Large Electron-Positron Collider
Encyclopedia
The Large Electron–Positron Collider (LEP) was one of the largest particle accelerator
s ever constructed.
It was built at CERN
, a multi-national centre for research in nuclear and particle physics near Geneva
, Switzerland
. LEP was a circular collider with a circumference
of 27 kilometres built in a tunnel straddling the border of Switzerland
and France
. It was used from 1989 until 2000. To date, LEP is the most powerful accelerator of leptons ever built.
each to enable production of the Z boson, which has a mass of 91 GeV. The accelerator was upgraded later to enable production of a pair of W bosons, each having a mass of 80 GeV. LEP collider energy eventually topped at 209 GeV at the end in 2000. At a Lorentz factor
( = particle energy/rest mass = [104.5 GeV/0.511 MeV]) of over 200,000, LEP still holds the particle accelerator speed record, extremely close to the limiting speed of light. At the end of 2000, LEP was shut down and then dismantled in order to make room in the tunnel for the construction of the Large Hadron Collider
(LHC).
The Super Proton Synchrotron
(an older ring collider) was used to accelerate electron
s and positron
s to nearly the speed of light
. These are then injected into the ring. As in all ring colliders, the LEP's ring consists of many magnet
s which force the charged
particles into a circular trajectory
(so that they stay inside the ring), RF accelerators which accelerate the particles with radio
frequency
(RF) waves, and quadrupole
s that focus the particle beam (i.e. keep the particles together). Rather than increasing the particles' velocities (which are already very close to the speed of light), the function of the accelerators is really to increase the particles' energies so that heavy particles can be created when the particles collide. When the particles are accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch is made to collide with each other at one of the collision points of the detector. When an electron and a positron collide, they annihilate to a virtual particle
, either a photon
or a Z boson. The virtual particle almost immediately decay
s into other elementary particles, which are then detected by huge particle detector
s.
, momentum
and charge, thus allowing physicists to infer the particle reaction that had happened and the elementary particles involved. By performing statistical analysis
of this data, knowledge about elementary particle physics is gained. The four detectors of LEP were called Aleph, Delphi, Opal, and L3. They were built differently to allow for complementary experiments
.
value of 3. The running of the quantum chromodynamics
(QCD) coupling constant
was measured at various energies and found to run in accordance with perturbative
calculations in QCD.
equipment. The lead glass
blocks from the OPAL barrel electromagnetic calorimeter
are currently being re-used by the NA62 experiment
at CERN. The name was a pun since some of the founding members of the scientific collaboration which first proposed the design had previously worked on the JADE detector at DESY
in Hamburg
.
—most importantly the mass of the Z boson and the W boson (which were discovered in 1983 at an earlier CERN
collider) to be obtained—and so confirm the Model and put it on a solid basis of empirical data.
of current high-energy physics. The run-time was extended for a few months, to no avail. The strength of the signal remained at 1.7 standard deviation
s which translates to the 91% confidence level, much less than the confidence expected by particle physicists to claim a discovery. Subsequent experiments at the Tevatron
have not yet been sensitive enough to confirm or refute these hints.
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s ever constructed.
It was built at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
, a multi-national centre for research in nuclear and particle physics near Geneva
Geneva
Geneva In the national languages of Switzerland the city is known as Genf , Ginevra and Genevra is the second-most-populous city in Switzerland and is the most populous city of Romandie, the French-speaking part of Switzerland...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
. LEP was a circular collider with a circumference
Circumference
The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....
of 27 kilometres built in a tunnel straddling the border of Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. It was used from 1989 until 2000. To date, LEP is the most powerful accelerator of leptons ever built.
History
When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45 GeVGEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
each to enable production of the Z boson, which has a mass of 91 GeV. The accelerator was upgraded later to enable production of a pair of W bosons, each having a mass of 80 GeV. LEP collider energy eventually topped at 209 GeV at the end in 2000. At a Lorentz factor
Lorentz factor
The Lorentz factor or Lorentz term appears in several equations in special relativity, including time dilation, length contraction, and the relativistic mass formula. Because of its ubiquity, physicists generally represent it with the shorthand symbol γ . It gets its name from its earlier...
( = particle energy/rest mass = [104.5 GeV/0.511 MeV]) of over 200,000, LEP still holds the particle accelerator speed record, extremely close to the limiting speed of light. At the end of 2000, LEP was shut down and then dismantled in order to make room in the tunnel for the construction of the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
(LHC).
Operation
Super Proton Synchrotron
The Super Proton Synchrotron is a particle accelerator of the synchrotron type at CERN. It is housed in a circular tunnel, in circumference, straddling the border of France and Switzerland near Geneva, Switzerland. The SPS was designed by a team led by John Adams, director-general of what was...
(an older ring collider) was used to accelerate electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s to nearly the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. These are then injected into the ring. As in all ring colliders, the LEP's ring consists of many magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s which force the charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
particles into a circular trajectory
Trajectory
A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...
