DE LAS INFRACCIONES, MEDIDAS DE SEGURIDAD Y SANCIONES CAPÍTULO ÚNICO

During the Run 1 data taking period (see Sec. 4.6), the LHC provided a peak in- stantaneous luminosity of 6.8× 1033cm−2s−1, which is fully used by the general pur- pose detectors ATLAS and CMS. However, to enable the proposed precision ana- lyses the LHCb detector runs at a significantly lower instantaneous luminosity of 4× 1032cm−2s−1. It is adjusted by reducing the overlap between the proton beams, which effectively reduces the amount of pp collisions per bunch crossing. However, even with the reduced rate the LHCb detector is running at, it is not possible to keep information from all collisions and to save everything at the full bunch crossing rate. Moreover, it would also be not efficient, as a bb quark pair is on average only pro- duced every 200 to 350th ppcollision. Therefore, the LHCb detector features a trig- ger system that brings the event rate down to about 5 kHz (3.5 kHz) in 2012 (2011), which is then low enough to be fully saved. Since 2015, i.e. the start of Run 2, it was possible to further increase the total trigger output rate to 12.5 kHz. The following subsections will describe the hardware and the software trigger, which are the two trigger stages of the LHCb experiment.

4.3.1 Hardware trigger

The hardware stage of the trigger (L0) runs synchronously to the full bunch crossing frequency and delivers a reduced event rate of around 1 MHz. It deduces informa- tion from the calorimeters and the muon chambers in order to decide whether an

4.3 The LHCb trigger system event is rejected or maintained. The selection criteria from the L0 trigger aim to fa- vour events that contain particles with high transverse energy, which are likely to be daughter particles of b hadrons. Thus, the L0 searches for clusters of high transverse energies in the calorimeters, which originate from electron, photon or hadron hits (L0Electron, L0Photon, L0Hadron). Only a limited cluster multiplicity in the SPD is allowed to avoid storing events in which the detector occupancy is too high. Fur- thermore, an event is kept if either the transverse momentum p_T(L0Muon) of at least one muon reaches a threshold, or if the p_Tproduct of the two muons with the highest momenta is large enough (L0DiMuon). The specific limits are stored for every run period in trigger configuration keys (TCKs) as they are changed from time to time due to optimisations. Efficiencies of the L0 differ between muon and hadrons. The former are triggered more than 90 % efficient, while decays with fully hadronic final states (like B_s0→ Ds∓K

±_{) are triggered at lower efficiencies of about 60 %[96, 97].}

4.3.2 Software trigger

The high level trigger (HLT) is the second trigger stage and is fully implemented in software. It is further divided into the two stages HLT1 and HLT2, which are running on the event filter farm (EFF). The EFF is a computer cluster with about 30 000 processor cores. Both HLT1 and HLT2 are organised in so-called trigger lines. An event survives, if it is accepted by at least one of the included lines in each of the stages.

At the 1 MHz output rate of the L0 trigger it is possible to fully read out all de- tector components. However, the available per-event time at the HLT1 stage is only sufficient to perform a partial event reconstruction in order to reconsider the L0 de- cisions. Hence, the HLT1 reconstructs the VELO tracks for all events and identifies vertices using at least five tracks. Primary vertices are considered to be vertices closer than 300 µm to the primary pp interaction, which itself is established at the begin- ning of a data taking run. Lines that do not use information from the muon system look for VELO tracks based on their smallest impact parameter with respect to any PV. For example, the Hlt1TrackAllL0 line selects tracks with a good quality by ap- plying the requirement of a transverse momentum p_T larger than 1.6 GeV/c and a displacement from the primary vertex. This line takes the dominant part of the HLT1 bandwidth, with about 58 kHz in 2012 [97]. It is especially important for hadronic modes. Important HLT1 lines for decays with muons are the Hlt1TrackMuon and the Hlt1DiMuonHighMass. The former selects good muon candidates by requiring

p_T> 1 GeV/c and a displacement from the primary vertex. The latter selects dimuon

candidates by requiring their invariant mass to be larger than 2.5 GeV/c and demand- ing their tracks to originate from a common vertex. Altogether, the HLT1 lines had a rate of about 80 kHz in 2012[97].

The HLT2 is able to fully reconstruct all incoming events with a minimum trans- verse momentum of 300 MeV/c. It contains more than 100 lines that implement inclusive and exclusive selections, further tightening the requirements of HLT1. For hadronic decays there are lines that make use of multivariate algorithms to find two-,

three- or four-track secondary vertices with large transverse momenta and significant displacements from the PV. All events accepted by HLT2 are stored on disk.

A novel feature of the LHCb experiment is its capability to exploit the times in which the LHC is not delivering collisions to run a so-called deferred trigger [98]. For this purpose, about 20 % of L0 triggered events are saved on the EFF’s disks and evaluated by the HLT in between data taking runs. This strategy guarantees an optimal usage of the available processing power and exploits the storage opportunities of the EFF nodes.