Tony LissDecember 6, 1997

What is the World Made of?

Air

Fire

Water

Earth

Since ancient times humankind has looked for fundamental constituents

AtomsAtomsThe first modern fundamental particle was the atom, from the Greekwork atmos, meaning indivisible.

The organization of the known types of atoms into the Periodic Tablewas a hint that there was a deeper underlying structure.

Electrons Are the ReasonElectrons Are the Reason

Mendeleev laid out this beautiful picture that classified the Mendeleev laid out this beautiful picture that classified the elements and predicted their properties. But elements and predicted their properties. But why?why?

In 1897 Thompson discovered the electron. The In 1897 Thompson discovered the electron. The negatively charged electron came from the atom provingnegatively charged electron came from the atom proving– The atom is The atom is notnot elementary and, elementary and,– There must be a positive charge in there too.There must be a positive charge in there too.

Rutherford: The First High Rutherford: The First High Energy PhysicistEnergy Physicist

The atomic nucleus is discovered!The atomic nucleus is discovered!

Gold foil

particles

Four fundamental particles describe it all: The proton, Four fundamental particles describe it all: The proton, neutron, electron and photon.neutron, electron and photon.

What Are the Forces That What Are the Forces That Hold It Together?Hold It Together?

Force Relative Strength

Gravitational 1

Weak 1029

Electromagnetic 1040

Strong 1043

With each force goes a quantum field theory to describe it (but we don’t yet have a working theory for the oldest known force, gravity).

Each force has a particle associated with it, a gauge boson, that allows the force to act at a distance. For the electromagnetic force the theory is quantum electrodynamics and the gauge boson is the photon.

Keeps you in your seat

Nuclear beta decay: np e-

Binds electrons and protons

Holds the nucleus together

Too Tiny To See?Too Tiny To See?

A typical nucleus has a radius of about 10-15 meters, yet Rutherford was able to “see” the nucleus of a gold atom. How did he do it?

We usually see by detecting light that has bounced off objectsinto our eyes. If the object is very small compared to the wavelengthof the light, then the light diffracts around the object and the imageis lost. To see very tiny objects we need very short wavelengths.

<d

>d

Enter Quantum MechanicsEnter Quantum Mechanics

With the development of quantum mechanics in the 1920s, it was shown that a particle with momentum p behaves like a wave with wavelength

h/pRutherford’s particles had a wavelength similar to the size of a

gold nucleus.

Today, particle physicists still use this principle: To resolve structure on a smaller and smaller scale, we need higher and higher energies

AndAnd Relativity! Relativity!

There’s another reason for high energies too:

E=Mc2

If we want to create a particle that isn’t found in normalmatter in the laboratory, we need an energy at least equalto its mass times the speed of light squared.

A Natural Source of High A Natural Source of High Energy ParticlesEnergy Particles

Early particle physicists discovered that nature provides an excellent source of Early particle physicists discovered that nature provides an excellent source of high energy particles: high energy particles: cosmic rayscosmic rays

Cosmic rays are high energy charged particles, mostly protons, that impinge on the earth’s atmosphere from outer space. Wanna bang a projectile on a target? Nature does it for you all the time!

Mostly Muons

collision w/ airmolecule

protons from outer space

The Particle Explosion The Particle Explosion BeginsBegins

The study of cosmic ray interactions brought the discovery of a host of new The study of cosmic ray interactions brought the discovery of a host of new particlesparticles::

– 1931 - The positron (e1931 - The positron (e++))

– 1936 - The muon (m)1936 - The muon (m)

– 1947 - Pions, kaons, hyperons1947 - Pions, kaons, hyperons

Meanwhile, Ernest Lawrence was learning how to build powerful accelerators

Intensities MUCH higher than cosmic rays!

Where’s the Order?Where’s the Order?

