The Standard The Standard Model Model

of of Particle Particle PhysicsPhysics

Jesse Jesse ChvojkaChvojkaUniversity of RochesterUniversity of Rochester

PARTICLE ProgramPARTICLE Program

LetLet’’s look at what it iss look at what it is……Description of fundamental Description of fundamental particles particles –– quarksquarks and and leptonsleptonsThree out of Four (Forces) Three out of Four (Forces) ainain’’tt half badhalf bad►► ElectromagneticElectromagnetic►► StrongStrong►► WeakWeakElectroweak forceElectroweak force

EMEM and and WeakWeak force force are two sides of a coin are two sides of a coin Rule BookRule Book –– Conservation of Conservation of Energy, Charge, momentumEnergy, Charge, momentum

A quick lookA quick look

Building Blocks of RealityBuilding Blocks of Reality

Our ToolboxOur Toolbox•• ParticlesParticles interact by exchanging interact by exchanging

other other particlesparticles•• QuantizationQuantization –– same thingsame thing•• SpinSpin►► Bosons Bosons –– Stack Stack emem upup►► Fermions Fermions –– CanCan’’t be in same t be in same place at the same time (place at the same time (PauliPauliPrinciplePrinciple))

• Antimatter►► Everyone Everyone

needs a needs a brotherbrother

QuarksQuarksAffected byAffected by►► strong force strong force ►► weak forceweak force►► electromagnetismelectromagnetism►► gravitygravityFractional chargeFractional chargeSpin 1/2Spin 1/2Three generations Three generations differing only by differing only by massmassUpUp and and DownDownquarks make up quarks make up protons & neutrons!protons & neutrons!

LeptonsLeptonsAffected byAffected by►► weak forceweak force►► electromagnetismelectromagnetism►► gravitygravityCharged LeptonsCharged Leptons –– 3 copies3 copies►► electronelectron, , muonmuon, , tautauNeutrinosNeutrinos –– 3 copies3 copies►► no chargeno charge►► tiny masstiny mass►► OscillateOscillateHave Have spin 1/2spin 1/2ElectronElectron is component of is component of ATOMS!ATOMS!

The Four (or Three) The Four (or Three) Fundamental ForcesFundamental Forces

GravityGravity

Strong ForceStrong ForceElectromagnetismElectromagnetism

Weak ForceWeak Force

ElectromagnetismElectromagnetismMediated by Mediated by photonphoton (light) (light) exchangeexchangeInfinite Range: actsInfinite Range: actson on cosmiccosmic and and atomic scalesatomic scales, , responsible for responsible for chemistry chemistry Acts upon objects with Acts upon objects with electric electric chargecharge

Weak ForceWeak ForceResponsible for Responsible for decays of massive decays of massive particlesparticlesinto lighter onesinto lighter onesCause of Cause of ββ –– decaydecayand and fusion fusion in in SunSunShort range force mediated Short range force mediated by by MASSIVEMASSIVE WW++, , WW--, and , and ZZ00 particlesparticles

Massive W and Z particles Massive W and Z particles –– just how massive?just how massive?

More More MASSIVEMASSIVE than than anything weanything we’’ve ever found ve ever found except except Top quarkTop quark

QQ: So how can a particle : So how can a particle decay by something more decay by something more massivemassive than itselfthan itself??

AA: : Nature CheatsNature Cheats: : HeisenbergHeisenberg’’s Uncertainty s Uncertainty principleprincipleΔΔppΔΔx x ≥≥ ħħ/2 or /2 or ΔΔEEΔΔt t ≥≥ ħħ/2/2

Strong ForceStrong Force

Strongest force, Strongest force, quarks/gluons quarks/gluons are only particles that it affectsare only particles that it affectsForce mediated by Force mediated by gluonsgluonsQuarks/gluonsQuarks/gluons have have color color chargecharge, analogous to , analogous to electric electric chargecharge with a few with a few differncesdiffernces

So how does So how does colorcolor work?work?

Quarks have CQuarks have CoolloorrThree types of Three types of color chargecolor charge, , RedRed, , GreenGreen, , BlueBlue and associated and associated antianti--color chargecolor charge

AndAnd……..

