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The nature of science, the definition almost is this: The test of all scientific questions is experiment. Experiment is the sole judge of scientific truth.

--Richard Feynman

Nature’s Deepest Questions

People have wondered about the nature of matter at least since time of the Greeks.

Democritis , around 450 BC coined the term atom for what he considered the fundamental particles of nature…the smallest bits of matter that could

not be divided further.

Atom comes from a Greek word meaning “indivisible”

Atoms, the building blocks of matter

The Greeks had the idea that there were only a few different types of atoms, and all matter was made

up of different combinations of them.

Mendeleev and the periodic table

Fast forward to the 1800s.

As more and more different kinds of atoms were discovered, Mendeleev noticed a pattern that

led him to arrange the different kinds of atoms into a table that we now call the

periodic chart of the elements

Periodic chart of the elements

But there are over 100 elements! Seems like too many…

The 20th century revolutions

There were many revolutions in physics during the 20th century, but one of the most

important was the realization that atoms were not indivisible…

JJ Thompson discovered the electron in 1897

The constituents of the atom

In 1912 Rutherford discovered the atomic nucleus

Oh well, I guess we can tolerate three...

In 1932, Chadwick discovered the neutron, the neutral partner of the proton. It wasn’t really

expected, but didn’t seem to complicate things too much...

Atoms were then understood as made from a nucleus of protons and neutrons surrounded

by the electrons. The nucleus is 100,000 times smaller than the atom.

But once again the simple picture broke down...

During the 1930s and 40s, cosmic ray experiments and the first accelerators

discovered more particles:

In 1937 another particle, which we now call the muon was discovered in cosmic ray

experiments. I. I. Rabi “Who ordered that?”

In 1946 the pion was discovered the same way

Discovery of the Kaon

In 1947 a new type of particle called “strange” particles was discovered in cosmic rays

Too many fundamental particles !!

By about 1965 there were over 100 fundamental particles…again too many.

Here is an example page from the Particle Data Book…this is one of several hundred pages of particle listings.

Enter the quark

Around 1965 Zweig and Gell-Mann suggested that protons, neutrons, and most of the other particles

were made of constituents which he called quarks.

“Three quarks for Muster Mark…” Finnegan’s Wake

The proton is made of three quarks, and so is the neutron.

But the electron is still fundamental.

The fundamental particles circa 1965:

Three quarks: u, d, s with charges +2/3 and -1/3 in units of the electron charge.

Three leptons: electron (e) and muon ()and a mysterious, neutral, very weakly

interacting particle called the neutrino, .

Six…maybe not too bad

The November revolution

In November of 1974, the world of physics was stunned by the discovery of a fourth quark, the

c or “charm” quark

This is a picture of the decay of a particle containing a c quark and anti-quark pair.

And then there were five.

In 1977 yet another quark was discovered, the b or bottom quark at Fermilab

The detector Leon

Now we know there are six quarks

Are we done yet?

Our current understanding of the fundamental particles:

Six quarks and six leptons! These particles have mass and electric charge but apparently no size!

But wait…where does the proton fit in?

The quarks combine in two (and only two) ways to form particles that we observe in the lab.

Quark-antiquark pairs from mesons:

+ = u anti-d K+=u anti-s

Three quarks from baryons:

Proton=uud neutron=udd

So the proton is not fundamental, but is a composite particle, like a nucleus.

The Importance of Energy

New discoveries often follow the opening of a new energy regime:

Discovery of the electron 1 eVDiscovery of the nucleus 5 MeVDiscovery of pion and muon 100 MeVDiscovery of the kaon 500 MeVDiscovery of the proton substructure 20 GeVDiscovery of the top quark 2 TeVDiscovery of ?? at the LHC 14 TeV

The importance of energy

To achieve ever higher and higher energies, larger and larger machines have been built, with Fermilab and CERN currently being the largest.

Fundamental question #1

Is there another layer of substructure?

We don’t know, but there is no sign of further substructure yet.

The spectrum of masses

Let’s look more closely at the masses of the quarks and leptons:

Units are 1 GeV, an energy unit equal to the mass of the proton

u,d 0.3 GeV e 0.0005 GeV s 0.5 GeV 0.1 GeV c 1.5 GeV 1.8 GeV b 4.5 GeV ? t ?

Let’s look more closely at the masses of the quarks and leptons:

Units are 1 GeV, an energy unit equal to the mass of the proton

u,d 0.3 GeV e 0.0005 GeV s 0.5 GeV 0.1 GeV c 1.5 GeV 1.8 GeV b 4.5 GeV .000000000003 GeV t 175 GeV

The spectrum of masses

Fundamental question #2

How can something with no physical size have mass and charge?

How do these fundamental particles acquire mass, and what determines the values of the masses?

Why is the top quark so massive?

The origin of mass

What we think we understand about the origin of mass is tied up in our understanding of the force carriers.

Force carriers: photons(massless)gluons (massless)W and Zs (80-90 times the proton mass )

The fundamental interactions

We view the interactions as occurring through the exchange of one of the force carriers.

Feynman diagrams showing weak and electromagnetic interactions

The origin of mass

In the theories we can explain the mass of the W and Z, but at the expense of predicting yet another particle which is called the Higgs particle, after Peter Higgs.

Peter Higgs

The origin of mass

The Higgs generates a field, a bit like an electric field, which permeates all space gives mass to all the particles.

The Higgs field is like a roomful of scientists, milling around randomly

A famous scientist walks in

A crowd forms around him, and it is difficult for him to move! He has acquired a large mass!

