With preparations for the ATLAS and CMS large general-purpose detectors for CERN’s LHC collider now advancing, the initial cast for the LHC experimental programme is extended with the publication of a full technical proposal for the LHCb experiment. The aim of this experiment is to study in detail the physics of the Standard Model’s third (and final) generation of particles, particularly the beauty, or “b” quark contained in B mesons. This third generation of quarks makes possible the mysterious mechanism of CP violation.
When component quarks mutate under the action of the weak force, subtle effects come into play. The first to be discovered was the violation of parity (left–right mirror symmetry) in standard nuclear beta decay. This parity violation is seen even with the up–down quark doublet that makes up protons and neutrons.
Searching for a more reliable mirror to reflect particle interactions, physicists proposed CP symmetry. As well as switching left and right, such a mirror also switches particles and antiparticles – the CP mirror image of a right-handed particle is a left-handed antiparticle. However, having six quarks (arranged pair-wise in three generations) opens up the possibility of violating CP symmetry as well. Such effects had been seen in 1964 with neutral kaons. But these kaon phenomena are only a tiny corner of the Standard Model’s CP violation potential. Much larger effects should happen in the B sector. The race is now on to collect enough B particles to become the first to glimpse this additional CP violation.
While these will surely reveal more CP violation effects, the full picture will probably only emerge with the interaction rates and energy conditions of the LHC, which will considerably extend the B physics reach. As well as investigating all aspects of CP violation, LHCb would also consolidate our knowledge of particle reactions and explore fully all quark and lepton sectors of the Standard Model.
The LHCb experiment, which so far has attracted some 340 physicists from 40 research centres in 13 countries, aims to exploit the luminosity of 2×1032 per cm2/s which should be available from the LHC from Day 1. For the other experiments, the LHC’s collision luminosity will be cranked up to 1034. LHCb expects to harvest about 1012 b quark–antiquark pairs each year. LHCb is a large single-arm spectrometer covering an angular range from 10 out to 300 mrad and will be housed in the 27 km LHC/LEP tunnel in the Intersection 8 cavern nearest Geneva airport, currently the site of the Delphi experiment at the LEP electron–positron collider.
At the heart of the detector is the vertex detector, studied by a CERN/Amsterdam/Glasgow/Heidelberg/Imperial College London/Kiev/Lausanne/Liverpool/MPI Heidelberg/NIKHEF Amsterdam/Rome 1 team. The vertex detector will record the decays of the B particles, which travel only about 10 mm before decaying. Each of the 17 planes of silicon (radius 6 cm) spaced over a metre consists of two discs to measure radial and polar coordinates. The arrangement should provide a hit resolution between 6–18 microns and 40 microns for the impact parameter of high momentum tracks.
Downstream of the vertex detector, the tracking system reconstructs the trajectories of emerging particles. Using 11 stations spaced over about as many metres, this tracking uses a honeycomb of drift chambers on the outside (where the particle fluxes are lower), enclosing a finer granularity arrangement on the inside. Microstrip gas chambers with gaseous electron multiplication is the prime contender for this part of the detector, but silicon strips and micro-cathode strips are also being investigated. The inner tracker is being investigated by Heidelberg (University and MPI), PNPI St Petersburg and Santiago (Spain), and the outer by Dresden, Free University of Amsterdam, Freiburg, Humboldt Berlin, IHPE Beijing, NIKHEF Amsterdam and Utrecht.
LHCb’s 1.1 tesla superconducting dipole spectrometer magnet (studied by CERN and PSI Villigen) would benefit from the infrastructure developed for the Delphi magnet at LEP. The magnet polarity is reversible to help the systematic study of CP violation effects.
Particle identification is carried out using the ring-imaging Cerenkov (RICH) technique, with the first RICH equipped with a 5 cm silica aerogel and 1 m C4F10 gas radiators behind the vertex detector and the second station with 2 m of CF4 gas radiator behind the tracker. Cerenkov photons would be picked up by a hybrid photodiode array, the subject of a vigorous ongoing R&D programme. The RICH study group consists of Cambridge, CERN, Genoa, Glasgow, Imperial College London, Milan and Oxford.
Following the second RICH is the electromagnetic calorimeter for identifying and measuring electrons using a ‘shashlik’ structure of scintillator and lead read out by wavelength-shifting fibres. It has three annular regions with different granularities to optimize readout. Identification of these electromagnetic particles is facilitated by a lead-scintillator preshower detector. Electromagnetic calorimetry is studied by a Bologna/Clermont Ferrand/lNR Moscow/lTEP Moscow/Lebedev Moscow/Milan/Orsay/Rome l/Rome 2 team.
