Particle PhysicsParticle Physics

True or False?

The fundamental particles of nature are protons,

neutrons, and electrons.

What IS Matter ?• Matter is all the “stuff” around you!

• Here’s the picture we’re going to uncover(not all today though)

HadronsHadrons

MatterMatter

LeptonsLeptons

BaryonsBaryons MesonsMesons ChargedCharged NeutrinosNeutrinos

ForcesForces

WeakWeak EMEM

StrongStrongGravityGravity

QuarksAnti-Quarks

QuarksAnti-Quarks

What are we made of ?

We’re made of cells whichcontain DNA. - Different cells serve different functions in your body.

Cells contain a nucleus, which holds your DNA !

And the DNA is simply a complex chain of molecules which contains your genetic code!

And what are molecules made of ? 0.0002”

The Elements Molecules are complex structures of the elements

Elements are made of…?

0.0000000002 m(2 x 10-10 m)

5x10-15 m

Electrons Nucleus

What’s in the Nucleus?

ProtonsNeutrons

Protons arepositively chargedand that amountof charge is exactlyequal (and opposite)to the charge of theelectron

Neutrons are similar to protons(ie., similar mass), buthave a net charge ofzero.

Recall: 1 [fm] = 10-15 [m]

Atomic Model

• To give you another perspective on the size of the atom. If the nucleons (protons/neutrons) were one cm in diameter…

• what is the diameter of the nucleus?– 10 cm

• what is the diameter of the electron?– less than the diameter of your hair

• What is the diameter of the entire atom?

– 30 football fields!30 football fields!

Are protons and neutrons fundamental?

(By fundamental, I mean are they indivisible?)

The answer is NO ! Protons and neutrons are made of smaller objects called quarks!

(1.6 x 10-15 m)

1x 10-18 m(at most) Protons

2 “up” quarks 1 “down” quark

Neutrons 1 “up” quark 2 “down” quarks

Three Families of Quarks

Generations

I II III

Charge =

-1/3d

(down)

s(strange)

b (bottom)

Charge =

+2/3u

(up)

c (charm)

t(top)

Also, each quark has a corresponding antiquark.The antiquarks have opposite charge to the quarks

Increasing mass

The 6 Quarks, when & where…

Quark Date WhereMass

[GeV/c2]Comment

up, down

- -~0.005, ~0.010

Constituents of hadrons, most prominently, proton and

neutrons.

strange 1947 - ~0.2 discovered in cosmic rays

charm 1974SLAC/

BNL~1.5

Discovered simultaneously in both pp and e+e- collisions.

bottom 1977Fermi-

lab~4.5

Discovered in collisions of protons on nuclei

top 1995Fermi-

lab~175 Discovered in pp collisions

How do we know

any of this?High energy particlesprovide a way to probe, or“see,” matter at the very smallestsizes.

Today, high energy accelerators produce energetic beams which allow us to probe matter at its most fundamental level.

As we go to higher energy particle collisions:1) Wavelength probe is smaller see finer detail2) Can produce more massive objects, via E=mc2

Major High Energy Physics LabsFermilabFermilab

SLACSLAC

KEKKEKCERNCERN

DESYDESY

BNLBNL

CESRCESR

Fermilab Accelerator (30 miles from Chicago)

1.25 miles

Main Injector

Tevatron

Experimental areasExperimental areas

Top Quarkdiscovered here at FNALin 1995.

Top Quarkdiscovered here at FNALin 1995.

“Typical” Particle Detector

A bit more on quarks

6 different kinds of quarks.

Matter is composed mainly of up quarks and down quarks bound in the nuclei of atoms.

The masses vary dramatically(from ~0.005 to 175 [GeV/c2])

Why is matter made only ofup & down quarks ?

This will be answered later!

Mas

s [G

eV/c

2 ]

Gold atom

Silver atom

Proton

Anti-particles too ! We also know that every particle has a corresponding antiparticle!

That is, there are also 6 anti-quarks, they have opposite charge to the quarks.

So, the full slate of quarks are:

, ,

, ,

u c t

d s b

Q= +2/3

Q= -1/3

Quarks

Particle

, ,

, ,

u c t

d s b

Q= -2/3

Q= +1/3Anti-

Particle

Protons & Neutrons

To make a proton:We bind 2 up quarks of Q = +2/3and 1 down quark of Q = -1/3. The total charge is 2/3 + 2/3 + (-1/3) = +1 !