Eight different Eight different color/anticolor/anti--color color combinations that combinations that gluonsgluons can makecan make

Quarks Unite!Quarks Unite!QuarksQuarks exchange exchange massive amounts massive amounts of of gluonsgluons creatingcreatinga a color fieldcolor fieldEach Each gluongluon exchange and exchange and absorption changes the color absorption changes the color of a of a quarkquarkSo how does this hold quarks So how does this hold quarks

together?together?Important!Important! GluonsGluons are selfare self--interacting. So what?! Wellinteracting. So what?! Well……this leads tothis leads to…….. ..

Stuck TogetherStuck Together

More More EnergyEnergy in pulling in pulling quarksquarks apart = apart = stronger force holding stronger force holding emem’’ togethertogether

Proton Proton ►►

ConfinementConfinement

Energy used to Energy used to pullpull two two quarksquarks apart literally apart literally pullspulls particles into particles into existenceexistence

Assembling the AtomAssembling the Atom

Residual forcesResidual forces felt between felt between protons/neutronsprotons/neutrons from from gluon gluon fieldfield. It. It’’s this that binds s this that binds nucleusnucleustogethertogetherElectronsElectronsorbit orbit nucleusnucleus

AndAnd……Atoms!!Atoms!!

End of End of Not So Not So Short Short StoryStory

CCoolloorrThree types of color charge, Red, Three types of color charge, Red, Green, Blue and associated antiGreen, Blue and associated anti--colorcolor

AndAnd……..

Eight different color, Eight different color, anticoloranticolorcombinations that gluons can makecombinations that gluons can make

CCoolloor r ccoonntt......Color has to be Color has to be ““neutralneutral”” for quarks to for quarks to

combinecombineA color and A color and anticoloranticolor cancel each cancel each other out (other out (““neutralneutral””))RedRed, , GreenGreen, and , and BlueBlue make make ““neutralneutral”” or or ““whitewhite””

So, the following can formSo, the following can formmesonsmesons: : quarkquark--antiquarkantiquark pair (e.g. pair (e.g.

pionspions))baryonsbaryons::►► Three quarks, different colorsThree quarks, different colors

(e.g. protons, neutrons)(e.g. protons, neutrons)►► Three Three antiquarksantiquarks, different , different

anticolorsanticolors(e.g. anti(e.g. anti--protons, antineutrons)protons, antineutrons)

Stuck TogetherStuck TogetherAs two quarks are separated, As two quarks are separated, energy used creates a lot of energy used creates a lot of gluongluon--gluongluon activityactivityIncreasing Increasing gluongluon--gluongluon activity activity causes more force between causes more force between particles particles Why the Why the Strong forceStrong forceincreases with distanceincreases with distance

ConfinementConfinement

Assembling the AtomAssembling the Atom

Residual forces are felt Residual forces are felt between nucleons from the between nucleons from the gluon field. It is this that binds gluon field. It is this that binds the nucleus the nucleus togethertogetherElectronsElectronsorbit the orbit the nucleus nucleus

AndAnd……Atoms!!Atoms!!

End of End of Long Long StoryStory

Electroweak forceElectroweak forceWeakWeak and and electromagneticelectromagneticforces unified into forces unified into electroweakelectroweak forceforceTheory predicted the Theory predicted the WW++, , WW--, , and and ZZ00 bosons, relates them bosons, relates them to to photonphotonRequires Requires another another particle called particle called Higgs bosonHiggs bosonwhich gives which gives particles particles massmass

Pop QuizPop QuizParticles interact by:Particles interact by:

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriers

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:Photon/lightPhoton/light

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:Photon/lightPhoton/lightThings with color interact by:Things with color interact by:

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:Photon/lightPhoton/lightThings with color interact by:Things with color interact by:GluonsGluons

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:Photon/lightPhoton/lightThings with color interact by:Things with color interact by:GluonsGluonsFor For ββ –– decay, the particle decay, the particle that is exchanged is:that is exchanged is:

Pop QuizPop QuizParticles interact by:Particles interact by:Exchanging other Exchanging other particles/force carriersparticles/force carriersThings with electric chargeThings with electric chargeinteract by:interact by:Photon/lightPhoton/lightThings with color interact by:Things with color interact by:GluonsGluonsFor For ββ –– decay, the particle decay, the particle that is exchanged is:that is exchanged is:WW++ or Wor W--

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:61 inches in Rochester area61 inches in Rochester area

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:61 inches in Rochester area61 inches in Rochester areaStrong Force holds together: Strong Force holds together:

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:61 inches in Rochester area61 inches in Rochester areaStrong Force holds together: Strong Force holds together: Neutrons and protons, quarks, Neutrons and protons, quarks, nucleusnucleus

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:61 inches in Rochester area61 inches in Rochester areaStrong Force holds together: Strong Force holds together: Neutrons and protons, quarks, Neutrons and protons, quarks, nucleusnucleusWeak force is responsible for:Weak force is responsible for:

Pop QuizPop Quiz –– part IIpart II

Protons and Neutrons are Protons and Neutrons are made up of:made up of:Quarks (up and down quarks)Quarks (up and down quarks)How many inches of snow did How many inches of snow did we get in January:we get in January:61 inches in Rochester area61 inches in Rochester areaStrong Force holds together: Strong Force holds together: Neutrons and protons, quarks, Neutrons and protons, quarks, nucleusnucleusWeak force is responsible for:Weak force is responsible for:ββ –– decay, fusion in sun, decay decay, fusion in sun, decay of massive particlesof massive particles

Unsolved MysteriesUnsolved Mysteries

GravityGravity

Why three generationsWhy three generations

CanCan’’t predict a particles mass, t predict a particles mass, oops, Higgs?oops, Higgs?

Matter/AntiMatter/Anti--Matter asymmetryMatter asymmetry

Dark MatterDark Matter

Kinks in the Standard Kinks in the Standard ModelModel

The HorizonThe HorizonSupersymmetrySupersymmetryString TheoryString TheoryExtra DimensionsExtra DimensionsDark MatterDark MatterDark EnergyDark EnergyGrand Unified TheoriesGrand Unified Theories

……and a look at what it and a look at what it is notis not

A complete theoryA complete theoryDescription Description of of gravitygravityExplanation Explanation of heavy of heavy generations generations of of leptonsleptons and and quarksquarksUnification of Unification of strongstrong and and electroweakelectroweak forcesforcesDefinitive explanation on the Definitive explanation on the origins of massorigins of mass

But what does all this But what does all this mean?mean?

What are What are quarksquarks and and leptonsleptons??What are the force carriers?What are the force carriers?What do they do?What do they do?And how do we get from weird And how do we get from weird sounding particles sounding particles to the world around to the world around us?us?How did anyone How did anyone

come up with all this?!come up with all this?!

WeWe’’ll need some tools and then ll need some tools and then we can dive inwe can dive in

SpinSpin

Analogous to spinning top, but Analogous to spinning top, but nothing is really nothing is really ““spinningspinning””Intrinsic Property of all Intrinsic Property of all Fundamental particles Fundamental particles All have All have magnetic momentsmagnetic momentswhich is what helped lead to the which is what helped lead to the idea of spinidea of spinCan be integer (Can be integer (bosonboson) or odd ) or odd halfhalf--integer (integer (fermionfermion))In the case of fermions, spin can In the case of fermions, spin can be up (be up (↑) or downor down (↓)Conserved quantityConserved quantity

AntimatterAntimatterEvery Every particleparticle has an has an antiparticleantiparticleAll properties the same except All properties the same except spin, charge, and color oppositespin, charge, and color opposite

Particle and its antiparticle Particle and its antiparticle annihilate upon contact into pure annihilate upon contact into pure energyenergyProblem of why more matter than Problem of why more matter than antianti--matter in the universematter in the universe

Bosons and FermionsBosons and FermionsBosonBoson = particle of integer spin= particle of integer spin

E.g., 0,1,2,E.g., 0,1,2,……Examples: Examples: Photon, W, Z, gluonPhoton, W, Z, gluonHeHe--4 nuclei, Oxygen 164 nuclei, Oxygen 16Multiple particles can be in the Multiple particles can be in the same statesame state

FermionFermion = odd half= odd half--integer spininteger spinE.g., E.g., --1/2, 1/2, 3/2,1/2, 1/2, 3/2,……..Examples: Electron (all Examples: Electron (all leptonsleptonsfor that matter), for that matter), quarksquarks, He, He--33PauliPauli Exclusion principleExclusion principle –– one one particle per quantum particle per quantum configurationconfiguration

The Wild World of The Wild World of Conservation LawsConservation Laws

•• SymmetriesSymmetries exist in the exist in the equations of the Standard equations of the Standard Model Model –– theoremtheorem: for each : for each symmetry a conservation law symmetry a conservation law

A few most of us are familiar withA few most of us are familiar with•• MassMass--energy, momentum energy, momentum And some a little less familiarAnd some a little less familiar•• Charge, Color, Spin, Angular Charge, Color, Spin, Angular

Momentum, baryon #, lepton #Momentum, baryon #, lepton #

These limit what is possibleThese limit what is possible……..

