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Το CERN διδάσκει: ένα πολύ όμορφο ταξίδι…

Copyright CERN 2008 – Web Communications, DSU-CO

1. Recipe for a Universe

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Take a massive explosion to create plenty of stardust and a raging heat. Simmer for an eternity in a background of cosmic microwaves. Let the ingredients congeal and leave to cool and serve cold with cultures of tiny organisms 13.7 billion years later.

To understand the basic ingredients and the ‘cooking conditions’ of the cosmos, from the beginning of time to the present day, particle physicists have to try and reverse-engineer the ‘dish’ of the Universe. Within the complex concoction, cryptic clues hide the instructions for the cosmic recipe.

Slowly simmer

Space, time, matter… everything originated in the Big Bang, an incommensurably huge explosion that happened 13.7 billion years ago. The Universe was then incredibly hot and dense but only a few moments after, as it started to cool down, the conditions were just right to give rise to the building blocks of matter – in particular, the quarks and electrons of which we are all made. A few millionths of a second later, quarks aggregated to produce protons and neutrons, which in turn were bundled into nuclei three minutes later.

Then, as the Universe continued to expand and cool, things began to happen more slowly. It took 380,000 years for the electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the Universe.

Another 1.6 million years later, gravity began to take control as clouds of gas began to form stars and galaxies. Since then heavier atoms, such as carbon, oxygen and iron, of which we are all made, have been continuously ‘cooked’ in the hearts of the stars and stirred in with the rest of the Universe each time a star comes to a spectacular end as a supernova.

The mystery ingredient

So far so good but there is one small detail left out: cosmological and astrophysical observations have now shown that all of the above accounts for only a tiny 4% of the entire Universe. In a way, it is not so much the visible things, such as planets and galaxies, that define the Universe, but rather the void around them!

Most of the Universe is made up of invisible substances known as ‘dark matter’ (26%) and ‘dark energy’ (70%). These do not emit electromagnetic radiation, and we detect them only through their gravitational effects. What they are and what role they played in the evolution of the Universe are a mystery, but within this darkness lie intriguing possibilities of hitherto undiscovered physics beyond the established Standard Model.

 

2. The standard package

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The theories and discoveries of thousands of physicists over the past century have resulted in a remarkable insight into the fundamental structure of matter: everything in the Universe is found to be made from twelve basic building blocks called fundamental particles, governed by four fundamental forces. Our best understanding of how these twelve particles and three of the forces are related to each other is encapsulated in the Standard Model of particles and forces. Developed in the early 1970s, it has successfully explained a host of experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments by many physicists, the Standard Model has become established as a well-tested physics theory.

Matter particles

Everything around us is made of matter particles.These occur in two basic types called quarks and leptons.

Each group consists of six particles, which are related in pairs, or ‘generations’. The lightest and most stable particles make up the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter in the Universe is made from particles that belong to the first generation; any heavier particles quickly decay to the next most stable level.

The six quarks are paired in the three generations – the ‘up quark’ and the ‘down quark’ form the first generation, followed by the ‘charm quark’ and ‘strange quark’, then the ‘top quark’ and ‘bottom quark’. The six leptons are similarly arranged in three generations – the ‘electron’ and the ‘electron-neutrino’, the ‘muon’ and the ‘muon-neutrino’, and the ‘tau’ and the ‘tau-neutrino’. The electron, the muon and the tau all have an electric charge and a mass, whereas the neutrinos are electrically neutral with very little mass.

Forces and carrier particles

There are four fundamental forces at work in the Universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force is, as the name says, the strongest among all the four fundamental interactions.

We know that three of the fundamental forces result from the exchange of force carrier particles, which belong to a broader group called ‘bosons’. Matter particles transfer discrete amounts of energy by exchanging bosons with each other. Each fundamental force has its own corresponding boson particle – the strong force is carried by the ‘gluon’, the electromagnetic force is carried by the ‘photon’, and the ‘W and Z bosons’ are responsible for the weak force. Although not yet found, the ‘graviton’ should be the corresponding force-carrying particle of gravity.

