Accelerator Overview (SLAC)
A particle accelerator uses electro-magnetic fields to accelerate charged particles to very high speeds, contain them in well-defined beams, collide them with other particles, and then analyze the results. Particle accelerators are used as a research tool in particle physics by accelerating elementary particles to very high energies and forcing them to impact other particles. Analysis of the by products of these collisions gives scientists evidence of the structure of the subatomic world and the laws of nature governing it. These results are apparent only at high energies, for tiny periods of time, and are impossible to study in other ways.
Early accelerators fired a stream of particles at fixed targets. Typical early experiments shot a beam of electrons at a plate of material such as gold. Physicists soon learned that to uncover smaller and smaller particles, they needed higher and higher energy impacts which led to "beam to beam" accelerators. Beam to beam accelerators generate far more than twice the energy that "beam to target" types do. The initial beam to beam colliders were linear accelerators, beams that were a
straight line. See the picture to the left of the SLAC accelerator at Stanford, CA. (SLAC originally stood for Stanford Linear Accelerator Center.) Scientists then graduated to circular beams because the beams could make many trips around the accelerator gaining more and more energy each trip before being smashed. Accelerators built in recent years are all circular.
What kind of particles make good beam colliders? First, the colliding particles must have a charge so that they can be accelerated by an electro-magnetic field. The obvious choices are electrons and protons. Because the electron does not divide after a collision, it makes a nice clean collision. However, light particles radiate energy when they are accelerated around a circle. Electrons lose a significant portion of their energy when accelerated in a circle and that is why the SLAC accelerator was a "linear accelerator". Protons are 2,000 times heavier than an electron. The nucleus of a proton contains many gluons and many quark anti-quark pairs besides the three valence quarks that give it its positive charge. So in a proton collision there are many loose particles flying around afterwards that make it messy to decipher.
On the other hand, the heavy mass of the proton when accelerated to 99.9999% of the speed of light has a tremendous amount of energy. And, from Einstein we know that mass and energy are interchangeable. So, when two protons collide, a tremendous amount of energy is released that can convert itself into possible new particles that physicists would like to analyze. Therefore, modern accelerators use protons and circular beams to maximize the probability of creating exciting new particles.
Detectors are the instruments that contain the collision area and the surrounding recording equipment. The SLAC SLD detector is shown at the left. Detectors are huge, note the size of the two men in the center. The SLD detector operated from 1992 to 1998. The SLAC PEP II detector operated from 1198 to 2008. The SLAC accelerator facilities are currently used mainly for medical radiation research.
Collision Operation. The copper pipes that contain the colliding beams are maintained in an extremely strong vacuum. The protons are released in "bunches". Each bunch is about 10 centimeters (4 inches) long and one millimeter wide. Bunches are separated by about 10 meters. The collisions take place at the center of the detector, called the interaction point. Powerful magnets focus the two opposing beams down to an extremely small area to increase the probability of collisions. About 20 collisions are recorded from each bunch. This is a manageable number for the computer equipment to handle. The detectors measure the energy, charge, and momentum of photons, electrons, and hopefully some new particles. The tau particle was discovered by the SLAC team between 1974 and 1978. Top
|Collider||Location||First Year||Last Year||Shape||Length||Energy||Particles|
|SLAC||Stanford, CA||1966||2008||Linear||3.2 km||100 GeV||electron/positron|
|Tevatron||Fermilab, IL||1983||2011||Circular||6.3 km||1,960 GeV||proton/anti-proton|
|LEP||CERN, Geneva||1989||2000||Circular||26.6 km||209 GeV||electron/positron|
|LHC||CERN, Geneva||2008||----||Circular||26.6 km||13,000 GeV||proton/proton|
The Tevatron is located at Fermilab in Batavia, Illinois outside of Chicago and was the most powerful accelerator in the world for 25 years. The Tevatron accelerated heavy proton particles at record energy levels until the Large Hadron Collider (LHC) came on stream in 2008 and set the new world record in 2009. The Tevatron was shut down for good on September 30, 2011. The Tevatron is perhaps best known for the discovery of the heavy "top" quark in addition to other more complex particles.
