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Copy file name to clipboardExpand all lines: episodes/01-introduction.md
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@@ -28,6 +28,7 @@ The detector is like a giant filter, where each layer is designed to stop, track
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The detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable, cooled to -268.5oC, that generates a magnetic field of 4 Tesla, about 100,000 times that of the Earth.
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Detectors consist of layers of material that exploit the different properties of particles to catch and measure the energy and momentum of each one. CMS needed:
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* a high performance system to detect and measure muons,
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* a high resolution method to detect and measure electrons and photons (an electromagnetic calorimeter),
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* a high quality central tracking system to give accurate momentum measurements, and
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Within the LHC, bunches of particles collide up to 40 million times per second, so a “trigger” system that saves only potentially interesting events is essential. This reduces the number recorded from one billion to around 100 per second.
Copy file name to clipboardExpand all lines: episodes/02-tracker.md
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Each of the four layers is composed of individual silicon modules, splitted into little silicon sensors, like tiny kitchen tiles: the pixels. Each of these silicon pixels is 100µm by 150µm, about two hairs widths. When a charged particle passes through a pixel, it gives enough energy to eject the electrons from silicon atoms. A voltage applied to the sensor allows collecting these charges as a small electric signal, which is amplified by an electronic readout chip (for a total of 16 chips per module).
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*Above: A schematic of a pixel detector.*
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Knowing which pixels have been touched allows us to deduce the particle's trajectory. And because the detector is made of 2D tiles and has four layers, we can create a three-dimensional picture.
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After the pixels and on their way out of the tracker, particles pass through ten layers of silicon strip detectors, reaching out to a radius of 130 centimetres.
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The silicon strip detector (see the figure below) consists of four inner barrel (TIB) layers assembled in shells with two inner endcaps (TID), each composed of three small discs. The outer barrel (TOB) – surrounding both the TIB and the TID – consists of six concentric layers. Finally two endcaps (TEC) close off the tracker on either end. Each has silicon modules optimised differently for its place within the detector.
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*Above: A projected event display view of the CMS tracker (contained within the ECAL barrel and endcaps) looking perpendicular to the beam pipe.*
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This part of the tracker contains 15,200 highly sensitive modules with a total of about 10 million detector strips read by 72,000 microelectronic chips. Each module consists of three elements: one or two silicon sensors, its mechanical support structure and readout electronics.
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The silicon detectors work in much the same way as the pixels: as a charged particle crosses the material it knocks electrons from atoms giving a very small pulse of current lasting a few nanoseconds. This small amount of charge is then amplified by Analogue Pipeline Voltage (APV25) chips, giving us “hits” when a particle passes, allowing us to reconstruct its path. Four or six such chips are housed within a “hybrid”, which also contains electronics to monitor key sensor information, such as temperature, and provide timing information in order to match “hits” with collisions.
*Above left: An event display view of the CMS tracker (contained within the ECAL barrel) looking along the beam pipe. Above right: An event display view of reconstructed tracks.*
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*Above left: A projected event display view of the CMS tracker (contained within the ECAL barrel and endcaps) looking perpendicular to the beam pipe. Above right: A projected event display view of reconstructed tracks.*
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Copy file name to clipboardExpand all lines: episodes/03-ecal.md
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Photodetectors, which have been especially designed to work within the high magnetic field, are glued onto the back of each of the crystals to detect the scintillation light and convert it to an electrical signal that is amplified and sent for analysis. Avalanche photodiodes or APDs are used in the the crystal barrel, and vacuum phototriodes (VPTs) for the endcaps.
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*Above: Lead tungstate crystals. One can see an APD attached to the end of one of the crystals at the bottom of the image.*
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For extra spatial precision, the ECAL also contains a preshower detector that sits in front of the endcaps. These allow CMS to distinguish between single high-energy photons (often signs of exciting physics) and the less interesting close pairs of low-energy photons.
*Above left: An event display of two electron tracks (green lines). The ECAL barrel and endcaps are shown by the blue volumes and especially in the endcaps one can make out individual crystals. The green volumes respresent the energy deposits in the crystals. Above right: A close up of the energy deposits in each crystal.*
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Copy file name to clipboardExpand all lines: episodes/04-hcal.md
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Lastly, the two hadronic forward calorimeters (HF) are positioned at either end of CMS, to pick up the myriad particles coming out of the collision region at shallow angles relative to the beam line. These receive the bulk of the particle energy contained in the collision so must be very resistant to radiation and use different materials to the other parts of the HCAL.