The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains extremely well how these forces act on all the matter particles. However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model. In fact, fitting gravity comfortably into the framework has proved to be a difficult challenge. The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are like two children who refuse to play nicely together. No one has managed to make the two mathematically compatible in the context of the Standard Model. But luckily for particle physics, when it comes to the minuscule scale of particles, the effect of gravity is so weak as to be negligible. Only when we have matter in bulk, such as in ourselves or in planets, does the effect of gravity dominate. So the Standard Model still works well despite its reluctant exclusion of one of the fundamental forces.

So far so good, but…

…it is not time for physicists to call it a day just yet. Even though the Standard Model is currently the best description we have of the subatomic world, it does not explain the complete picture. The theory incorporates only three out of the four fundamental forces, omitting gravity. Alas, Newton would be turning in his grave! There are also important questions it cannot answer, such as what is dark matter, what happened to the missing antimatter, and more.

Last but not least, an essential ingredient of the Standard Model, a particle called the Higgs boson, has yet to be found in an experiment. The race is on to hunt for the Higgs – the key to the origin of particle mass. Finding it would be a big step for particle physics, although its discovery would not write the final ending to the story.

So despite the Standard Model’s effectiveness at describing the phenomena within its domain, it is nevertheless incomplete. Perhaps it is only a part of a bigger picture that includes new physics that has so far been hidden deep in the subatomic world or in the dark recesses of the Universe. New information from experiments at the Large Hadron Collider are sure to help us find more of these missing pieces.

3. Towards a superforce

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Our understanding of the workings of the Universe often progress when unexpected connections are found between what appeared at first to be separate entities. A major breakthrough occurred in the 1860s when James Clerk Maxwell recognized the similarities between electricity and magnetism and developed his theory of a single electromagnetic force. A similar breakthrough came a century later, when theorists began to develop links between electromagnetism, with its obvious effects in everyday life, and the weak force, which normally hides within the atomic nucleus. Vital support for these ideas came first from the Gargamelle experiment at CERN, and then with the Nobel prize winning discovery of the W and Z particles, which carry the electroweak force. But take note – it is only at the higher energies explored in particle collisions at CERN and other laboratories that the electromagnetic and weak forces begin to act on equal terms.

So will other forces join the club at even higher energies? Experiments already show that the effect of the strong force becomes weaker as energies increase. This is a good indication that at incredibly high energies, the strengths of the electromagnetic, weak and strong forces are probably the same. The energies involved are at least a thousand million times greater than particle accelerators can reach, but such conditions would have existed in the very early Universe, almost immediately (10-34 s) after the Big Bang. Pushing the concept a step further, theorists even contemplate the possibility of including gravity at still higher energies, thereby unifying all the fundamental forces into a single ‘super force’. This would have ruled the first instants of the Universe, before its different components separated out as the Universe cooled.

Enter superparticles

Although at present we cannot recreate conditions with energy high enough to test these ideas directly, we can look for the consequences of ‘grand unification’ at lower energies, for instance at the Large Hadron Collider. A very popular idea suggested by such a unification is called supersymmetry, or SUSY for short. SUSY provides a symmetry between matter and forces, and predicts that for each known particle there is a ‘supersymmetric’ partner. If this is correct, then supersymmetric particles should appear in collisions at the LHC.

4. Antimatter detectives

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The antimatter is missing – not from CERN, but from the Universe! At least that is what we can deduce so far from careful examination of the evidence. Matter and antimatter have the same mass, but opposite electric charge. For each basic particle of matter, there exists an antiparticle; for example, the negatively charged electron has a positively charged antiparticle called the positron. When a particle and its antiparticle come together, at the blink of an eye they both disappear in a flash as the annihilation process transforms their mass into energy.