Fermilab was initially known as the National Accelerator Laboratory. It was renamed in 1974 in honor of Enrico Fermi, a naturalized American physicist. In 1938, Fermi received the Nobel Prize in Physics at the age of 37 and soon afterwards emigrated from Italy to New York City and Columbia University. Enrico Fermi was well known for
his work on the first nuclear reactor, his contributions to particle physics, and for his contributions to quantum theory.
The Tevatron accelerates protons and anti-protons in a 3.9 mile (6.3 km) ring to energies of one TeV (trillion electron volts), hence its name. The Tevatron was first completed in 1983 with significant upgrades incorporated since. The 3.9 mile circular Tevatron (see the large ring at the left) accelerator uses super-conducting magnets cooled to -267 °C (-450 °F) as cold as outer space to move particles at nearly the speed of light. The Tevatron produces about 10 million proton-antiproton collisions per second. Each collision produces hundreds of particles. About 200 collisions per second are recorded at each detector for further analysis.
The cooling system, which circulates liquid helium to the Tevatron's super-conducting magnets (at -450 °F), was the largest low temper-ature cooling system in existence when it was completed in 1978. It keeps the coils of the magnets (which bend and focus the particle beams) in a super-conducting state (minimum resistance) so that they consume only one-third of the power the magnets would require at normal temperatures. See the Tevatron tunnel at the left.
Acceleration occurs in a number of stages. The first stage is the 750 keV (thousand electron volts) pre-accelerator which ionizes hydrogen gas. The ions then pass into the 150 meter long linear accelerator (linac) which uses oscillating electrical fields to accelerate the particles to 400 MeV (million electron volts). The ions then pass through a carbon foil
to remove the electrons. The charged protons then move into the Booster ring. The Booster is a small circular accelerator, around which the protons circulate about 20,000 times to attain an energy of around 8 GeV (billion electron volts). From the Booster the particles pass into the Main Injector, which was completed in 1999. The Tevatron can accelerate the particles in the Main Injector up to 980 GeV. The protons and anti-protons are accelerated in opposite directions colliding in the CDF detector (shown at the left) at 1.96 TeV. Note the size of the worker near the bottom of the picture by the ladder.
During the 1990's Fermilab had the only accelerator in the world able to generate a 2 TeV energy collision, and in order to be able to "confirm" a new particle discovery, the DZero detector was added to the ring. In 1995, the CDF and DZero detectors confirmed the discovery of the "top quark", and by 2007 they had measured its mass of 176 GeV to a remarkable precision of 1%.
Because of its enormous mass, the top quark is extremely short-lived with a life of only 5×10^−25 seconds As a result, top quarks do not have time to form hadrons before they decay as other quarks do. (This provides physicists with the unique opportunity to study the behavior of a "bare quark". As of this writing, no "bare quarks" have ever been observed.) By the end of its life on September 30, 2011, the CDF detector had analyzed more than 10 inverse femtobarns of collision data, while the DZero detector had scrutinized up to 11 inverse femtobarns. (One inverse femtobarn represents about 70 trillion proton anti-proton collisions.) That is a humongous amount of data that was analyzed. Top
CERN LEP And LHC
Large Electron Positron (LEP) Collider. The LEP collider was built at CERN, the multi-national center for research in particle physics near Geneva, Switzerland. The LEP was a circular collider with a circumference of 26.6 kilometers (16.5 miles) at depths ranging from 50 to 175 meters (160 to 575 feet) underground. The 3.8 meter (12 foot) wide concrete lined tunnel crosses the border between Switzerland and France at four points, with most of it in France. Surface buildings hold auxiliary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.
It was operational from 1989 until 2000. The LEP was the most powerful accelerator of leptons (electrons and positrons) ever built. At the end of 2000, the LEP was shut down and dismantled in order to make room in the tunnel for the construction of the new Large Hadron Collider (LHC). See the LHC tunnel picture at the lower left with the
super-conducting magnets in blue. (Super-conducting magnets are also found in MRI machines.)
