episode content

Author: Julie Hogan

episodes/01-introduction.md

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+---
+title: "Introduction"
+teaching: 5
+exercises: 0
+---
+
+:::::::::::::: questions
+- What is the CMS detector?
+- What are the design objectives of the CMS detector?
+- What are the main detector components of the CMS detector?
+::::::::::::::
+
+:::::::::::::: objectives
+- Learn about the CMS detector and how it works.
+::::::::::::::
+
+## Introduction and overview
+
+The CMS experiment is 21 m long, 15 m wide and 15 m high, and sits in a cavern that could contain all the residents of Geneva; albeit not comfortably.
+The detector is like a giant filter, where each layer is designed to stop, track or measure a different type of particle emerging from proton-proton and heavy ion collisions. Finding the energy and momentum of a particle gives clues to its identity, and particular patterns of particles or “signatures” are indications of new and exciting physics
+
+![](../fig/cms_160312_02.png){:width="100%"}
+
+*Above: A schematic view of the CMS detector.*
+
+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.
+
+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:
+* a high performance system to detect and measure muons,
+* a high resolution method to detect and measure electrons and photons (an electromagnetic calorimeter),
+* a high quality central tracking system to give accurate momentum measurements, and
+* a “hermetic” hadron calorimeter, designed to entirely surround the collision and prevent particles from escaping.
+
+Particles emerging from collisions first meet a tracker, made entirely of silicon, that charts their positions as they move through the detector, allowing us to measure their momentum. Outside the tracker are calorimeters that measure the energy of particles. In measuring the momentum, the tracker should interfere with the particles as little as possible, whereas the calorimeters are specifically designed to stop the particles in their tracks.
+
+The Electromagnetic Calorimeter (ECAL) - made of lead tungstate, a very dense material that produces light when hit – measures the energy of photons and electrons whereas the Hadron Calorimeter (HCAL) is designed principally to detect any particle made up of quarks (the basic building blocks of protons and neutrons). The size of the magnet allows the tracker and calorimeters to be placed inside its coil, resulting in an overall compact detector.
+
+As the name indicates, CMS is also designed to measure muons. The outer part of the detector, the iron magnet “return yoke”, confines the magnetic field and stops all remaining particles except for muons and neutrinos. The muon tracks are measured by four layers of muon detectors that are interleaved with the iron yoke. The neutrinos escape from CMS undetected, although their presence can be indirectly inferred from the “missing transverse energy” in the event.
+
+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.
+
+![](../fig/CMSslice_whiteBackground.png){:width="75%"}
+
+*Above: A transverse slice of the CMS detector and the particles detected by each subdetector.*
+
+Below is an interactive 3D model of the CMS detector:
+<p>
+  <div class="embed-responsive embed-responsive-16by9">
+    <iframe class="embed-responsive-item"
+            src="https://cms3d.web.cern.ch/detector-embedded/"
+	    allowfullscreen>
+    </iframe>	
+  </div>
+</p>
+  
+Move around the image below to see what CMS looks like in the experimental cavern:
+<p>
+  <div id="aframe">
+    <a-scene embedded>
+      <a-sky src="../fig/0pOgmYSEETO5_equirectangular_16384.jpg" rotation="0 -120 0">
+      </a-sky>
+    </a-scene>
+  </div>
+</p>
+
+
+:::::::::::::: keypoints
+- The CMS detector is a large general-purpose detector at the LHC, CERN.
+- CMS consists of layers of detector material that exploit the different properties of particles to catch and measure the energy or momentum of each one.
+::::::::::::::