The evidence spoke for itself

The ‘case file’ of antimatter was opened in 1928 by physicist Paul Dirac. He developed a theory that combined quantum mechanics and Einstein’s special relativity to provide a more full description of electron interactions. The basic equation he derived turned out to have two solutions, one for the electron and one that seemed to describe something with positive charge (in fact, it was the positron). Then in 1932 the evidence was found to prove these ideas correct, when the positron was discovered occurring naturally in cosmic rays.

For the past 50 years and more, laboratories like CERN have routinely produced antiparticles, and in 1995 CERN became the first laboratory to create anti-atoms artificially. But no one has ever produced antimatter without obtaining the corresponding matter particles also. The scenario must have been the same during the birth of the Universe, when equal amounts of matter and antimatter must have been produced in the Big Bang.

“Just one more thing…”

So if matter and antimatter annihilate, and we and everything else are made of matter, why do we still exist? This mystery arises because we find ourselves living in a Universe made exclusively of matter. Didn’t matter and antimatter completely annihilate at the time of the Big Bang? Perhaps this antimatter still exists somewhere else? Otherwise where did it go and what happened to it in the first place?

Such questions have led to speculative theories, from a break in the rules to the existence of an entire anti-Universe somewhere else! The way to solve the baffling disappearance of antimatter, and to learn more about this substance in general, is by studying both particles and antiparticles to find and decipher the subtle clues. The mystery demands teams of ‘scientific Sherlock Holmeses’ to conduct thorough detective work, to uncover a logic that is ultimately “elementary”.

5. Clues to the early Universe

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The Universe has changed a great deal in the 13.7 billion years since the Big Bang, but the basic building blocks of everything from microbes to galaxies were signed, sealed and delivered in the first few millionths of a second. This is when the fundamental quarks became locked up within the protons and neutrons that form atomic nuclei. And there they remain, stuck together by gluons, the carrier particles of the strong force. This force is so strong that experiments have not been able to prise individual quarks or gluons out of protons, neutrons or other composite particles.

Primordial soup

Suppose, however, you could reverse the process. The current theory of the strong interaction predicts that at very high temperatures and very high densities, quarks and gluons should no longer be confined inside composite particles. Instead they should exist freely in a new state of matter known as ‘quark-gluon plasma’.

Such a transition should occur when the temperature goes above a value around 2000 billion degrees – about 100 000 times hotter than the core of the Sun! For a few millionths of a second after the Big Bang the temperature of the Universe was indeed above this value, so the entire Universe would have been in a state of quark-gluon plasma – a hot, dense ‘soup’ of quarks and gluons. Then as the Universe cooled below the critical value, the soup condensed into composite particles, including the building blocks of atomic nuclei.

Experiments at CERN’s Super Proton Synchrotron reported tantalising evidence for quark-gluon plasma in 2000. The next big step will be with the Large Hadron Collider and the ALICE experiment in particular.

6. Dark secrets of the Universe

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It’s perhaps natural that we don’t know much about how the Universe was created – after all, we were never there ourselves. But it’s surprising to realise that when it comes to the Universe today, we don’t necessarily have a much better knowledge of what is out there. In fact, astronomers and physicists have found that all we see in the Universe – planets, stars, galaxies – accounts for only a tiny 4% of it! In a way, it is not so much the visible things that define the Universe, but rather the void around them.

Cosmological and astrophysical observations indicate that most of the Universe is made up of invisible substances that do not emit electromagnetic radiation – that is, we cannot detect them directly through telescopes or similar instruments. We detect them only through their gravitational effects, which makes them very difficult to study. These mysterious substances are known as ‘dark matter’ and ‘dark energy’. What they are and what role they played in the evolution of the Universe are a mystery, but within this darkness lie intriguing possibilities of hitherto undiscovered physics beyond the established Standard Model.

Dark matter

Dark matter makes up about 23% of the Universe. The first hint of its existence came in 1933, when astronomical observations and calculations of gravitational effects revealed that there must be more ‘stuff’ present in the Universe than telescopes could see.

