Large Electron–Positron Collider
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The Large Electron–Positron Collider (LEP) was one of the largest
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...
s ever constructed. It was built at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
, a multi-national centre for research in nuclear and particle physics near
Geneva Geneva ( ; french: Genève ) frp, Genèva ; german: link=no, Genf ; it, Ginevra ; rm, Genevra is the List of cities in Switzerland, second-most populous city in Switzerland (after Zürich) and the most populous city of Romandy, the French-speaki ...
,
Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
. LEP collided
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s with positrons at energies that reached 209 GeV. It was a circular collider with a
circumference In geometry, the circumference (from Latin ''circumferens'', meaning "carrying around") is the perimeter of a circle or ellipse. That is, the circumference would be the arc length of the circle, as if it were opened up and straightened out to ...
of 27 kilometres built in a tunnel roughly 100 m (300 ft) underground and passing through
Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
and
France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
. LEP was used from 1989 until 2000. Around 2001 it was dismantled to make way for the
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundred ...
, which re-used the LEP tunnel. To date, LEP is the most powerful accelerator of leptons ever built.


Collider background

LEP was a circular lepton collider – the most powerful such ever built. For context, modern colliders can be generally categorized based on their shape (circular or linear) and on what types of particles they accelerate and collide (leptons or hadrons). Leptons are point particles and are relatively light. Because they are point particles, their collisions are clean and amenable to precise measurements; however, because they are light, the collisions cannot reach the same energy that can be achieved with heavier particles.
Hadrons In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
are composite particles (composed of quarks) and are relatively heavy; protons, for example, have a mass 2000 times greater than electrons. Because of their higher mass, they can be accelerated to much higher energies, which is the key to directly observing new particles or interactions that are not predicted by currently accepted theories. However, hadron collisions are very messy (there are often many unrelated tracks, for example, and it is not straightforward to determine the energy of the collisions), and therefore more challenging to analyze and less amenable to precision measurements. The shape of the collider is also important. High energy physics colliders collect particles into bunches, and then collide the bunches together. However, only a very tiny fraction of particles in each bunch actually collide. In circular colliders, these bunches travel around a roughly circular shape in opposite directions and therefore can be collided over and over. This enables a high rate of collisions and facilitates collection of a large amount of data, which is important for precision measurements or for observing very rare decays. However, the energy of the bunches is limited due to losses from synchrotron radiation. In linear colliders, particles move in a straight line and therefore do not suffer from synchrotron radiation, but bunches cannot be re-used and it is therefore more challenging to collect large amounts of data. As a circular lepton collider, LEP was well suited for precision measurements of the
electroweak interaction In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
at energies that were not previously achievable.


History

Construction of the LEP was significant undertaking. Between 1983–1988, it was the largest civil engineering project in Europe. When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45 
GeV GEV may refer to: * ''G.E.V.'' (board game), a tabletop game by Steve Jackson Games * Ashe County Airport, in North Carolina, United States * Gällivare Lapland Airport, in Sweden * Generalized extreme value distribution * Gev Sella, Israeli-Sou ...
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 The Lorentz factor or Lorentz term is a quantity expressing how much the measurements of time, length, and other physical properties change for an object while that object is moving. The expression appears in several equations in special relativit ...
( = particle energy/rest mass = 04.5 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 The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundred ...
(LHC).


Operation

LEP was fed with
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s and positrons delivered by CERN's accelerator complex. The particles were generated and initially accelerated by the
LEP Pre-Injector The LEP Pre-Injector (LPI) was the initial source that provided electrons and positrons to CERN's accelerator complex for the Large Electron–Positron Collider (LEP) from 1989 until 2000. LPI comprised the LEP Injector Linac (LIL) and the Electro ...
, and further accelerated to nearly the speed of light by the
Proton Synchrotron The Proton Synchrotron (PS, sometimes also referred to as CPS) is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It ...
and the Super Proton Synchrotron. From there, they were injected into the LEP ring. As in all ring colliders, the LEP's ring consisted of many
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, steel, nickel, ...
s which forced the charged particles into a circular
trajectory A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete traj ...
(so that they stay inside the ring), RF accelerators which
accelerate In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
d the particles with radio frequency waves, and
quadrupole A quadrupole or quadrapole is one of a sequence of configurations of things like 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 refl ...
s that focussed the particle beam (i.e. keep the particles together). The function of the accelerators was to increase the particles' energies so that heavy particles can be created when the particles collide. When the particles were accelerated to maximum energy (and focused to so-called bunches), an electron and a positron bunch were 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 A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in the perturbat ...
, either a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
or a Z boson. The virtual particle almost immediately
decay Decay may refer to: Science and technology * Bit decay, in computing * Software decay, in computing * Distance decay, in geography * Decay time (fall time), in electronics Biology * Decomposition of organic matter * Tooth decay (dental caries ...
s into other elementary particles, which are then detected by huge
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 ionizing particles, such as those produced by nuc ...
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
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
,
momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
and charge, thus allowing physicists to infer the particle reaction that had happened and the
elementary particles In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, anti ...
involved. By performing statistical analysis of this data, knowledge about
elementary particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and b ...
is gained. The four detectors of LEP were called Aleph, Delphi, Opal, and L3. They were built differently to allow for
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 (ideally non-overlapping) properties or aspects can be investigated. For example, X-ray scatter ...
.


