Transcript A Brief Tour of High Energy Physics

More on the Elementary Particles
and Forces in the Universe
Dr. Mike Strauss
The University of Oklahoma
Two Questions Asked for Centuries
1) What are the fundamental objects from which
everything else in the universe is made?
2) What are the forces or interactions that hold
these objects together and how do these
forces work?
What are the fundamental objects in the universe from
which everything else is made?
• This question has been pondered for over 2500 years
– Ancient Greece (followers of Thales)
–Ancient Greece (Democritus)
•Indivisible particles called  - atomos
How are the fundamental objects held together?
or in more precise scientific language
What are the fundamental forces of nature?
• At the turn of the century, (that is in 1900) two fundamental
forces were known:
– Gravity
– Electromagnetism
The Fundamental Particles in the Universe
(Current Model)
Particles
• Leptons
– Latin for “Light”
– Usually found alone
• Quarks
– A nonsense word in Finnegan’s Wake by James Joyce
– Always found in groups
The Atom
These electrons
are fundamental
particles (leptons).
Other fundamental
particles (quarks)
are buried deep
inside the nucleus.
The Fundamental Forces in the Universe
(Current Model)
Forces
• Gravity
• Electromagnetic Force
• Weak Nuclear Force
• Strong Nuclear Force
Electroweak
– Only quarks and particles made from quarks
(hadrons) interact via this force
The Standard Model:
A Theory of Everything (except gravity)
The Fundamental Particles: (Fermions)
six quarks
(and antiquarks)
u
d
c
s
t
b
six leptons
(and antileptons)
ee

Charge = +2/3e
Charge = -1/3e
The Fundamental Forces: (Bosons)
Strong force:
8 gluons
Weak force:
W+, W-, Z0
Electromagnetic force: 
And: Higgs Boson:
H
Not yet discovered
(plus a lot of Nobel Prize winning math)
Quarks are very bizarre objects
• They have no size, but they do have mass.
– (All “elementary” particles have no apparent size)
• They have charges that are fractions of the proton
and electron charge.
• They cannot be isolated
– No quark has ever been discovered by itself.
– They are always found in groups of three quarks or
antiquarks (baryons) or one quark and one antiquark
(mesons).
Terminology Review
Antiparticle: Every particle, including quarks, has an
antiparticle. The charge and “quantum numbers”
of the antiparticle are opposite that of the particle,
and the mass is the same.
Hadron: Any particle made of quarks and/or antiquarks.
Baryon: Any particle made of three quarks.
(Antibaryons are made up of three antiquarks.)
Meson: Any particle made of a quark and an antiquark.
Selected Hadrons
(Hundreds of hadrons have been discovered)
Baryons
p: uud
n: udd
 uds
: sss
c:udc
p: uud
(electric charge)
2/3+2/3-1/3=+1
2/3-1/3-1/3=0
2/3-1/3-1/3=0
-1/3-1/3-1/3=-1
2/3-1/3+2/3=+1
-2/3-2/3+1/3=-1
Mesons
: ud
: uu
-: ud
K: us
D: cu
(electric charge)
2/3-(-1/3)=+1
2/3-2/3=0
-2/3-1/3=-1
2/3-(-1/3)=+1
2/3-2/3=0
• Properties of hadrons can be explained from the
properties of their constituents.
• Most of the visible matter in the universe is made of up
and down quarks and electrons.
• Most of the known objects in the universe are made of
matter and not antimatter.
The Forces of Nature
• Gravity: All objects in the universe are attracted to
each other by this force.
• Electromagnetic*: Holds atoms and molecules
together. Most of the phenomena we experience
everyday is a result of this force.
• Weak Nuclear Force*: Responsible for radioactive
decay.
• Strong Nuclear Force: Holds quarks together in
hadrons and holds the nucleus together.
*A theory combining these two into an “electroweak”
force was developed in the 1960’s and verified in
1983.
The Forces of Nature (continued)
Force
Carrier(s)
Particles
Affected
Gravity
Graviton*
All
EM
Photon
Weak
W+, W-, Z0
Strong
Gluons (8)
Charged
All
Quarks/Gluons
Hadrons
Relative
Strength
Range
10-38