Researchers now believe that the gravitational effect of dark matter makes galaxies spin faster than expected, and that its gravitational field deviates the light of objects behind it. Measurements of these effects show that dark matter exists, and they can be used to estimate the density of dark matter even though we cannot directly observe it.

But what is dark matter? One idea is that it could contain ‘supersymmetric particles’ – hypothesized particles that are partners to those already known in the Standard Model. Experiments at the Large Hadron Collider may be able to find them.

Dark energy

Dark energy makes up approximately 73% of the Universe and appears to be associated with the vacuum in space. It is homogenously distributed throughout the Universe, not only in space but also in time – in other words, its effect is not diluted as the Universe expands.

The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the Universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the Universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.

7. Loose ends

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Will the string tie the Standard package? Hot on the heels of the Standard Model, some physicists are working to support a new idea called string theory. This attempts to tie up the loose ends in the Standard Model by explaining all the fundamental particles and forces (including gravity) in a unified framework.

Underlying string theory is the radical idea that fundamental particles are not really like points or dots, but rather small loops of vibrating strings. All the different particles and forces are just different oscillation modes of a unique type of string. Bizarrely, the theory also implies that besides the familiar three–dimensional world and the fourth dimension of time, there are six additional spatial dimensions! These extra dimensions are apparently ‘curled up’ so small that we do not see them.

Which string?

String theory is conceptually complex, with a fascinating but very difficult mathematical structure. This has so far prevented researchers from deriving concrete hypotheses from the theory for comparison with experimental results. Not only does string theory involve the complex study of the geometry of extra dimensions, but the way the structure of the dimensions are chosen appears arbitrary and can lead to different outcomes.

For instance, there seem to be many possible ways to curl up the extra dimensions, by choosing different shapes and sizes. This leads to many alternative versions of the theory. In certain cases, the sizes of the extra dimensions are very small and it will be difficult to obtain direct evidence for them. In others, the sizes are far larger and could be observed at new accelerators such as CERN’s Large Hadron Collider.

8. Secret dimensions

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In everyday life, we inhabit a space of three dimensions – a vast ‘cupboard’ with height, width and depth, well known for centuries. Less obviously, we can consider time as an additional, fourth dimension, as Einstein famously revealed. But just as we are becoming more used to the idea of four dimensions, some theorists have made predictions wilder than even Einstein had imagined.

String theory intriguingly suggests that six more dimensions exist, but are somehow hidden from our senses. They could be all around us, but curled up to be so tiny that we have never realized their existence.

Beyond the third dimension

Some string theorists have taken this idea further to explain a mystery of gravity that has perplexed physicists for some time – why is gravity so much weaker than the other fundamental forces? Does its carrier, the graviton, exist and where? The idea is that we do not feel gravity’s full effect in the everyday world. Gravity may appear weak only because its force is being shared with other spatial dimensions.

To find out whether these ideas are just products of wild imaginations or an incredible leap in understanding will require experimental evidence. But how?

High-energy experiments could prise open the inconspicuous dimensions just enough to allow particles to move between the normal 3D world and other dimensions. This could be manifest in the sudden disappearance of a particle into a hidden dimension, or the unexpected appearance of a particle in an experiment. Who knows where such a discovery could lead!

9. Glossary

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Accelerating cavity

Accelerating cavities produce the electric field that accelerates the particles inside particle accelerators. Because the electric field oscillates at radio frequency, these cavities are also referred to as radio-frequency cavities.

Accelerator

A machine in which beams of charged particles are accelerated to high energies. Electric fields are used to accelerate the particles while magnets steer and focus them. Beams can be made to collide with a static target or with each other.

  • A collider is a special type of circular accelerator where beams travelling in opposite directions are accelerated and made to interact at designated collision points.
  • A linear accelerator (or linac) is often used as the first stage in an accelerator chain.
  • A synchrotron is an accelerator in which the magnetic field bending the orbits of the particles increases with the energy of the particles. This makes the particles move in a circular path.
AD

The Antiproton Decelerator, the CERN research facility that produces the low-energy antiprotons.