The much smaller circular feeder accelerator, 6.9 kilometers (4.3 miles) of the LEP, called the SPS, Super Proton Synchrotron, is famous for the discovery of the twin W bosons (W+ and W-) and the neutral Z boson in May, 1983. They had been predicted in 1968 by the theory that unified the electro-magnetic force and the weak force into the electro-weak force. For the unification theory, Sheldon Glashow, Steven Weinberg and Abdus Salam shared the 1979 Nobel Prize in Physics. Their electro-weak theory predicted not only the W bosons necessary to explain beta decay, but also the Z boson. The accelerator leaders who discovered the W and Z bosons, Carlo Rubbia and Simon van der Meer, also were awarded the Nobel Prize in 1984.
Large Hadron Collider (LHC). The LHC, the world's largest and most powerful particle accelerator, is located in the same circular tunnel built for the LEP. The accelerator was built in collaboration with over
10,000 scientists and engineers from over 100 countries as well as hundreds of universities and laboratories.
The LHC contains two adjacent parallel beam pipes that intersect at four points where the main detectors are located (high-lighted in blue names at the left, such as ATLAS and CMS). Each pipe contains a proton beam which travels around the ring in the opposite direction from the other beam. Some 1,232 dipole magnets, each weighing about 27 tons, keep the proton beams on their circular path. An additional 392 quadrupole magnets are used to keep the beams narrowly focused. (See the center of the "quadrupole" illustration at the below left.)
Also, 96 tons of liquid helium are used to keep the magnets at their operating temperature of −271 °C (1.9 K), making the LHC the largest cryogenic (super cold) facility in the world and colder than outer space. A strong vacuum is maintained in the beam pipes in order to prevent the circulating protons from crashing into stray particles. The
pressure in the pipes is 10 times less than on the surface of the moon. It will take less than 90 microseconds (millionths of a second) for a proton to travel once around the main ring, which works out to be about 11,000 revolutions per second.
When the two beams of protons collide they generate temperatures of more than a billion times those in the center of the sun, but in a very tiny volume of space. These temperatures approach those in the very early moments (one billionth of a second) of the Big Bang. The extreme temperatures help in creating exciting new particles.
The first beam was circulated through the collider on the morning of September 10th, 2008. It took less than one hour to guide the stream of particles clockwise sector by sector around its inaugural circuit. CERN next sent a beam of protons in a counter clockwise direction in the adjacent beam pipe which also only took one and a half hours. These first tests of the proton beams were extremely successful.
However, nine days later on September 19th, a minor explosion occurred and approximately six tons of liquid helium were ejected into the tunnel. The cause of the problem was a faulty electrical connection between two of the magnets. A total of 14 quadrupole and 39 dipole magnets were damaged, and 2,000 feet of beam pipe were contaminated with soot. In the end, 53 magnets were repaired or replaced and 4 kilometers of beam piping had to be cleansed.
It took over a year for the repairs and new safety systems to be installed. Nine hundred pressure release ports and 6,500 pressure detectors were added to the magnetic system. All the other joints of the tunnel system were also closely inspected while the LHC was down. Fourteen months later on November 20th, 2009 the LHC came back online and was extremely successful. Not only did the proton beams circulate without problems, but a few days later on November 23rd, the beams collided with an energy of 1.8 TeV spraying particles to the excitement of the experimenters. On November 30th, 2009 the collision energies were increased to 2.36 TeV breaking Fermilab's eight year old record for the highest collision energy ever. On March 30th, 2010, the two beams collided at 7 TeV initiating the LHC advanced research program. The LHC continued at 7 TeV until the scheduled short upgrade period at the end of December, 2011.
Starting in April, 2012 the collision energy was increased by 15% to 8 TeV. Other small operational measures were also implemented to increase the number of collisions per unit of time (the luminosity) to maximize the opportunities for scientific breakthroughs. The LHC ran at 8 TeV until February of 2013. Then it shut down for 24 months while a major upgrade to the super-conducting magnets was installed so that the LHC could run at its full design collision energy of 13 TeV. After the LHC upgrade was completed in May, 2015, its next research run with protons colliding at 13 TeV commenced. The LHC "second run" will operate at 13 TeV until at least 2018.
