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An Experimentalist's View of the Standard Model

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An Experimentalist's View of the Standard Model The lrr[cn[ ot’ the lourlh cxpcnment IS to measure Inlcrfercnce C’lTCCIS be[wecn !hc neutral and charged weak currcn~s via sca\- An Experiments ist’s mnng cxpcnmcrrts w!lh nculnnos and clcc- (rorrs. It’ dcstructl\ c lnlcrfcrcncc is dclcc[ed. ~hen lhc present clcclrowcak [hcor) should View of the bc applicable cwn at h]ghcr rncrgles: If con- s~ructlI c interference IS cietcctcd. then the thcor> v.111need to be expanded. say by Standard Model Includlng Icc[or bosons bc~ond Ihose (the # and the lt’~) already In the standard model. he dream of physicists to produce a electroweak theory appears to account for unified field theory has. at different the apparent difference, at low energies, be- Tritium Beta Decay T times in the hisiory of physics, ap- tween the weak interaction and the elec- peared in a different light. For example, one tromagnetic interaction. As the energy of an In 1930 Pault argued thal Ihc continuous of the most astounding intellectual achieve- interaction increases. a unification is hlncllc cnL,rg\ spcclrum ofclcclrons cmlttcd ments in nineteenth century physics was the achieved. In bcla deca} would he explained hy a Ilght. realization that electric forces and magnetic So far, at energies accessible to modem ncu[ral parltclc. Th[s partlclc. the ncutrino. forces (and their corresponding fields) are high-energy accelerators, the theory is sup- was used b} Fermi in [934 to accoun[ quan - different manifestations of a single elec- ported by experiment, In fact, the discovery Iltatlicl} tclrlhc klncmallcsofbe[a decay. In tromagnetic field. Maxwell’s construction of at CERN in 1983 of the heavy vector bosons 1953. th~, c}usI\c, neu[rlno was observed the differential equations relating these two W+. W’–. and ZO, whose large mass (com- dlrcc[l! b! a LOS Alamm team. Fred Rclncs fields paved the way for their later relation to pared to the photon) accounts for ~he rel- and (’l>dc [.. (’owan. using a reactor at Han- special relativity. atively ‘“weak” nature of the weak force. ford. beautifully confirms and reinforces the new Though tbc ncutrino has generally been QED. The most successful field theory to theory. [akcn 10 br masslcss. no ~heory requires ncu- date, quantum electrodynamics (QED), ap- The electroweak theory has many ex- (nrros [o have zero mass. Tbc current ex- pears to have provided us with a complete perimental triumphs, but experimental pcnmcnial upper limit on the clcc[ron ncu- description of the electromagnetic force. physicists have been encouraged to press Irlnc) mass IS 55 c.lcc-iron volis (cV). and Ihc This theory has withstood an extraordinary ever harder to test the theory. to explore its Ruwlan [c>am responsible for this Ilmit array of precision tests in atomic, nuclear, range of validity. and to search for new fun- clalmsa lov. crllnlll 01’20c V.The mass of[hc and particle physics, and at low and high damental interactions and particles, The ex- ncutrlno IS \IIll gcnL.r~ll> uskcn to bc zero, Ior energies. A generation of physicists has perience with QED, which has survived hlsloncal rcaw)n~, hccausc Ihc cxpcrimcnls yearned for comparable field theories de- decades of precision [ests. is the standard by dorw b> [hc RussIan Icam are c\trcmcl} scribing the remaining forces: the weak inter- which to judge tests of ~he newest field ~he- complc~, and bc’cause masslmsncss Icads Ioa action, the strong interaction, and gravity, ories. plcaslng slmplltica(ion olthc Iheor>, An even more romantic goal has been the A more carclul look. houc~er. shows thal notion that a single field theory might de- QCD. A recent, successful field theory that no rcspcctablc [heur> rcqulrcs a mass [hat IS scribe all the known physical interactions. describes the strong force is quantum Idcntlcull! Icro Slncc wc l~arc man} ncu - chromodynamics (QCD). In this Iheory the Inno llaiors (clcclrorr. muon and (au ncu - Electroweak Theory. In the last two dec- strong force is mediated by the exchange of Innos. a[ Icas[). a nonzcrn mass would lm - ades we have come a long way towards realiz- color gluons and a coupling constant is de- medlawl! open possibilil]es for mlxlng be- ing this goal. The electromagnetic and weak termined analogous to the tine structure con- Iuccn [hcsc three known lep[on tamilics, interactions appear to be well described by stant of the electroweak theory. U’llhoul rc~gard 10 [he mlnlmal standard the Weinberg-Salam-Glashow model that model or an! unliica{lon schcmcs. the unifies the two fields in a gauge theory. (See Standard Model. QCD and the elec- posstblc c\lstcncc of massltc ncu[nnos “Particle Physics and the Standard Model” troweak theory are now embedded and pnlrrls OUIour basic ignorance of’lbcongin 01 for a discussion of gauge theories and other united in the minimal standard model. This Ihc known parlwlc masses and Ihc Iamll) details just briefly mentioned here.) This model organizes all three fields in a gauge structure ofparlicles. 