The The Standard Standard

ModelModel

Jesse ChvojkaJesse ChvojkaUniversity of RochesterUniversity of Rochester

PARTICLE ProgramPARTICLE Program

A quick lookA quick look

These are the ingredients you These are the ingredients you need to make our world minus need to make our world minus

a few of the detailsa few of the details

Is this the Standard Is this the Standard Model?Model?

Yes….and No,Yes….and No, the Standard the Standard Model is more than just a list of Model is more than just a list of particle, but what is it?particle, but what is it?

Let’s look at what it is…Let’s look at what it is… Description of the Description of the

fundamental particlesfundamental particles Description of three of the Description of three of the

fundamental forces fundamental forces ► ►StrongStrong ► ► WeakWeak ► ► ElectromagneticElectromagnetic

Union of Union of weakweak & & electromagneticelectromagnetic as the as the electroweak forceelectroweak force

Conservation laws, e.g. Conservation laws, e.g. matter-energy, momentum, matter-energy, momentum, charge, etc…charge, etc…

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

A complete theoryA complete theory Description Description

of of gravitygravity Explanation Explanation

of heavy of heavy generations generations of leptons and quarksof leptons and quarks

Unification of Unification of strongstrong and and electroweakelectroweak forces forces

Definitive 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?!

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

Our ToolboxOur Toolbox

Concepts and MethodsConcepts and Methods• SpinSpin

►► BosonsBosons►► FermionsFermions

• QuantizationQuantization• AntimatterAntimatter• Conservation LawsConservation Laws• Feynman DiagramsFeynman Diagrams

►► Real ParticlesReal Particles►► Virtual particlesVirtual particles

SpinSpin

Analogous to spining top, but Analogous to spining top, but nothing is really spinningnothing is really spinning

Intrinsic Property of all Intrinsic Property of all Fundamental particlesFundamental particles

Can be integer (Can be integer (bosonboson) or ) or odd half-integer (odd half-integer (fermionfermion))

In the case of fermions, spin In the case of fermions, spin can be up (can be up () or down or down ()

Conserved quantityConserved quantity

Bosons and FermionsBosons and FermionsBosonBoson = particle of integer spin = particle of integer spin E.g., 0,1,2,…E.g., 0,1,2,… Examples: Photon, W, Z, gluon Examples: Photon, W, Z, gluon

He-4 nuclei, Oxygen 16He-4 nuclei, Oxygen 16 Multiple particles can be in the Multiple particles can be in the

same statesame stateFermionFermion = odd half-integer spin = odd half-integer spin

E.g., -1/2, 1/2, 3/2,….E.g., -1/2, 1/2, 3/2,….Examples: Electron (all leptons Examples: Electron (all leptons for that matter), quarks, He-3for that matter), quarks, He-3Pauli Exclusion principle – one Pauli Exclusion principle – one particle per configurationparticle per configuration

QuantizationQuantizationEnergy, charge, spin, matter, Energy, charge, spin, matter, etc. come in quantized etc. come in quantized amountsamounts

Einstein (1905) – light Einstein (1905) – light quantized, thus the photonquantized, thus the photon

Logical ConclusionLogical Conclusion

Force carries — quantization of Force carries — quantization of a force a force

AntimatterAntimatter Every particle has an Every particle has an

antiparticleantiparticle All properties the same except spin All properties the same except spin

and charge oppositeand charge opposite

Particle and its antiparticle Particle and its antiparticle annihilate upon contact into pure annihilate upon contact into pure energyenergy

Problem of why more matter than Problem of why more matter than anti-matter in the universeanti-matter in the universe

The Wild World of The Wild World of Conservation LawsConservation Laws

• Symmetries exist in the Symmetries 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• Mass-energy, momentum Mass-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 possible….These limit what is possible….