Major Discovery - The Higgs Boson. CERN physicists announced in early 2012 with 99% certainty that they had found a new elementary particle weighing about 126 times the mass of a proton. After collecting two and a half times more data, the physicists said the particle was the Higgs Boson. The Higgs Boson, the particle whose field explains how other particles get their mass. was definitively announced in July, 2012.
The ATLAS image to the left is a collision resulting in a Higgs particle temporarily being formed and then decaying.
The scientists were not sure whether this particular Higgs Boson is the one predicted by the Standard Model or perhaps the lightest of several bosons predicted to exist by other theories. The Higgs particle should have zero spin, its parity should be positive, and its mass should be between 120 and 130 GeV. All of these attributes were confirmed by data from both the ATLAS and CMS detectors.
For much more Higgs information, see Hunting The Higgs Page.
ATLAS and CMS. For background information, ATLAS stands for "A Toroidal LHC ApparatuS" and CMS stands for "Compact Muon Solenoid". ATLAS and CMS are designed as general purpose detectors. (See the drawings at the left, ATLAS is above and CME is below.) When the proton beams produced by the LHC collide in the center of the detectors, many different particles with a wide range of energies will be produced. Particles that are produced in accelerators must also be observed and that is the role of the particle detectors. Rather than focusing on one speculated outcome, they are designed to measure the broadest possible range of signals. This is to ensure that whatever new particles are formed, the detectors will be able to sense them and measure their properties. ATLAS and CMS were designed to discover new heavy particles that might be produced by the
enormous energy of the LHC collisions. The other four detectors are designed for specific purposes to expand the knowledge of known forces and particles. The goals for both ATLAS and CMS include the search for The Higgs Boson, extra dimensions, and particles that could make up dark matter.
When the LHC is operational large amounts of radiation are present and hence this is why all beam related systems are buried deep underground. Since the detectors were finished, there is no more human access to them because of the radiation and also because they were designed to last a decade without any maintenance. However, during the long shut downs, many parts of the detectors could be accessed if need be.
As with Fermilab, two independent detectors are needed to confirm any findings. While in principle the two detectors use similar scientific approaches in their detection schemes, the detailed designs of the two
detectors were done by two completely different teams. They also differ greatly in size as the CMS detector is much more compact than ATLAS. ATLAS is 45 meters long and 25 meters wide versus only 21 meters long and 15 meters wide for CMS. (See the picture of the ATLAS detector at the left and note the size of the man standing above the white box on the bottom of the picture. Wow!) However, CMS weighs considerably more than ATLAS, 12,500 tons versus 7,000 tons. The two detectors are located in enormous caverns about 100 meters below ground level. The concrete caverns were started in the year 2000 and finished in 2004. Both have huge concrete vertical channels to the surface at ground level. Large cranes were used to lower the extremely delicate sections of the detectors which were then assembled on the platforms below. It is amazing that such large instruments are necessary to measure miniscule particles.
During 2010 and 2011, ATLAS and CMS each had analyzed 5 inverse femtobarns of data. During 2012 CERN set a goal of 15 inverse femtobarns to be analyzed. (One inverse femtobarn represents about 70 trillion or 70 million million or 70 x 10^12 collisions.) Top
Analyzing Particles (CMS)
The above illustration is a slice of the CMS detector from the collision point to the perimeter as shown in the lower left hand corner. The various colored curved lines represent paths that a particle might take, for example, the red line represents an electron.
The following is a discussion of each of the major CMS layers:
The Interaction Point - This is the point in the center of the detector where collisions occur between the two counter rotating beams. (See the actual CMS collision photo at the left. The 4 green lines are electrons.) The protons are released in "bunches" around the ring, about 2,800 total bunches at a time, with each bunch containing about 115 billion protons. A bunch is released every 50 nanoseconds and is about 10 centimeters (4 inches) long and one millimeter wide. Bunches are separated by about 10 meters. At opposite ends of the detector, powerful magnets focus the two opposing beams down to an extremely small diameter of about 16 microns (millionths of a meter - about a fifth the size of a human hair) at the Interaction Point to increase the probability of collisions. Even though the the size of the collision area is 16 x 10^-6 meters, the diameter of a hydrogen proton is 2 x 10^-15 meters (2 femtometers or 2 millionths of a nanometer), meaning
The Silicon Tracker (pictured at the left) - Immed-iately around the "Interaction Point" the inner tracker serves to identify the tracks of individual particles and match them to the locations where they originated. The curved trail of charged particle tracks in the magnetic field allows their charge and momentum to be measured. This part of the CMS detector is the world's largest silicon detector. It has 205 square meters of silicon sensors (approximately the area of a tennis court).