Feynman DiagramsFeynman DiagramsThe BasicsThe Basics

Embodies Quantum Theory in Simple Embodies Quantum Theory in Simple DiagramsDiagrams

•• Arrow of time Arrow of time →→ either points up or either points up or to the right (conventions)to the right (conventions)

•• Arrow in direction ofArrow in direction of……time = time = particleparticleopposite = opposite = antiparticleantiparticle

•• Events can be rotated in any Events can be rotated in any direction to represent different direction to represent different processesprocesses

More on Feynman More on Feynman DiagramsDiagrams

Arrangements limited by Arrangements limited by conservation lawsconservation laws……..i.e. cannot replace the photon i.e. cannot replace the photon with an electronwith an electron

Electrons in this case Electrons in this case represent represent real particlesreal particlesPhoton in this case is a Photon in this case is a virtual virtual particleparticle

GravityGravityAttractive force between any Attractive force between any object with mass or energy object with mass or energy Outside of the Standard Model, Outside of the Standard Model, described by described by General RelativityGeneral RelativityInfinite Range, weakest of the Infinite Range, weakest of the forces, dominates astronomical forces, dominates astronomical scalesscalesGravitonGraviton predicted as force predicted as force carriercarrier

Differences between Differences between Weak & EM forceWeak & EM force

•• Range of EM = Range of EM = ∞∞Range of Weak = atomic scaleRange of Weak = atomic scale

•• Photon is Photon is masslessmasslessWs, Z are MASSIVEWs, Z are MASSIVE

•• EM conserves parityEM conserves parityWeak violates parityWeak violates parity

•• EM isEM is……uhhuhh, a strong force, a strong forceWeak force is, err, well, weakWeak force is, err, well, weak

Why people buy thisWhy people buy this……

MathematicsMathematics••The The EM forceEM force is proportional is proportional to to ‘‘ee’’, the electric charge, the electric charge

••The The Weak forceWeak force is is prortionalprortionalto to ‘‘gg’’, which behaves the , which behaves the same way in equations as same way in equations as ‘‘ee’’

••Both forces can be described Both forces can be described by the same equations by the same equations (Called (Called LagrangianLagrangian))

AndAnd……..

Not so DifferentNot so DifferentWW++, , WW--, , ZZ00 and and photonphoton are are very similar except for huge very similar except for huge mass differencemass differenceWW++, , WW--, , ZZ00 predicted by this predicted by this theory and found (and the Z theory and found (and the Z with no experimental backing!)with no experimental backing!)Ws, Z, and photon interact very Ws, Z, and photon interact very similarly at higher energies and similarly at higher energies and short distancesshort distances

Why do EM & the Weak Why do EM & the Weak Force look so different?Force look so different?

Electroweak symmetry Electroweak symmetry breakingbreaking……This is broke by theThis is broke by theHiggs MechanismHiggs MechanismMechanism explains Mechanism explains why why WW++, , WW--, and , and ZZ00 have masshave mass►►Predicts Predicts Higgs BosonHiggs Boson as as particle that does this particle that does this Mass and few other properties Mass and few other properties generated by this mechanism generated by this mechanism create the riftcreate the rift

A little HistoryA little HistoryThe foundations for this framework The foundations for this framework

born at the end of 19born at the end of 19thth centurycentury

•• 18951895 –– Radioactive decay Radioactive decay discovered by Becquereldiscovered by Becquerel

•• 18971897 –– J.J. Thomson discovers J.J. Thomson discovers the the electronelectron

•• 19001900 –– PlanckPlanck’’s idea of energy s idea of energy quantizationquantization

•• 19051905 –– Einstein: Brownian motion Einstein: Brownian motion suggests atoms (oh, photoelectric suggests atoms (oh, photoelectric effect and relativity too)effect and relativity too)

•• 19111911 –– Rutherford, using alpha Rutherford, using alpha particles demonstrates small, particles demonstrates small, dense, positive nucleus dense, positive nucleus