ALICE (A Large Ion Collider Experiment)

One of the four large experiments that will study the collisions at the LHC.

Antimatter

Every kind of matter particle has a corresponding antiparticle. Charged antiparticles have the opposite electric charge to their matter counterparts. Although antiparticles are extremely rare in the Universe today, matter and antimatter are believed to have been created in equal amounts at the Big Bang.

Antiproton

The antiparticle of the proton.

ATLAS

One of the four large experiments that will study the collisions at the LHC.

Atom

All ordinary matter is made up of atoms, which are themselves composed of a nucleus and electrons. The protons and neutrons in the nucleus are made of quarks, the smallest known matter particles.

Beam

The particles in an accelerator are grouped together in a beam. Beams can contain billions of particles and can be divided into discrete portions called bunches. Each bunch is typically several centimetres long and just a few microns wide.

Big Bang

The name given to the explosive origin of the Universe.

Boson

The collective name given to the particles that carry forces between particles of matter. (See also Particles.)

Calorimeter

An instrument for measuring the amount of energy carried by a particle. In particular, the electromagnetic calorimeter measures the energy of electrons and photons, whereas the hadronic calorimeter determines the energy of hadrons, that is, particles such as protons, neutrons, pions and kaons.

CARE (Co-ordinated Accelerator Research in Europe)

An EU-supported activity to generate a structured and integrated area in accelerator research and development in Europe.

Cherenkov radiation

Light emitted by fast-moving charged particles traversing a dense transparent medium faster than the speed of light in that medium.

CLIC (Compact LInear Collider)

A site-independent feasibility study aiming at the development of a realistic technology at an affordable cost for an electron–positron linear collider for physics at multi-TeV energies.

CMS (Compact Muon Solenoid)

One of the four large experiments that will study the collisions at the LHC.

CNGS (CERN Neutrinos to Gran Sasso)

A project that aims at the first observation of the tau neutrino by sending a beam of muon neutrinos from CERN to the Laboratori Nazionali del Gran Sasso in Italy.

Collider

Special type of acclerator where counter-rotating beams are accelerated and interact at designated collision points. The collision energy is twice that of an individual beam, which allows higher energies to be reached than in fixed target accelerators. 

Cosmic ray

A high-energy particle that strikes the Earth’s atmosphere from space, producing many secondary particles, also called cosmic rays.

CP violation

A subtle effect observed in the decays of certain particles that betrays Nature’s preference for matter over antimatter.

Cryogenic distribution line (QRL)

The system used to transport liquid helium around the LHC at very low temperatures. This is necessary to maintain the superconducting state of the magnets that guide the particle beam.

Cryostat

A refrigerator used to maintain extremely low temperatures.

Dark matter

Only 4% of the matter in the Universe is visible. The rest is known as dark matter —26%, and dark energy —70%. Finding out what it consists of is a major challenge for modern science.

Detector

A device used to measure properties of particles. Some detectors measure the tracks left behind by particles, others measure energy. The term ‘detector’ is also used to describe the huge composite devices made up of many smaller detector elements. In the large detectors at the LHC each layer has a very specific task.

Dipole

A magnet with two poles, like the north and south poles of a horseshoe magnet. Dipoles are used in particle accelerators to keep particles moving in a circular orbit. In the LHC there are 1232 dipoles, each 15 m long.

Enabling Grids for E-SciencE (EGEE) project

An EU-funded project led by CERN, now involving more than 90 institutions in over 30 countries worldwide, to provide a seamless Grid infrastructure that is available to scientists 24 hours a day.

Electronvolt (eV)

A unit of energy or mass used in particle physics. One eV is extremely small, and units of a million electronvolts, MeV, or thousand million electronvolts, GeV, are more common. The latest generation of particle accelerators reaches up to several million million electronvolts, TeV. One TeV is about the energy of motion of a flying mosquito.

