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Par$clesandInterac$ons Prof.MarinaCobal FisicaSperimentaleNucleareeSubnucleare 2016/2017 Whatistheworldmadeof? • Intheancient*me:4elements • 19°century–atoms • Beginning20thcentury– electrons,protons,neutrons • Today–quarksandleptons Allthingsaroundusaremadeofatoms Human Hair ~ 50 µm = 50 10-6 m = 0.000050 m Atom ~ 10-10 m = 0.0000000001 m Magritte Theatominthe20thcentury... • Atoms reacts through chemical reactions • More than 100 atoms known (H, He, Fe …) • The internal structure is not well known Tableofelements • Atomsaregroupedinfamilieswhichpresentsimilarproper*es Theelementstableisdone • Thissymmetrysuggestsastructurewithsimplercons*tuents. AtomicModel • Withtheexperimentsnowweareableto “breakup”atoms • Lightpar*cles(electrons)withnega*ve chargearoundaposi*veandheavy nucleus • Prac*cally,theatomisempty! Rutherford:atomsarenotelementary par*cles! 1911 Rutherford found a nucleus in the atom by firing alpha particles at gold and observing them bounce back Precursor of modern scattering experiments at accelerator Atoms Atoms are all similarly made of: - protons and neutrons in the nucleus - electrons orbiting around proton The electron was the first elementary particle to be discovered (JJ Thomson 1897) Protons, neutrons are made up of quarks electron neutron Nucleus • Thenucleusissmallanddense.For awhileitwasthoughttobepointlike. • However,thereweresomany differentnucleaasmanyatoms • Simplifica*on:allnucleaaremade ofneutronsandprotons! NewPar$cles • CollisionsofeleYronsandnucleainthecosmicraysandin thepar*cleacceleratorsatthebeginningofthe30ies, broughttothediscoveryofmanyotherpar*cles. • Someofthemwerepredicted,otherwerediscoveredas surprises,beingcompletelyunexpected. • Atthebeginningitwasthoughtthatallthesepar*cles werefundamentals. Atthebeginningjustafew… And many others… Introducingthequarks.. "Youngman,ifIcouldremember thenamesofthesepar8cles, Iwouldhavebeenabotanist!“ E.Fermitohisstudent L.Lederman(bothNobellaureates) ThePar*clePhysicist’sBible: Par*cleDataBook hYps://pdg.lbl.gov Most particles are not stable and can decay to lighter particles.. Quarks • Todayweknowthatalsoprotonsand neutronsarenotfundamentalunits. • Theyaremadeofsmallerpar*clescalled quarks • Forthemomentlookslikequarksare pointlike Sub-atomicdimensions TheModernAtom • Acloudofelectronsmovingconstantly aroundthenucleus • Protonsandneutronsmovinginthe nucleus • Quarksmovinginprotonsand neutrons Whatisfundamental? • Physicistshavefoundhundredsofnewpar*cles. • Todayweknowthatmostofthemarenotfundamental • Atheoryhasbeendevelopedthatseemstoexplainquitewell whatwedoobserveinnature:thetheoryiscalledStandard Model • Thismodelincludes6quarks,6leptonsand13par*cleswhich carrytheforceinbetweenquarksandleptons. IsthewholeUniversemadeonlyof quarksandelectrons? No!Therearealsoneutrinos! ν Electron, proton and neutrons are rarities! For each of them in the Universe there is 1 billion neutrinos Neutrinos are the most abundant matter-particles in the Universe! Within each cm3 of space: ~300 neutrinos from Big Bang ν ν ν ν ν νννννννν νν ν ν νν ν ννννννν ν νν νν νν ν ν ν ν ν νν ν ν νν ν νννννν ν νν νν νν ννννννν νν νν ν ννν ννννννν νν νν ν ννν ννννννν 1 cm 1 cm Neutrinos are everywhere! in the outer space, on Earth, in our bodies.. Neutrinosgetunderyourskin! Every cm2 of Earth surface is crossed every second by