectors mounted on both sides of vertical structures called staves, providing mechanical support and CO2 evaporative cooling.
Four different silicon sensor designs are used to handle the varying occupancy over the detector acceptance.
The three tracking stations after the magnet have been replaced by a new type of station based on scintillating fibres (SciFi), read out at one extremity by silicon photomultiplier (SiPM) arrays.
As the name indicates, the detector is made of scintillating fibres – optical fibres that emit light when a particle interacts with them. Each scintillating fibre making up the sub-detector is 0.25 mm in diameter and nearly 2.5 m in length.
The SciFi tracker represents a major challenge for the collaboration, not only due to its complexity, but also because the technology has never been used for such a large area in such a radiation environment. Scientists ordered more than 11 000 km of fibre, which they meticulously verified and even cured of a few rare and local imperfections.
6. Front-end electronics
Within the LHC, the beams cross one another in a detector every 25 nanoseconds, corresponding to a frequency of 40 MHz (40 million times per second). In previous years, LHCb filtered down this “event rate” to 1 MHz, using fast electronics to select the most interesting events. Those events were then processed and sifted further.
But from 2022 onwards, this will change radically: the whole detector will instead read at the full rate of 40 MHz to allow event selection to be done more precisely and flexibly by the software. For this reason, the electronics of essentially all the subdetectors have been modified and the computing power of the LHCb event selection system (trigger) will increase.
For this purpose, the Front-End electronics will make extensive use of a radiation resistant chipset, the Gigabit Transceiver (GBT), for readout as well as for slow control, monitoring and synchronization.
In addition: New data centre
Six new data-centre modules have been installed at LHCb. They host together 132 racks for a total power of more than 2 MW.
The two central modules will be home to the readout system for Run 3, comprised of about 500 servers with special readout cards developed by LHCb and used also by ALICE.
Over 14 000 optical fibres enter these two modules from the detector. They bring about 40 terabit/s of raw data and are distributed to the readout servers (each module can host more than 1000 servers).
The remaining four modules will host the servers of the high-level trigger farm. LHCb will deploy at least 2000 servers at the start of Run 3 and at least 20 PB of storage.