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DETECTORS AT THE LHC: ATLAS AND CMS UNIVERSIDAD COMPLUTENSE MADRID- SPAIN INTRODUCTION Beatriz González Merino and Clara González-Santander de la Cruz The aim of this work is to summarize the main characteristics of the detectors that will be used at the LHC accelerator. We will study their principal components and some experiments that will be carried out. Física Nuclear y de Partículas (347). Facultad de Ciencias Físicas WHAT IS THE LHC? ATLAS AND CMS LHC stands for Large Hadron Collider. Large due to its size, Hadron because it accelerates protons or ions (Pb), which are hadrons, and Collider because these particles form two beams, travelling in opposite directions, which collide at four points around the machine’s circumference. The energy of the collision is the sum of the energies of the two beams, bigger than the one produced at accelerators where a beam collides with a stationary target. The collision at such high energy could produce new particles to study. • Located at CERN, Geneva (Switzerland). • Currently under construction, it is scheduled to begin operation in 2008. • Funded and built in collaboration with over two thousand physicists from thirty-four countries, universities and laboratories. • Experiments at the LHC: ATLAS, CMS, ALICE, LHCb, TOTEM, LHCf. They are general purpose detectors designed to cover the widest possible range of physics at the LHC, from the search for the Higgs boson, supersymmetry (SUSY) and extra dimensions. The difference between them is the technical solution. This modern general-purpose high-energy physics detector needs to be hermetic, so most of the particles are detected. For engineering convenience, ATLAS and CMS adopt the barrel plus end-caps design where a cylindrical detector covers the central region and two flat circular end-caps cover the angles close to the beam (the forward region). Function (from the inner to the outer region) Carries the beam of particles- bunches of protons (100 billion approximately) TRACKING DEVICE Reveal the paths of electrically charge particle when they interact with detector's atom's substance → measures the momentum under a magnetic field due to the curvature of the trajectory Measures the energy lost by the particle while it travels through it, and it is designed to fully stop or absorb most of the particles coming from a collision event. Provides the main way to identify neutral particles CALORIMETERS MUON DETECTOR Semiconductor detectors: the passing particle creates electrons and holes in a reversely-biased semiconductor, usually silicon. The devices are divided intro strips or pixels (resolution 10 microns) Tile calorimeter Difference between charged and not charged particles Number of quadrupoles 858 Nominal energy, protons 7 TeV Weight 7000 tonnes 12500 tonnes Nominal energy, ions 2.76 TeV/u Location Meyrin, Switzerland Cessy, France Design luminosity 1034 cm-2s-1 Numbers of turns per second 11245 Number of collisions per second 600 million Figure 4 Hadronic calorimeters (HCAL): measure the energy of the particles made of quarks (hadrons) when they interact with atomic nuclei by the production of charged particles. Pions, protons, or neutrons Drift Tubes (DT) Cathode Strip Chambers (CSC) Thin Gap Chambers (TGC) Resistive Plate Chambers (RPC) Figure 6 Tracking Electron. Hadronic Muon device Calorimeter calorimeter detector Photon Photon, electrons or positrons Consists of a steel plate structure spread with 500000 plastic tiles of 3 mm thickness, it senses the hadronic shower particles. It is formed by modules which are controlled and read by three optical fibres. It also has a liquid argon end cap formed by a liquid argon device made of copper plates. Muons are the only charged particle that can travel through all of the calorimeter material. The Muon System determines the signs and momenta of muons with better precision than the inner tracking system does. It has some different parts: MDT (tube resolution 80µm), CSC (resolution 60µm), TGC and RPC (electric field of 5000V/mm). 1232 21 m log, 15 m high, 15 m wide Figure 5 Electromagnetic calorimeters (ECAL): measure the energy of light particles when they interact with the electrically charged particles inside matter (from the electromagnetic shower) Tile calorimeter (hadronic) Number of dipoles 46 m long, 25 m high, 25 m wide Figure 3 Electrons or positrons Muons Figure 8. DATA ANALYSIS: Trigger and data acquisition system: an event has to pass two independent sets of tests or trigger levels, for selecting 100 interesting events per second out of 1000. The data acquisition system channels the data from the detector to the storage. Physicists from around the world use this data for their studies. Pions or protons Neutrons Muons Figure 7. Onion-like structure of experiments. The particles trough the detector and their interaction in each of the regions from the innermost to the outermost layer as explained in the left. ATLAS: A Toroidal LHC ApparatuS Muon detector 9300 Particles detected Gaseous chambers: the medium ionized is a gas and the ions or electrons are collected on electrodes