With high energy accelerators and a new particle detector calledthe bubble chamber, particle physicists went to town in the 1950s

K 0

K-K0

K+

e

p

n

e

QuarksQuarks In 1961 Gell-Mann & Ne’eman did for “fundamental” particles what In 1961 Gell-Mann & Ne’eman did for “fundamental” particles what

Mendeleev had done 100 years earlier for “fundamental” atoms.Mendeleev had done 100 years earlier for “fundamental” atoms.

n p

- 0

K0 K+

K- K0

Q=-1Q=0

Q=+1

Q=-1

Q=0

Q=+1

Q=+2

S=0

S=-1

S=-2

S=-3S=+1

S=0

S=-1

A missing piece!The : S=-3, Q=-1

OrderOrderConstituentsConstituents

Just as the order of the periodic table was due to the three constituents, so Just as the order of the periodic table was due to the three constituents, so Gell-Mann and Zweig proposed that all the hadrons were made up from just 3 Gell-Mann and Zweig proposed that all the hadrons were made up from just 3 “quarks”“quarks”

Which, oddly enough, had electric charges (in units of e) of 2/3, -1/3. -1/3

p uudn udd+ ud0 uu- du

uuu uud udd ddd sss

K+ usK0 dsK- suK0 sd

Where are the Quarks?Where are the Quarks?

This is a nice picture, but no one had (or has) ever seen a free quark.This is a nice picture, but no one had (or has) ever seen a free quark. Repeat Rutherford’s experiment at Repeat Rutherford’s experiment at MUCHMUCH higher energies! higher energies!

Protons

electrons

Evidence that the proton has something hard inside!

Quantum Quantum ChromodynamicsChromodynamics

In the 1970s a quantum theory of strong interactions was developed called In the 1970s a quantum theory of strong interactions was developed called quantum chromodynamicsquantum chromodynamics (QCD). It includes a new gauge boson called the (QCD). It includes a new gauge boson called the gluongluon..

The “charge” of the strong force is called The “charge” of the strong force is called colorcolor, and each quark comes in one , and each quark comes in one of three colors: red, green or blue. The color force is transmitted by the gluon.of three colors: red, green or blue. The color force is transmitted by the gluon.

The observed hadrons are white, i.e.they have all 3 colors, or a color andand anti-color.

No Free Quarks (ever)No Free Quarks (ever)

The color force between quarks decreases with decreasing The color force between quarks decreases with decreasing distance (distance (asymptotic freedomasymptotic freedom) and grows large at large ) and grows large at large distances (distances (infrared slaveryinfrared slavery))

Here’s what happens if you try to pull a (colorless) baryon Here’s what happens if you try to pull a (colorless) baryon apart:apart:

Energy in the field increasesuntil...

Enough E=Mc2

for a quark-antiquarkpair to be produced

The Standard ModelThe Standard Model

ElectricMagneticWeakStrong

ElectromagneticElectroweak

Quarks

Leptons

ud

cs

tb

e

e-

GaugeBosons

W+ W- Z0 g

W W W

W W W

Simplicity regained!

On To Higher Energies!On To Higher Energies!

To test these new theories, physicists needed higher energiesCOLLIDING BEAMS.COLLIDING BEAMS.

“Fixed Target”

Amount of energy for new particle production~ Ebeam

Colliding BeamsAmount of energy for new particleproduction ~Ebeam

Detecting the DebrisDetecting the Debris

A typical collidingbeam detector

Tracking chamber - Inside magnetic field, measures charged particle momenta

Electromagnetic calorimeter - Measuresenergy of electrons and photons

Hadronic calorimeter - Measures energyof hadrons

Muon tracking - If it has an electric charge and itmakes it out here, it’s a muon

Evidence for QCDEvidence for QCD

QCD tells me I can’t see a free quark. So, what happens if I whack twoprotons together so hard that one of the constituent quarks goes flying away?