To make a neutron:We bind 2 down quarks of Q= -1/3with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 !

BARYONSThe forces which hold the protons and neutrons together in thenucleus are VERY strong.

Protons and neutrons are among a class of particles called “hadrons”(Greek for strong). Hadrons interact very strongly!

Baryons are hadrons which contain 3 quarks (no anti-quarks).Anti-baryons are hadrons which contain 3 anti-quarks (no quarks).

Wow, I’m somebody… I’m

a Baryon!Me too,

me too…

Are there baryons other than protons and neutrons?

Good question, my dear Watson…

The answer is a resounding YES !

Other quarks can combine to form other baryons. For example:

us d

This combination is called a Lambda baryon, or 0 for shortWhat is the charge of this object?

u This combination is called a Delta baryon, or ++ for shortWhat’s this one’s charge?uu

Note: The neutron can be turned into a proton by simply replacing a“d” quark by a “u” quark!

Let’s make baryons!

u d s

Charge Q

Mass

+2/3 -1/3 -1/3

~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]

Quark up down strange

u u d d ss

uu

d

ProtonQ = +1

M=938 MeV/c2

du

d

NeutronQ = 0

M=940 MeV/c2

Let’s make some more baryons !

su

d

Lambda ()Q = 0

M=1116 MeV/c2

su

u

Sigma ()Q = +1

M=1189 MeV/c2

su

d

Sigma ()Q = 0

M=1192 MeV/c2

sd

d

Sigma ()Q = -1

M=1197 MeV/c2

u d s

Q

Mass

+2/3 -1/3 -1/3

Quark up down strange

u u d d ss

~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]

ColorI have been showing quarks as being either RED, GREEN or BLUE.

It turns out that quarks have a property called “COLOR”. This property is as intrinsic to the quarks as “electric charge”.

This intrinsic property is not really a visible color, but it helps to visualize the property, which can have3 values.

We will discuss this in greater detail when we start talking about the strong force.

I have been showing quarks as being either RED, GREEN or BLUE.

It turns out that quarks have a property called “COLOR”. This property is as intrinsic to the quarks as “electric charge”.

This intrinsic property is not really a visible color, but it helps to visualize the property, which can have3 values.

We will discuss this in greater detail when we start talking about the strong force.

Color (cont)Color (cont)

For now, assume that quarks come in 3 colors:

RED, GREEN, BLUE

Anti-quarks have “anti-color”

ANTIRED, ANTIGREEN, ANTIBLUE

Baryons always have 1 of each color. Antibaryons have one of each anti-color.

For now, assume that quarks come in 3 colors:

RED, GREEN, BLUE

Anti-quarks have “anti-color”

ANTIRED, ANTIGREEN, ANTIBLUE

Baryons always have 1 of each color. Antibaryons have one of each anti-color.

Spin

There’s another intrinsic property which Quarks have known as “spin”.

For convenience, you can think of it as a spinning top, but this is just a conceptual aid…

Quarks have spin S = ½.

There’s another intrinsic property which Quarks have known as “spin”.

For convenience, you can think of it as a spinning top, but this is just a conceptual aid…

Quarks have spin S = ½.

Mesons Mesons are also in the hadron family.

They are formed when a quark and an anti-quark “bind” together.

Because they are hadrons, they must be colorless. So, the quarkhas color, and the antiquark has “anticolor”

ud

What’s the charge of this particle?

cd

What’s the charge of this particle?

Q=+1, and it’s called a +

Q= -1, and this charmmeson is called a D-

sd

What’s the charge of this particle?

Q= 0, this strangemeson is called a K0

Summary Up & down quarks make up protons & neutrons

Quarks have an intrinsic property known as color, of whichthere are 3 varieties: red, green or blue.

Quarks also have a property known as Spin, and have Spin = 1/2.

Hadrons refer to strongly interacting particles: Baryons & Mesons

Baryons contain 3 quarks: 1 red + 1 green + 1 blue colorlessThey may have spin 1/2 or spin 3/2.

Mesons contain 1 quark & 1 antiquark: rr, gg, or bb colorlessThey may be spin 0, or spin 1

What IS Matter ?• Matter is all the “stuff” around you!