under strong electric fields (spatial resolution 50-100 microns) Determines the signs and momenta of muons Number of magnets CMS Size Figure 5 and 6. Tracks of a simulated event detected at ATLAS and CMS, respectively Types BEAM PIPE-COLLISION REGION 26659 m Figure 3 and 4. Simulated event in the ATLAS and CMS detector, respectively The particle detectors are made to record and visualize the particles which come from the collisions at accelerators (LHC: collision between protons at 14 TeV). They give information about their charge, mass or speed. The shape, the size, the length, the shape, the direction and the depth are only some parameters we can study. Detector zone Circumference (previous LEP) ATLAS Figure 2. General detector shape DETECTOR GENERALITIES Figure 1. The LHC system Important quantities for the LHC: CMS: Compact Muon Solenoid Tracker Liquid Argon calorimeter (electromagnetic) Consists of thin lead plates (about 1,5 mm thick), immersed in a bath of liquid argon (-183ºC), separated by sensing devices. Particle showers in the liquid argon produce ions which are read out as electric pulses by Kapton electrodes. Cylinder of a length of 6m and a diameter of 2.6m. Fine pitch silicon strip detectors provide precise hits (25000 silicon strip sensors over 210m2). In the central region the momentum resolution is given by ∆p/pi=0.005+0.15pi (pi in TeV) Electromagnetic calorimeter (ECAL) Nearly 80000 crystals of scintillating lead tungstate (PbWO4) are used to measure accurate energies of electrons and photons. A preshower detector, which contains two thin lead converters followed by silicon strip detector planes placed in front of the ECAL, helps in the γ-π0 separation. The inner tracker and its parts. Silicon tracker Toroid Magnets There are an additional set of magnets located in the regions outside of the calorimeters, they produces a magnetic field which lines are circles centred on, and perpendicular to the beam line, and encircling it. The toroidal field deflects the muons in the plane defined by the beam axis and the muon position Electromagnetic calorimeter Transition Radiation Tracker (TRT) Solenoid Magnet It is located outside the inner detector. A magnetic field (2T) is produced by a large electrical current (8000A) inside a hollow cylindrical coil. It deflects each charged particle coming from the collision: Pixel detector Solenoid magnet •Particle perpendicular to the beam: travels in a circle. •Not perpendicular to the beam: the field changes the trajectory to a circular helix. Semiconductor Tracker (SCT) - Strip Detectors It is used to give additional position measurements further from the collision point. It has layers of silicon subdivided into narrow strips, each layer has two sets of strips. On the cylinders, the strips run parallel to the the beam axis, on the disks, the sets go radially. When a charged particle goes through the strip detector, signals identify which strip in each set has been touched. The dissipated power in the detector, more than 30kW, is removed by and evaporating cooling system, keeping the silicon temperature at –7ºC. The intersection of those two strips gives a 3-dimensional position measurement. This is very expensive, that is why this is not used for larger detector areas. Is the closest sensor to the collision point. These devices are made of thin layers of silicon subdivided into pixels of dimensions 50×400µm2. The ATLAS detector will have approximately 140 million of these tiny pixel sensors. Closer to the collision, pixels are placed cylindrically, whereas they are located on disks further away. Each time a charged particle traverses a layer a signal is produced, that gives a precise measure of particle position to know if the particle has been originated at the collision point, or a few millimetres from it as a decay product of another. They are formed by gas-wire drift detectors made of tubes with thin wires running through the centres. They are filled with an appropriate gas, and high voltage is applied between the wire and the metallized tube wall. When a particle traverses a tube, its wire produces a discharge and that determines how far from the wire has the particle passed. In the centre section the tubes run parallel to the beam pipe, and in the two end sections the tubes are positioned radially. TRT Magnet system: superconducting coil+ magnet yoke (barrel and encap) + vacuum tank + cryogenics. Hadronic calorimeter (HCAL) Hadronic barrel (HB) and hadronic endcap (HE) calorimeters are sampling calorimeters with 50mm thick copper (selected because its density) absorber plates which are interleaved with 4mm scintillator sheets. HB is made of two 4.4m length half-barrels, and HE has two large structures, situated at each end of the barrel detector and within the region of high magnetic field. It is 13m long and 6m in diameter, and its refrigerated superconducting niobium-titanium coils, cooled at -270ºC, produces a magnetic field of 4 Teslas. The magnetic field is obtained by a solenoid because with the field parallel to the beam, the bending of the muon tracks is in the transverse plane (with an accuracy of 