(so that they stay inside the ring), RF accelerators which accelerate the particles with radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
(RF) waves, and quadrupole
Quadrupole
A quadrupole or quadrapole is one of a sequence of configurations of—for example—electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.-Mathematical...
s that focus the particle beam (i.e. keep the particles together). Rather than increasing the particles' velocities (which are already very close to the speed of light), the function of the accelerators is really to increase the particles' energies so that heavy particles can be created when the particles collide. When the particles are accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch is made to collide with each other at one of the collision points of the detector. When an electron and a positron collide, they annihilate to a virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
, either a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
or a Z boson. The virtual particle almost immediately decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
s into other elementary particles, which are then detected by huge particle detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
s.
Detectors
The Large Electron–Positron Collider had four detectors, built around the four collision points within underground halls. Each was the size of a small house and was capable of registering the particles by their energyEnergy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
and charge, thus allowing physicists to infer the particle reaction that had happened and the elementary particles involved. By performing statistical analysis
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
of this data, knowledge about elementary particle physics is gained. The four detectors of LEP were called Aleph, Delphi, Opal, and L3. They were built differently to allow for complementary experiments
Complementary experiments
In physics, two experimental techniques are often called complementary if they investigate the same subject in two different ways such that two different properties or aspects can be investigated...
.
ALEPH
ALEPH stands for Apparatus for LEP PHysics at CERN. The detector determined the mass of the W-boson and Z-boson to within one part in a thousand., The number of families of particles with light neutrinos was determined to be , which is consistent with the standard modelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
value of 3. The running of the quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD) coupling constant
Coupling constant
In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian of a system can be separated into a kinetic part and an interaction part...
was measured at various energies and found to run in accordance with perturbative
Perturbation theory
Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem...
calculations in QCD.
DELPHI
DELPHI stands for DEtector with Lepton, Photon and Hadron Identification.OPAL
OPAL stands for Omni-Purpose Apparatus for LEP. The detector was dismantled in 2000 to make way for LHCLarge Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
equipment. The lead glass
Lead glass
Lead glass is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40 weight% lead oxide , while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO...
blocks from the OPAL barrel electromagnetic calorimeter
Calorimeter (particle physics)
In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its...
are currently being re-used by the NA62 experiment
NA62 experiment
The NA62 experiment is a planned particle physics experiment at the North Area of the SPS accelerator at CERN. It aims to start physics data taking in 2014, when it will become the first in the world to probe the decays of the charged kaon with probabilities down to 10−12...
at CERN. The name was a pun since some of the founding members of the scientific collaboration which first proposed the design had previously worked on the JADE detector at DESY
DESY
The DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....
in Hamburg
Hamburg
-History:The first historic name for the city was, according to Claudius Ptolemy's reports, Treva.But the city takes its modern name, Hamburg, from the first permanent building on the site, a castle whose construction was ordered by the Emperor Charlemagne in AD 808...
.
Results
The results of the LEP experiments allowed precise values of many quantities of the Standard ModelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
—most importantly the mass of the Z boson and the W boson (which were discovered in 1983 at an earlier CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
collider) to be obtained—and so confirm the Model and put it on a solid basis of empirical data.
Dr. Bagger on precision and the mass of the Z boson at CERN: "The experimenters found that the Z boson got heavier at certain times of the day. This was a very high-precision experiment. They discovered that the patterns of the particle getting heavier corresponded to the tides. The gravitational adjustments due to tides slightly changed the shape of the collider over the course of the day. After adjusting for tidal effects, they found that the Z boson was heavier in spring and lighter in fall. This was because there's a lake in Geneva near the detector, that is drained in Fall to make room for the spring snow-melt. So the bigger lake in the Spring was making the particle heavier. After correcting for both of these factors, they found that the particle got suddenly heavier multiple times during the day, at the same times. This was because a train runs near the detector whose electromagnetic fields were disturbing the experiment. This is how precise the experiment was."
An unfinished discovery of the Higgs boson
Precision measurements of the shape of the Z boson mass peak constrained the number of light neutrinos in the standard model to exactly three. Near the end of the scheduled run time, data suggested very tentative but inconclusive hints that the Higgs particle of a mass around 115 GeV might have been observed, a sort of Holy GrailHoly Grail
The Holy Grail is a sacred object figuring in literature and certain Christian traditions, most often identified with the dish, plate, or cup used by Jesus at the Last Supper and said to possess miraculous powers...
of current high-energy physics. The run-time was extended for a few months, to no avail. The strength of the signal remained at 1.7 standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
s which translates to the 91% confidence level, much less than the confidence expected by particle physicists to claim a discovery. Subsequent experiments at the Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
have not yet been sensitive enough to confirm or refute these hints.
External links
- LEP Working Groups
- The LEP Collider from Design to Approval and Commissioning excerpts from the John AdamsJohn Adams (physicist)Sir John Bertram Adams KBE FRS was a British nuclear physicist and administrator.During World War II, Adams worked in the Radar laboratories of the British Ministry of Aircraft Production where he learned physics and engineering on the job. After the war he moved to Harwell and the Atomic Energy...
memorial lecture delivered at CERN on 26 November 1990 - A short but good (though slightly outdated) overview (with nice photographs) about LEP and related subjects can be found in this online booklet of the British Particle Physics and Astronomy Research CouncilParticle Physics and Astronomy Research CouncilThe Particle Physics and Astronomy Research Council was one of a number of Research Councils in the United Kingdom. It directed, coordinated and funded research in particle physics and astronomy for the people of the UK...
.