• Here’s the picture we’re going to uncover(not all today though)

MatterMatter

LeptonsLeptons

ChargedCharged NeutrinosNeutrinos

ForcesForces

WeakWeak EMEM

StrongStrongGravityGravity

HadronsHadrons

BaryonsBaryons MesonsMesons

QuarksAnti-Quarks

QuarksAnti-Quarks

The Quarks – a Recap

Proton

uu

d

Quarks Antiquarks

Q = +2/3 Q = -1/3 Q = -2/3 Q = +1/3

u d

c s

t b

u

c

t

d

s

b

Quarks can have 3 color values: red, green & blue Quarks have total spin S = ½ (SZ = -½ or +½) Anti-quarks have the same mass as their quark does.

Hadrons = Baryons + Mesons Baryons (antibaryons) contain 3 quarks (3 antiquarks) Mesons contain a quark and an antiquark

Why quarks?

Murray Gell-Mann 1969 Nobel Prize

in Physics

Why should nature be this complicated? To simplify the picture, and still account for this plethora of particles which were observed, Murray Gell-Mann proposed all these particles were composed of just 3 smaller constituents, called quarks.

If neutrons & protons are not fundamental, what about

electrons?

Are they made up of smaller constituents also ?

As far as we can tell, electrons appear to be indivisible.

As far as we can tell, electrons appear to be indivisible.

Leptons

Electrons belong to a general class of particles, called “Leptons”

As far as we can tell, the leptons are “fundamental”.

Each charged lepton has an uncharged partner called the “neutrino”

The leptons behave quite differently than the quarks

- They don’t form hadrons (no binding between leptons)

Are there other types of charged leptons (like the electron) ?

1932: Discovery of the positron,the “anti-particle” of the electron.

Anti-particles really exist !!!!!

1937 – Muons (and ) discovered in cosmic rays.

M() ~ 200*M(e)

The muon behaves very similarly to the electron (i.e., it’sa lepton).

1932: Discovery of the positron,the “anti-particle” of the electron.

Anti-particles really exist !!!!!

1937 – Muons (and ) discovered in cosmic rays.

M() ~ 200*M(e)

The muon behaves very similarly to the electron (i.e., it’sa lepton).

Neutrinos

Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino ( ).

1934: To account for the “unseen” momentum in the reaction (decay):

Nobel Laureate: Enrico Fermi

np

e X

n p + e- + X

(means “little neutral one”)

Lepton Picture continues…

Family Leptons Antileptons

Q = -1 Q = 0 Q = +1 Q = 0

1 e- e e+ e

2

1962: An experiment at Brookhaven National Lab showed that there were in fact at least 2 types of neutrinos.

Three happy families… In 1975, researchers at the Stanford Linear Accelerator discovered

a third charged lepton, with a mass about 3500 times that of theelectron. It was named the -lepton.

In 2000, first evidence of the ’s partner, the tau-neutrino () was announced at Fermi National Accelerator Lab.

Family Leptons Antileptons

Q = -1 Q = 0 Q = +1 Q = 0

1 e- e e+ e

3 families, just like the quarks… interesting !!!

This all looks Greek to me ?

e

e

electron

muon-minus

tau-minus

electron neutrino

muon neutrino

tau neutrino

Lepton (particle)

e

e

positron

muon-plus

tau-plus

electron anti-neutrino

muon anti-neutrino

tau anti-neutrino

Anti-lepton (anti-particle)

So here’s the big picture Quarks and leptons are the most fundamental particles of nature that we know about.

Up & down quarks and electrons are the constituents of ordinary matter.

The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators.

Each particle has a correspondingantiparticle.

The Standard Model of Particle Physics

EM

STRONG

WEAK

Interactions and Decays

Forces are responsible for:

Interactions

Decays Decays are really a type ofinteraction though…

What do I mean by an Interaction?

12

12

12

21 BAM 43

43

43

34

1 + 2 3 + 4

What do I mean by a Decay ?1 3 + 4

1 1 1 1 BAM 43

43

43

34

Conserved Quantities

By conserved, we simply mean that the value on the left hand sidemust equal the value on the right hand side.

Consider the process where two particles A and B collide, and produceparticles C and D:

A + B C + D

Some of the most important examples are:

Total energy is ALWAYS conserved – energy cannot be created nor destroyed, only transformed

Total momentum is ALWAYS conserved Total charge is ALWAYS conserved

- cannot simply create net charge

Some of the most important examples are:

Total energy is ALWAYS conserved – energy cannot be created nor destroyed, only transformed

Total momentum is ALWAYS conserved Total charge is ALWAYS conserved

- cannot simply create net charge

Energy Conservation (I)

A + B C + D Energy conservation means:

Energy of particle “A” + Energy of particle “B”

= Energy of particle “C” + Energy of particle “D”

If you knew any 3 of the energies, you could compute the fourth!

