Large Electron–Positron Collider

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The former LEP tunnel at CERN being filled with magnets for the Large Hadron Collider. Inside the CERN LHC tunnel.jpg
The former LEP tunnel at CERN being filled with magnets for the Large Hadron Collider.

The Large Electron–Positron Collider (LEP) was one of the largest particle accelerators ever constructed.

Contents

It was built at CERN, a multi-national centre for research in nuclear and particle physics near Geneva, Switzerland. LEP collided electrons with positrons at energies that reached 209 GeV. It was a circular collider with a circumference of 27 kilometres built in a tunnel roughly 100 m (300 ft) underground and passing through Switzerland and France. LEP was used from 1989 until 2000. Around 2001 it was dismantled to make way for the Large Hadron Collider, 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 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 lots of 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.

A section of the LEP Particle beam tube LEP Particle beam tube.JPG
A section of the LEP Particle beam tube

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 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. [1]

When the LEP collider started operation in August 1989 it accelerated the electrons and positrons to a total energy of 45  GeV each to enable production of the Z boson, which has a mass of 91 GeV. [1] 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).

Operation

An old RF cavity from LEP, now on display at the Microcosm exhibit at CERN Radio frequency cavity-IMG 5781-white (cropped).jpg
An old RF cavity from LEP, now on display at the Microcosm exhibit at CERN

LEP was fed with electrons and positrons delivered by CERN's accelerator complex. The particles were generated and initially accelerated by the LEP Pre-Injector, and further accelerated to nearly the speed of light by the Proton Synchrotron 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 magnets which forced the charged particles into a circular trajectory (so that they stay inside the ring), RF accelerators which accelerated the particles with radio frequency waves, and quadrupoles 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, either a photon or a Z boson. The virtual particle almost immediately decays into other elementary particles, which are then detected by huge particle detectors.

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, 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.

ALEPH

ALEPH stands for Apparatus for LEPPHysics 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 2.982±0.013, which is consistent with the standard model 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. [2]

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. [3] 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 blocks from the OPAL barrel electromagnetic calorimeter are currently being re-used in the large-angle photon veto detectors at the NA62 experiment at CERN.

L3

L3 was another LEP experiment. [4] 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—most importantly the mass of the Z boson and the W boson (which were discovered in 1983 at an earlier CERN collider, the Proton-Antiproton Collider) to be obtained—and so confirm the Model and put it on a solid basis of empirical data.

A not quite discovery of the Higgs boson

Near the end of the scheduled run time, data suggested tantalizing but inconclusive hints that the Higgs particle of a mass around 115 GeV might have been observed, a sort of Holy Grail 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 deviations which translates to the 91% confidence level, 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 had not been sensitive enough to confirm or refute these hints. [5] Beginning in July 2012, however, the ATLAS and CMS experiments at LHC presented evidence of a Higgs particle around 125 GeV, [6] and strongly excluded the 115 GeV region.

See also

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Tevatron particle accelerator

The Tevatron was a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory, east of Batavia, Illinois, and is the second highest energy particle collider ever built, after the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. The Tevatron was a synchrotron that accelerated protons and antiprotons in a 6.28 km (3.90 mi) ring to energies of up to 1 TeV, hence its name. The Tevatron was completed in 1983 at a cost of $120 million and significant upgrade investments were made in 1983–2011.

Compact Muon Solenoid One of the two main purposes experiment at the CERNs Large Hadron Collider

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Large Hadron Collider particle collider

The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider and the largest machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres (17 mi) in circumference and as deep as 175 metres (574 ft) beneath the France–Switzerland border near Geneva.

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ATLAS experiment CERN LHC experiment

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Compact Linear Collider

The Compact Linear Collider (CLIC) is a concept for a future linear particle accelerator that aims to explore the next energy frontier. CLIC would collide electrons with positrons and is currently the only mature option for a multi-TeV linear collider. The accelerator would be between 11 and 50 km long, more than ten times longer than the existing Stanford Linear Accelerator (SLAC) in California, USA. CLIC is proposed to be built at CERN, across the border between France and Switzerland near Geneva, with first beams starting by the time the Large Hadron Collider (LHC) has finished operations around 2035.

