From March 29 until 31, the LHCb collaboration held its 6th Upgrade II workshop in the Faculty of Physics in Barcelona with 147 participants. It was the first time the collaboration gathered in-person after the pandemic to discuss plans for the Upgrade II of the LHCb detector.

The LHCb Upgrade II is planned during the long shutdown 4 (LS4) of the LHC. Its aim is to have a unique forward detector with an unprecedented potential for heavy-flavour physics during the High-Luminosity (HL) LHC phase. The Upgrade II detector will operate at a luminosity of up to 1.5 x 10³⁴ cm^-2 s^-1, posing new challenges to the detector and the reconstruction of the physics events due to high particle densities. Although the general layout of the detector is expected to remain unchanged, every single sub-detector will undergo a major modification to adapt to the harsher environment expected after LS4.

After the presentation of the Physics case [1] and the Framework TDR [2], it is now time to prepare a so-called scoping document with the aim to analyse and compare the physics potential of several scenarios based on different overall cost envelopes for the Upgrade II. Techical Design Reports on the individual sub-detectors will follow afterwards.

During the two and a half days of the workshop, new developments concerning all aspects related to the LHCb Upgrade II were discussed. This includes global detector optimisation as well as the designs of the upgraded tracking and particle identification (PID) detectors. Furthermore, the future online system, offline software and the software trigger were covered.

Schematic side-view of the Upgrade II LHCb detector

Consolidation of the experiment in LS3

Some limited-size enhancements are proposed for LS3 in order to bring physics benefits already in Run 4. These are also important to enable the installation of the Upgrade II during LS4 by performing infrastructure work and tests of key elements earlier. Along these lines, the RICH detector presented a plan to upgrade the readout electronics to significantly improve the time information. This will reduce the amount of out-of-time photons which impacts the data rate and enhances the particle identification capabilities for hadrons.

Due to radiation damage, it is planned to exchange the innermost modules of the electromagnetic calorimeter (ECAL) in LS3 using SpaCal technology with dense tungsten and lead absorber. This approach is also fully compatible with the LS4 Upgrade II. The other modules of the ECAL will be rearranged in a rhomboid shape for modules with equal cell size to optimise the overall occupancy distribution over the ECAL surface resulting, for example, in a better performance for the reconstruction of neutral pions.

Another opportunity are the magnet stations. These could to be installed on the internal surfaces of the LHCb magnet with the aim to measure the particles that do not reach the downstream exit of the magnet due to their low momenta.

The LHCb collaboration is currently discussing the benefit of operating the HCAL in Run 4 and 5. While not required any more for the hardware trigger, the HCAL also plays a role for the overall particle identification capabilities of the experiment.

Plans for Upgrade II in LS4

Planning further into the future, each sub-detector project presented recent developments in the R&D towards the Upgrade II in LS4 as well as considerations on down-scoping scenarios in view of the scoping document.

The Upgrade II conditions impose stringent requirements in terms of radiation damage and timing resolution on the VELO. New scenarios to meet these challenges were presented and first simulations to illustrate the physics loss from down-scoping the VELO design were shown. 
The current Upstream Tracker (UT) needs to be replaced with pixel technology in Upgrade II due to the much higher particle density. For this purpose, MAPS-based technologies are being studied. As a down-scoping option, the reduction of planes from 4 to 3 was investigated.

The Mighty Tracker downstream of the magnet combines HV-CMOS pixel sensors in the central part with a SciFi tracker further outside. A possible option for down-scoping would be to increase the SciFi area. Cryogenic cooling of the SiPMs is being studied to increase the radiation hardness.

The particle identification system could be complemented by a novel time-of-flight detector, TORCH, in the Upgrade II. In Barcelona, recent results from test beam measurements in 2022 were presented.

Building on the consolidation in LS3, major upgrades of the RICH and ECAL sub-detectors are foreseen in Upgrade II to handle the increased particle fluxes. For the RICH, new photon detectors are being studied to improve the time and spacial resolutions. The considered options include SiPMs and MCP-PMTs.

The upgraded ECAL is referred to as PicoCal indicating the foreseen timing capabilities with tens of picosecond precision. The innermost SpaCal modules will be equipped with radiation-hard crystal scintillating fibres in LS4. In the baseline configuration, all cells will feature double-sided readout. Down-scoping could be achieved by omitting double-sided readout in the outer regions.

Finally, the MUON detector group presented new physics performance and design studies for Upgrade II. An important parameter is the amount of shielding in front of the MUON detector. High-granularity μ-RWELL technology is being investigated for the inner regions in this context.

Common DAQ and infrastructure issues were addressed in a dedicated session. The PCIe400 project is developing a new DAQ board. It represents an evolution of the  current PCe40 board with higher bandwidth and better timing capabilities.

New ideas for the usage of FPGAs in the high-level trigger were discussed. In particular, the RETINA approach aims to develop tracking algorithms inside FPGAs freeing resources in the event builder for other tasks.

An urgent topic is the development and refinement of simulation software for the Upgrade II detector which was reflected in many of the presentations by the individual sub-detectors. Moreover, developments of faster simulation solutions were shown. These could turn out to be very useful tools for detector optimisation purposes. In view of large simulation campaigns in the future, hybrid architectures of CPUs and GPUs are being investigated.

Final words

The vision for the LHCb detector in the HL-LHC era discussed at the workshop is originating from several years of R\&D. At the same time, unique and interesting challenges remain for the Upgrade II. To address them, interesting and innovative ideas in different areas were presented. With the scoping document ahead, exciting opportunities exist for new groups to join this effort.

References

[1] LHCb Collaboration, Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era. arXiv:1808.08865.

[2] LHCb Collaboration, Framework TDR for the LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era. CERN LHCC-2021-012.