Feynman DiagramsFeynman Diagrams

The BasicsThe BasicsEmbodies 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 of…Arrow in direction of… time = particletime = particle opposite = antiparticleopposite = antiparticle• 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 laws….conservation 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 real particlesrepresent real particles

Photon in this case is a virtual Photon in this case is a virtual particleparticle

So what are RealSo what are Real and and Virtual Particles?Virtual Particles?

Real particlesReal particlesCan be observed directly or Can be observed directly or indirectly in experimentindirectly in experimentSatisfy the relativity equation Satisfy the relativity equation EE22 == p p22cc22+ m+ m22cc44

Virtual particlesVirtual particlesCannot be observed directly, Cannot be observed directly, represents intermediate stage represents intermediate stage of a processof a processEE22 ≠≠ p p22cc22+ m+ m22cc44 !!! !!!Allowed by Heisenberg’s Allowed by Heisenberg’s Uncertainy principleUncertainy principle

ΔΔppΔΔx ≥ x ≥ /2 or /2 or ΔΔEEΔΔt ≥ t ≥ /2/2

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

GravityGravity

Strong ForceStrong Force

ElectromagnetismElectromagnetism

Weak ForceWeak Force

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 Relativity

Infinite Range, weakest of the Infinite Range, weakest of the forces, dominates astronomical forces, dominates astronomical scalesscales

GravitonGraviton predicted as force predicted as force carriercarrier

ElectromagnetismElectromagnetismMediated by Mediated by photonphoton exchange exchange

Described by Described by QEDQED

Infinite Range: actsInfinite Range: acts

on astronomical on astronomical

and atomic scales, responsible and atomic scales, responsible for chemical propertiesfor chemical properties

Attractive or repulsive force that Attractive or repulsive force that acts upon objects with electric acts upon objects with electric chargecharge

Strong ForceStrong Force

Strongest force, but Strongest force, but quarksquarks are are only fermions that it affectsonly fermions that it affects

Force mediated by Force mediated by gluonsgluons

Quarks and gluons have Quarks and gluons have color color chargecharge which is analogous to which is analogous to electric charge, but with electric charge, but with differences that we’ll exploredifferences that we’ll explore

So how So how does does

color work?color work?

CCoolloorr Three types of color charge, Red, Three types of color charge, Red,

Green, Blue and associated anti-Green, Blue and associated anti-colorcolor

And….And….

Eight different color, anticolor Eight different color, anticolor combinations that gluons can makecombinations that gluons can make

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

combinecombineA color and anticolor cancel each A color and anticolor cancel each other out (“neutral”)other out (“neutral”)RedRed, , GreenGreen, and , and BlueBlue make make “neutral” or “white” “neutral” or “white”

So, the following can formSo, the following can formmesonsmesons: : quark-antiquark pair (e.g. quark-antiquark pair (e.g.

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

(e.g. protons, neutrons)(e.g. protons, neutrons) ►► Three antiquarks, different Three antiquarks, different anticolorsanticolors (e.g. anti-protons, antineutrons)(e.g. anti-protons, antineutrons)

Quarks Unite!Quarks Unite!

QuarksQuarks exchange exchange massive amounts massive amounts of gluons creatingof gluons creatinga color fielda color fieldEach gluon exchange and Each gluon exchange and absorption changes the color absorption changes the color of a quarkof a quarkSo how does this hold quarks So how does this hold quarks

together?together?Important!Important! Gluons are self- Gluons are self-interacting. So what?! Well…interacting. So what?! Well…this leads to this leads to Confinement!!!Confinement!!!

Stuck TogetherStuck TogetherAs two quarks are separated, As two quarks are separated, the energy used creates a lot the energy used creates a lot of of gluon-gluongluon-gluon activity activity

Until enough energy is present Until enough energy is present in the gluon interactions to in the gluon interactions to produce another quark pairproduce another quark pair

So quarks can’t be separatedSo quarks can’t be separated

And increasing And increasing gluon-gluongluon-gluon activity is why the Strong force activity is why the Strong force increases with distanceincreases with distance

Assembling the AtomAssembling the Atom

Residual forces are felt Residual forces are felt between nucleons. It is this between nucleons. It is this that binds the nucleus togetherthat binds the nucleus together

And electronsAnd electrons

orbit the orbit the

nucleus nucleus

Atoms!!Atoms!!