more than 10 billion (1010) neutrinos produced in the Sun 1014 neutrinos per second from Sun are zipping through you Within your body at any instant: roughly 30 million neutrinos from the Big Bang ν No worries! Neutrinos do not harm us. Our bodies are transparent to neutrinos QuarksandLeptons Structurelessbuildingblocksdowntoaspa*alextensionof10-18m Welldefinedspinandcharge Leptonshavewelldefinedmassaswell LowMass MaYer Cons*tuentsunder ordinarycondi*ons (lowenergy/T) Cons*tuentsofunstable par*cle(producedathigh energy,inastrophysical systems).Theydecayto lowermasspar*cles HighMass20 Families • The6quarksandleptonsare organizedinfamilies • The3familieshaveanalogies • Quarkshavecharge+2/3e-1/3. Leptonshavecharge-1and0. Fermions Elementarypar*cles:fermions 1st generation 2nd generation Massa (MeV) Why3families?Are theremore? 3rd generation Charge (e) 2/3 2/3 -1/3 -1/3 0 0 -1 -1 Quarks Leptons Whatistheworldmadeof? • Realworldisnotdonebysinglequarks • Quarksexistonlyingroups,toformtheso-calledhadrons (protonsandneutronsarehadrons) • Example:aprotonismadeoftwoquarksofuptypeand onequarkoftypedown. • ThemaYeraround,andeveneachofus,ismadeof quarksupanddownandofelectrons. Areduc*onistexample: theDeuteriumAtom p = (u, u, d ) n = (u, d , d ) 10-15 m 10-10 m Andtheleptons? • Thereare6leptons:3chargedand3neutral. • Theylooklikepointlikepar*clewithoutaninternal structure. • Electronsarethemostcommonandarefoundinthe ordinarymaYer. • Muons(µ)andtaus(τ)areheavierandchargedasthe electrons. • Neutrinos(ν)havenochargeandtheyhaveanextremely smallmass. Wave-par$cledualityofNature Central concept of quantum mechanics: all particles present wave-like properties λ Not only light has a dual nature De Broglie showed that moving particles have an equivalent wavelength λ 1 λ∝ p So high momentum gives us short wavelengths so we can make out small details Example: electron microscope Electron Microscope Image Gold atoms (0.2 nm apart) Copyright © FEI Energyanddistance Probingtheproton Quarksdetectedwithinprotons Freeway 280 2 miles long accelerator End Station A experimental area Stanford (SLAC), California, late 1960s Fire electrons at proton: big deflections seen! Quarkmasses 180 160 140 120 Mass 100 (GeV) 80 60 40 20 0 175 GeV Top (discovered 1995) E= mc2 1 proton mass ~ 1GeV (10-27 Kg) 0.003 0.006 0.095 1.2 4.5 Quarks Up Down Strange Charm Bottom Top The mass grows larger in each successive family QuantumMechanics • Atomsandpar*clesbehaviorisdescribedbytheQuantum Mechanics • Someproper*es,likeenergyforexample,canonlyassume somediscretevalues,theydon’tbelongtoacon*nuum • Par*cleproper*esaredescribedbythesevalues(quantum numbers).Someexamples: – Electriccharge – Colourcharge – Flavour – Spin Pauliprinciple • Wecanusethepar*clequan*s*c proper*estoclassifythem • Somepar*cles,calledfermions, obeytothePauliprinciple.While someothers–thebosons–do not. TheSpin/Sta$s$csTheorem Half-integer Spin Particles 1 3 ! , ! , .... Fermions 2 2 Fermi-Dirac Statistics (W. Pauli, 1940) Bose-Einstein Statistics 0, ! , 2 ! , .... Bosons IntegerSpinPar*cles ConsequencesoftheSpin/Sta*s*csTheorem: • formal:wavefunc*ons,fieldoperatorscommuta*onrules • experimental:nuclearandatomicstructure,Bose-Einstein condensates 33 FermionsandBosons TheWaveFunc*onmusthavethecorrectsymmetryunder interchangeofiden*calpar*cles.If1,2areiden*calpar*cles: 2 ψ ( x1 , x2 ) = ψ ( x2 , x1 ) ψ → +ψ (1↔ 