Big Bang Science Big Bang Science Particle physicists from most of Europe and beyond have joinedtogether in a remarkable attempt to seek the answers to thesequestions.
Their base is CERN, the European Laboratory for ParticlePhysics, on the outskirts of Geneva, straddling the French/Swissborder. Their experiments lie underground, observing high-energy collisions provided by LEP, the Large Electron-Positronmachine - presently the world's largest particle acceleratorCERN is probably the best example ofEuropean co-operation in any field, not onlyin science. It was founded in 1954, at a timewhen many European physicist~s began to realise that co-operation provided theonly way forward for a project as complex as a large particle accelerator. TheUK is one of the founder states, along with Belgium, Denmark, France, the Federal Republic of Germany, Greece, Italy, the Netherlands, Norway, Sweden, Switzerland and Yugoslavia. Since 1954, CERN has grown in size untiltoday it houses several accelerators which serve a community of more than 4,000physicists worldwide. The number of member states now stands at 14, withAustria, Portugal and Spain adding to the original list, while Yugoslavia has left.
Physicists from countries outside CERN, such as Japan, the USA and the USSR, alsocollaborate in experiments by contributing pieces of equipment.
Many members of this international community are now embarking on CERN'smost exciting journey yet: exploring the high-energy realm collisions from LEP thelaboratory's latest machine.
LEP collides together bunches of electronswith bunches of positrons, as they travel in opposite directions around a ring 27km incircumference, at velocities close to the speed of light. When the bunches ofparticles meet electrons and positrons annihilate, creating, for a fraction of asecond bursts of high energy which echo the state of the early Universe, but are quiteharmless. The energy soon rematerialises as streams of subatomic particles. Four hugedetector assemblies record the tracks of particles created in this way and provide thephysicists with glimpses of the behaviour of matter at high energies.
LEP is a circular machine, as big as the Circle line on London'sUnderground, and a direct descendant of the first acceleratorsthat Lawrence and Livingston built at Berkley almost 60 yearsago. It has a ring of magnets to guide the bunches of particles ona circular path around a narrow pipe, so that they pass repeatedlythrough regions where they are given small accelerating boosts.But in LEP the two types of particle - four bunches of electronsand four bunches of positrons - travel in opposite directionsaround the ring. Once the particles have reached maximumenergy the paths of the particles are allowed to cross at fourpoints so that some of the electrons and positrons can collide(although most of the particles from each pair of collidingbunches do not come close enough to annihilate). So, once it hasaccelerated the particles, LEP stores the bunches allowing them tospeed around the machine for several hours colliding every 22millionths of a second.
At LEP's high energies, the curvature must be as small aspossible to minimise the radiation losses.
The electrons and positrons enter LEP after a journey throughsmaller accelerators, which raise their energy in stages. Two of these machines were built originally to accelerate protons, underthe guidance of John Adams, a British engineer who was twicedirector general of CERN. The smaller of the two started up in1959, and continues to run as the centrepiece in a network ofmachines that can supply beams of protons, antiprotons, sulphuror oxygen nuclei, as well as electrons and positrons. Following inAdams's footsteps, British engineers and physicists continue tofigure prominently in the running of CERN's accelerators, as wellas in successive new developments.
The major role of British physicists at LEP lies with theexperiments that record the aftermath of the electron-positronannihilations.
There are four large detector arrays which envelope the beampipe at points where the electron and positron bunches cross.Physicist and engineers from 15 British universities, as well asfrom the Science and Engineering Research Council's RutherfordAppleton Laboratory (RAL) are involved in three of theseexperiments, codenamed ALEPH DELPHI and OPAL.
The main aim of each of the large experiments at LEP is to trapas much of the debris from each annihilation as possible. To dothis requires apparatus that surrounds the annihilation point.In addition, the physicists need to know which kinds of particleemerge and with what energies. They therefore use a variety ofdetectors that can assist in identifying different particles as wellas in measuring their energies. These detectors are wrapped inlayers around the beam pipe where the annihilations occur.
Together the detectors form a huge structure, typically 10 to12 metres high, wide and long - the size ~ a reasonably largehouse-and weighing several thousand tonnes.
The detector assemblies have a similar basic design, although thedetails vary so that they complement each other by havingdifferent strengths. The first layers of detectors (tracking detectors), closest to the beam pipe, reveal the tracks of chargedparticles (neutral particles do not leave tracks). A largeelectromagnet provides a magnetic field to bend these tracks,sothat a particles momentum can later be calculated from theamount of bending.
Outside the tracking detectors, the next layer traps andidentifies all electrons, positrons and photons as they plough intoa dense material such as lead. This material is interleaved withdetectors to measure the energy that the particles lose as theycome to a halt. The aim is to create an electromagnetic calorimeterthat measures all the energy the electrons,positrons and photons. This helps in identifying particles such asneutral pions, which leave no tracks in the inner detectors, butwhich decay to photons.
A third layer incorporates the iron that forms the outer part ofthe electromagnet. It stops the strongly interacting particles, orhadrons -that is, particles (mesons and baryons) built fromquarks and antiquarks - and measures their energy. This layerforms hadron calorimeter, since it measures the total energy ofthe particles.
Two kinds of particle are likely to penetrate beyond the hadroncalorimeter: muons and neutrinos. The outermost layer ofdetectors reveals the tracks of penetrating charged particles,mainly muons. Only the neutrinos escape the apparatus withoutdirect detection. But the physicists can infer their existence. Theyknow the total energy of the electron and positron thatannihilated. So if they add up all the energy deposited by particlesin the various pieces of the apparatus, they can calculate, usingthe principle of conserving energy and momentum, how muchenergy has gone missing in the form of neutrinos, and in whichdirections the neutrinos have gone.
Electronics and computing play crucial roles in experiments of this kind. All the detectors used produce electrical signals whichelectronic circuits convert into a form that can be fed intocomputers and stored on magnetic tapes. Other sophisticatedcircuits are necessary to process signals and make fast 'decisions'as to whether the information from an annihilation is worthrecording. Such circuits act as triggers which set off the wholecomplex chain for recording data from the experiment. Last, butby no means least, computers are necessary to take the recordedinformation and reconstruct what happened immediately afteran annihilation, as the newly made particles flew out through theapparatus. It is from these 'events' that the physicists caneventually build up a picture of the underlying physicalprocesses.