eaa.iop.orgDOI: 10.1888/0333750888/2104 Standard Model of Particle PhysicsJohn Ellis FromEncyclopedia of Astronomy & AstrophysicsP. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888Downloaded on Tue Jan 31 17:11:35 GMT 2006 [127.0.0.1]Institute of Physics PublishingBristol and PhiladelphiaTerms and ConditionsStandard Model of Particle PhysicsENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSStandard Model of Particle PhysicsThe Standard Model describes all the confirmed dataobtained using particle accelerators and has enabled manysuccessful theoretical predictions to be made. It has beentested with per mille accuracy, at which level its calculablequantum corrections play an essential role. Its onlymissing feature is a particle, called the Higgs boson, whosecoupling to the other particles is believed to generate theirmasses. However, the Standard Model is theoreticallyunsatisfactory despite its many successes, and possibleextensions to provide a more unified picture of thedifferent particle interactions, the apparent proliferationof different particle species and the origin of their massesare being proposed. Evidence has recently been presentedthatNEUTRINOS may alter their nature when they propagateover long distances. This is expected if neutrinos havemasses, which are not predicted by the Standard Model.If confirmed, this may be the first evidence for particlephysics beyond the Standard Model.TheFUNDAMENTAL PARTICLE interactions described bythe Standard Model are the electromagnetic, weak andstrong nuclear forces. Electromagnetic forces have longbeen known, since the discovery of electromagnetic waveradiation by Hertz in 1885 and the early days of quantumphysics, to be mediated by the exchange of the photon(see figure 1), a massless boson of unit angular momentum(spin 1) with two polarization states. The long-establishedquantum theory of electrodynamics is called QED.Yukawa conjectured that the weak interactions such asβ decay were mediated analogously by the exchange ofmassive intermediate bosons, as also shown in figure 1.This hypothesis was put on a firm theoretical basis byGlashow, Weinberg and Salam in their unified theoryof the weak and electromagnetic interactions, and theintermediate bosons (weighing about 80 and 91 GeV)were discovered at CERN in 1983. The strong nuclearinteractions are known also to be mediated by masslessbosons called gluons, see also figure 1, which werediscovered at DESY in 1979. Thus all the fundamentalinteractions have very similar structures, but why only theweak bosons are massive is a puzzle to which we returnlater.The first elementary matter particle to be identifiedwas the spin-1/2 electron (weighing about 1/2 MeV),followed by the unstable muon (weighing about 100 MeV)that was first detected inCOSMIC RAYS. These were showneach to have an associated neutrino, the electron neutrinobeing produced in weak β decay and the muon neutrinobeing produced either in association with a muon or inmuon decays. These particles do not have strong nuclearinteractions, only weak and (in the case of the electronand muon, which are charged particles) electromagnetism,and are calledLEPTONS. A third type of charged lepton(the τ weighing about 1.78 GeV) was discovered inaccelerator experiments at SLAC in 1975 and is alsobelieved to have its own associated neutrino. Acceleratordata have established upper limits on the possible massesquarks, leptons quarks, leptons quarksquarks, leptons quarks, leptons quarksphoton gluonW±, Z0Figure 1. The fundamental forces between elementary particlesare mediated by intermediate bosons: the photon ofelectromagnetism, the massive W±and Z0bosons of the weakinteractions and the gluons of the strong interactions.of the neutrinos, which are much less than those of thecorresponding charged leptons.From the 1940s onwards, many strongly interactingparticles (HADRONS) were discovered. It was proposedin 1964 that these could all be understood as compositebound states of more elementary entities calledQUARKS.Atthat time, all the known hadrons could be constructed outof just three quark species of spin 1/2 and fractional chargeQ:up(Q = 2/3), down and strange (Q =−1/3). Suchpointlike constituents inside protons were discovered byexperiments at SLAC in 1968. Initially it was a puzzlehow quarks could appear almost free at high energies,corresponding to short distances, but were never seensingly, only confined inside hadrons. This puzzle wasresolved when it was demonstrated that the couplings ofgluons became weaker at high energies (short distances),a property known as asymptotic freedom.Shortly afterwards, a key step in the establishmentof the Standard Model was the discovery in a neutrinoexperiment at CERN in 1973 of a novel type of weakinteraction, called neutral currents, in which the chargesof the participating particles do not change, in contrastto the familiar weak interactions, called charged currents,in which the charges of participating quarks and leptonsdo change. Such neutral-current interactions werepredicted by many unified theories of the weak andelectromagneticinteractions, and their discovery providedthe first circumstantial evidence that some such unifiedtheory might be correct. The second piece of evidence forsuch a theory was provided by the discovery in acceleratorexperiments at BNL and SLAC in 1974 of hadronscontaining a fourth quark constituent weighing about112GeV. This was baptized charm and had been predictedby unified electroweak theories, in order to explain the factthat neutral-current interactions that would change quarktype, e.g. strange → down, are very suppressed, whereasthere do exist charged-current interactions: strange →up. The charm discovery briefly pre-dated the τ leptondiscovery mentioned earlier. Consistency of the simplestunified electroweak theory then required the existence oftwo more quark types, and the first of these (bottom),weighing about 5 GeV, was discovered in an acceleratorCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Standard Model of Particle PhysicsENCYCLOPEDIA OF ASTRONOMY AND A STROPHYSICSexperiment at FNAL in 1977. The spectrum of quarks wasapparently completed by the discovery at FNAL in 1995of the top quark, which weighs about 175 GeV.The mathematical framework for theories of particleinteractions is provided by gauge