The LHCb experiment has had a remarkable year, moving from first results to world-beating measurements of B-hadron properties, such as the oscillation frequency of the Bs meson, CP and forward-backward asymmetries, as well as limits on rare decays, for example Bs → μ+μ– (CERN Courier March 2011 p8, April 2011 p9, July/August 2011 p9, September 2011 p13, October 2011 p8). Even though the physics harvest is now in full flow, the collaboration is already planning for the eventual upgrade of the experiment, which is scheduled to be ready for data-taking in 2019.
The instantaneous luminosity delivered to LHCb has steadily increased throughout the year, reaching 4 × 1032 cm2 s–1 by the end of the run, already twice the original design luminosity for the experiment. Unlike the general-purpose detectors at the LHC, ATLAS and CMS, LHCb has been specifically designed for the optimal study of B hadrons, covering an angular range of 10–300 mrad from the beam axis (the forward region). This gives it different constraints concerning the luminosity. The track density increases in this region, so detectors suffer from higher occupancy, and are potentially more prone to radiation damage. In addition, because the experiment is tuned for the precise study of B-hadron decay vertices, too many overlapping events can confuse the picture. Finally, the experiment’s trigger has the special feature that it can select fully hadronic decays, rather than only relying on electron or muon signatures, and this trigger cannot handle too high an input rate without reducing its efficiency.
As a result, the luminosity cannot be pushed much higher in the current experiment. This has the positive aspect that next year should be one of continuous operation with the experiment in stable conditions, but eventually it means that the time taken to double the data-set will become long. The goal for this year was 1 fb–1 of integrated luminosity, which (thanks to the excellent performance of the LHC) was comfortably passed with a few weeks to spare; it represents more than 30 times as much data as last year. The expectation is to at least double that sample again in 2012, but for the longer term the collaboration plans to upgrade the experiment so that it can operate at higher luminosity and accumulate an order of magnitude more data. This will allow even higher precision in the search for new physics in the flavour sector.
The key to the upgrade will be to read out the full experiment at 40 MHz, the design bunch-crossing rate of the LHC, and to perform the trigger in software in a powerful computer farm. For this to succeed, collisions will indeed have to be provided by the LHC at 40 MHz at the time of the upgrade, rather than at the current rate of 20 MHz.
The LHCb Collaboration submitted a Letter of Intent describing the proposed upgrade to the LHC Committee (LHCC) in March, and the committee endorsed the physics programme. A review panel looked into the proposed 40 MHz readout scheme and gave a positive report, so that the LHCC has now encouraged LHCb to proceed with preparing Technical Design Reports for the upgrade components. This will ensure the future of the experiment into the next decade.
Upgrades to the B-factory experiments are under consideration in Japan and Italy on the same timescale, and have a complementary reach for this physics. While they have strong performance for neutral decay products, they cannot compete with the enormous production rate at the LHC for charged modes, and the time-dependent study of Bs states will remain the province of LHCb. The upgrade will also allow the LHCb experiment to act as a general-purpose detector in the forward region, with the ability to search for exotic particles that might give long decay lengths, or to study in detail the influence of any new physics states that might be discovered at the LHC over the same period. The collaboration is now pressing ahead with the R&D necessary to ensure the upgrade’s success.
• For more information see Letter of Intent for the LHCb Upgrade, CERN-LHCC-2011-001.