episodes/02-tracker.md

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+---
+title: "Tracker detector"
+teaching: 5
+exercises: 0
+---
+
+:::::::::: questions
+- What is the tracker and how does it work?
+::::::::::
+
+:::::::::: objectives
+- Learn about the tracker, what it measures, and how it works.
+::::::::::
+
+
+## Tracker
+
+![](../fig/cms_tracker.png)
+
+### Overview
+
+Measuring the momentum of particles is crucial in helping us to build up a picture of events at the heart of the collision. One method to calculate the momentum of a particle is to track its path through a magnetic field; the more curved the path, the less momentum the particle had. The CMS tracker records the paths taken by charged particles by finding their positions at a number of key points.
+
+The tracker can reconstruct the paths of high-energy muons, electrons and hadrons as well as see tracks coming from the decay of very short-lived particles such as beauty or “b quarks” that will be used to study the differences between matter and antimatter.
+
+The tracker needs to record particle paths accurately yet be lightweight so as to disturb the particle as little as possible. It does this by taking position measurements so accurate that tracks can be reliably reconstructed using just a few measurement points. Each measurement is accurate to 10 µm, a fraction of the width of a human hair. It is also the inner most layer of the detector and so receives the highest volume of particles: the construction materials were therefore carefully chosen to resist radiation.
+
+The final design consists of a tracker made entirely of silicon: the pixels, at the very core of the detector and dealing with the highest intensity of particles, and the silicon microstrip detectors that surround it. As particles travel through the tracker the pixels and microstrips produce tiny electric signals that are amplified and detected. The tracker employs sensors covering an area the size of a tennis court, with 75 million separate electronic read-out channels: in the pixel detector there are some 6000 connections per square centimetre.
+
+### Pixels
+
+The pixel detector, though only about the size of a small suitcase, contains 124 million pixels, allowing it to track the paths of particles emerging from the collision with extreme accuracy. It is also the closest detector to the beam pipe, with cylindrical layers roughly at 3cm, 7cm, 11cm and 16cm and disks at either end, and so will be vital in reconstructing the tracks of very short-lived particles.
+
+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).
+
+![](../fig/Pixelement.gif)
+
+*Above: A schematic of a pixel detector.*
+
+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.
+
+However, being so close to the collision means that the number of particles passing through is huge: the rate of particles received at 3cm from the beamline is around 600 million particles per square centimetre per second! The pixel detector is able to disentangle and reconstruct all the tracks particles leave behind, and withstand such a pummeling over the duration of the experiment. 
+
+### Strips
+
+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.
+
+![](../fig/CMS_photo_3_courtesy_of_CERN.jpg){:width="75%"}
+
+*Above: Silicon strips in the tracker barrel.*
+
+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.
+
+![](../fig/tracker_rz_labelled.png)
+
+*Above: A projected event display view of the CMS tracker (contained within the ECAL barrel and endcaps) looking perpendicular to the beam pipe.*
+
+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.
+
+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.
+
+![](../fig/tracker_rphi.png) | ![](../fig/tracker_rphi_tracks.png)
+
+*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.*
+
+![](../fig/tracker_rz.png) | ![](../fig/tracker_rz_tracks.png)
+
+*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.*
+
+
+:::::::::: keypoints
+- A particle emerging from the collision and travelling outwards will first encounter the tracking system, made of silicon pixels and silicon strip detectors.
+- The tracker accurately measures the positions of passing charged particles allowing physicists to reconstruct their tracks.
+::::::::::

episodes/03-ecal.md

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+---
+title: "Electromagnetic Calorimeter (ECAL)"
+teaching: 5
+exercises: 0
+---
+
+::::::::::: questions
+- What is the ECAL?
+:::::::::::
+
+::::::::::: objectives
+- Learn about the ECAL and how it works.
+:::::::::::
+
+
+## ECAL
+
+![](../fig/cms_ecal.png)
+
+The energies of electrons and photons are measured using the CMS electromagnetic calorimeter (ECAL). Measuring their energies with the necessary precision in the very strict conditions of the LHC - a high magnetic field, high levels of radiation, and only 25 nanoseconds between collisions - requires dedicated detector materials. 
+
+Lead tungstate crystal is made primarily of metal and is heavier than stainless steel, but with a touch of oxygen in this crystalline form, it is highly transparent and “scintillates” when electrons and photons pass through it. This means it produces light in proportion to the impinging particle’s energy. These high-density crystals produce light in fast, short, well-defined photon bursts that allow for a precise, fast, and fairly compact detector.
+
+The ECAL, made up of a “barrel” section and two “endcaps”, forms a layer between the tracker and the HCAL. The cylindrical barrel consists of 61,200 crystals formed into 36 “supermodules”, each weighing around three tonnes and containing 1700 crystals. The flat endcaps seal off the barrel at each end and are made up of almost 15,000 more crystals.
+
+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.
+
+![](../fig/ECALcrystals_0.jpg){:width="75%"}
+
+*Above: Lead tungstate crystals. One can see an APD attached to the end of one of the crystals at the bottom of the image.*
+
+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.
+
+![](../fig/ecal_0.png) | ![](../fig/ecal_1.png)
+
+*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.*
+
+
+::::::::::: keypoints
+- The ECAL is designed to measure the energies of electrons and photons with great precision.
+:::::::::::

