Elementary Particle Physics
Lecture 1
Preliminary Notions
A particle is considered to be elementary only if
there is no evidence that it is made up of smaller
constituents
Fundamental particles have no internal
structure, which is one of their most
distinguishing characteristics.
.
References
History of Particle Physics
4
Late 1800’s – Early 2000’s:
Discoveries of standard model particles,
particle properties,
realization of fundamental symmetries,
experimental triumphs,
but also a lot of puzzles, frustration, confusion...
From Classical Physics…
…to Modern Physics…
…and Particle Physics today
(Standard Model)
At the begging
It is natural to ask what protons, neutrons, and electrons are made of?.
With today's particle accelerators, one can "look inside" these objects for an internal
structure. The proton and neutron are found to be made up of "quarks."
Two quarks with a positive electric charge of 2/3 (of the electron charge), called u
quarks, and one quark with a negative electric charge of -1/3, called the d quark, make
up the proton. Similarly, the neutron is made up of one u quark and two d quarks. There
are many other particles that can be built out of the quarks combined in particular ways;
these are called hadrons..
With high-energy accelerators, particle physicists can effectively "trade" energy for mass,
allowing them to directly produce particles that weigh many times more than the particles
being accelerated. This follows from relativity, which says that a particle with mass m that is
at rest has an energy E given by the famous equation E = m𝒄𝟐 . Thus, if two protons each
having an energy of 1,000 GeV can be brought together, it would in principle be possible to
produce in such collisions two new particles (at rest) each weighing 1,000 GeV, or about
1,000 times as much as the initial protons. This is the means by which very heavy members
of subsequent particle generations were discovered.
Introduction
Quantum theory
proton + neutron
contain quarks, up quarks, down quarks 
(elementary particles)
In molecule the electron’ three degrees of freedom ( charge, spin, orbital) can 
separate via the wavefunction into three quasiparticles (holon, spinon, orbiton)
Quantum statistics
Elementary particles
are either bosons or fermions
Fermions(half-integer spin) obey Fermi-Dirac statistics 
Bosons(integer spin)
obey Bose-Enstein statistics 
Around the turn of the 20th century, atomic nuclei were shown to consist of
protons and neutrons. Then, throughout the 1950s and '60s, particle
accelerators kept revealing a bevy of exotic subatomic particles, such as
pions and kaons.

Elementary Particle Physics
(scales)
The search for the origin of matter means the understanding of elementary particles.
More than 200 subatomic particles have been discovered
so far, all detected in sophisticated particle accelerators.
However, most are not fundamental, most are composed
of other, simpler particles. For example, Rutherford showed
that the atom was composed of a nucleus and orbiting
electrons. Later physicists showed that the nucleus was
composed of neutrons and protons. More recent work has
shown that protons and neutrons are composed of quarks.
What Are Elementary Particles?
Elementary particles are the smallest known building blocks of the universe.
They are thought to have no internal structure, meaning that researchers
think about them as zero-dimensional points that take up no space.
Standard Model of physics
(describes the interactions of particles and almost all
forces) recognizes 10 total elementary particles.
Fundamental particles called quarks come in six different
flavors.
Protons are made of two up quarks and one down quark
neutrons contain two down quarks and one up quark.
Structure within the Atom
The atom contains a nucleus
surrounded by a cloud of
negatively charged electrons.
The nucleus is composed of
neutral neutrons and positively
charged protons. The opposite
charge of the electron and
proton
binds
the
atom
together with electromagnetic
forces.
The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3)
combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the
nuclear strong force (that overcomes the electromagnetic repulsion of like-charged protons)
Quarks and Leptons:
The two most fundamental types of particles are quarks and leptons. The quarks and
leptons are divided into 6 flavors corresponding to three generations of matter. Quarks
(and antiquarks) have electric charges in units of 1/3 or 2/3's. Leptons have charges in units
of 1 or 0.
Electrons and related particles
Electrons are probably the most familiar
elementary particles
The electron has two heavier cousins, called the muon and the
tau. Muons can be created when high-energy cosmic rays from
outer space hit the top of Earth's atmosphere, generating a
shower of exotic particles. Taus are even rarer and harder to
produce, as they are more than 3,400 times heavier than
electrons. Neutrinos, electrons, muons and taus make up a
category of fundamental particles called leptons.
According to a historical report from SLAC National Accelerator Laboratory in
California. Residing inside protons and neutrons are tiny particles called quarks, which
come in six possible types or flavors: up, down, strange, charm, bottom and top.
By 1977, physicists had isolated five of the six quarks in the lab — up, down, strange,
charm and bottom — but it wasn't until 1995 that researchers at Fermilab National
Accelerator Laboratory in Illinois found the final quark, the top quark. Searching for it
had been as intense as the later hunt for the Higgs boson. The top quark was so hard to
produce because it's about 100 trillion times heavier than up quarks, meaning it
required a lot more energy to make in particle accelerators.
