LHCb: First year of running for the LHCb calorimeter system

The LHCb experiment is dedicated to precision measurements of CP violation and rare decays of B hadrons at the Large Hadron Collider (LHC) at CERN (Geneva) [1, 2]. LHCb is a single-arm spectrometer with a forward angular coverage from approximately 10 mrad to 300 mrad. It comprises a calorimeter system composed of four subdetectors [3]. It selects transverse energy hadron, electron and photon candidates for the first trigger level (L0), which makes a decision 4µs after the interaction. It provides the identification of electrons, photons and hadrons as well as the measurement of their energies and positions. The set of constraints resulting from these functionalities defines the general structure and the main characteristics of the calorimeter system and its associated electronics. A classical structure of an electromagnetic calorimeter (ECAL) followed by a hadron calorimeter (HCAL) has been adopted. In addition the system includes in front of them the Scintillating Pad Detector (SPD) and Pre-Shower (PS), which are two planes of scintillating pads separated by a 2.5 radiation length lead sheet, aimed at tagging the electric charge and the electromagnetic nature of the calorimeter clusters for the first level of trigger. ECAL, PS and SPD account for about 6000 channels each while HCAL has 1500. Each layer exhibits three degrees of granularity, concentric around the beam pipe, namely, the inner, the middle and the outer parts. All the four detectors are arranged in pseudo-projective geometry and follow the general principle of reading the light from scintillator tiles with wave-length shifting fibers, and transporting the light towards photomultipliers, all following the 25 ns readout. The scintillator pad and preshower detectors (SPD/PS) consist of a 15 mm thick lead converter, that is sandwiched between two almost identical planes of rectangular scintillator pads of high granularity with a total of 12032 detection channels. In order to achieve a one-to-one projective correspondence with the ECAL segmentation, each PS and SPD plane is subdivided into inner (3072 cells), middle (3584 cells) and outer (5376 cells) sections with approximately 4 x 4, 6 x 6 and 12 x 12 cm2 cell dimensions. The cells are packed in ~ 48 x 48 cm2 boxes (detector units) that are grouped into 26 supermodules. The space available for the SPD/PS detector between the first muon chamber and the electromagnetic calorimeter is only 180 mm. The detector units are optically connected to the 64 channels multi anode PMTs by bundles of 32 clear fibres, enclosed in a light-tight plastic tube, by means of a phototube coupler. The HV is provided by a Cockroft-Walton voltage multiplier. The PMT gain is set to ~10000. The shashlik calorimeter technology, i.e. a sampling scintillator/lead structure readout by plastic WLS fibres, has been chosen for the electromagnetic calorimeter. This decision was made taking into account modest energy resolution, fast time response, acceptable radiation resistance and reliability of the shashlik technology. Specific features of the LHCb shashlik ECAL are a very high uniformity and an advanced monitoring system. The light from the scintillator tiles is absorbed, reemitted and transported by 1.2 mm diameter WLS Kuraray Y-11(250)MSJ fibres, traversing the entire module. In order to improve light collection efficiency the fibres are looped such that each traverses the module twice, the looped part remaining in a housing outside the front of the module. The light is read out with Hamamatsu R7899-20 phototubes where the high voltage is provided by a Cockcroft-Walton (CW) base. The LHCb hadron calorimeter (HCAL) is a sampling device made from iron and scintillating tiles, as absorber and active material respectively. The special feature of this sampling structure is the orientation of the scintillating tiles that run parallel to the beam axis. In the lateral direction tiles are interspersed with 1 cm of iron, whereas in the longitudinal direction the length of tiles and iron spacers corresponds to the hadron interaction length in steel. The light in this structure is collected by WLS fibres running along the detector towards the back side where photomultiplier tubes (PMTs) are housed. The HCAL is segmented transversely into square cells of size 131.3 mm (inner section) and 262.6 mm (outer section). Readout cells of different sizes are defined by grouping together dfferent sets of fibres onto one photomultiplier tube that is fixed to the rear of the sampling structure. The basic structure of the electronics is dictated by the need to handle the data for the Level 0 trigger as fast as possible. The front-end electronics and the PS/SPD photomultipliers are located at the detector periphery. The HCAL and ECAL phototubes are housed directly on the detector modules. The signals are shaped directly on the back of the photomultiplier for the PS/SPD or after 12 m and 16 m long cables for ECAL and HCAL respectively. They are then digitized in crates positioned on top of the detectors and the trigger circuits, hosted in the same crates, perform the clustering operations required by the trigger. For each channel, the data, sampled at the bunch crossing rate of 40 MHz, are stored in a digital pipeline until the Level-0 trigger decision. In order to exploit the intrinsic calorimeter resolution over the full dynamic range, ECAL and HCAL signals are digitized by a 12-bit flash ADC. Ten bits are enough for the preshower, and the SPD information is only one bit, a simple discriminator recording whether a cell has been hit or not. The calorimeter has been pre-calibrated before installing in the pit, and the calibration techniques have been tested with the data taken in 2010. During 2010 operation, hadronic, leptonic and photon triggers of particular interest for hadronic B decays and radiative decays was given by the calorimeter system. The design and construction characteristics of the LHCb calorimeter will be recalled. Strategies for monitoring and calibration during data taking will be detailed in all aspects. The performances achieved will be illustrated by selected channels of interest for the B physics.