Electromagnetic force

The electromagnetic force binds negative electrons to the positive nuclei in atoms, and underlies the interactions between atoms that give rise to molecules and to solids and liquids. Unlike gravity, it can produce both attractive and repulsive effects. Opposite electric charges (positive and negative) and opposite magnetic poles (north and south) attract, but charges or poles of the same type repel each other.

End-cap

Detector placed at each end of a barrel-shaped detector to provide the most complete coverage in detecting particles.

Forces

There are four fundamental forces in nature. Gravity is the most familiar to us, but it is the weakest. Electromagnetism is the force responsible for thunderstorms and carrying electricity into our homes. The two other forces, weak and strong, are confined to the atomic nucleus. The strong force binds the nucleus together, whereas the weak force causes some nuclei to break up. The weak force is important in the energy-generating processes of stars, including the Sun. Physicists would like to find a theory that can explain all these forces. A big step forward was made in the 1960s when the electroweak theory uniting the electromagnetic and weak forces was proposed. This was later confirmed in a Nobel-prize-winning experiment at CERN.

Fundamental particle

One of the smallest known particles, from which all the other particles are made of.

Gluon

Gluon is a special particle, called boson, that carries the strong force, one of the four fundamental forces, or interactions, between particles.

Hadron

A subatomic particle that contains quarks, antiquarks, and gluons, and so experiences the strong force. (See also Particles.)

Higgs boson

A particle predicted by theory. It is linked with the mechanism by which physicists think particles acquire mass.

Injector

System that supplies particles to an accelerator. The injector complex for the LHC consists of several accelerators acting in succession.

Ion

An ion is an atom with one or more electrons removed (positive ion) or added (negative ion).

Isotope

Slightly different versions of the same element, differing only in the number of neutrons in the atomic nucleus—the number of protons is the same.

Kaon

A meson containing a strange quark (or antiquark). Neutral kaons come in two kinds, long-lived and short-lived. The long-lived ones occasionally decay into two pions, a CP-violating process. (See also Particles.)

Kelvin

A unit of temperature. One kelvin is equal to one degree Celsius. The Kelvin scale begins at absolute zero, –273.15°C, the coldest temperature possible.

LCG (LHC Computing Grid)

The mission of the LCG is to build and maintain a data-storage and analysis infrastructure for the entire high-energy physics community that will use the LHC.

LEP

The Large Electron–Positron Collider, which ran at CERN until 2000.

Lepton

A class of elementary particle that includes the electron. Leptons are particles of matter that do not feel the strong force. (See also Particles.)

LHC

The Large Hadron Collider, CERN’s biggest accelerator.

LHCb (Large Hadron Collider beauty)

One of the four large experiments that will study the collisions at the LHC.

Linac

An abbreviation for linear accelerator.

Model / Scientific model

The scientific model is a widely used tool in many fields of modern science. Scientists construct and develop ‘models’ to describe a scientific theory in the context of related phenomena. In general, a model is based on a theory (a set of hypothesis), acting on a set of parameters obtained from actual experimental data and/or from observations.
Computer simulations may sometimes be used to test the reliability of a model. If it was found to be reasonably reliable, the simulation can even be used to predict what would happen if the initial parameters were different.

Muon

A particle similar to the electron, but some 200 times more massive. (See also Particles.)

Muon chamber

A device that identifies muons, and together with a magnetic system creates a muon spectrometer to measure momenta.

Neutrino

A neutral particle that hardly interacts at all. Neutrinos are very common and could hold the answers to many questions in physics. (See also Particles.)

Neutron

A baryon with electric charge zero; it is a hadron with a basic structure of two down quarks and one up quark (held together by gluons).

Nucleon

The collective name for protons and neutrons.