Weak ForceWeak Force Facilitates decay of massive Facilitates decay of massive

particles into lighter particlesparticles into lighter particles Mediated by Mediated by WW++, , WW--, and , and ZZ00

bosonsbosons

Electroweak forceElectroweak force

Another look at Another look at leptons and quarks leptons and quarks

Holds quarks together to form Holds quarks together to form protons, neutrons, pions, etc…protons, neutrons, pions, etc…

GluonsGluons act on something act on something called called colorcolor charge, which only charge, which only quarksquarks and and gluonsgluons have have

Where we are….Where we are….

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 century century

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

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

• 19001900 – Planck’s idea of energy – Planck’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 atom – Bohr model of the atom

History Marches OnHistory Marches On

Theses accomplishments gave Theses accomplishments gave birth to other discoveries:birth to other discoveries:

• SpinSpin – deduced from Zeeman – deduced from Zeeman 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 deBroglie, deBroglie, Schrödinger, Schrödinger, Heisenberg, DiracHeisenberg, Dirac

Breakthroughs during Breakthroughs during the 1930sthe 1930s

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

• Neutron deduced from Neutron deduced from unaccounted unaccounted

for mass in for mass in

nucleus, observed nucleus, observed

19321932 PositronPositron (antimatter) predicted (antimatter) predicted

by QED and foundby QED and found MuonMuon found in Cosmic Ray found in Cosmic Ray

Experiments!!Experiments!!

Enter the Weak ForceEnter the Weak Force

• Enrico Fermi – postulates Enrico Fermi – postulates weakweak forceforce to to explain explain beta beta decaydecay

• Hans Bethe – sun and other Hans Bethe – sun and other stars burn through reverse stars burn through reverse beta decay, i.e. via the beta decay, i.e. via the weakweak forceforce

Other Breakthroughs of Other Breakthroughs of the 1930sthe 1930s

Yukawa’s hypothesis ofYukawa’s hypothesis of

strongstrong nuclear force – template nuclear force – template for later theories of the standard for later theories of the standard

model (also predicts pion) model (also predicts pion)

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

And then….And then….

Particle Particle Explosion!Explosion!

The 40s, 50s, early 60s Particle explosion begins,

many new particles discovered (lambda, kaon, pion, etc...)

Property of strangeness observed

Electron neutrino and then muon neutrino found as well

Post WWII – SLAC evidence that protons are composite

Quarks!!Quarks!!

19641964 – Breakthrough: Murray – Breakthrough: Murray Gell-Mann and George Zweig Gell-Mann and George Zweig independently put forward independently put forward quarkquark model model

► ► 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 isn’t acceptedmodel still isn’t 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 quarks

19741974 – BNL and SLAC both – BNL and SLAC both observe the observe the CharmCharm (# 4) quark, (# 4) quark, quark model finally exceptedquark model finally excepted

19781978 – – BottomBottom quark (# 5) found, quark (# 5) found, Top qurak predictedTop qurak predicted

1970s1970s – – 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 understanding – Finally some understanding• Glashow, Weinberg, and SalamGlashow, Weinberg, and Salam put put

forth electroweak theory which….forth 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 weakweak

and and electromagneticelectromagnetic

force 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 up…The Last Round up…

19771977 – – TauTau lepton observed lepton observed suggesting a third generation suggesting a third generation of quarks tooof quarks too

19831983 – – WW++ & & WW-- bosons found bosons found 19841984 – – ZZoo boson found boson found

((note:note:

bosonboson = particle of integer spin = particle of integer spin

whilewhile

fermionfermion = half integer spin) = half integer spin) 20002000 – – Tau neutrinoTau neutrino found found