2) ψ → −ψ (1↔ 2) 2 (probabilitymustbe conserveduponexcange ofiden*calpar*cle) Iden*calBosons (symmetric) Iden*calFermions (an*symm.) AconsequenceoftheSpin/Sta*s*csTheorem:fortwoiden*cal Fermions1,2inthesamequantumstatex: ψ (x1, x2 )= ψ (x2 , x1 )=− ψ (x1, x2 ) ⇒ ψ (x1, x2 ) = 0 Because identical Spin/Statistics Theorem Pauli Exclusion Principle! Par$cles/An$par$cles:the“birth”ofPar$clePhysics 1928:DiracEqua*on,mergingSpecialRela*vityandQuantumMech. Arela*vis*cinvariantEqua*onforspin½par*cles.E.g.theelectron (i γ µ ∂ µ − m) ψ = 0 • E>0, s=+1/2 • E>0, s=-1/2 • E<0, s= +1/2 • E<0, s= -1/2 Electron, s=+1/2 Electron, s=-1/2 Positron, s=1/2 Positron, s=-1/2 Rest frame solutions: 4 independent states: Uponreinterpreta*on ofnega*ve-energy statesasan*par*cles oftheelectron: Thepositron,apar*cleiden*caltotheelectron e-butwithaposi*vecharge:e+.Thefirst predic*onoftherela*vis*cquantumtheory. An$-maXer • Foreveryfundamentalpar*cleofmaYerthereisanan*-par*clewith samemassandproper*esbutoppositecharge Matter Anti-Matter +2/3 0 u νe d +1 e+ -2/3 u νe -1/3 -1 e- 0 Bar on top to indicate anti-particle +1/3 d positron • Correspondent anti-particles exist for all three families • Anti-matter can be produced using accelerators An$par$cles MaXer-an$maXerpaircrea$on • Electron-positron pair created out of photons hitting the bubble-chamber liquid • Example of conversion of photon energy into matter and anti-matter • Matter and anti-matter spiral in opposite directions in the magnetic field due to the opposite charge • Energy and momentum is conserved 4Forces • Thereare4fundamental interac*onsinNature • Allforcescanbebroughtbackto theseinterac*ons • ThegravityisaYrac*ve,allother forcescanalsoberepulsive • Theinterac*onsarealso responsibleofthenucleusdecays Howdopar$clesinteract? • Objectscaninteractwithoutbeingin contact • HowdomagnetsaYractorrepulse? • HowdoestheSunaYracttheEarth? • Aforceissomethingwhichis propaga*nginbetweenobjects TheconceptofForce In Classical Physics : In Quantum Physics : • Instantaneous action at a distance • Field (Faraday, Maxwell) • Exchange of Quanta Inverse square law k F= 2 r Constituents properties: Quarks: • Electric charge • Color • Effective mass • Spin (1/2) Leptons: • Electric charge • Mass • Spin (1/2) 3 families All Contituents (Quarks, Leptons) are Fermions. Costituents of Matter Force Carriers 43 ForcesinNature Force Intensity Strong Nuclear ~ 1 Carriers Happens in Gluons (massless) Atomic nuclea Elettromagnetic ~ 10-3 Photons (massless) Atomic levels Weak Nuclear ~10-5 W+,W-,Z0 (heavy) Beta radioactive decay Gravitation ~10-38 Gravitons (?) Heavy bodies Classical/QuantumForce Letusconsidertwopar*clesatadistancer . r . Ifasourcepar*cleemitsaquantumthatreachestheotherpar*cle, thechangeinmomentumwillbe: r Δp ≈ ! Andsince: Wehave: c Δt ≅ r Δp ! c ≈ Δt r r Δp k F = ≈ 2 Δt r 45 Rangeofinterac$ons Forcesanddistances R>106m(gravita*onalforce) R~10-10m(forzaelettromagne*ca) R~10-15m(forzaforte) Electromagne$sm • Theelectromagne*cforcesaresuch thatoppositechargesaYractand equalchargesrepulse • Theforcecarrieristhephoton(γ) • • Thephotonismasslessandmoveat thespeedoflight Electromagne$cforce The repulsive force that two approaching electrons “feel” e- e- Photon is the particle associated to the electromagnetic force “smallest bundle” of force Photon Photonexchange FeynmanDiagram e- e- γ e- e- Electromagne$cresidualforce • Normallytheatomsare neutralhavingthesame numberofprotonsand neutrons • Thechargedpartofanatom canaYractthechargedpart ofanotheratom • Theatomscanlink themselvestoform molecules Whynucleadonotexplode? • Aheavynucleuscontains manyprotons,allwith posi*vecharges • Theseprotonsrepulse eachothers • Whythenucleusdoesnot explode? Strongforce • InaggiuntaallacaricaeleYrica,i quarksportanoancheunnuovo*po dicarica,deYa“caricadicolore” • Laforzatralepar*cellechehanno caricadicoloree’deYaforzaforte Thegluon • Thestrongforcekeepsthequark together,toformthehadrons. • Theforcecarriersarethegluons: thereare8differentgluons • Thestrongforceactsonlyonshort distances Colourandan$-colour • Thereare3colourchargesand3an*-colourcharges • Notethatthesecoloursarenotatallrelatedwiththe standardcolourandwiththevisiblelight.Itisjustaway todescribethephysics(quantumnumbers) Quarksandcolour All quark flavours come in 3 versions, called “colours” uu u up +2/3 dd d down -1/3 Quarks combine together to form colourless particles - Baryons (three quarks: red+ green + blue = white) Strong forces “glue” quarks together in bound states proton p - Mesons (quark-antiquark pair) pion such as red+anti-red u-ubar state π u u Buildingmorepar$cles B mesons (bq) c c b u u b J/ψ B- B+ b d d b B0 B0 b b Y Many more mesons and baryons… Colouredquarksandgluons • Everyquarkhasoneofthethree colourchargesandeveryan*quark hasoneofthethreean*colour charges • Barionsandmesonsareneutralin colour Quarksconfinement • Thecolourforceincreasewiththe distance • Partcileswithcolourchargecannot existasisolatedpar*cles • Quarksareconfinedwithother quarkstoformhadrons • Hadronsareneutralincolour Quarksemitgluons • Whenaquarkemitsorabsorbsagluon, thecolourofthequarkchanges,in ordertosavethecolourcharge • Aredquarkemitsared/an*-bluegluon, andbecomesblu Strongforce:gluons Gluons interact with quarks Gluons interact with other gluons Quarkconfinement • There are no free quarks, quarks and antiquarks are “confined” in colourless doublet (mesons) or triplets (baryons) by the exchange of gluons Deca y Z 0 Gluon hold quarks together as they move further apart until the gluon connection snaps, and other quark-antiquark pairs are created out of the energy released The new quarks bound to the old quarks and form new mesons Residualstrongforce • Thestrongforcebetweenthequarksofaprotonandthe quarksofanotherprotonisstrongenoughtowinonthe repulsiveelectromagne*cforce Weakforce • Weakinterac*onsareresponsibleforthe decaysofheavyquarksandleptons • Example:theneutrondecaysinaproton+ electrons+neutrino • ThisexplainswhyallmaYeriscomposedby thelighterleptonsandquarks Weakforce:betadecay n p Antineutrin o Electron Neutronβ-decay At quark level: d→ u e- νe u d d n 15 min p u u d e- νe A (free) neutron decays after 15 min Long life time (15min is an eternity in particle physics!) ⇒ “weak” Electroweakforce • IntheSMtheelectromagne*candweak forceshavebeenunifiedinasingle electro-weakforce • Atveryshortdistances(~10-18meters),the weakandelectromagne*cinterac*ons havethesameintensi*es • Thecarriersarephotons,WandZs. Weakforce:W-,W+,Z0 β-decay n→peνe WElectric charge conserved at each vertex Muondecay • Exampleofapar*cledecay • Heretheproducedpar*clesare notpiecesoftheini*alpar*cle, butarecompletelynewones. Howdoesapar$cledecay? • Whenapar*cledecay,itistransformedinalighter par*cleandinapar*clewhichisoneofthecarriersofthe