What the Universe is Made Of
Physics 103
Notes on Particle Physics for Sept 2 and Sept 5 classes
Stuff
• electrons, protons, neutrons
• quarks, muons, neutrinos,....
David Nice
Glue that holds stuff together
• gravity (gravitons??)
• electromagnetic force (photons)
• weak force (Z±, W0)
• strong force (gluons)
Disclaimers!!!!
• I will not usually distribute lecture notes. Don’t get used to it!!
• These notes supplement the text and class discussions. They do
not contain all of this week’s material.
What is Stuff Made Of?
Cosmic Ray Shower
Electrons
Primary cosmic ray
Electrons are cannot be broken
into small particles. They
belong to a class of particles
called leptons.
Protons & Neutrons
Protons & Neutrons are each
made up of three quarks. Things
made up of quarks are called
hadrons.
Secondary rays produced
in the atmosphere
ht
l heig
typica m
0
10 00
muons and other particles
1
Fermi National Accelerator Laboratory, Batavia, Illinois
Some Particles....
Accelerate particles so they have
very high energy...
e
...then smash them together;
massive, short-lived particles may
form, but often they rapidly decay
into other particles...
...so that many particles emerge.
By measuring the trajectories,
energies, and charge of these
particles, it is possible to
reconstruct what happened in the
collision, and study the properties
of all these particles.
CERN: European Center for Nuclear Research
p
γ, g, W+, W−, Z0, u, d, c, s, t, b, e−, e+, µ−, µ+, τ−, τ+, υe, υµ,
υτ, π−, π0, π+, η, σ, ρ(770) ,ω(782), η'(958), f0(980),
a0(980), φ(1020), h1(1170), b1(1235), a1(1260), f2(1270),
f1(1285), η(1295), π(1300), a2(1320), f0(1370), f1(1420), ω
(1420), η(1440), a0(1450), ρ(1450), f0(1500), f2'(1525), ω
(1650), ω3(1670), π2(1670), φ(1680), ρ3(1690) , ρ(1700),
f0(1710) , π(1800), φ3(1850), f2(2010), a4(2040), f4(2050),
f2(2300), f2(2340), K+, K0, K−, K0L, K0S, K*(892),
K1(1270), K1(1400), K*(1410), K0*(1430), K2*(1430),
K*(1680), K2(1770), K3*(1780), K2(1820), K4*(2045),
D+, D0, D−, D*(2007)0, D*(2010)+, D*(2010)−, D1(2420)0,
D2*(2460)−, D2*(2460)0, D2*(2460)+, DS*+, DS*−,
DS1(2536)+, DS1(2536), D8J(2573)+, D8J(2573)−, B+, B0, B−
, B*, BS0, BC+, BC−, ηC(1S), J/ψ(1S), χC0(1P), χC1(1P), ....
2
Quarks
Quarks were named by Murray Gell-Mann, who
first theorized their existence. There are three
quarks in every proton and every neutron, and the
name “quark” is attributed to Finnegan’s Wake
(James Joyce, 1939):
“Three quarks for Muster Mark!”
The quarks that make up protons and neutrons are
called up quarks (u) and down quarks (d):
proton:
neutron:
Murray Gell-Mann
p= u+u+d
n= u+d+d
Individual quarks are never seen—they are always
combined with other quarks into particles called
hadrons.
Charge of Quarks
Charge is a fundamental property of all particles. In particle
physics terminology, the charges on some familiar particles are:
electron
proton
neutron
photon
0
0
The charges of the up and down quarks are:
up quark +2/3
down quark −2/3
These values are consistent with p = u+u+d and n = u+d+d.
Although quarks have fractional charge, all particles that are
directly seen have integer charge (0, −1, +1, etc.)
Anti-Matter!
For almost every particle, there is a corresponding antiparticle with....
• The same mass
• The opposite charge
• The same spin
A few anti-particles have their own names:
• The anti-particle of the electron is called the positron
But most don’t have special names:
• The anti-particle of the proton is called the anti-proton
They are indicated by a “bar” over the particle symbol:
• An anti-proton is written p
• An anti-up-quark is written u
−1
+1
Anti-Matter, continued
In most cases, bringing matter and anti-matter together
will cause them to annihilate rapidly. Example: electron
and anti-electron (positron) become two photons:
e
e
Why is the universe made mostly of matter and not antimatter? We have no idea.