The Higgs mechanism

But what is the Higgs particle?

If someone shouts a rumor into the room...

The scientists form a cluster as the rumor passes through the room. This cluster is like the Higgs particle!

But even if we find the Higgs particle, we won’t know why particles have the specific masses that they do.

Antimatter

I wasn’t complete when I listed all the particles…I should have

mentioned that all particles have their antiparticles !

When matter and antimatter meet, they annihilate each other in a burst

of energy !

Fire up the antimatter drive, Mr. Scott.

Dirac predicted antimatter in 1927

Antimatter

We create antimatter all the time at places like Fermilab, and we can store it for days. But we

always make it as matter-antimatter pairs.

Antiproton accumulator at Fermilab

Fundamental question #3

If we always produce matter and antimatter together, in equal amounts…why is the universe today dominated by matter? This is a 40-year old puzzle that is yet to be solved!

Electron-positron production from a photon

e-

e+

photon

The cosmic connection

There is a very close connection between particle physics and astrophysics.

I’m going to show two examples:

Type II supernovas

Dark matter

SN1994

Particle decay

Heavier particles decay into lighter ones, which is why the world is made of up and down quarks and not top quarks.

These decays occur because systems go to their lowest energy state.

As an example, free neutrons decay into protons, electrons and electron anti-neutrinos:

np e e

This example is called beta decay.

Question: why don’t neutrons within an atomic nucleus decay?

_

Particle decay

In such decays, several quantities are conserved:

Energy, linear momentum, angular momentum

Electric charge

The total number of leptons (anti-leptons count as -1)

The total number of baryons (anti-baryons count as -1)

np e e

We have one baryon to start, one baryon to end.We have zero leptons to start and zero to end (the antineutrino counts as -1, the electron as +1)

The “bar” means “antiparticle”

_

Particle Decay

All decays can go in reverse, too (inverse beta decay)

epne or enpe

General rule: When you move a particle from one side of the decay equation to the other, it changes into it’s antiparticle

Verify that lepton number and baryon number are conserved in this decay.

Beta decay and supernovas

Why is this important in astrophysics?

Beta decay is crucial in the generation in stars of all the chemical elements.

And inverse beta decay plays a central role in Type II supernovas

Supernova 1064 remnant

A Type II supernova occurs due to gravitational collapse

Type II supernovas

In type II supernovas (stars much more massive than our sun), when all the nuclear fuel is used up, the gravitational pressure is so great that the atoms collapse!

The electrons and protons undergo inverse beta decay:

epne

Every proton in the star becomes a neutron, forming a neutron star. The neutrinos escape(1057 of them!) and carry away a large part of the supernova energy.

Galaxy M31 with a SN on the outter edge

Type II Supernovas

The neutron star resulting from a Type II supernova is about 10 km in radius and rotates with periods of seconds or less.

In 1987 neutrinos from a nearby supernova were detected in two detectors on earth!

Here’s a problem for students: If a star the size of our sun with a rotation period of 10 hours collapses to a radius of 10km, how fast would it be rotating to conserve angular momentum?

Dark Matter

As early as 1933 Zwicky showed that gravitational effects in galaxy clusters could not be explained by the visible matter.

In the 1980’s, rotational curves of galaxies gave more evidence for the existence of unseen or dark matter.

Rotational velocity vs distance from the galactic center

Gravitational Lensing

Einstein predicted in 1936 that light from distant objects could be bent by massive objects between us and the distant object.

Multiple, distorted images are evidence for gravitational lensing.

Dark matter

Quantitative calculations of gravitational lensing give strong evidence that galaxies have much more matter than we see.

Galactic Haloes

The rotational curves of galaxies and the observed gravitational lensing can be explained if galaxies are surrounded by a huge halo of dark matter.

The dark matter makes up 80% of the matter in galaxies!

What is the dark matter?

We don’t know, but the answer almost certainly lies in particle physics.

In the 1980’s particle theorists developed what is still a leading candidate for a theory beyond the Standard Model. Supersymmetry predicts that every particle we know has a supersymmetric partner which is much more massive and interacts only weakly with regular matter.

Supersymmetry solves deep problems in the mathematics of the Standard Model. It was developed for this reason, and not because it solves the dark matter problem.

Is Supersymmetry real?

“Supersymmetry is an offer nature can’t refuse” Dmitri Nanopolis, theorist

“There ain’t no supersymmetry”, Leo Bellantoni, experimental physicist

“Experiment is the sole judge of scientific truth”, Richard Feynman

We are pushing hard to find supersymmetry. But as yet there is no direct experimental evidence for it.

Dark Energy

It gets weirder…around 2000 it was found that the expansion of the universe is speeding up.

The stuff in the universe is dominated by dark energy, which acts like anti-gravity.

Dark energy is pushing the universe apart faster and faster.

It’s nature is unknown.

The cosmic connectionThe science of the very big and the science of the very small are in a close, synergistic relationship.

Aerial view of Fermilab

Deployment of the Hubble Space Telescope from the shuttle

Summary-what we don’t know (yet)

1. Have we reached the bottom yet? (is there another layer of substructure?)2. What is the origin of mass, and why do particles

have the masses they do?3. What is the origin of the matter-antimatter

asymmetry of the universe? 4. What is the dark matter making up galactic

halos? ( Not neutrinos)

Where do we do these experiments?

Aerial view of Fermilab, near Chicago, and Wilson Hall

One of the collider detectors

And a typical event

More views of Fermilab

Visit www.fnal.gov and pdg.lbl.gov to learn more about particle physics and the labs