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*Above: A schematic view of the HCAL detectors, looking "from the side", perpendicular to the beam pipe.*
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The CMS barrel and endcap sampling calorimeters are made of repeating layers of dense absorber and tiles of plastic scintillator. When a hadronic particle hits a plate of absorber, in this case brass or steel, an interaction can occur producing numerous secondary particles. As these secondary particles flow through successive layers of absorber they too can interact and a cascade or “shower” of particles results. As this shower develops, the particles pass through the alternating layers of active scintillation material causing them to emit blue-violet light. Within each tile tiny optical “wavelength-shifting fibres”, with a diameter of less than 1mm, absorb this light. These shift the blue-violet light into the green region of the spectrum, and clear optic cables then carry the green light away to readout boxes located at strategic locations within the HCAL volume.
Copy file name to clipboardExpand all lines: episodes/06-muon.md
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@@ -34,15 +34,11 @@ In total there are 1400 muon chambers: 250 drift tubes (DTs) and 540 cathode str
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DTs and RPCs are arranged in concentric cylinders around the beam line (“the barrel region”) whilst CSCs and RPCs, make up the “endcaps” disks that cover the ends of the barrel.
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*Above: A transverse slice of CMS slowing a muon passing through RPCs and DTs in the barrel.*
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*Above left: Installation of a wheel of drift tubes. Above right: event display of two muons seen in CMS with matching drift tubes.*
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The drift tube (DT) system measures muon positions in the barrel part of the detector. Each 4-cm-wide tube contains a stretched wire within a gas volume. When a muon or any charged particle passes through the volume it knocks electrons off the atoms of the gas. These follow the electric field ending up at the positively-charged wire.
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Each DT chamber, on average 2m x 2.5m in size, consists of 12 aluminium layers, arranged in three groups of four, each up with up to 60 tubes: the middle group measures the coordinate along the direction parallel to the beam and the two outside groups measure the perpendicular coordinate.
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*Above: An event display of a muon seen in DTs. The green volumes indicate the position of the triggered wires.*
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### Cathode Strip Chambers
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Cathode strip chambers (CSC) are used in the endcap disks where the magnetic field is uneven and particle rates are high.
*Above: A double muon event seen in CMS with highlighted matching CSCs (in red).*
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CSCs consist of arrays of positively-charged “anode” wires crossed with negatively-charged copper “cathode” strips within a gas volume. When muons pass through, they knock electrons off the gas atoms, which flock to the anode wires creating an avalanche of electrons. Positive ions move away from the wire and towards the copper cathode, also inducing a charge pulse in the strips, at right angles to the wire direction.
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Because the strips and the wires are perpendicular, we get two position coordinates for each passing particle.
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In addition to providing precise space and time information, the closely spaced wires make the CSCs fast detectors suitable for triggering. Each CSC module contains six layers making it able to accurately identify muons and match their tracks to those in the tracker.
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*Above: Event display of a muon seen in CSCs. The pink lines running along the long end of the chambers indicate the triggered strips and the shorter pink lines represent the triggered wires.*
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### Resistive Plate Chambers
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*Above: Resistive plate chambers installed on one of the CMS muon endcaps.*
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Resistive plate chambers (RPC) are fast gaseous detectors that provide a muon trigger system parallel with those of the DTs and CSCs.
*Above: The positions of the RPCs in the barrel and endcaps highlighted in green*
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RPCs consist of two parallel plates, a positively-charged anode and a negatively-charged cathode, both made of a very high resistivity plastic material and separated by a gas volume.
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*Above: The layers of an RPC*
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When a muon passes through the chamber, electrons are knocked out of gas atoms. These electrons in turn hit other atoms causing an avalanche of electrons. The electrodes are transparent to the signal (the electrons), which are instead picked up by external metallic strips after a small but precise time delay. The pattern of hit strips gives a quick measure of the muon momentum, which is then used by the trigger to make immediate decisions about whether the data are worth keeping. RPCs combine a good spatial resolution with a time resolution of just one nanosecond (one billionth of a second).
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