episodes/04-hcal.md

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+---
+title: "Hadron Calorimeter (HCAL)"
+teaching: 5
+exercises: 0
+---
+
+:::::::::: questions
+- What is the HCAL?
+::::::::::
+
+:::::::::: objectives
+- Learn about the HCAL and how it works.
+::::::::::
+
+
+## HCAL
+
+![](../fig/cms_hcal.png)
+
+The Hadron Calorimeter (HCAL) measures the energy of “hadrons”, particles made of quarks and gluons (for example protons, neutrons, pions and kaons). Additionally it provides indirect measurement of the presence of non-interacting, uncharged particles such as neutrinos.
+
+Measuring these particles is important as they can tell us if new particles such as the Higgs boson or supersymmetric particles (much heavier versions of the standard particles we know) have been formed.
+As these particles decay they may produce new particles that do not leave record of their presence in any part of the CMS detector. To spot these the HCAL must be “hermetic”, that is make sure it captures, to the extent possible, every particle emerging from the collisions. This way if we see particles shoot out one side of the detector, but not the other, with an imbalance in the momentum and energy (measured in the sideways “transverse” direction relative to the beam line), we can deduce that we’re producing “invisible” particles.
+
+To ensure that we’re seeing something new, rather than just letting familiar particles escape undetected, layers of the HCAL were built in a staggered fashion so that there are no gaps in direct lines that a familiar particle might escape through.
+
+The HCAL is a sampling calorimeter [see explanation below] meaning it finds a particle’s position, energy and arrival time using alternating layers of “absorber” and fluorescent “scintillator” materials that produce a rapid light pulse when the particle passes through. Special optic fibres collect up this light and feed it into readout boxes where photodetectors amplify the signal.   When the amount of light in a given region is summed up over many layers of tiles in depth, called a “tower”, this total amount of light is a measure of a particle’s energy.
+As the HCAL is massive and thick, fitting it into “compact” CMS was a challenge, as the cascades of particles produced when a hadron hits the dense absorber material (known as showers) are large, and the minimum amount of material needed to contain and measure them is about one metre.   
+
+To accomplish this feat, the HCAL is organised into barrel (HB and HO), endcap (HE) and forward (HF) sections. There are 36 barrel “wedges”, each weighing 26 tonnes. These form the last layer of detector inside the magnet coil whilst a few additional layers, the outer barrel (HO), sit outside the coil, ensuring no energy leaks out the back of the HB undetected.  Similarly, 36 endcap wedges measure particle energies as they emerge through the ends of the solenoid magnet.
+
+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.
+
+![](../fig/Figure_001.png)
+
+*Above: A schematic view of the HCAL detectors, looking "from the side", perpendicular to the beam pipe.*
+
+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.
+
+A megatile is a layer of tiles whose sizes depend on their spatial location and orientation relative to the collision, chosen so that each receives roughly the same number of particles. Optic fibres fit into grooves cut into the individual tiles. Because the light picked up gives a measure of energy, the gaps between tiles must be filled with a reflective paint to ensure that light produced in each tile cannot escape into others and vice versa.
+
+
+:::::::::: keypoints:
+- The HCAL measures the energy of “hadrons”, particles made of quarks and gluons (for example protons, neutrons, pions and kaons).
+- The HCAL is hermetic, made up of a barrel, endcaps, and forrward and outer detectors.
+::::::::::