The Large Hadron Collider[1]
Have you read the preceding page on the Higgs and its conceptual framework?
This symbol which adorns a variety of poster displays at the travelling London Science Museum’s Large Hadron Collider (LHC) exhibition, on display from August to October 2016 at the Sydney Powerhouse Museum, says just about all that need be said about the LHC - at least in a nutshell.
Hadrons, may I repeat, are matter particles - protons, neutrons - composed of quarks. The rectangular shape at the bottom of the graphic represents the gas bottle which contains the hydrogen, the source of the gas which contains the protons, trillions of them in each of two beams, which are ultimately injected into the LHC tunnel in opposite directions before being brought forcefully into collision on multiple occasions in the LHC tunnel. One ordinary such gas bottle contains enough gas to supply the LHC for several months.
The protons and neutrons in the gas are separated by means of a magnetic field. They are only loosely bound and so can easily be separated, unlike the case of lead atoms, where some 82 electrons are tightly bound to the lead nucleus. The protons are isolated into groups before travelling in a linear direction along the Lineac 2 accelerator (the straight line after the rectangular shape representing the gas bottle). They are then successively injected into the proton synchroton (628m circumference), the proton synchroton booster (157m circumference), and the super proton synchroton (7 km circumference) - the three dark circles at the lower end of the graphic.
As they flow through each synchroton, their speed is successively ramped up with the aid of electric fields which move the charged particles through the tunnel until the beams enter the 27 kilometre LHC ring (the blue circle), which is colder than deep space, where they travel around in opposite directions each at 99.9999991% of the speed of light (670,616,623 miles per hour), before being brought into collision. The faster the protons travel before they collide, the higher the energy of the collision, which has been described as the energy equivalent of a jumbo passenger jet at full speed.
The four black dots on the blue circle are huge underground cathedral-sized detectors where the beams cross over and collide.at the rate of 40 million times per second in each detector at temperatures hotter than the sun. In the process, the protons are destroyed, energy is converted into matter, and hundreds of new particles created, but because collisions are a quantum process, no one can tell in advance which particles will emerge. Most of the particles produced are well known and are not very interesting to physicists. However occasionally an exotic particle such as the Higgs (150 times heavier than a proton) is created, because the faster these collisions occur as the beams cross, the heavier the particles likely to be created. Many particles thus created are unstable and decay into other particles before they can be detected.
Inside the LHC tunnel
Each time the protons in the beams complete a circuit of the LHC (11,000 times per second) they get an energy kick from 16 accelerating cavities, 8 for each beam, sitting on one straight section of the LHC beam. The electric fields in the beams switch from positive to negative at a frequency that means the particles are constantly nudged forward, gaining energy with each push[2]
Accelerating the protons increases their energy and keeps them in tight bunches. These cavities only work at a chilly -268.7 degrees, not much above absolute zero, and sit in a bath of liquid helium to keep them cool. Operating at such low temperatures, these superconductors carry huge electric currents.
Most of the LHC ring is made of incredibly powerful magnets which steer the beams in a curved direction around the ring because the protons must travel in a circle. The LHC magnets number some 1232 in total, each being 16.6 metres long. Their total weight is 40,000 tonnes. Smaller magnets squeeze and focus the beams, keeping them narrower than a human hair and bring them into collision. These magnets have four poles: north-south-north-south. Colliding the beams inside the detectors has been compared with firing two knitting needles from the opposite sides of the Atlantic and having them meet halfway.
Each time the protons in the beams complete a circuit of the LHC (11,000 times per second) they get an energy kick from 16 accelerating cavities, 8 for each beam, sitting on one straight section of the LHC beam. The electric fields in the beams switch from positive to negative at a frequency that means the particles are constantly nudged forward, gaining energy with each push[2]
Accelerating the protons increases their energy and keeps them in tight bunches. These cavities only work at a chilly -268.7 degrees, not much above absolute zero, and sit in a bath of liquid helium to keep them cool. Operating at such low temperatures, these superconductors carry huge electric currents.
Most of the LHC ring is made of incredibly powerful magnets which steer the beams in a curved direction around the ring because the protons must travel in a circle. The LHC magnets number some 1232 in total, each being 16.6 metres long. Their total weight is 40,000 tonnes. Smaller magnets squeeze and focus the beams, keeping them narrower than a human hair and bring them into collision. These magnets have four poles: north-south-north-south. Colliding the beams inside the detectors has been compared with firing two knitting needles from the opposite sides of the Atlantic and having them meet halfway.
