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The nature of science, almost 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 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 eV

Discovery of the nucleus 5 MeV

Discovery of pion and muon 100 MeV

Discovery of the kaon 500 MeV

Discovery of the proton substructure 20 GeV

Discovery of the top quark 2 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.

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

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?

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 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?

Electron-positron production from a photon

e-

e+

photon

Origin of the matter-antimatterasymmetry

We know that the violation of some simple symmetries is a

necessary condition. And we do see that these symmetries are

violated by nature, but at a rate 1,000,000 times too small!

This is a 40 year old puzzle which we are still working to solve!

The dark matter problem

For about 20 years astronomers have known from the way galaxies rotate, that we don’t see most of the matter in them.

Whatever the dark matter is, we know it is not ordinary matter.

Fundamental question #4

What is the dark matter controlling the rotation of galaxies?

The most popular theory is that there is an entirely new from of matter that

we haven’t discovered yet…much heavier than the particles we know

so far, which is why we haven’t seen it yet.

Well, yes, this does make a lot of new particles, but it solves problems in both particle physics and astrophysics

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

Fermilab is a wildlife refuge as well as a national lab