The Electromagnetic Calorimeter (ECAL) - The Electromagnetic Calorimeter is designed to measure the energies of electrons and photons with high accuracies. The ECAL is constructed from crystals of lead tungstate. This is an extremely dense but optically clear material ideal for stopping high energy particles.
The Hadronic Calorimeter (HCAL) - The purpose of the Hadronic Calorimeter is to to provide wide angle coverage and record the energy and continuous position of individual hadrons (protons and neutrons). This will enable CMS physicists to be able to observe as many particles as possible from an interaction and allow collisions with missing energy and particles to be reconciled. The HCAL contains layers of brass and steel alternating with plastic tiles that record the energy and position of hadrons passing through. See the insertion of the CMS HCAL into the Solenoid Magnet at the left.
The Super-Conducting Solenoid Magnet - The large muon solenoid magnet (which gives the detector its name - "Compact Muon Solenoid") is not part of the detection scheme per se, but part of the identification process. The strong magnetic field causes charged particles to curve allowing the experimenters to calculate the charge of the particle as well as its momentum. This allows the charge/mass ratio of particles to be calculated which is critical in identifying some of the particles.
The superconducting magnet is 13 meters long and 6 meters in diameter (the circular silver item in the middle at the left). Superconductivity is achieved by chilling coils to a temperature of almost absolute zero. The CMS solenoid generates a magnetic field of an incredible 4 Tesla, some 100,000 times stronger than the earth's
The Muon Detectors - The outer most elements in any general purpose detector are the muon chambers (in silver at the left). Muons are charged particles like electrons except they are 200 times heavier. They barrel through the inner layers and then the outer most layers too. Muons are relatively easy to detect and measure.
Muons are more likely to be associated with interesting collisions. The muon detectors will also be useful for detecting any other "heavy" charged particles that make it to the outer reaches of the detector, which are the particles physicists are most interested in. Very high energy is necessary to create heavy particles previously unknown to physicists.
The Return Yoke - The large red bars, shown at the left between the muon detectors (silver) , are a huge iron structure that forms the "return yoke" which provides a return route for the magnetic field. The return yoke uses approximately 10,000 tons of iron, more iron than is used in the Eifel Tower. Top
that the collision area is still mostly empty space. So the probability of a collision is still very small even with 115 billion protons from each beam passing one another. The beams collide at a very slight angle. Bunches collide every 50 nanoseconds and about 20 collisions are recorded from each confronting bunch.
The crystals are backed by silicon photo diodes for readout. The barrel region consists of 61,200 crystals, with 7,324 more crystals in each of the end caps.
magnetic field. The nominal current for the solenoid magnet is 18,500 amps which stores the equivalent energy of about a half ton of TNT in the magnet.
Major Upgrade - The High-Luminosity LHC (HL-LHC).
During October 2015 scientists and engineers from around the world met at CERN to discuss the High-Luminosity LHC (HL-LHC), a big upgrade to the Large Hadron Collider (LHC) that will be a major increase in the accelerator’s performance starting in 2025. It will increase the LHC’s luminosity by a factor of 10. Luminosity is the number of collisions that occur in a given amount of time. The higher the luminosity, the more data the experiments will generate allowing scientists to observe very rare collisions that create new particles. After a four year design study, the HL-LHC project is now moving into its second phase, which is the development of industrial level prototypes for various parts of the accelerator.
The beam will be more intense and more concentrated than at present. One particular challenge will be maintaining luminosity at a constant level throughout the lifespan of the beam. At present, the beam decreases over time as the protons collide. In the HL-LHC, the beam focusing (the concentration of the beam before impact) will be designed in such a way that the number of collisions remains constant.
To achieve this major upgrade, scientists and engineers are optimizing the collider’s parameters as shown in the image below.
The Alice Detector
Alice (A Large Ion Collider Experiment) is one of the seven detectors at the Large Hadron Collider at CERN. Alice is optimized to study heavy-ion lead to lead collisions as opposed to much lighter proton to proton collisions. The resulting temperature and energy density are high enough to produce a very short quark-gluon-plasma, a state of matter where quarks and gluons are freed for about a micro-second. (Similar conditions are believed to have existed a fraction of a second after the Big Bang before quarks and gluons bound together to form heavier particles.)
During head-on collisions of lead ions, hundreds of protons and neutrons smash into one another at energies of a few TeVs. Lead ions are accelerated to more than 99.9999% of the speed of light and lead collisions at the LHC are more than 40 times more energetic than those of protons. This heats matter at the interaction point to a temperature almost 100,000 times higher than the temperature in the core of the sun.
When the two lead nuclei slam into each other, matter undergoes a transition to form for a brief instant a droplet of primordial matter, the quark-gluon-plasma (QGP), which is believed to have filled the universe a few microseconds after the Big Bang. The QGP is formed as protons and neutrons "melt" into their elementary constituents in which quarks and gluons become free (an extremely rare condition).
This creates - for a tiny moment, a small volume of matter consisting of quarks, antiquarks and gluons that has a temperature of over 4,000 billion degrees. The droplet of QGP instantly cools and the individual quarks and gluons recombine into a blast of ordinary matter that speeds away in all directions.
Each lead nucleus contains 82 protons, and the LHC accelerates each proton to an energy of 5 TeV, thus resulting in a total collision energy density of an unprecedented 20 GeV. About 30,000 charged particles are emitted from each collision, a sample shown at the left above as lines radiating from the collision point. The colors of the lines indicate how much energy each particle carried away from the collision.
In order to identify all the particles that are coming out of the the QGP, Alice is using a set of 18 detectors that give information about the mass, the velocity and the electrical sign of the particles. The short lived heavy particles cover a very small distance before decaying. The system aims at identifying these phenomena by measuring the location where they occur with a precision of a tenth of a millimeter.
Alice has confirmed that the matter created in lead to lead collisions behaves like a fluid, with strong collective motions that are well described by modern hydrodynamic equations.
Data Collection And Processing
New particles will typically be unstable and rapidly cascade into lighter, more stable, known particles. These secondary particles will then leave behind characteristic "signatures" in the different detector layers which will identify them. The presence of new particles can then be inferred. Most collision events are "soft" and do not produce interesting results.
The amount of raw data from collisions is about 40 terabytes a second, a humongous amount that cannot possibly be processed and stored. So a series of "trigger" levels are deployed in the detector to reduce the data rate to manageable levels. All data is held in temporary buffers while a small amount of critical information is quickly processed to identify key features - such as high energy jets, muons, or missing energy. (See the CMS 6 Jets in the picture below left.) This "Level 1" calculation is completed in about one microsecond reducing the "event rate" by a factor of a thousand. All these calculations are done on very fast, field programmable circuits located in the detector itself.
If an event is "passed" by the Level 1 trigger, all the data still buffered in the detector, is sent over fiber optic links to the "High Level" trigger. The High Level trigger is a software program that runs on normal computer servers. The lower data rate in the High Level trigger allows time for much more detailed analysis of events than in the Level 1 trigger. The High Level trigger further reduces the "event rate" by another factor of a thousand down to a manageable 100 events per second. These events are then stored on tape for future analysis. Data that has passed the triggering stages and has been stored on tape is then sent to additional sites around the world.. The LHC produces roughly 15 million gigabytes of data annually.
Thousands of scientists around the world want to analyze the LHC data, so CERN is collaborating with institutions in 34 countries to organize a distributed data storage structure called the Worldwide LHC Computing Grid (WLCG). After initial processing, data is distributed to 11 large computer centers with sufficient storage. These “Tier-1” centers then make the data available to over 160 “Tier-2” centers for specific scientific analysis. Individual scientists can access the LHC data in their home countries using local computer servers or individual PCs.