Quarks, which make up protons and neutrons, are another type of
fundamental particle. Together with the leptons, quarks make up the
stuff we think of as matter.
Then there are the four fundamental forces of nature: electromagnetism, gravity, and the
strong and weak nuclear forces. Each of these has an associated fundamental particle.
Photons are the most well-known; they carry the electromagnetic force.
Gluons carry the strong nuclear force and reside with quarks inside of protons and
neutrons. The weak force, which mediates certain nuclear reactions, is carried by two
fundamental particles, the W and Z bosons.
Neutrinos, which only feel the weak force and gravity, interact with these bosons, and so
physicists were able to first provide evidence for their existence using neutrinos
Gravity is an outsider here. It isn't incorporated into the Standard Model, though physicists suspect that
it could have an associated fundamental particle, which would be called the graviton. If gravitons
exist, it might be possible to create them at the Large Hadron Collider (LHC) in Geneva, Switzerland,
but they would rapidly disappear into extra dimensions, leaving behind an empty zone where they
would have been
So far, the LHC has seen no evidence of gravitons or extra dimensions.
The king of the elementary particles
The Higgs boson, the king of the elementary particles, which is responsible for giving all other
particles their mass. Hunting for the Higgs was a major endeavor for scientists striving to
complete their catalog of the Standard Model. When the Higgs was finally spotted, in 2012,
physicists rejoiced, but the results have also left them in a difficult spot.
The Higgs looks pretty much exactly like it was predicted to look, but scientists were
hoping for more. The Standard Model is known to be incomplete; for instance, it lacks a
description of gravity, and researchers thought finding the Higgs would help point to
other theories that could supersede the Standard Model.
Simulation showing the production of the Higgs
boson in the collision of two protons at the
Large Hadron Collider. The Higgs boson quickly
decays into four muons, which are a type of
heavy electron that is not absorbed by the
detector. The tracks of the muons are shown in
yellow. (Image credit: Lucas Taylor/CMS)
How quarks usually fit into our understanding of tiny particles.
(Image credit: udaix/Shutterstock)
Note that for every quark or lepton there is a corresponding antiparticle. For example,
there is an up antiquark, an anti-electron (called a positron) and an anti-neutrino.
Bosons do not have antiparticles since they are force carriers
Fundamental Forces
Matter is effected by forces or interactions
There are four fundamental forces in the Universe:
gravitation (between particles with mass)
electromagnetic (between particles with charge/magnetism)
strong nuclear force (between quarks)
weak nuclear force (operates between neutrinos and electrons)
Gravity is the attractive force between all matter, electromagnetic force describes the
interaction of charged particles and magnetics. Light (photons) is explained by the
interaction of electric and magnetic fields.
The strong force binds quarks into protons, neutrons and mesons, and holds the nucleus of
the atom together despite the repulsive electromagnetic force between protons. The
weak force controls the radioactive decay of atomic nuclei and the reactions between
leptons (electrons and neutrinos).
Note that , although the strong force has the greatest
strength, it also has the shortest range.
Baryons and Mesons:
Quarks combine to form the basic
building blocks of matter, baryons and
mesons. Baryons are made of three
quarks to form the protons and neutrons
of atomic nuclei (and also anti-protons
and anti-neutrons). Mesons, made of
quark pairs, are usually found in cosmic
rays. Notice that the quarks all combine
to make charges of -1, 0, or +1.
Color Charge:
Quarks in baryons and mesons are bound together by the strong force in the
form of the exchange of gluons. Much like how the electromagnetic force
strength is determined by the amount of electric charge, the strong force
strength is determined by a new quantity called color charge.
Quarks come in three colors, red, blue and green (they are not actually
colored, we just describe their color charge in these terms). So, unlike
electromagnetic charges which come in two flavors (positive and negative or
north and south poles), color charge in quarks comes in three types. And, just
to be more confusing, color charge also has its anti-particle nature. So there is
anti-red, anti-blue and anti-green.
Gluons serve the function of carrying color when they interact with quarks.
Baryons and mesons must have a mix of colors such that the result is white. For
example, red, blue and green make white. Also red and anti-red make white.
Quark Confinement
There can exist no free quarks, i.e. quarks by
themselves. All quarks must be bound to another
quark or antiquark by the exchange of gluons. This
is called quark confinement. The exchange of
gluons produces a color force field, referring to the
assignment of color charge to quarks, similar to
electric charge.
The color force field is unusual in that separating
the quarks makes the force field stronger (unlike
electromagnetic or gravity forces which weaken
with distance). Energy is needed to overcome the
color force field. That energy increases until a new
quark or antiquark is formed (energy equals mass,
E=mc2).