So, in such a reaction, you only need to measure 3 particles, and energy conservation allows you to compute the fourth! BUT, it really is not quite that easy!

If you knew any 3 of the energies, you could compute the fourth!

So, in such a reaction, you only need to measure 3 particles, and energy conservation allows you to compute the fourth! BUT, it really is not quite that easy!

Energy Conservation (II)

A B + C Energy conservation “really” means:

Rest Mass and Kinetic Energy of particle “A” ≥

= Rest Mass and Kinetic Energy of particle “B”+ Rest Mass and Kinetic Energy of particle “C”

What does all that craziness mean???What does all that craziness mean???

Decays easy…interactions not so muchDecays easy…interactions not so much

Energy Conservation (III)

So, energy conservation helps in 2 ways:

1. It allows you to predict the energy of particles which you do not, or cannot measure.

2. It may tell you that a process cannot occur, since energy conservation cannot be violated!

1. It allows you to predict the energy of particles which you do not, or cannot measure.

2. It may tell you that a process cannot occur, since energy conservation cannot be violated!

Momentum Conservation (I)

Momentum conservation is used just as frequently as energy conservation

The classic scenario is the case of neutron decay:

np mP

eme

neutron at rest decays to a proton + electron

Momentum Conservation

np mP

eme

neutron at rest appears todecay to a proton + electron

Total Momentum Before Decay = Total Momentum after Decay

P(neutron) = P(proton) + P(electron)

0 = P(proton) + P(electron)

We don’t know what the value is, but we know it is NOT zero. In other words the proton and electrons momentum cannot cancel, because they are in the same direction!

We don’t know what the value is, but we know it is NOT zero. In other words the proton and electrons momentum cannot cancel, because they are in the same direction!

This is precisely what lead to the conjecture that there must bean undetected particle, called the neutrino!

Charge Conservation (I)

A + B C + DAgain, let’s consider the process:

Charge conservation implies that:

Charge of particle “A” + Charge of particle “B”

= Charge of particle “C” + Charge of particle “D”

So, if you know the charge of any 3 of the particles, you canimmediately say what the charge of the 4th MUST BE!

So, if you know the charge of any 3 of the particles, you canimmediately say what the charge of the 4th MUST BE!

Charge Conservation (II)A + B C + D

A B C D

-1 +1 0 ?

0 0 +1 ?

+1 +1 ? 0

? -1 -2 +1

0 ? 0 0

? +1 +1 +1

0

-1

+2

0

0

+1

Other conserved quantitiesBaryon Number ConservationWhen we collide particles together, we find that the number ofbaryons is conserved.

A + B C + D

For each baryon, we simply assign B = +1 (protons, neutrons,for example) For each anti-baryon ,we assign B = -1 (antiprotons, antineutrons,for example)

Compute the total baryon number on each side and they must be equal!

For each baryon, we simply assign B = +1 (protons, neutrons,for example) For each anti-baryon ,we assign B = -1 (antiprotons, antineutrons,for example)

Compute the total baryon number on each side and they must be equal!

Baryon Number ConservationA + B C + D

A B C D

p ? n

?

p n ?

? n

Assume the only particles we know about are:p, n, p, n, +, -, and 0.

p

p n

π0

n p

p

π0

n

Lepton Number Conservation (I)Electron, Muon and Tau Lepton Number

LeptonConserved Quantity

Lepton Number

e-

Le

+1

e +1

L

+1

+1

L

+1

+1

Anti-Lepton

Conserved Quantity

Lepton Number

e+

Le

-1

e -1

L

-1

-1

L

-1

-1

We find that Le , L and L are each conserved quantities

Lepton Number Conservation (II)Let’s look at how this works:

LL

00 -1-1 +1+1

ee +

LL

LeLe

-1-1

00

00

-1-1

00

+1+1

-1-1

00

eLL

LeLe

-1-1

00

00

-1-1

00

00 X

X

Lepton & Baryon Number ConservationMore examples:

ee

LeLe00 -1-1 +1+1

n pe- +

e

eLL

LeLe

00

00

-1-1

00

00

-1-1 X

X

LeLe

BB

00

+1+1

00

+1+1

+1+1

00

-1-1

00

There is no such thing as“Meson Number

CONSERVATION”

There is no such thing as“Meson Number

CONSERVATION”

Summary of Conservation Laws

1. Conservation of Total Energy

2. Conservation of Total Momentum

3. Conservation of Baryon Number

4. Conservation of Lepton Number

1. Conservation of Total Energy

2. Conservation of Total Momentum

3. Conservation of Baryon Number

4. Conservation of Lepton Number

These conservation laws always apply

There are other conservation laws, but they only apply to certain forces.

These conservation laws always apply

There are other conservation laws, but they only apply to certain forces.

What IS Matter ?• Matter is all the “stuff” around you!

• Here’s the picture we’re going to uncover(the last piece!)

MatterMatter

LeptonsLeptons

ChargedCharged NeutrinosNeutrinos

ForcesForces

WeakWeak EMEM

StrongStrongGravityGravity

HadronsHadrons

BaryonsBaryons MesonsMesons

QuarksAnti-Quarks

QuarksAnti-Quarks

The Four Fundamental Forces

1. Gravity

2. Weak Force

3. Electromagnetic force

4. Strong Force

Wea

ker

Stronger

All other forces you know about can be attributed to one of these!

GravityGravity is the weakest of the 4 forces. The gravitational force between two objects of masses m1 and m2, separated by a distance d is:

F = Gm1m2/d2

G = gravitational constant = 6.7x10-11[N*m2/kg2]d = distance from center-to-center

The units of each are:[Force] = [Newton] = [N] [mass] = [kg] [distance] = [meters]

Gravity is only an attractive force

Electric Forces

Opposites charges attract

+ -

+ +

Like charges repel

- -

Strong Force

The strong force is the strongest of the known forces.

It is responsible for the binding of quarks intobaryons and mesons. Its residual effects also account for the binding of protons & neutrons in the nucleus.

This force behaves more like a “spring”. That is, the the force actually gets stronger as quarks move apart!

This in striking contrast to the EM & Grav. Force. Their forces decreases with separation (recall F 1/d2)

The strong force is the strongest of the known forces.

It is responsible for the binding of quarks intobaryons and mesons. Its residual effects also account for the binding of protons & neutrons in the nucleus.

This force behaves more like a “spring”. That is, the the force actually gets stronger as quarks move apart!

This in striking contrast to the EM & Grav. Force. Their forces decreases with separation (recall F 1/d2)

Weak Force

The weak force is the weakest of the known forces.

It is responsible for neutron decay, and decays of heavyquarks to the lighter quarks

It’s interaction is very short range (as opposed to thelong range interactions of the EM and gravitational force).

The weak force is the weakest of the known forces.

It is responsible for neutron decay, and decays of heavyquarks to the lighter quarks

It’s interaction is very short range (as opposed to thelong range interactions of the EM and gravitational force).

Forces

Electromagnetic: the photon ()

Strong: the gluon (g)

Weak: the W+, W- & Z0

Forces are the due to the exchange of force carriers.

For each fundamental force, there is a force carrier (or set of them).

The force carriers only “talk-to” or “couple to”particles which carry the proper charge.

Electric Charge (+, -)

Color Charge (r,g,b)

Weak Charge

Particles & Forces

quarksCharged leptons(e,)

Neutral leptons

()

ColorCharge ?

EMCharge ?

WeakCharge ?

Y

Y

Y

Y

Y Y

N N

N

Quarks can participate in Strong, EM & Weak Interactions ! All quarks & all leptons carry weak charge

In other words…

Since quarks have color charge, EM charge & weak charge, they can engage in all 3 types of interactions !

Charged leptons (e,,) carry EM and weak charge, but no strong charge. Therefore, they can participate in the EM & weak interaction, but they cannot participate in the strong interaction.

Neutrinos only carry weak charge, and therefore they onlyparticipate in the weak interaction they can pass through the earth like it wasn’t even there !

Why should we believe that forces are the result of force carriers?

The Standard Model (SM) which I have described to you is just that, it’s a model, or better yet, a theory.

All forces are described by exchange of force carriers, period !

It’s is an extremely successful theory.

It explains all subatomic phenomenon to extraordinary precision!

One example is in a quantity referred to as the electron’s “g-factor”

“g” from experiment: 2.0023193043768“g” from theory (SM): 2.0023193043070

They agree to better than 1 part in 10 billion ! Coincidence ?

Summary

EM

STRONG

WEAK