LHCb has significantly improved the trigger for the experiment during Run 2 of the LHC. The detector is now calibrated in real time, allowing the best possible event reconstruction in the trigger, with the same performance as the Run 2 offline reconstruction. The improved trigger allows event selection at a higher rate and with better information than in Run 1, providing a significant advantage in the hunt for new physics in Run 2.
The trigger consists of two stages: a hardware trigger that reduces the 40 MHz bunch-crossing rate to 1 MHz, and two high-level software triggers, HLT1 and HLT2 (figure 1). In HLT1, a quick reconstruction is performed before further event selection. Here, dedicated inclusive triggers for heavy-flavour physics use multivariate approaches. HLT1 also selects an inclusive muon sample, and exclusive lines select specific decays. This trigger typically takes 35 ms/event and writes out events at about 150 kHz.
In Run 1, 20% of events were deferred and processed with the HLT between fills. For Run 2, all events that pass HLT1 are deferred while a real-time alignment is run, so minimizing the time spent using sub-optimal conditions. The spatial alignments of the vertex detector – the VELO – and the tracker systems are evaluated in a few minutes at the beginning of each fill. The VELO is reinserted for stable collisions in each fill, so the alignment could vary from one fill to another; figure 2 shows the variation for the first fills of Run 2. In addition, the calibration of the Cherenkov detectors and the outer tracker are evaluated for each run. The quality of the calibration allows the offline performance, including the offline track reconstruction, to be replicated in the trigger, thus reducing systematic uncertainties in LHCb’s results.
The second stage of the software trigger, HLT2, now writes out events for offline storage at about 12.5 kHz (compared to 5 kHz in Run 1). There are nearly 400 trigger lines. Beauty decays are typically found using multivariate analysis of displaced vertices. There is also an inclusive trigger for D* decays, and many lines for specific decays. Events containing leptons with a significant transverse momentum are also selected.
A new trigger stream – the “turbo” stream – allows candidates to be written out without further processing. Raw event data are not stored for these candidates, reducing disk usage. All of this enables a very quick data analysis. LHCb has already used data from this stream for a preliminary measurement of the J/ψ cross-section in √s = 13 TeV collisions (CERN Courier September 2015 p11).