Probably the universe started with lots of matter and lots of
anti-matter, but a tiny bit more matter. Then all the antimatter annihilated, taking most of the matter with it, leaving
the matter that remains with us today.
3
Anti-Quarks; Baryons, Mesons
Quarks: u, d
Anti-quarks: u, d
Combine quarks and anti-quarks to make observed particles:
• Three quarks:
uud
udd
ddd
uuu
p
n
Δ−
Δ++
• Three anti-quarks:
uud
p
M
M
(proton)
(neutron)
mass 120 ± 50 MeV
charge –1/3
MESONS
p
Σ+
Ω−
M
mass 4200 ± 200 MeV
charge –1/3
mass 6 ± 3 MeV
charge +2/3
charm (c)
mass 1250 ± 100 MeV
charge +2/3
top (t)
mass 175,000 ± 5,000 MeV
charge +2/3
About Quark Names
• Up quark, Down quark: named after their “spin”:
(Is the “north pole” of the quark pointing up or down as
it “spins”?)
(proton)
BARYONS
• Three anti-quarks:
uus
Σ+
M
M
• Quark & anti-quark:
db
B0
M
M
bottom (b)
up (u)
Anti-quarks: d, u, s, c, b, t
We still have the same three ways of making particles:
• Three quarks:
uud
uus
sss
M
mass 3 ± 2 MeV
charge –1/3
strange (s)
(anti-proton)
Baryons & Mesons, revisited
Quarks: d, u, s, c, b, t
After developing the quark theory in the mid-1960’s, physicists
continued to crank up the energy available at accelerators. More
quarks were discovered. The most recently discovered, the top quark,
was first detected in 1995, though its existence was predicted in the
1970s. We know about six quarks, which we pair into three families.
down (d)
BARYONS
• Quark & anti-quark:
ud
π−
M
M
But wait, there’s more!
• Strange quark: named because particles made up of it
had strange properties (e.g., fast decay times), which
were not expected from particles made of up and down
quarks.
• Charm quark: just a silly name , no excuse for it really.
MESONS
• Top quark, Bottom quark: started out being called
“truth” and “beauty,” but those names were just too silly.
4
Quark Summary
What is Stuff Made Of?, Revisisted
• there are six quarks: u, d, c, s, t, b
• three families: u & d, c & s, t & b
• within each family, one has charge +2/3, one charge −1/3
Not made of quarks
Electrons...
...a type of lepton
• each has an anti-quark with the same mass but opposite charge
• for example, u has charge +2/3, u has charge −2/3
• combinations of quarks are hadrons; there are two types:
• three quarks or three anti-quarks make a baryon
• a quark and an anti-quark make a meson
Stuff made of quarks
• mass of a baryon is much greater than the masses of its quarks
Protons, neutrons, ....
• quarks have “color”—we’ll get to that later.
Is that it? Is every sort of stuff made of quarks? Not quite.
There is a second type of stuff— particles called leptons.
Leptons
Leptons are:
• Electrons and heavier things sort-of-like electrons (muons, tau); charge −1
• Three types of neutrino, corresponding to electron, muon, and tau; charge 0
• Anti-electrons, anti-muons, anti-tau
charge +1
• Anti-neutrinos
charge 0
As far as we can tell, leptons are not composed of other things.
electron
(e−)
mass 0.5 MeV
charge –1
muon (µ−)
mass 106 MeV
charge –1
tau (τ−)
mass 1777 MeV
charge –1
electron neutrino (νe)
mass < 0.000003 MeV?
charge 0
muon neutrino(νµ)
mass < 0.19 MeV
charge 0
tau neutrino (ντ)
mass < 18.2 MeV
charge 0
How Stuff Interacts with other Stuff
We can see stuff interact in several ways:
• Particles feel forces from each other (e.g., two electrons repel
each other because of their charge)
• Particles collide with each other and turn into other particles
• Particles decay into other particles
Each of these interactions can be ascribed to one of four
fundamental forces:
Gravity, Electromagnetic, Weak, Strong
In each case*, the interaction happens by exchanging particles
between the things that are interacting. These exchange particles
are yet another type of particle (besides quarks & leptons).