Two new quarks form and bind to the old quarks to
make two new mesons. Thus, none of the quarks
were at anytime in isolation. Quarks always travel in
pairs or triplets.
Quantum Chromodynamics
Quantum chromodynamics is the
subfield of physics that describes
the strong or ``color'' force that
binds quarks together to form
baryons and mesons, and results
in the complicated the force that
binds atomic nuclei together.
The Standard Model
The Standard Model is a way of making sense of the multiplicity of elementary particles and
forces within a single scheme. The Standard Model is the combination of two schemes; the
electroweak force (unification of electromagnetism and weak force) plus quantum
chromodynamics. Although the Standard Model has brought a considerable amount of
order to elementary particles and has led to important predictions, the model is not without
some serious difficulties.
For example, the Standard Model contains a large number of arbitrary constants. Good
choice of the constants leads to exact matches with experimental results. However, a good
fundamental theory should be one where the constants are self-evident.
The Standard Model does not include the unification of all forces and, therefore, is
incomplete. There is a strong expectation that there exists a Grand Unified Field Theory
(GUTS) that will provide a deeper meaning to the Standard Model and explain the missing
elements.
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What is matter made of (c. 2000 CE)
The story is far from being complete!
• “Normal” matter, described by the Standard Model, makes up only 4% of the total
matter/energy in the Universe!
• We know almost nothing about
the other 96%... more on
this later in the course.
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Are photons real?
 In order to explain blackbody emission spectra, Planck
needed to assume that thermal radiation is emitted in
bundles whose energy comes in integral multiples of hn.
 This suggested that light could actually be quantized
(it’s a particle). But most of the experimental evidence
(and Maxwell’s Equations) at the time said that light is
a wave.
 So is light a particle, or a wave? As it turns out, light
can behave like a particle if you are performing the right kind of
experiment!
 At first, Planck did not really believe in the light quantum, and most
physicists did not accept its existence until faced with undeniable
evidence from two phenomena:
1) The photoelectric effect
2) Compton scattering
Evidence for particle nature of light
M. Planck
NobelPrize.org
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Photoelectric effect (1905)
In the 1800’s, it was discovered that shining light onto certain metals 
liberated electrons from the surface.
Experiments on this photoelectric effect showed odd results:
1) Increasing the intensity of the light increased the number of electrons,
but not the maximum kinetic energy of the electrons.
2) Red light did not liberate electrons, no matter how intense it was!
3) Weak violet light liberated few electrons, but their maximum kinetic
energy was greater than that for more intense long-wavelength beams!
In 1905, A. Einstein showed that these results made perfect sense in 
the context of quantization of the EM field, where photon energy
is proportional to frequency. If photons of energy E=hn strike
electrons in the surface of the metal, the freed electrons have a
kinetic energy:
The work function f is a constant that depends on the metal. 
A. Einstein
NobelPrize.org
Compton scattering (1923)
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 In 1923, A.H. Compton found that light scattered from a particle at rest is shifted in
wavelength by an amount:
A. H. Compton
NobelPrize.org
Here, lc=h/mc is the Compton wavelength of the target mass m.
 There is no way to derive this formula if you assume light is a wave, but if you treat
the incoming light beam like a particle with energy E=hn, Compton’s formula drops
right out!
 Hence, the Compton Effect proved to be the decisive evidence in favor of the
quantization of the EM field into photons.
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Field quantization in nuclear physics
 Field quantization, once accepted for the electromagnetic field,
was quickly applied to other calculations.
 One was the physics of the atomic nucleus, which gets very
complicated after hydrogen.
QUESTION: How are protons in heavy
atoms bound inside the 1 fm “box” of
the nucleus?
Shouldn’t the electrostatic repulsion of
the protons blow the nucleus apart?
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Nuclear force model (1934)
 Evidently, some force is holding the nucleus
together: the “strong force.”
 Inside the nucleus, the strong force has to
overwhelm the EM force, but outside, on the
atomic scale, it should have almost no effect.
 How to accomplish this? Assume the strong force
has a very short range, falling off rapidly to
zero for distances greater than 1 fm.
 H. Yukawa: force may vary as:
H. Yukawa
Image: NobelPrize.org
where a
1 fm is the range.
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+Neutrinos…
 Postulated to save conservation of energy!
 In the study of radioactive decays (esp. b-decay), physicists
found that many reactions appeared to violate energy
conservation.
 Conclusion 1 (Bohr): nuclear decays do actually violate energy
conservation.
 Conclusion 2 (W. Pauli): the missing energy is carried off by
another neutral particle which hadn’t been detected (as of
1930).
W. Pauli
 In 1932, E. