Transcript presentation source

Tony Liss
December 6, 1997
What is the World Made of?
Air
Fire
Water
Earth
Since ancient times humankind has looked for
fundamental constituents
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.
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.
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.
What Are the Forces That
Hold It Together?
Force
Relative Strength
Gravitational
1
Weak
1029
Nuclear beta decay: np e- n
Electromagnetic
1040
Binds electrons and protons
Strong
1043
Holds the nucleus together
Keeps you in your seat
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.
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.
l<d
l
l>d
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
l=h/p
Rutherford’s 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
And Relativity!
There’s another reason for high energies too:
E=Mc2
If we want to create a particle that isn’t found in normal
matter in the laboratory, we need an energy at least equal
to its mass times the speed of light squared.
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 earth’s 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
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!
Where’s the Order?
With high energy accelerators and a new particle detector called
the bubble chamber, particle physicists went to town in the 1950s
e
p
K
nm
0
Sn
S+
K0
K-
p
L
Quarks

In 1961 Gell-Mann & Ne’eman did for “fundamental” particles what
Mendeleev had done 100 years earlier for “fundamental” atoms.
n
p
S0
L
-
S

-
D-
S=0
S+

0
S*-
S=-1
Q=+1
Q=0
S=+1
S=0
S=-1
K
p
0
K
p0
h
-
K-
S=-3
+
p+
K0
S*0
D++
S*+
*- *0
S=-2
Q=-1
D+
D0
?
Q=+2
Q=+1
Q=0
Q=-1
A missing piece!
The W- : S=-3, Q=-1
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”
Which, oddly enough, had electric charges (in units of e) of 2/3, -1/3. -1/3
p
n
p+
p0
p-
uud
udd
ud
uu
du
D++
D+
D0
DW-
uuu
uud
udd
ddd
sss
K+
K0
KK0
us
ds
su
sd
Where are the Quarks?


This is a nice picture, but no one had (or has) ever seen a free quark.
Repeat Rutherford’s experiment at MUCH higher energies!
electrons
Protons
 

 

 

 

 
 
 







Evidence that the proton has something hard inside!
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.
No Free Quarks (ever)


The color force between quarks decreases with decreasing
distance (asymptotic freedom) and grows large at large
distances (infrared slavery)
Here’s what happens if you try to pull a (colorless) baryon
apart:
Energy in
the field increases
until...
Enough E=Mc2
for a quark-antiquark
pair to be produced
The Standard Model
Electric
Magnetic
Weak
Strong
Electromagnetic
Quarks
u
d
W
c
s
Leptons
ne
e-
W
nm
m
W+
W-
Gauge
Bosons
g
Electroweak
W
t
b
W
W
nt
t
W
Z0
g
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
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, it’s a muon
Evidence for QCD
QCD tells me I can’t see a free quark. So, what happens if I whack two
protons together so hard that one of the constituent quarks goes flying away?
A Real Two “Jet” Event
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
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 by
their decay modes, also predicted by electroweak theory
ene
mnm
W ud
cs
tb(?)
e+e-
m +m Z t+tqq
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
Heisenberg’s uncertainty principle can be written
DEDt/2
Coupled with E=Mc2, this says that the mass of the Z when it decays
is determined only up to a constant proportional to its lifetime
DMz  1
Dt
Number of Z decays
The Width of the Z
Z width (DE)
Z mass (=E/c2)
So….Six quarks
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
q, e, m, t
But there is no direct prediction of
its mass.
q , n e , n m , nt
Making Top Quarks in Batavia, IL

The Fermilab Tevatron is the world’s highest energy
accelerator. Protons and antiprotons collide at an energy of
1.8 TeV (1800000000000 electron volts).
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
MTOP c 2=
And only 1 out of 1010 collisions produced a top-antitop pair.
We’re Done! (NOT!)
OK, 3 generations, all six quarks, Ws, Zs, gluons, photons.
What’s missing?





The t neutrino (we know it’s there, it’s 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? We’re doing the Rutherford experiment
yet again, this time with quark-antiquark collisions! Do
quarks have constituent parts? Maybe!
Are there fundamental particles?
Or is nature just an onion?