The beam pipes in which the protons travel are emptier than deep space, as any contaminants would reduce the intensity of the beam. However, just occasionally specks of dust may pass through the beam creating sudden bursts of collision.
The LHC experiments
The four detectors are used to conduct experiments run by collaborations of scientists from institutes all over the world. Each experiment is distinct, and characterised by its detectors:
The LHC experiments
The four detectors are used to conduct experiments run by collaborations of scientists from institutes all over the world. Each experiment is distinct, and characterised by its detectors:
- The Atlas experiment (ATLAS) is a general purpose detector designed to study a wide range of particles and phenomena including Higgs bosons, dark matter and extra dimensions. The Transition Radiation tracker (TRT) is part of that experiment and it records the tracks of charged particles as they fly out from the collision and helps to identify the type of particle detected.
- The Compact Muon Solenoid Experiment (CMS) hunts for the same types of particles as ATLAs but uses different technologies. The two detectors are run independently of each other so that they can countercheck each other’s findings. Having two independently designed detectors is vital for cross-confirmation of any new discoveries made. ATLAS (the A Toroidal LHC ApparatuS experiment) and CERN (the Conseil Européen pour la Recheche Nucléaire) were instrumental in the discovery of the Higgs boson. When a particles passes through the silicon tracker, it creates an electrical signal, allowing its location to be determined.
- The Large Hadron Collider Beauty experiment (LHCb) is designed to make very precise measurements of particles involving beauty particles or bottom quarks. This might help us to learn how matter and not antimatter came to dominate the universe. The Vertical Locator (VELO) modules which form part of this experiment sit just 7 mm from the collision point, closer than any other detector at the LHC, affording scientists the opportunity to identify beauty particles among the hundreds of other particle tracks. There are 42 such modules in the LHCb. While the beams are being injected, the VELO modules are pulled back to avoid them being vaporised.
- A Large ION Collider Experiment (ALICE) is a powerful detector that studies an exotic super-hot state of matter called a quark-gluon plasma produced when the LHC collides with the nuclei of lead atoms. The Time of Flight modules which forms part of this experiment measure the time it takes for particles to travel from the collision point, with a precision of ten billionths of a second. “They’re incredibly precise, but the Time of Flight modules were built from materials you can buy in a hardware store: window glass and fishing line!”
How the detectors do their work[3]
The detectors gather clues about the particles – including their speed, mass and charge – from which physicists can work out a particle's identity. Particles produced in collisions normally travel in straight lines, but in the presence of a magnetic field their paths become curved. Electromagnets around particle detectors generate magnetic fields to exploit this effect. Physicists can calculate the momentum of a particle – a clue to its identity – from the curvature of its path: particles with high momentum travel in almost straight lines, whereas those with very low momentum move forward in tight spirals inside the detector.
Modern particle detectors consist of layers of subdetectors, each designed to look for particular properties, or specific types of particle. Tracking devices reveal the path of a particle; calorimeters stop, absorb and measure a particle’s energy; and particle-identification detectors use a range of techniques to pin down a particle's identity. Tracking devices reveal the paths of electrically charged particles as they pass through and interact with suitable substances. Most tracking devices do not make particle tracks directly visible, but record tiny electrical signals that particles trigger as they move through the device. A computer program then reconstructs the recorded patterns of tracks.
One type of particle, the muon, interacts very little with matter – it can travel through metres of dense material before it is stopped. For this reason, muon chambers – tracking devices specialized for detecting muons – usually make up the outermost layer of a detector. Collating all these clues and others[4] from different parts of the detector, physicists build up a snapshot of what was in the detector at the moment of a collision. The next step is to scour the collisions for unusual particles, or for results that do not fit current theories.
What happens next?
In particle physics, a trigger is a system that uses criteria to rapidly decide which events in a particle detector to keep when only a small fraction of the total can be recorded. So much data is captured from the four main particle detectors at the LHC that it is not possible to simply transfer it to "offline" data facilities, much less to permanently store it for future processing[5]. For this reason the LHC detectors are equipped with real-time analysis systems, called triggers, which process this volume of data in real-time and select the most interesting proton-proton collisions – about 2000 per second.