A Real Two “Jet” EventA Real Two “Jet” Event

Tests of Electroweak Tests of Electroweak TheoryTheory

With the advent of electroweak theory, three new particles With the advent of electroweak theory, three new particles were needed: the gauge bosons Wwere needed: the gauge bosons W++, W, W-- and Z and Z00. The . The masses were predicted by the theory to bemasses were predicted by the theory to be

– MMWWcc22 80 GeV 80 GeV

– MMZZcc22 90 GeV 90 GeV

That’s about the mass of bromine (z=35) and zirconium (z=40).Not badd for a pair of elementary particles!

The W and Z are extremely short-lived, but can be identified bytheir decay modes, also predicted by electroweak theory

W

ee

udcstb(?)

Z

e+e-

qq

How Many Generations?How Many Generations?

The Z decays to the leptons and antileptons of each The Z decays to the leptons and antileptons of each generation, as well as the quarks and antiquarks.generation, as well as the quarks and antiquarks.

The more ways there are for the Z to decay, the easier it is The more ways there are for the Z to decay, the easier it is for it to do so and the shorter its lifetimefor it to do so and the shorter its lifetime

Heisenberg’s uncertainty principle can be writtenHeisenberg’s uncertainty principle can be written

Et/2

Coupled with E=Mc2, this says that the mass of the Z when it decaysis determined only up to a constant proportional to its lifetime

M

tz 1

The Width of the ZThe Width of the Z

Z width (E)

Z mass (=E/c2)

Num

ber

of Z

dec

ays

So….Six quarksSo….Six quarks

And the sixth one is

The Hunting of the Top The Hunting of the Top QuarkQuark

After the bottom quark was discovered (Fermilab, After the bottom quark was discovered (Fermilab, 1977) it was known that a top quark had to exist 1977) it was known that a top quark had to exist because the Standard Model requires the quarks to because the Standard Model requires the quarks to come in pairs.come in pairs.

The Standard Model tells us exactly how the top The Standard Model tells us exactly how the top decaysdecays

q, e,

q e, , , But there is no direct prediction ofBut there is no direct prediction ofits mass.its mass.

Making Top Quarks in Making Top Quarks in Batavia, ILBatavia, IL

The Fermilab Tevatron is the world’s highest energy The Fermilab Tevatron is the world’s highest energy accelerator. Protons and antiprotons collide at an energy accelerator. Protons and antiprotons collide at an energy of 1.8 TeV (1800000000000 electron volts).of 1.8 TeV (1800000000000 electron volts).

Success !!Success !! In 1994, after 17 years of trying at various accelerators, In 1994, after 17 years of trying at various accelerators,

and nearly 10 years at the Tevatron, we finally found a and nearly 10 years at the Tevatron, we finally found a grand total of 12 collisions that looked like they produced grand total of 12 collisions that looked like they produced a top-antitop pair. Now there are more than 100.a top-antitop pair. Now there are more than 100.

Why was it so hard?Why was it so hard?

– BecauseBecause

MTOP c 2=

And only 1 out of 1010 collisions produced a top-antitop pair.

Now that’s a needle in a haystack!

We’re Done! (NOT!)We’re Done! (NOT!)

The The neutrino (we know it’s there, it’s just neutrino (we know it’s there, it’s just reallyreally hard to hard to see directly)see directly)

The Higgs Boson (Not The Higgs Boson (Not anotheranother one?!?) one?!?) Why Why are there three generations?are there three generations? How can we explain the weird pattern of masses?How can we explain the weird pattern of masses? Are the fundamental particles Are the fundamental particles reallyreally so? Might they too so? Might they too

have constituents? We’re doing the Rutherford have constituents? We’re doing the Rutherford experiment yet again, this time with quark-antiquark experiment yet again, this time with quark-antiquark collisions! Do quarks have constituent parts? collisions! Do quarks have constituent parts? Maybe!Maybe!

OK, 3 generations, all six quarks, Ws, Zs, gluons, photons. What’s missing?

Are there fundamental particles?

Or is nature just an onion?