ALEPH

ALEPH stands for ''Apparatus for LEP pHysics at CERN''. The detector determined the mass of the
W-boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
and
Z-boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
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 Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
value of 3. The running of the
quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...
(QCD) coupling constant was measured at various energies and found to run in accordance with
perturbative In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea is to start with a simple system for whi ...
calculations in QCD.


DELPHI

DELPHI stands for ''DEtector with Lepton, Photon and Hadron Identification''.


OPAL

OPAL stands for ''Omni-Purpose Apparatus for LEP''. The name of the experiment was a play on words, as 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 (male), (female) en, Hamburger(s), Hamburgian(s) , timezone1 = Central (CET) , utc_offset1 = +1 , timezone1_DST = Central (CEST) , utc_offset1_DST = +2 , postal ...
. OPAL was a general-purpose detector designed to collect a broad range of data. Its data were used to make high precision measurements of the Z boson lineshape, perform detailed tests of the Standard Model, and place limits on new physics. The detector was dismantled in 2000 to make way for LHC equipment. The
lead glass Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically al ...
blocks from the OPAL barrel electromagnetic calorimeter are currently being re-used in the large-angle photon veto detectors at the
NA62 experiment The NA62 experiment (known as P-326 at the stage of proposal) is a fixed-target particle physics experiment in the North Area of the SPS accelerator at CERN. The experiment was approved in February 2007. Data taking began in 2015, and the exper ...
at CERN.


L3

L3 was another LEP experiment. Its enormous octagonal magnet return yoke remained in place in the cavern and became part of the ALICE detector for the LHC.


Results

The results of the LEP experiments allowed precise values of many quantities of the
Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
—most importantly the mass of the Z boson and the
W boson In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , and ...
(which were discovered in 1983 at an earlier
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
collider, the Proton-Antiproton Collider) to be obtained—and so confirm the Model and put it on a solid basis of empirical data.


Higgs boson

Near the end of the scheduled run time, data suggested tantalizing but inconclusive hints that the
Higgs particle The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...
of a mass around 115 GeV might have been observed, a sort of
Holy Grail The Holy Grail (french: Saint Graal, br, Graal Santel, cy, Greal Sanctaidd, kw, Gral) is a treasure that serves as an important motif in Arthurian literature. Various traditions describe the Holy Grail as a cup, dish, or stone with miracu ...
of current
high-energy physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) a ...
. The run-time was extended for a few months, to no avail. The strength of the signal remained at 1.7 standard deviations which translates to the 91%
confidence level In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...
, much less than the confidence expected by particle physicists to claim a discovery, and was at the extreme upper edge of the detection range of the experiments with the collected LEP data. There was a proposal to extend the LEP operation by another year in order to seek confirmation, which would have delayed the start of the LHC. However, the decision was made to shut down LEP and progress with the LHC as planned. For years, this observation was the only hint of a Higgs Boson; subsequent experiments until 2010 at the
Tevatron The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as ''Fermilab''), east of Batavia, Illinois, and is the second highest energy particle collider ...
had not been sensitive enough to confirm or refute these hints. Beginning in July 2012, however, the
ATLAS An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...
and
CMS CMS may refer to: Computing * Call management system * CMS-2 (programming language), used by the United States Navy * Code Morphing Software, a technology used by Transmeta * Collection management system for a museum collection * Color manag ...
experiments at LHC presented evidence of a Higgs particle around 125 GeV, and strongly excluded the 115 GeV region.


See also

*
Electron–positron annihilation Electron–positron annihilation occurs when an electron () and a positron (, the electron's antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and positron, and the creation of energetic photo ...
*
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundred ...


References


External links

*
LEP Working Groups

The LEP Collider from Design to Approval and Commissioning
excerpts from the
John Adams John Adams (October 30, 1735 – July 4, 1826) was an American statesman, attorney, diplomat, writer, and Founding Fathers of the United States, Founding Father who served as the second president of the United States from 1797 to 1801. Befor ...
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 i
this online booklet
of the British ''
Particle Physics and Astronomy Research Council The Particle Physics and Astronomy Research Council (PPARC) 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. Its head office w ...
''.
Experiment Record for LEP
on
INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1970 ...
{{DEFAULTSORT:Large Electron-Positron Collider CERN accelerators Particle experiments Particle physics facilities CERN facilities CERN experiments