Directions in High Energy Physics

arXiv:hep-ph/9606437v126Jun1996MPI-Ph/94-76November1994DirectionsinHighEnergyPhysicsBodoLampeMax-Planck-Institutf¨urPhysik,Werner-Heisenberg-InstitutF¨ohringerRing6,D-80805M¨unchene-mail:[e-mail]ofparticlephysicsareclassifiedfromatheorist’spointofview.TheprospectsofmassandmixingangledeterminationandofthetopquarkandHiggsbosondiscoveryarediscussed.ItisshownthatthemostimportantprogresswillcomefromLHCandNLC.Thesemachinesshouldbeplannedanddevelopedasquicklyaspossible.1.IntroductionPhysicsisthescienceinwhichmatterandratioaremosttightlyconnected.Highenergyphysicsisitsmostfundamentalbranchbecausewetrytofindeversmallerconstituentsofmatter.Itwasbornfromnuclearphysicsashigherenergiesbecameavailable.Itsgoalhasalwaysbeentodiscovernewparticlesanddeterminetheirpropertiesinhighenergycollisions.Thepropertiestobedeterminedaremass,quantumnumbersandcouplingstootherparticles.Whenitcomestostillhigherenergiesfurtherquestionscanbeexamined,likecompositenessormorefundamentalsymmetries.Finally,acompletetheoreticalmodelfortheparticlecanbedeveloped,basedinthebestcasesonsimpleandfundamentalmathematicalprinciples.Iamsurethatthisconceptofhighenergyphysicswillsuccessfullypersistinthefuturealthoughnewexperimentsbecomeincreasinglyexpensive.MostimportantitemsarethediscoveryandexaminationofthetopquarkandoftheHiggsparticlebecausethiswillopennewfrontiersinourunderstandingofthefundamentalinter-actions.Itwouldbedesastrousifwewouldwaitandwithdrawtocheaperbutlessimportantprojects.Lessimportantprojectsalsocostmoney,bindenergyand,mostofall,theydistortthedirectionofresearch.IcanseeatendencyforthisinsomerecentdecisionsandIwanttowarnthatthiscouldjeopardizethefutureofparticlephysics.Oncemoneyislostinwrongdirectionsitisdifficulttoattainnewmoney,evenforimportantexperiments.Itisthemainaimofthisarticletodiscusstheseissuesindetailandtoratethevariousprojectsaccordingtotheirpriority.Astandardpopularobjectiontobasicscienceisthatthenumberofphysicallawsislimitedandmostofthemarealreadyknown.Icannotseethatthisisanargumentagainsthighenergyphysics.Aswillbediscussedinthenextsectionitisabsolutelyclearthatthedeepestphysicallawshavenotyetbeenfound.Infact,thenextsectionwillstartwithasummaryforandagainsttheso-calledstandardmodelofelementaryparticlephysics.Afterwards,asetofexperimentsassuggestedbythestandardmodelwillbevaluated.Weshallseethattheheavyparticlesofthestandardmodelareatthecentreofinterestandshouldbetreatedwiththehighestpriority.Finally,Ishallcometophysicsbeyondthestandardmodelandtothequestionhowreasonablecertainspecificnullexperimentsarewhichsearchfordeviationsfromthestandardmodel.Usually,thetheoreticalideasonwhichsuchexperimentsarebasedareratherexcitingbuttheperformanceisboringwhennothingisfound.Notethatthispaperisnotintendedasanexhaustivereviewbutasagradingofpresentandfuturehighenergyphysicsexperiments.Itsaimistoinitiatediscussionsaboutthefuturedirectionsandtosharpentheviewforwhatisimportantandwhatnot.Theknownelementaryparticlesdonotformaverycomplexsystem.Ascomparedtobiologicalsystems,forexample,theyareremarkablysimple.Complexsystemsarealwaysmultiparticlecharacterwhereashighenergyphysicsiseventuallysearchingforthemostfundamentalentitiesandtheirinteractionsandisthusdifferentfromotherbranchesofsciencelikechemistry,astrophysicsorevenmathematics.Thisshouldbekeptinmindwhenthesignificanceofexperimentalandtheoreticalideasisreflected.2.ThestandardmodelThestandardmodelofelementaryparticlephysicsdescribestheinteractionsofthequarksandleptonsasmediatedbythevectorbosonsofthestrong(gluons),weak(W±,Z)andelectromagnetic(photon)interactions,c.f.fig.1.Quarksandleptonsarefermions(=spin12particles)whereasthevectorbosonshavespin1.Finally,thereistheHiggsboson,aspin0particle,whichisassociatedwiththegenerationofallparticlemasses.Itiscertainlythemostmysteriousobjectofthestandardmodelbecauseitisasyetundiscoveredandreliesonapurelytheoreticalconstructionaboutsymmetrybreaking.Oncediscovered,thenatureoftheHiggsparticlewillbeestablishedbydemonstratingthatitscouplingstootherparticlesgrowwiththeirmasses.Thestandardmodelhasbeenverysuccessfulphenomenologically.Iknowofnoexperimentwhichdefinitelycontradictsthestandardmodel.However,itisnotbelievedtobetheultimatetheoryofnature,becauseithastoomanyfreeparame-ters,namely,allthefermionmasses.Thestandardmodeldescribesparticleswhosemassesrangebetween0(neutrinosandphoton)and100-1000GeVandonewouldliketounderstandbetter,howthesevastlydifferentmassesariseandwhetherthereareotherimportantmassscalesathigherenergies.Somerelatedquestionsare:themasslessnessoftheneutrinoandthestabilityoftheproton.Theseitemswillbediscussedinlatersections.Thegaugesectorofthestandardmodelisremarkablyclearandextremelyrestricted,withonly3dimensionlesscouplingconstantsdescribingtheinteractionsofthevec-Figure1:torbosonswiththemselvesandwiththefermions.Throughthisonewasabletopredictmanypropertiesofc-,b-andt-quarkandtheτ-leptonlongbeforethesepar-ticleswereactuallydiscovered.Itisevenpossiblethatthestandardmodelgaugegroupisembeddedinalargermoresimplyconnectedgroup.Thisisthe“Gr