10-2

10-1
<10-18 m
1
10-15 m
*Not yet discovered. Not part of the “Standard Model”
How Do We Know the Fundamental Structure of
Anything?
(How Do You Know How Your Car Works?)
• Be taught by someone who already knows
• Take it apart (or look inside)
• Put it together
Looking Inside Very Small Objects
Earnest Rutherford’s 1911 Experiment
“Pudding”
“Plum Pudding”
“The Results”
Rutherford proposed the “Nucleus” to explain the results.
Early Evidence for Quarks (late 1960’s)
(Looking Inside the Proton)
Incoming
electron (e-)
Proton (p)
Deep Inelastic Scattering
The Wave Nature of Matter
The de Broglie Wavelength
 = h/p
h = 6.63  10-34 Js
p = mv (momentum)
In order to “see” an object, the wavelength of the probe must
be smaller than the object being observed.
But How Do You Put Protons (or other particles)
Together?
E = m0c2
E2 = m02c4
E2 = m02c4 + c2p2
Answer: Mass is a form of energy. If I can concentrate
enough energy at any point (even energy of motion—kinetic
energy), I can create any particle(s) with mass.
Particle accelerators can create matter
(from other forms of energy)
Step 1: Accelerate two particles towards each other. They
have a lot of energy from their motion, kinetic energy.
e-
e+
Step 2: Let them collide and annihilate each other to create
energy or other particles.
Step 3: That energy can create any particle and its antiparticle
with mass less than or equal to the total energy (E=mc2).
“Feynman” Diagram of e+e-Annihilation
any
fundamental
particle
e.g. -
Space
e+
Photon or Z0
e-
Time
the
corresponding
antiparticle
e.g. +
Creating Hadrons
1. Quarks created from initial annihilation
2. Strong nuclear force acts like a rubber band
3. Eventually the “rubber band” breaks creating new quarks
Production of Hadrons
q meson
q
e+
Space
q
meson
q
Photon or Z0
e-
q
q meson
q
meson
q
Time
So Let’s Review
• What are the two classes of fundamental particles?
• Which class of fundamental particles are always bound
together to make other subatomic particles?
• What are the four fundamental forces?
• Which force is so weak that it plays little role in the
interactions of fundamental particles?
• Which principle of physics allows scientist to probe the
structure of matter with high energy particles?
• Which principle of physics allows fundamental particles
to be created in the laboratory?
Let’s Look at a Few Topics in More Detail
• Forces as Particles
• Quarks and Protons
• Benefits
What about the forces?
Why are they described by particles?
The interaction between two particles can be thought
of as the two particles exchanging another particle. In
this case, the two people throw a basketball back and
forth to change their momentum. The basketball is the
“carrier” of the force or interaction.
Now consider an electron (with a negative charge)
and a positron (with a positive charge) approaching
each other at a rapid rate.
e+
e-
This can be thought of as the two particles
exchanging a “photon” which, in turn, changes their
direction as indicted in this Feynman Diagram
Space
e+
e+
Photon
e-
eTime
Different quarks have different masses
The equation E=mc2 is used to define the mass of an
object. In these units, a proton has a mass of about 1
billion electron volts (1 GeV/c2).
(The following masses are in GeV/c2)
Up quark (u): 0.0004
Charm quark (c): 1.5
Top quark (t): 175
Down quark (d): 0.0007
Strange quark (s): 0.15
Bottom quark (b): 4.7
The mass of just one top quark is more than the entire
mass of a gold nucleus which has 79 protons and 118
neutrons, or more than 591 up and down quarks!
Quarks have fractional charge
In a very basic model:
A neutron is made of 3 quarks: up, down, down (udd)
Charge: +(2/3) - (1/3) - (1/3) = 0
A proton is also made of 3 quarks: up, up, down (uud)
Charge: +(2/3) + (2/3) - (1/3) = 1
All the properties of the neutron and proton can be
derived from the properties of its constituent
particles.
Why are quarks always bound together?
• The force that holds quarks together is called the strong
nuclear force.
• There are 3 types of strong nuclear charge which can
attract quarks to each other and cause them to bind
together.
Strong charge
• Objects with strong charge interact via the strong
force
• Three types of strong charge
Larry, Curly, Moe
anti-larry, anti-curly, antimoe
knife, fork, spoon
Three strong charges
color
Quantum Chromodynamics (QCD)
Every color is attracted
to its anticolor
Hadrons in nature are colorless
Baryons:
Mesons
• 3 quarks
• 1 quark and 1 anti-quark
– 1 green, one red, one blue
– Constantly changing color
• Antibaryons have 3 anti-quarks
– With 3 different anti-colors
constantly changing
• Some Baryons
–
–
–
–
–
Proton
Neutron
Lambda
Sigma
Anti-proton
– Color and anticolor constantly
changing
• Some Mesons
– Pion
– Kaon
– Eta
Quark and Gluon Color
• At any “moment” in a baryon, the three quarks are
three different colors.
• At any moment in a meson, the quark is a particular
color and the antiquark is the corresponding anticolor.
• Gluons can also carry color so they can interact with
each other.
– When gluons are exchanged between quarks, they can
change the color of the quarks. The type of quark, or
flavor, cannot be changed by a gluon.
Space
A model of the Structure of a Proton
valence
quarks
u
u
u
u
gluons
d
d
Time
Virtual Particles Exist!
It’s as if a tennis ball changed into a bowling ball and an
“anti”-bowling ball for a brief moment, before turning
back into a tennis ball.
E1
E1=E3
DE = E2 - E1
DEDt  h/2
E2
E3
A more complete model of the Structure of a Proton
Space
valence
quarks
u
virtual
“sea” quarks
q
q
u
u
u
gluons
d
d
Time
Neutron Decay and the Weak Force Described Using
Particles
e
d
u
d
d
u
u
Time
Proton
W-
Neutron
Space
e-
Question: The neutron has a mass of about 1 GeV/c2 and
the W has a mass of about 84 GeV/c2. How is energy
conserved in neutron decay?
Answer: During the very brief period of time that the
W exists, energy is not conserved? ...How can this be?
Heisenberg’s Uncertainty Principle:
DEDt ≥ h/2
mc2(d/c) ≥ h/2
mc2 ≥ hc/2d
d ≥ h/2mc
So if d < h/2mc a “virtual” particle can be produced.
(h = 6.63  10-34 Js)
Benefits of High Energy Physics
• Answers questions about the structure and origin of the
universe that have been pondered for millennia.
• Leads to future technology. Technological advances can
only be made when the underlying physical principles are
understood.
– e.g. Electricity, Semi-conductors, Superconductors
• “Spin-off” applications result from technologies developed
to accelerate, collide and detect particles.
– CT scans, Proton Therapy, World Wide Web
• Builds a foundation for other areas of science.
• Develops an educated work force.
• Economic benefits (30% return on investment).