weakforce(theWboson) • Apar*cledecaysifitstotalmassishigherthanthesumof themassesofthedecayproductsandifthereisaforce whichmediatesthedecay Missingmass • Inmostofthedecays,thepar*cles andthenucleuswhichareleohavea totalmasssmallerthanthatofthe ini*alpar*cleoroftheorigina*ng nucleus • Themissingmassitistransformedin kine*cenergyofthedecayproducts Theunstablenucleus • Wesawthatstrongforceskeepthe nucleustogether,againsttherepulsive forcebetweenprotons. • However,notallnucleaarestable • Someofthemdecay • Thenucleuscansplitinsmallernuclea • Thishappensforexampleinanuclear reactor VirtualPar$cles • Par*clesdecayviapar*cleswhichareforce-carriers • Insomecases,apar*clemaydecayviaaforce-carrierthat ismoremassivethantheini*alpar*cle • Theforce-carrierpar*cleisimmediatelytransformedinto lower-masspar*cles • Theshort-livedmassivepar*cleappearstoviolatethelaw ofenergyconserva*on Annihila$on • Annihila*onsarenotdecaysbut,inthesameway,they takeplacethankstovirtualpar*cles • Theannihila*onoflightquarksathighenergiescanbring totheproduc*onofheavyquarksinthelaboratory • Thisbubblechambershows anan*-protonwhichhits againstaproton, annihilatesandproduces8 pions.Oneofthepionthen decaysinamuonanda neutrino(neutrinodoesnot leaveanytrace) Electron-Positronannihila$on Andthegravity? • Gravityisveryweak • Isimportantatmacroscopicdistances • Theforcecarriers(gravitons)ispredicted bytheorybutnotyetobserved MysteriesandfailuresofSM • TheSMisatheoryoftheUniverse • Itgivesagooddescrip*onofthephenomenawhichwe observeexperimentally • Undermanyrespectsisnotcomplete:whythereare3 genera*ons?WhatisthedarkmaYer? • AsEnsteinhasextendedthelawsofmechanicsofNewton withtheRela*vitytheory,wenowhavetogobeyondthe SM • Wehavetodoittoexplainmasses,gravityetc.. Threefamilies • Thereare3familiesof fundamentalpar*cles • Whyonly3? • Andwhywejustseeone ofthemintherealworld? Whatcanwesayaboutmass? • TheSMcannotexplainwhya givenpar*cleischaracterizedby itsmass. • Physicists“invented”anewfield, calledHiggsfield,whichinteracts withalltheotherpar*clestogive theirmasses • TheHiggsbosonhasbeen discoveredwiththeLHC experimentin2013aoer~70 yearsofsearch!. BeyondtheStandard Model:Unifica*onofforces ELECTROMAGNETIC GRAVITY UNIFIED FORCE? STRONG WEAK Looking for a simple elegant unified theory TheoryofGrandUnifica$on • ItisbelievedthataGUTwill unifythestrong,weak,and electromagne*cforces. • These3forceswillthenbe visibleasdifferent manifesta*ons–atlowenergy –ofauniqueforce • The3forceswillunifyatavery highenergy Supersymmetry • Somephysicists,intheaYempt tounifygravitywiththeother fundamentalforces,have suggestedthatevery fundamentalpar*cleshould haveashadowpar*cle.Itis morethan20yearsthatweare lookingforthese supersymmetricpar*cles! • Today,physicshasatheoryforquantummechanic,for rela*vityandforgravity,butthese3theoriesarenot unified.. • Ifwewouldliveinaworldwithmorethan3spa*al dimensions,maybethisproblemcouldbeoverpassed. • Thestringtheorysuggeststhatinaworldwherethereare the3standarddimensionsandsomeaddi*onal(but smaller)dimensions,thepar*cleswouldbelikestrings. ExtraDimensions • TheStringtheoryrequiresmorethanjust3dimensions • Theseextra-dimensionscanbesosmallthatwecannotseethem • Experimentsnowlookforevidenceoftheseextra-dimensions DarkMaXer • ItseemsthattheUniverseis madebyadifferentmaYer thanourSunandstars • ThedarkmaYeraYracts normalmaYer,butithasnot beeniden*fiedyet