International Linear Collider

The International Linear Collider (ILC) is a proposed linear particle accelerator. It is planned to have a collision energy of 500 GeV initially, with the possibility for a later upgrade to 1000 GeV. Although early proposed locations for the ILC were Japan, Europe (CERN) and the USA (Fermilab), the Kitakami highland, in the Iwate prefecture of northern Japan, has been the focus of ILC design efforts since 2013. The Japanese government is willing to contribute half of the costs, according to the coordinator of study for detectors at the ILC.

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Super Proton Synchrotron Particle accelerator at CERN

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HERA (particle accelerator)

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Proton Synchrotron CERNs first synchrotron accelerator

The Proton Synchrotron (PS) 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 has since served as a pre-accelerator for the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS), and is currently part of the Large Hadron Collider (LHC) accelerator complex. In addition to protons, PS has accelerated alpha particles, oxygen and sulphur nuclei, electrons, positrons and antiprotons.

OPAL was one of the major experiments at CERN's Large Electron–Positron Collider. OPAL studied particles and their interactions by collecting and analysing electron-positron collisions. LEP was the largest particle accelerator in the world. There were three other experiments at LEP: ALEPH, DELPHI and L3.

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Search for the Higgs boson

The search for the Higgs boson was a 40-year effort by physicists to prove the existence or non-existence of the Higgs boson, first theorised in the 1960s. The Higgs boson was the last unobserved fundamental particle in the Standard Model of particle physics, and its discovery was described as being the "ultimate verification" of the Standard Model. In March 2013, the Higgs boson was officially confirmed to exist.

The Circular Electron Positron Collider is an electron positron collider first proposed by the Chinese high-energy physics community in 2012. This machine could later be upgraded to a high-energy proton-proton collider, with potential far beyond the current production of the Higgs boson. The low Higgs mass of ~125 GeV makes possible a Circular Electron Positron Collider (CEPC) as a Higgs Factory, which has the advantage of higher luminosity to cost ratio and the potential to be upgraded to a proton-proton collider to reach unprecedented high energy and discover new physics. The underground particle-smashing ring aims to be at least twice the size of the globe's current leading collider - the Large Hadron Collider (CERN) outside Geneva. With a circumference of 80 kilometres, the Chinese accelerator complex would encircle the entire island of Manhattan.

Future Circular Collider

The Future Circular Collider (FCC) is a proposed post-LHC particle accelerator with an energy significantly above that of previous circular colliders. After injection at 3.3 TeV, each beam would have a total energy of 560 MJ. With a centre-of-mass collision energy of 100 TeV the total energy value increases to 16.7 GJ. These total energy values exceed the present LHC by nearly a factor of 30.

Super Proton–Antiproton Synchrotron particle accelerator at CERN

The Super Proton–Antiproton Synchrotron was a particle accelerator that operated at CERN from 1981 to 1991. To operate as a proton-antiproton collider the Super Proton Synchrotron (SPS) underwent substantial modifications, altering it from a one beam synchrotron to a two-beam collider. The main experiments at the accelerator were UA1 and UA2, where the W and Z boson were discovered in 1983. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for their decisive contribution to the SppS-project, which led to the discovery of the W and Z bosons. Other experiments conducted at the SppS were UA4, UA5 and UA8.

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.

References

  1. 1 2 Myers, S.; Picasso, E. (2006). "The design, construction and commissioning of the CERN large Electron–Positron collider". Contemporary Physics. 31 (6): 387–403. doi:10.1080/00107519008213789. ISSN   0010-7514.
  2. "Welcome to ALEPH" . Retrieved 2011-09-14.
  3. "The OPAL Experiment at LEP 1989–2000" . Retrieved 2011-09-14.
  4. "L3 Homepage" . Retrieved 2011-09-14.
  5. CDF Collaboration, D0 Collaboration, Tevatron New Physics, Higgs Working Group (2010-06-26). "Combined CDF and D0 Upper Limits on Standard Model Higgs-Boson Production with up to 6.7 fb−1 of Data". arXiv: 1007.4587 [hep-ex].CS1 maint: multiple names: authors list (link)
  6. "New results indicate that new particle is a Higgs boson - CERN". home.web.cern.ch. Archived from the original on 20 October 2015. Retrieved 24 April 2018.