Positioning system of the ANTARES neutrino telescope

Miguel Ardid
2009 Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment  
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). The initial configuration and expected performance of the detector and associated systems, as established by test beam measurements and simulation studies, is described. LHCb is an experiment dedicated to heavy flavour physics at the LHC [1, 2] . Its primary goal is to look for indirect evidence of new physics in CP violation and rare
more » ... of beauty and charm hadrons. The current results in heavy flavour physics obtained at the B factories and at the Tevatron are, so far, fully consistent with the CKM mechanism. On the other hand, the level of CP violation in the Standard Model weak interactions cannot explain the amount of matter in the universe. A new source of CP violation beyond the Standard Model is therefore needed to solve this puzzle. With much improved precision, the effect of such a new source might be seen in heavy flavour physics. Many models of new physics indeed produce contributions that change the expectations of the CP violating phases, rare decay branching fractions, and may generate decay modes which are forbidden in the Standard Model. To examine such possibilities, CP violation and rare decays of B d , B s and D mesons must be studied with much higher statistics and using many different decay modes. With the large bb production cross section of ∼ 500 µb expected at an energy of 14 TeV, the LHC will be the most copious source of B mesons in the world. Also B c and b-baryons such as Λ b will be produced in large quantities. With a modest luminosity of 2 × 10 32 cm −2 s −1 for LHCb, 10 12 bb pairs would be produced in 10 7 s, corresponding to the canonical one year of data taking. Running at the lower luminosity has some advantages: events are dominated by a single pp interaction per bunch crossing (simpler to analyse than those with multiple primary pp interactions), the occupancy in the detector remains low and radiation damage is reduced. The luminosity for the LHCb experiment can be tuned by changing the beam focus at its interaction point independently from the other interaction points. This will allow LHCb to maintain the optimal luminosity for the experiment for many years from the LHC start-up. The LHCb detector must be able to exploit this large number of b hadrons. This requires an efficient, robust and flexible trigger in order to cope with the harsh hadronic environment. The trigger must be sensitive to many different final states. Excellent vertex and momentum resolution are essential prerequisites for the good proper-time resolution necessary to study the rapidly oscillating B s -B s meson system and also for the good invariant mass resolution, needed to reduce combinatorial background. In addition to electron, muon, γ, π 0 and η detection, identification of protons, kaons and pions is crucial in order to cleanly reconstruct many hadronic B meson decay final states such as B 0 → π + π − , B → DK ( * ) and B s → D ± s K ∓ . These are key channels for the physics goals of the experiment. Finally, a data acquisition system with high bandwidth and powerful online data processing capability is needed to optimise the data taking. -1 - JINST 3 S08005 LHCb is a single-arm spectrometer with a forward angular coverage from approximately 10 mrad to 300 (250) mrad in the bending (non-bending) plane. The choice of the detector geometry is justified by the fact that at high energies both the b-and b-hadrons are predominantly produced in the same forward or backward cone. The layout of the LHCb spectrometer is shown in figure 2.1. The right-handed coordinate system adopted has the z axis along the beam, and the y axis along the vertical. Intersection Point 8 of the LHC, previously used by the DELPHI experiment during the LEP Figure 2.1: View of the LHCb detector. -2 - JINST 3 S08005 time, has been allocated to the LHCb detector. A modification to the LHC optics, displacing the interaction point by 11.25 m from the centre, has permitted maximum use to be made of the existing cavern for the LHCb detector components. The present paper describes the LHCb experiment, its interface to the machine, the spectrometer magnet, the tracking and the particle identification, as well as the trigger and online systems, including front-end electronics, the data acquisition and the experiment control system. Finally, taking into account the performance of the detectors as deduced from test beam studies, the expected global performance of LHCb, based on detailed MonteCarlo simulations, is summarized. The interface with the LHC machine is described in section 3. The description of the detector components is made in the following sequence: the spectrometer magnet, a warm dipole magnet providing an integrated field of 4 Tm, is described in section 4; the vertex locator system (including a pile-up veto counter), called the VELO, is described in section 5.1; the tracking system made of a Trigger Tracker (a silicon microstrip detector, TT) in front of the spectrometer magnet, and three tracking stations behind the magnet, made of silicon microstrips in the inner parts (IT) and of Kapton/Al straws for the outer parts (OT) is described in sections 5.2 and 5.3; two Ring Imaging Cherencov counters (RICH1 and RICH2) using Aerogel, C 4 F 10 and CF 4 as radiators, to achieve excellent π-K separation in the momentum range from 2 to 100 GeV/c, and Hybrid Photon Detectors are described in section 6.1; the calorimeter system composed of a Scintillator Pad Detector and Preshower (SPD/PS), an electromagnetic (shashlik type) calorimeter (ECAL) and a hadronic (Fe and scintillator tiles) calorimeter (HCAL) is described in section 6.2; the muon detection system composed of MWPC (except in the highest rate region, where triple-GEM's are used) is described in section 6.3. The trigger, the online system, the computing resources and the expected performance of the detector are described in sections 7, 8, 9, and 10, respectively. Most detector subsystems are assembled in two halves, which can be moved out separately horizontally for assembly and maintenance, as well as to provide access to the beampipe. Interactions in the detector material reduce the detection efficiency for electrons and photons; multiple scattering of pions and kaons complicates the pattern recognition and degrades the momentum resolution. Therefore special attention was paid to the material budget up to the end of the tracking system. Estimations of the material budget of the detector [3] using realistic geometries for the vacuum chamber and all the sub-detectors show that at the end of the tracking, just before entering RICH2, a particle has seen, on average, about 60% of a radiation length and about 20% of an absorption length. Architecture of the front-end electronics The front-end architecture chosen for LHCb [4, 5] has to a very large extent been determined by the requirement of making a hardware-based short latency trigger, with an efficient event selection, for complicated B events. A fast first level trigger has been found capable of making an event rate reduction of the order of 1 in 10. This has for the chosen LHCb luminosity enforced the use of a front-end architecture with a first level trigger rate of up to 1 MHz. This was considered to be the highest rate affordable for the data acquisition system (DAQ) and required readout links. The general front-end electronics architecture and data flow in the DAQ interface are shown in figure 2.2. All sub-detectors store sampled detector signals at the 40 MHz bunch crossing rate in -3 - JINST 3 S08005
doi:10.1016/j.nima.2008.12.033 fatcat:5bimskasxvbklhymlxxzj7ugom