Particles

There are two groups of elementary particles, quarks and leptons. The quarks are up and down, charm and strange, top and bottom. The leptons are electron and electron neutrino, muon and muon neutrino, tau and tau neutrino. There are four fundamental forces, or interactions, between particles, which are carried by special particles called bosons. Electromagnetism is carried by the photon, the weak force by the charged W and neutral Z bosons, the strong force by the gluon; gravity is probably carried by the graviton, which has not yet been discovered. Hadrons are particles that feel the strong force. They include mesons, which are composite particles made up of a quark–antiquark pair and baryons, which are particles containing three quarks. Pions and kaons are types of meson. Neutrons and protons (the constituents of ordinary matter) are baryons; neutrons contain one up and two down quarks; protons two up and one down quark.

Photon

The force carrier particle of electromagnetic interactions.

See Particles.

Pion

The least massive type of meson.

Positron

The antiparticle of the electron.

Proton

The most common hadron, a baryon with electric charge +1 equal and opposite to that of the electron. Protons have a basic structure of two up quarks and one down quark (bound together by gluons). The nucleus of a hydrogen atom is a proton.

PS

The Proton Synchrotron, backbone of CERN’s accelerator complex.

Quadrupole

A magnet with four poles, used to focus particle beams rather as glass lenses focus light. There are 392 main quadrupoles in the LHC.

Quantum electrodynamics (QED)

The theory of the electromagnetic interaction.

Quantum chromodynamics (QCD)

The theory for the strong interaction, analogous to QED.

Quark

A class of elementary particle. Quarks are particles of matter that feel the strong force.

Quark–gluon plasma (QGP)

A new kind of plasma in which protons and neutrons are believed to break up into their constituent parts. QGP is believed to have existed just after the Big Bang.

Quench

A quench occurs in a superconducting magnet when the superconductor warms up and ceases to superconduct.

Ring Imaging CHerenkov (RICH) counter

A kind of particle detector that uses the light emitted by fast-moving particles as a means of identifying them.

Scintillation

The flash of light emitted by an electron in an excited atom falling back to its ground state.

Sextupole

A magnet with six poles, used to apply corrections to particle beams. At the LHC, eight- and ten-pole magnets will also be used for this purpose.

Spectrometer

In particle physics, a detector system containing a magnetic field to measure momenta of particles.

SPS

The Super Proton Synchrotron. An accelerator that provides beams for experiments at CERN, as well as preparing beams for the LHC.

Standard Model

A collection of theories that embodies all of our current understanding about the behaviour of fundamental particles.

Strong force

The strong force holds quarks together within protons, neutrons and other particles. It also prevents the protons in the nucleus from flying apart under the influence of the repulsive electrical force between them (because they all have positive charge). Unlike the more familiar effects of gravity and electromagnetism where the forces become weaker with distance, the strong force becomes stronger with distance.

Superconductivity

A property of some materials, usually at very low temperatures, that allows them to carry electricity without resistance. If you start a current flowing in a superconductor, it will keep flowing forever—as long as you keep it cold enough.

Superfluidity

A phase of matter characterized by the complete absence of resistance to flow.

Supersymmetry

A theory that predicts the existence of heavy ‘superpartners’ to all known particles. It will be tested at the LHC.

Synchrotron

A particle accererator in which a magnetic field bends the orbits of the particles, which increases their energy. The particles travel in a circular path.

Technical Design Report (TDR)

The blueprint for an LHC sub-detector system.

Technology transfer

The promotion and dissemination to third parties of technologies developed, for example at CERN, for socio-economic and cultural benefits.

Transfer line

Carries a beam of particles, e.g., protons, from one accelerator to another using magnets to guide the beam.

Trigger

An electronic system for spotting potentially interesting collisions in a particle detector and triggering the detector’s read-out system to record the data resulting from the collision.

Vacuum

A volume of space that is substantively empty of matter, so that gaseous pressure is much less than standard atmospheric pressure.

Weak force

The weak force acts on all matter particles and leads to, among other phenomena, the decay of neutrons (which underlies many natural occurrences of radioactivity) and allows the conversion of a proton into a neutron (responsible for hydrogen burning in the centre of stars). It can be either an attractive or a repulsive force.

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