•• 19131913 –– Bohr model of the atomBohr model of the atom

History Marches OnHistory Marches OnTheses accomplishments gave Theses accomplishments gave

birth to other discoveries:birth to other discoveries:•• SpinSpin –– deduced from deduced from ZeemanZeeman

and Stark effectsand Stark effects•• Quantum theory:Quantum theory:

matter as discrete matter as discrete wave packets, wave packets, gives a more gives a more accurate view of accurate view of the atom courtesy the atom courtesy deBrogliedeBroglie, , SchrSchröödinger, dinger, Heisenberg, Heisenberg, DiracDirac

Breakthroughs during Breakthroughs during the 1930sthe 1930s

•• Quantum theory extended by Quantum theory extended by DiracDirac to include relativity which to include relativity which gave rise to gave rise to QEDQED

•• Neutron deduced from Neutron deduced from unaccountedunaccountedfor mass in for mass in nucleus, observed nucleus, observed 19321932

PositronPositron (antimatter) predicted (antimatter) predicted by QED and foundby QED and foundMuonMuon found in Cosmic Ray found in Cosmic Ray Experiments!!Experiments!!

Enter the Weak ForceEnter the Weak Force

•• EnricoEnrico Fermi Fermi –– postulates postulates weakweakforceforce to to explain explain beta beta decaydecay

•• Hans Hans BetheBethe –– sun and other sun and other stars burn through reverse stars burn through reverse beta decay, i.e. via the beta decay, i.e. via the weakweakforceforce

Other Breakthroughs of Other Breakthroughs of the 1930sthe 1930s

YukawaYukawa’’ss hypothesis ofhypothesis ofstrongstrong nuclear force nuclear force –– template template for later theories of the standard for later theories of the standard model (also predicts model (also predicts pionpion) )

Wolfgang Wolfgang PauliPauli predicts predicts neutrinoneutrino to preserve energy to preserve energy conservation in beta decayconservation in beta decay

And thenAnd then……..

Particle Particle Explosion!Explosion!

The 40s, 50s, early 60sParticle explosion begins, many new particles discovered (lambda, kaon, pion, etc...)Property of strangeness observed Electron neutrino and then muon neutrino found as wellPost WWII – SLAC evidence that protons are composite

Quarks!!Quarks!!

19641964 –– Breakthrough: Murray Breakthrough: Murray GellGell--Mann and George Mann and George ZweigZweigindependently put forward independently put forward quarkquark modelmodel

►► Three quark model put forth Three quark model put forth with the 3 flavors, with the 3 flavors, upup, , downdown, , and and strangestrange►► SLAC sees evidence, but SLAC sees evidence, but model still isnmodel still isn’’t acceptedt accepted

More quarks?More quarks?

Fourth Fourth quarkquark predicted out of predicted out of symmetrysymmetry►►There are four leptons, but There are four leptons, but

only three quarksonly three quarks19741974 –– BNL and SLAC both BNL and SLAC both observe the observe the CharmCharm (# 4) quark, (# 4) quark, quark model finally exceptedquark model finally excepted19781978 –– BottomBottom quark (# 5) found, quark (# 5) found, Top Top qurakqurak predictedpredicted1970s1970s –– QCDQCD formed formed to explain to explain strong forcestrong force,,gluon gluon predicted!predicted!19941994 –– TopTop Quark (# 6) found!Quark (# 6) found!

Shedding Light on the Shedding Light on the Weak ForceWeak Force

1960s1960s –– Finally some understandingFinally some understanding•• GlashowGlashow, Weinberg, and , Weinberg, and SalamSalam put put

forth electroweak theory whichforth electroweak theory which……..►► Describes the Describes the weakweak force in force in terms of quantum terms of quantum theory and relativitytheory and relativity►► Describes the Describes the weakweakand and electromagneticelectromagneticforce as two components force as two components of one electroweak forceof one electroweak force►► Predicts Predicts WW++, , WW--, and , and ZZ00 as as transmitters of the weak forcetransmitters of the weak force►► Implies Implies Higgs BosonHiggs Boson as a way to as a way to give Ws and Z massgive Ws and Z mass

The Last Round upThe Last Round up……

19771977 –– TauTau lepton observed lepton observed suggesting a third generation suggesting a third generation of quarks tooof quarks too19831983 –– WW++ & & WW-- bosons foundbosons found19841984 –– ZZoo boson foundboson found

((note:note:bosonboson = particle of integer spin= particle of integer spin

whilewhilefermionfermion = half integer spin)= half integer spin)

20002000 –– TauTau neutrinoneutrino foundfound