*Actually,
this might not be true for gravity. We have no idea how gravity really works.
5
Feynman Diagram
Another Feynman Diagram: “Beta Decay”
e−
e−
p+
n
γ
W−
e−
e−
e−
νe
e− + e− ⇒ e− + e−
This Feynman diagram shows two electrons coming in (from the
left); they experience an electromagnetic force by exchanging a
photon (γ); then they continue on. The electrostatic repulsion of
these two particles is a direct consequence of this happening over
and over and over again.
Richard Feynman
n ⇒ p + e− + νe
• A neutron (n) decays into a proton (p+), an electron (e−), and
an anti-electron-neutrino (νe).
• The decay is “mediated by” exchange of a W− particle.
• The anti-electron-neutrino is an outgoing product of the decay,
but it is drawn as inward-moving because it is an anti-particle.
• Charge is conserved at each “vertex” of the diagram.
Things to Learn from Feynman Diagrams
The probability of an interaction happening can be calculated
from the “coupling strength” at each vertex (e.g., coupling
strength of n, p+, W− vertex in beta decay diagram). A challenge
for present-day theory is the calculation of these coupling
strengths.
More straightforwardly, we can look for patterns in the
interactions:
Stuff
quarks
Increasing
mass
leptons
charge −1/3
charge +2/3
charge −1
charge 0
down (d)
up (u)
electron (e−)
electron
neutrino (νe )
strange (s)
charm (c)
muon (µ−)
muon
neutrino(νµ )
bottom (b)
top (t)
tau (τ−)
tau
neutrino (ντ )
• Which force carriers interact with which particles?
• Find conserved quantities (e.g., charge is always conserved,
but several new quantities are as well)
• Can force carriers themselves interact with each other? (yes!)
hadrons
baryons: qqq or qqq
mesons: qq
6
Glue that holds stuff together
Some Hadrons
(By no means a complete list)
particle
proton
neutron
pion
pion
pion
kaon
quarks
charge
baryon no. strangeness
p
n
Δ++
Δ−
Λ0
Ξ0
Ω−
uud
udd
uuu
ddd
uds
uss
sss
+1
0
+2
−1
0
0
−1
+1
+1
+1
+1
+1
+1
+1
0
0
0
0
−1
−2
−3
π+
π0
π−
K+
ud
uu, dd
ud
us
+1
0
−1
+1
0
0
0
0
0
0
0
−1
Electromagnetic Force
Fundamental Forces
Gravity:
holds large things together (planets, stars)
Electromagnetic:
holds atoms together
Weak:
nuclear reactions and deacys
Strong:
holds atomic nuclei together
Electromagnetic Example: pion decay
• Operates on all charged particles
γ
(Also neutral particles with “magnetic moments”, which act like small rotating magnets)
• Mediated by photons (γ)
• Infinite range
Example:
electron-electron scattering*
physics-speak, “scattering” means
particles impose forces on each other,
but they do not change into other
particles. Crudely, this means they
“bounce off” one another.
π0
e−
e−
γ
Electromagnetic Example: scattering of a photon off an electron.
*In
The vertex, where the photon
and other particles meet, is the
crucial part of the interaction.
π0 → γ + γ
γ
γ
e−
e−
e− + e− → e− + e−
e−
e−
e−
γ
e+ γ→ e+ γ
7
Weak Force
This one can’t happen—no charged particles
γ
•
•
•
•
•
n
γ
n→ γ+γ
Operates all charged particles
The only force that operates on neutrinos
Mediated by W−, W+, Z0
Heavy masses of mediating particles ⇒ weak, rare interactions
Short range
Example: muon neutrino scatters off an electron
νµ
This one can’t happen—it violates conservation of charge.
νµ
γ
Z0
π−
γ
π− → γ + γ
e−
Weak Force Example:
Two ways an electron neutrino can scatter off an electron
Mediated by Z0
νe
νe
Mediated by W+
νe
e−
Weak Force Example:
How can a muon neutrino can scatter off an electron?