20µm) and the momentum measurement starts at zero radial distance from the beam pipe. Inner tracker HIGGS PHYSICS The Standard Model (SM) of Particle Physics has unified the electromagnetic interaction (carrier: γ – massless-) and the weak interaction (carriers: Z0, W+, W – quite massive: 80-90GeV-). According to the SM, particles acquire mass trough their interaction with the Higgs field, which implies the existence of a new particle: the Higgs boson H0. MEDICAL The theory does not predict the H0 mass, but it predicts its production rates and decay modes for each possible mass. Measuring the decays at ATLAS and CMS detectors the MH could be determined. Figure 11 TECHNICAL Figure 11 and 12. Different simulations of the Higgs decays depending on its mass. The Higgs signal that will be measured is indicated in each case. H0→ZZ*→2l+2l- If the mass is in the range 130-700 GeV the most promising channels are or H0→ZZ→2l+2l-. The detection relies on the excellent performance from the muon chambers, the tracker and the electromagnetic calorimeter CULTURE SUPERSYMMETRY (SUSY) In SUSY each fermion (matter particle, spin -1/2) has a “superpartner” of spin 0 while each boson (force carrier, integer spin) has a spin -1/2 superpatner. Nowadays, no superpartners have been observed, it may be caused by the different mass of the superpartners relatively to their partner particles. If the scale is in the TeV it will be measured at the LHC. Figure 13. Production of sparticles may reveal itself REFERENCES Eyes on the LHC, F. Gianotti and C. Quigg, Physics Today, September 2007 http://public.web.cern..ch http://atlas.ch http://cms.cern.ch trough some kinematical spectra with a pronounced edge in the l+l- (leptons) mass spectrum, as in the inclusive dilepton mass spectrum image. Muon detectors They are placed behind the calorimeters and the coil, and consist of four muon stations interleaved the iron return yoke plates. In the barrel region they are arranged in concentric cylinders around the beam line, and in perpendicular disks to the beam line in the endcaps. CMS uses three types of detector: drift tubes (DT), cathode strip chambers (CSC) and resistive plate chambers (RPC). Drif tube cross section Parts of the muon chamber OTHER APLICATIONS FOR THE TECHNICAL ADVANCES USED IN ATLAS AND CMS Figure 10. Decay rates depending on the Higgs mass If the mass is in the range 80-140 GeV H0→γγ is the most promising channel. Hadronic forward (HF) calorimeter: two situated at each extreme of the CMS detector. It is made of steel absorbing plates and quartz fibers are inserted into them. The pass of charged particles trough the quartz fibers produces Cerenkov light signals, which are used to measure the energy of the jets. The inductance of the magnet is 14 henries and the nominal current is 19500 amperes, giving a total stored energy of 2.66GJ. There are dump circuits to safely dissipate this energy. The circuit resistance (0.1 milliohms) leads to a circuit time constant of nearly 39 hours, which is the longest time constant of any circuit at CERN. SCT Figure 9. Production of Higgs boson at the LHC Detail of the HB There are different types of HCAL: Superconducting magnet Pixel detector SOME PHYSICAL EXPERIMENTS AT THE LHC THAT WILL BE DETECTED AT ATLAS AND CMS Simulated event in the CMS Advances used in detectors Other applications Multi picture elementary counters Miniature electronic silicon chips Biomedical imaging (mammography) Computer tomography XPAD, photon counting detector Can be combined with Positron Emission Tomography (PET) Retina project Silicon microstrip detector technology Used to understand how living neural systems process X-Ray detector XPIX is an X-ray detector using the XPAD chip developed for ATLAS Studying the structure of proteins and solid state materials Ultrasound gas analysis Measure the fluorocarbon vapours in the cooling system of the inner detector Analyse the gas mixtures in semiconductor production Emergency personnel location System for finding and rescuing persons in case of an accident in the CERN area Is suitable in large areas such as mines where people are difficult to find Sound reproduction Measure and align the individual silicon detectors of the ATLAS Semiconductor Measure the shape of the groove on mechanical sound carriers. Grid computing Data taken are distributed and accessible to scientists of LHC experiments. Applications running on this infrastructure: Education Students are involved in the development of parts of the new detectors This work provides experience in modern laboratory work Figure 14. Ultrasound gas analysis Figure 15. Digital radiograph of a hornet. CONCLUSIONS Figure 12 The knowledge about Particle Physics is fundamental to understand the real nature of matter It requires the newest technology → challenge for physicists and for technological advances Detectors play a fundamental role because they are our eyes in the particles world, they are the instrument used to study and be near of them. They will be doing the same in the next years because with the new detectors we have broaden our possibilities of making new important discoveries.