The UT sub-detector is located in front of the LHCb dipole magnet. The images included in this news were taken during the UT closing process around the LHC beam pipe. The UT plays an essential role in achieving the desired processing speed in the innovative LHCb real-time event selection, by providing an estimate of the momentum and charge of the reconstructed tracks. Moreover, it significantly reduces the rate of fake tracks created by mismatched hits in the VErtex LOcator (VELO) and SciFi sub-detectors, and is an integral component of the outstanding LHCb tracking system.

The layout of the LHCb experiment differs from traditional ideas of particle physics detectors, in which the coordinates of charged particle tracks are measured multiple times inside the magnetic field by a set of tracking chambers. Instead, in LHCb, the volume inside the magnet is empty. The LHCb tracking algorithms compare track segments measured before and after the magnet, and use the measured deflection inside the magnet to define the particle momentum and charge. In addition, the UT, while not inside the magnet, still experiences a small fringe magnetic field. This allows it to make a crucial independent estimate of the particle momentum and charge by comparing its precisely measured points with the track segment in the VELO. These two methods of momentum measurement are critical to allow the real-time trigger to operate with the required speed and robustness. In addition, the UT provides reconstruction of particles decaying after the VELO, such as K0 mesons, or hypothetical long-lived particles which may be predicted by New Physics models.

In Run 3 the proton-proton collision rate inside the LHCb detector will be five times higher than before and the whole LHCb detector is read out at the proton-proton bunch collision rate of 40 MHz instead of the previous 1 MHz. The UT replaces the (Tracker Turicensis – TT), previously playing a similar role. However, the UT works at Run 3 at much higher data acquisition frequency and with greater particle density from the proton-proton collisions at LHC. Its design is based on 10 cm x 10 cm x 0.25 mm silicon sensors mounted on a lightweight carbon fiber support structure, called a stave. The staves are used to form 4 large planes of silicon. The UT was successfully installed on a very tight schedule during the 2022 end technical stop, working seven days per week. The final connections and closing around the beam-pip were performed in time for the start of the 2023 LHC Run.

The installation of the UT marked the completion of the new LHCb Upgrade 1 detector. The VErtex LOcator (VELO) has been upgraded and is now based on silicon pixels mounted on microchannel cooled silicon wafers. The tracking detectors have been replaced by the Upstream Tracker (UT) and by the three stations of the Scintillating Fibre Tracker (SciFi), which are placed downstream of the magnet, and consist of 2.5m-long scintillating plastic fibre mats read out by silicon photo-multipliers. The RICH1 is now a brand new sub-detector: the complete optics and mechanics have been re-designed and re-built, the mirrors have a larger radius of curvature, and Hybrid Photon Detectors (HPD) have been replaced by multi-anode photomultipliers (PMTs) in both RICH1 and RICH2. The scintillating pad detector (SPD), the preshower (PS) and the first muon chamber (M1) have been removed. The new ECAL, HCAL and muon chamber electronics were implemented in order to fulfill the requirement of the upgraded data acquisition and trigger systems. A suite of sub-detectors, PLUME, BCM and RMS, enable LHCb to instantaneously measure the collision rate and reconstruct the collision region. This information allows the LHC to ensure a constant luminosity for the collisions in LHCb. In addition, the SMOG2 system has been installed, allowing all noble gases plus hydrogen and deuterium to be injected into a dedicated cell upstream of the collision region. This allows LHCb to study collisions of LHC protons with these gases in a fixed-target mode simultaneously with the study of proton-proton collisions at the centre of the VELO. The collaboration has also made a revolutionary improvement to the data-taking and analysis process, called “Real-Time Analysis” (RTA). The final processing already takes place online, by performing the calibration and alignment processes automatically in the trigger system using a powerful new computer centre.

The VELO modules are operated under vacuum in a volume separated from the LHC vacuum by an ultra-thin aluminum shield called the “RF box” between the LHC and detector volumes. The thickness of this corrugated shield reaches less than two tenths of a millimetre in the central region to minimize interactions with the particles emerging from proton-proton collisions. In 2022, the volume containing the VELO modules and the LHC vacuum were filled with ultra pure Neon. A sophisticated pressure balancing system was operated to limit pressure differences between the two volumes to +2/-5 mbar (see “VELO closing news”). On January 10th, following a hardware component failure, this system functioned improperly, resulting in the pressure difference moving beyond the proper limits, and the RF boxes were accidentally deformed. During the accident, the two halves of the VELO detector were located in the OUT position, and the VELO modules themselves were not affected. However, since the RF boxes are mechanically linked to the VELO, a change in their shape impacts the degree to which the VELO sensors can be moved and centered around the colliding beams. The VELO collaboration has been working around the clock to repair the detector and prepare it for the upcoming Run 3 (read more HERE).

Fifteen years after the first full collaboration meeting, and nine years after construction was approved, the installation of the LHCb Upgrade I is complete! The LHCb collaboration is excited about the operation of this second era experiment and excited to see what discoveries may await.