126 Sumnlcr/F’all IYX.4 1.OS AI.AJIOS SC IE\CF -— .—-— Experiments To Test Unification Schemes vector bosons for the weak force and all the fermions (except perhaps the neutrinos) are Table known to be massive. the symmetry of the theory has to be broken. Such symmetV- The first three generations of elementary particles. breaking is accomplished by the Higgs mech- anism in which another gauge field with its Family: I 11 Ill yet unseen Higgs particle is built into the theory. However, no other Higgs-lype parti- ~, [ cles are included. Quarks: ~~ Many important features are built into Ihe t) (), (),.\ (), minimal standard model. For example. iow- Double\s energy. charged-current weak interactions are dominated by r’ – A (vector minus axial v,. Vp Lcp[ons: vector) currents: thus. only left-handed ~t”t (’ i ), ()PI bosons have been included. Also. since neu- trinos arc ~akcn to be rnassless, there are supposed to be no oscillations between neu- Slnglcls: fiK.(fK.(’u. trino flavors. There arc many possibilities for ex- tensions to the standard model. New bosons. families of particles. or fundamental interac- tions may be discovered, or new substruc- theory of electroweak and strong interac- that is. the photon y. For the weak force. tures or symmetries may be required. The tions. There are two classes of particles: spin- there are both neutral and charged currents standard model. al this moment. has no I/2 particles called fermions (quarks and lep- that involve, respectively, the exchange of demonstrated flaws. but there are many po- tons) that make up the parlicles of ordinary the neutral vector boson ZO and the charged tential sources of trouble (or enligh~enment), matter, and spin- 1 particles called bosons vector bosons W+ and W-, The color force that account for the interactions between the of QCD involves eight bosons called gluons GUT. One of the mos[ dramatic notions fermions. that carry the color charge. that goes beyond the standard model is the In this theory the fermions are grouped The coupling constants for the weak and grand unified theory (GUT). In such a the- asymmetrically according to the “handed- electromagnetic interactions, ~Wk and gc~, ory. the coupling constants in the elec- ness” of their spin to account for the ex- are related by [he Weinberg angle t3w. a mix- troweak and strong sectors run together at perimentally observed violation of C’P sym- ing angle used in the theory to parametrize extremely high cncrgics ( 101j to 101” giga- metry. Particles with right-handed spin are the combination of the weak and elec- electron volts (Ge V)), ,+1[the fields are uni- grouped in pairs or doublets; particles with tromagnetic gauge fields. Specifically, fied undera single group structure. and a ncw left-handed spin are placed in singlets. The object. the X, appears to generate this grand exchange ofa charged vector boson can con- symmetry group. This very high-energy mass sin t3w = &n,/gWk vert one particle in a given doublet to the scale is not directly accessible at any con- other, whereas the singlet particles have no ceivable accelerator. To explore the wilder- weak charge and so do not undergo such Only objects required by experimental re- ness between present mass scales and the transitions. sults are in the standard model. hence the GUT scale, alas, all high-energy physicists The Table shows how the model, using term minimal. For example, no right-handed will have to be content to work as low-energy this scheme, builds the first three generations neutrinos are included. Other minimal as- physicists. Some seers believe the wilderness ofleptons and quarks. Since each quark (u, d. sumptions are massless neutrinos and no will be a desert, devoid of striking new phys- c. s, [, and b) comes in three colors and all requirement for conservation of lotal Icpton ics. In the Iikely event thal the desert is found fermions have antiparticles, [he model in- number or of individual Iepton flavor (thal blooming with unexplored phenomena. the cludes 90 fundamental fermions. is. electron. muon, or [au number), journey through this terra incognita will be a The spin-1 boson mediating the elec- The theory, in fact, includes no mass for long and fruitful one. even if we are restricted tromagnetic force is a massless gauge boson, an!’ of the elementary particles. Since the to feasible tools. ■ 1.0S .At.4iloSS(’IK\(”E ‘Sunlmcr/Fdll 19X4 127 -—. .-.
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