The Particle Zoo

1
The Particle Zoo
And Who's Behind the Bars
Tony Liss December 6, 1997
2
What is the World Made of?
Air
Fire
Water
Earth
Since ancient times humankind has looked for
fundamental constituents
3
Atoms
The first modern fundamental particle was the
atom, from the Greek work atmos, meaning
indivisible.
The organization of the known types of atoms into
the Periodic Table was a hint that there was a
deeper underlying structure.
4
Electrons Are the Reason

• Mendeleev laid out this beautiful picture that
  classified the elements and predicted their
  properties. But why?
• In 1897 Thompson discovered the electron. The
  negatively charged electron came from the atom
  proving
• The atom is not elementary and,
• There must be a positive charge in there too.

5
Rutherford The First High Energy Physicist
a particles
Gold foil

• The atomic nucleus is discovered!

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

6
What Are the Forces That Hold It Together?
Keeps you in your seat
Nuclear beta decay n?p e- n
Binds electrons and protons
Holds the nucleus together
With each force goes a quantum field theory to
describe it (but we dont 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.
7
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 objects into our eyes. If the object is very
small compared to the wavelength of the light,
then the light diffracts around the object and
the image is lost. To see very tiny objects we
need very short wavelengths.
ll
ld
8
Enter 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
lh/p
Rutherfords a 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
9
And Relativity!
Theres another reason for high energies too
EMc2
If we want to create a particle that isnt found
in normal matter in the laboratory, we need an
energy at least equal to its mass times the speed
of light squared.
10
A Natural Source of High Energy Particles

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

Cosmic rays are high energy charged particles,
mostly protons, that impinge on the earths
atmosphere from outer space. Wanna bang a
projectile on a target? Nature does it for you
all the time!
protons from outer space
collision w/ air molecule
Mostly Muons
Terra Firma
11
The Particle Explosion Begins

• The study of cosmic ray interactions brought the
  discovery of a host of new particles
• 1931 - The positron (e)
• 1936 - The muon (m)
• 1947 - Pions, kaons, hyperons

• Meanwhile, Ernest Lawrence was learning how to
  build
• powerful accelerators

Intensities MUCH higher than cosmic rays!
12
Wheres the Order?
With high energy accelerators and a new particle
detector called the bubble chamber, particle
physicists went to town in the 1950s
p
e?
?-
nm
S0
S-
ne
K
n
S
r
K0
K-
p?
p0
L
m?
13
Quarks

• In 1961 Gell-Mann Neeman did for fundamental
  particles what Mendeleev had done 100 years
  earlier for fundamental atoms.

14
Order?Constituents

• 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 quarks

Down
UP
Strange
Which, oddly enough, had electric charges (in
units of e) of 2/3, -1/3. -1/3
D uuu D uud D0 udd D- ddd W- sss
15
Where are the Quarks?

• This is a nice picture, but no one had (or has)
  ever seen a free quark.
• Repeat Rutherfords experiment at MUCH higher
  energies!

electrons
Protons
Evidence that the proton has something hard
inside!
16
Quantum Chromodynamics

• In the 1970s a quantum theory of strong
  interactions was developed called quantum
  chromodynamics (QCD). It includes a new gauge
  boson called the gluon.
• The charge of the strong force is called color,
  and each quark comes in one 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 and and anti-color.
17
No Free Quarks (ever)

• The color force between quarks decreases with
  decreasing distance (asymptotic freedom) and
  grows large at large distances (infrared slavery)
• Heres what happens if you try to pull a
  (colorless) baryon apart

Energy in the field increases until...
Enough EMc2 for a quark-antiquark pair to be
produced
18
The Standard Model
Electric Magnetic Weak Strong
Electromagnetic
Electroweak
?
u d
c s
t b
Quarks
ne e-
nm m
nt t
Leptons
Gauge Bosons
g
W
W-
Z0
g
Simplicity regained!
19
On To Higher Energies!
To test these new theories, physicists needed
higher energies? COLLIDING BEAMS.
Fixed Target
Amount of energy for new particle production
?Ebeam
Colliding Beams
Amount of energy for new particle production
Ebeam
20
Detecting the Debris
A typical colliding beam detector
Tracking chamber - Inside magnetic field,
measures charged particle momenta
Electromagnetic calorimeter - Measures energy of
electrons and photons
Hadronic calorimeter - Measures energy of hadrons
Muon tracking - If it has an electric charge and
it makes it out here, its a muon
21
Evidence for QCD
QCD tells me I cant see a free quark. So, what
happens if I whack two protons together so hard
that one of the constituent quarks goes flying
away?
22
A Real Two Jet Event
23
Tests of Electroweak Theory

• With the advent of electroweak theory, three new
  particles were needed the gauge bosons W, W-
  and Z0. The masses were predicted by the theory
  to be
• MWc2 ? 80 GeV
• MZc2 ? 90 GeV

Thats about the mass of bromine (z35) and
zirconium (z40). Not badd for a pair of
elementary particles!
The W and Z are extremely short-lived, but can be
identified by their decay modes, also predicted
by electroweak theory
24
How Many Generations?

• The Z decays to the leptons and antileptons of
  each generation, as well as the quarks and
  antiquarks.
• The more ways there are for the Z to decay, the
  easier it is for it to do so and the shorter its
  lifetime
• Heisenbergs uncertainty principle can be written

DEDt??/2
Coupled with EMc2, this says that the mass of
the Z when it decays is determined only up to a
constant proportional to its lifetime
1
D
µ
M
z
D
t
25
The Width of the Z
of generations 3!
Number of Z decays
Z width (DE)
Z mass (E/c2)
26
So.Six quarks
And the sixth one is
The Top Quark
27
The Hunting of the Top Quark

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

t
q, e, m, t
But there is no direct prediction of its mass.
28
Making Top Quarks in Batavia, IL

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

29
Success !!

• In 1994, after 17 years of trying at various
  accelerators, and nearly 10 years at the
  Tevatron, we finally found a grand total of 12
  collisions that looked like they produced a
  top-antitop pair. Now there are more than 100.
• Why was it so hard?
• Because

175 GeV!!
MTOP c 2
And only 1 out of 1010 collisions produced a
top-antitop pair.
Now thats a needle in a haystack!
30
Were Done! (NOT!)
OK, 3 generations, all six quarks, Ws, Zs,
gluons, photons. Whats missing?

• The t neutrino (we know its there, its just
  really hard to see directly)
• The Higgs Boson (Not another one?!?)
• Why are there three generations?
• How can we explain the weird pattern of masses?
• Are the fundamental particles really so? Might
  they too have constituents? Were doing the
  Rutherford experiment yet again, this time with
  quark-antiquark collisions! Do quarks have
  constituent parts? Maybe!

31
Are there fundamental particles?
Or is nature just an onion?