CERN Accelerating science

The LHCb Upgrade

LHCb is a high precision experiment devoted to the search for new physics beyond the Standard Model by studying CP violation and rare decays in the b and c-quark sectors. By searching for deviations from the Standard Model due to virtual contributions of new heavy particles in loop diagrams, LHCb is sensitive to new particles above the TeV scale that are not accessible to direct searches. All results so far have shown that flavour changing processes are consistent with the CKM mechanism and that large sources of flavour symmetry breaking are excluded at the TeV scale. However, small deviations from the Standard Model are not yet excluded and necessitate very high precision measurements. In order to increase the statistical precision of the measured parameters significantly and to reach the level of uncertainty of the theory predictions, an upgrade of the LHCb subdetectors and trigger system is mandatory.

LHCb is currently running at a constant, levelled luminosity of 4×1032 cm-1s-1 at a bunch crossing rate of 40 MHz. The hardware trigger reduces the data rate to 1 MHz at which the full detector can be readout to provide information to the high level trigger (HLT). Following output of the HLT, the typical data rate storage is kept below 5 kHz. So far LHCb has recorded 3 fb-1 of data and expects to accumulate another ~5 fb-1 by mid-2018, before the start of the Long Shutdown 2 (LS2).

Figure 1. Overview of the LHCb detector.

To substantially increase the event yields, in particular for hadronic decay modes, after the upgrade in LS2, LHCb aims at running at a five times higher luminosity of 2×1033 cm-1s-1 and to remove the 1 MHz bottleneck of the hardware trigger. The complete detector information will be read out with a trigger-less data-acquisition system at the bunch crossing rate of 40 MHz, reducing the data rate in a fully flexible and efficient high level software trigger to about 20-100 kHz.

Upgrading the readout of the sub-detectors to 40 MHz requires not only a redesign of all front-end electronics but also involves replacement of those sub-detectors that have embedded electronics, like for example all silicon detectors. Furthermore, the detector components that suffer from increased particle occupancies or radiation damage also need to be adapted to the harsher running conditions.

The LHCb upgrade therefore involves replacement of the complete tracking system. The current silicon-strip Vertex Locator (VELO) will give way to a state-of-the-art silicon-pixel detector with fast 3D vertex reconstruction capability. Meanwhile, the first tracking plane upstream of the magnet will be upgraded to an upstream tracker (UT) with higher granularity and improved acceptance by means of silicon strips, arranged in a stave concept inspired by the ATLAS IBL design. The inner and outer tracker stations downstream of the magnet will be replaced by a single, novel tracker technology built from 250 micron scintillating fibres readout by silicon photomultipliers at the detector periphery. This Scintillating Fibre Tracker together with UT and the VELO will constitute a high performance tracking system that allows for a very fast track reconstruction in the high level trigger.

Figure 2. Layout of the new LHCb vertex locator with silicon pixels.

Upgrade of the LHCb particle identification system involves optimisation of the optics of the first Ring Imaging Cherenkov (RICH) detector and replacement of all RICH photo detectors (HPDs). The HPDs, which have encapsulated readout electronics, will be substituted by multi-anode photomultipliers read out by a dedicated 40 MHz electronics chip. Apart from upgrading to 40 MHz some relevant parts of the readout electronics of the calorimeters as well as of the muon system, these detectors are already compatible with the upgraded running conditions.  

The LHCb upgrade as outlined in the framework technical design report has been approved by the CERN management in 2012. Details of the detector upgrades are described in the already approved Technical Design Reports (TDRs) for the particle identification and the vertex locator that have already been approved as well as in the TDR of the tracker system that is presently under review by the LHCC committee. The last TDR in the upgrade series specifies all the details of the Trigger and Online architecture and is just being submitted to the committee.

The collaboration is confident that, following the upgrades during LS2, LHCb will be fully operational when running at an increased luminosity of up to 2×1033 cm-1s-1, reading data at the bunch crossing rate of 40 MHz with a trigger-less data acquisition system and a fully efficient software trigger. This will allow to accumulate more than 50 fb-1 of data in the high luminosity LHC era and to reduce the statistical errors for a wide range of decay channels in the b- and c-quark sectors down to the theoretical uncertainties and hopefully catching a glimpse of the elusive new physics.