The CERN Computer Centre then processes and stores the raw data, but there is too much to analyse in one place, so the data is sent out from CERN to computer centres across the world where it can be stored an analysed using the Worldwide LHC Computing Grid. Multiple copies of the data are sent across the Grid, so somewhere out there on the Grid lies a wealth of information, mined from the ground, so to speak, whose vast potential is as yet unrealised. It may yet harbour the secrets of dark matter, why gravity is so weak and whether there are more dimensions.
[1] Unless otherwise indicated, this is a condensed summary of material in the travelling Science Museum London's exhibition on display at the Powerhouse Museum, Sydney, on 17 August 2016. A comprehensive analysis also appears in Chapter 7 of Gavin Hesketh's The Particle Zoo - The search for the fundamantal nature of reality, Quercus, Hachette, London, 2016.
[2] The technique is described in Nature at http://www.nature.com/news/cern-prepares-to-test-revolutionary-mini-accelerator-1.18519
[3] https://home.cern/about/how-detector-works See also The Particle Zoo, op cit, Chapter 7, the segment entitled "Not your typical digital camera".
[4] Ibid.
[5] http://arxiv.org/abs/1509.06173 (the Cornell University site). (It may be necessary to copy and paste this link into your browser).
The detectors gather clues about the particles – including their speed, mass and charge – from which physicists can work out a particle's identity. Particles produced in collisions normally travel in straight lines, but in the presence of a magnetic field their paths become curved. Electromagnets around particle detectors generate magnetic fields to exploit this effect. Physicists can calculate the momentum of a particle – a clue to its identity – from the curvature of its path: particles with high momentum travel in almost straight lines, whereas those with very low momentum move forward in tight spirals inside the detector.
Modern particle detectors consist of layers of subdetectors, each designed to look for particular properties, or specific types of particle. Tracking devices reveal the path of a particle; calorimeters stop, absorb and measure a particle’s energy; and particle-identification detectors use a range of techniques to pin down a particle's identity. Tracking devices reveal the paths of electrically charged particles as they pass through and interact with suitable substances. Most tracking devices do not make particle tracks directly visible, but record tiny electrical signals that particles trigger as they move through the device. A computer program then reconstructs the recorded patterns of tracks.
One type of particle, the muon, interacts very little with matter – it can travel through metres of dense material before it is stopped. For this reason, muon chambers – tracking devices specialized for detecting muons – usually make up the outermost layer of a detector. Collating all these clues and others[4] from different parts of the detector, physicists build up a snapshot of what was in the detector at the moment of a collision. The next step is to scour the collisions for unusual particles, or for results that do not fit current theories.
What happens next?
In particle physics, a trigger is a system that uses criteria to rapidly decide which events in a particle detector to keep when only a small fraction of the total can be recorded. So much data is captured from the four main particle detectors at the LHC that it is not possible to simply transfer it to "offline" data facilities, much less to permanently store it for future processing[5]. For this reason the LHC detectors are equipped with real-time analysis systems, called triggers, which process this volume of data in real-time and select the most interesting proton-proton collisions – about 2000 per second.
The CERN Computer Centre then processes and stores the raw data, but there is too much to analyse in one place, so the data is sent out from CERN to computer centres across the world where it can be stored an analysed using the Worldwide LHC Computing Grid. Multiple copies of the data are sent across the Grid, so somewhere out there on the Grid lies a wealth of information, mined from the ground, so to speak, whose vast potential is as yet unrealised. It may yet harbour the secrets of dark matter, why gravity is so weak and whether there are more dimensions.
[1] Unless otherwise indicated, this is a condensed summary of material in the travelling Science Museum London's exhibition on display at the Powerhouse Museum, Sydney, on 17 August 2016. A comprehensive analysis also appears in Chapter 7 of Gavin Hesketh's The Particle Zoo - The search for the fundamantal nature of reality, Quercus, Hachette, London, 2016.
[2] The technique is described in Nature at http://www.nature.com/news/cern-prepares-to-test-revolutionary-mini-accelerator-1.18519
[3] https://home.cern/about/how-detector-works See also The Particle Zoo, op cit, Chapter 7, the segment entitled "Not your typical digital camera".
[4] Ibid.
[5] http://arxiv.org/abs/1509.06173 (the Cornell University site). (It may be necessary to copy and paste this link into your browser).