(Contract with electron neutrino in previous slide.)
This can happen:
νµ
Z0
e−
e−
νµ
This cannot happen:
νµ
e−
W+
e−
e−
Z0
νe
• Note that charge is conserved at each vertex
• To calculate the properties of a neutrino scattering off an electron,
both of these processes must be considered.
• Calculating things (like scattering rates) requires figuring out all
ways the process can happen (e.g., writing out all possible Feynman
diagrams), then doing the calculation for each diagram.
e−
W+
e−
e−
νµ
The right-hand diagram cannot happen because of a new
conservation law: lepton number conservation, which is
violated at each vertex of that diagram.
8
Weak Force Example:
“Beta decay” of a neutron
Lepton Number Conservation
• Leptons are divided into three families: electron, muon, tau
• The “lepton number” for each family is the number of
particles minus the number of anti-particles of that family
participating in a given reaction.
lepton no.:
n → p+ + e − + ν e
0 →
0
+1
n
−1
p+
• Example: lepton number for the electron family is
W−
# electrons + # electron-neutrinos − # anti-electrons − # anti-electron-neutrinos
• The lepton number for each family is separately conserved.
νe
• In the previous example, a vertex with an incoming muon
neutrino and and outgoing electron and W+ violates lepton
number conservation.
Weak Force Example:
“Beta decay” of a neutron:
n → p + e− + νe
n
p+
Weak Force Example:
Creating a new quark–anti-quark pair
d
n u
d
d
u
u
p
Λ0 → p+ + π −
dus → duu
W−
W−
νe
e−
e−
νe
+ ud
d
d
u
u
Λ0 u
s
baryon number 0
p+
W−
e−
• Weak interaction can change the type of quark (e.g., down to up)
• “Baryon number” is conserved
• +1/3 for each quark, −1/3 for each anti-quark
• B = +1 for baryons (3 quarks)
• B = −1 for anti-baryons (3 anti-quarks)
• B = 0 for mesons (quark and anti-quark)
u
d
π−
9
Strong Force
Strong Force Example:
Annihilation and creation of quark/anti-quark pairs
•
•
•
•
Operates on quarks only
Mediated by gluons
Very short range
Cannot change one type of quark into another
• New conservation laws for strong force only
• Example: S = # anti-strange-quarks − # strange quarks
• Can re-arrange quarks and/or create quark/anti-quark pairs
π−
ud
Strong Force Example: re-arranging quarks
π−
ud
+ p → Λ0 + K0
uud → uds
sd
The Feynman diagram for this reaction is:
+ p → n + π0
uud → udd
uu
u
g
s
s
u
u+ u→ s+ s
How Forces Work: A Few Last Words
• The electromagnetic force works by the
emission and absorption of photons by
charged particles.
• Therefore, we can say that the charge of a
particle is a measure of its ability to emit
photons.
• It follows that photons must be, in some
sense, the ultimate source of charge.
• What about other forces?
What about other characteristics of particles?
e−
How Forces Work: Last Words, continued
e−
γ
e−
• Gravity:
Acts on mass-energy of particle
• Electromagnetic force:
Photons acts on charge of particle
e−
• Weak force:
W, Z particles act on “weak charge” or “flavor charge,”
related to the “spin” of a particle
• Strong force:
Gluons act on color of quarks. Every quark is red, blue,
green. Gluons act between quarks of different colors.
10
Higgs Boson
• Big mystery: How does a particle get its rest mass?
(Example: why does an electron have mass 9 × 10−31 kg?)
• Possible solution:
Something interacts with particles to give them mass,
just like photons interact with particles to give them charge
• This purported “something” is the Higgs boson.
• If the Higgs theory is correct, it would have subtle effects on
interactions involving the W and Z particles at energies of
hundreds of GeV (1 GeV = 1000 MeV). Recent experiments
at CERN gave tantalizing hints of the existence of the Higgs
boson, but the evidence is not yet strong. Fermilab and
CERN are now upgrading their accelerators to search for this
particle.
11