The hadron calorimeter (Bucharest/IHEP Moscow/Kharkov/Rome 1) is of scintillator tiles embedded in iron. Like the electromagnetic calorimeter upstream, it has three zones of granularity. Readout tests with a full-scale module prototype in a beam have already exceeded the expected performance of 50 photoelectrons per GeV. Downstream, shielded by the calorimetry, four layers of muon detector (Beijing/CERN/Hefei/Nanjing/PNPI/Shandong/Rio de Janeiro/Virginia) uses multigap resistive plate chambers and cathode pad chambers embedded in iron, with an additional plane of cathode pad chamber muon detectors mounted in front of the calorimeters. As well as muon identification, this provides important input for the triggering.
Data handling will use four levels of triggering (event selection), with initial (level 0) decisions based on a high transverse-momentum particle and using the calorimeter and muon systems. This reduces the 40 MHz input rate by a factor of 40. The next level trigger (level 1) is based on information from the vertex detector (to look for secondary vertices) and from tracking (essentially to confirm high transverse momentum) and reduces the data by a factor of 25 to an output rate of 40 kHz. Level 2, suppressing fake secondary decay vertices, achieves another further 8-fold compression. Level 3 reconstructs B decays to select specific decay channels, achieving another compression factor of 25 and data are written to tape at 2OO Hz. Data handling and offline computing are being looked at by Bologna, Cambridge, CERN, Clermont Ferrand, Heidelberg, Lausanne, Lebedev, Marseille, NIKHEF, Orsay, Oxford, Rice and Virginia.
• May 1998 pp3–5 (abridged).
Beauty at the LHC
The Standard Model of physics, with its picture of six quarks and leptons grouped in pairs into three generations, is coming under detailed scrutiny as physicists try to understand what makes it work so well. This demands precision probes of all quark channels, rare as well as familiar.
The LHC will be a prolific source of B particles containing the fifth (beauty, b) quark, either in beam–beam collisions or using one of the high energy proton beams in a fixed-target set-up. Obvious aims of the B-physics programme at the LHC are to investigate the mixing of neutral B mesons, the particle lifetimes and the spectroscopy of beauty baryons. However the main goal will be observing CP violation in the neutral B system (neutral mesons containing b with either d or s quarks).
CP violation – the subtle disregard of an otherwise perfect symmetry of a combined particle–antiparticle and left-right switch – has been known for 30 years and only seen in the decays of neutral kaons. Its origin is still a mystery but it is widely believed to be responsible for the universe’s matter-antimatter asymmetry. The Big Bang initially produced equal amounts of matter and antimatter but the tiny CP-violation mechanism was enough to tilt the balance in favour of matter as we know it.
To complement the B physics capabilities of LHC’s big detectors (ATLAS and CMS), one dedicated B physics experiment is planned for the initial phase of the LHC experimental programme. Three groups submitted Letters of lntent based on different experimental approaches:
• colliding beams at the full LHC 14 TeV collision energy (the COBEX project)
• an internal gas jet target intercepting a circulating beam at the fixed target energy of 114 GeV (the GAJET project)
• a beam extracted from the beam halo by a bent crystal and a septum magnet for a fixed target experiment (the LHB project).
Considering these ideas, the LHC Experiments Committee pointed out that when LHC comes on line, initial measurements of CP violation in the B meson system will have been made by several ongoing projects. The LHCb will therefore be a second-generation study. While identifying attractive features in all three Letters of lntent, the Committee was of the view that an experiment using the collider approach, handling the full production rate, is the most attractive.
The Committee, whose view was subsequently endorsed by the Research Board, encouraged all participants in the three Letters of Intent to join together to submit a fresh design for a collider-mode B experiment.
• September 1994 p10.
Birth of a collaboration
The stage being set for CERN’s LHC proton–proton collider includes a place for an experiment – LHC-B – to study the physics of B particles. The Letter of Intent for this experiment has been reviewed by the appropriate committees, who recommend that the collaboration should now proceed to a vigorous research and development programme for the various detector components en route to a full technical proposal.
By the time the LHC is operational, the B meson system will have been extensively studied elsewhere – in the B factories being built at SLAC (Stanford) and at KEK, Japan, at Cornell’s revamped CESR ring, at the HERA-B experiment at DESY, Hamburg, and at Fermilab’s Tevatron. The LHC-B experiment will therefore be a second-generation study. While all three initially submitted approaches had different appealing features, the collider route, exploiting the full B production rate, was thought to be the most attractive for mature physics. CERN therefore encouraged all participants in the initial B-physics ideas to collaborate in a fresh design for a collider-mode experiment. The result is the LHC-B collaboration, which currently groups almost 200 researchers from 40 institutes in 15 countries, and is growing.
• April/May 1996 pp2–4 (extract).
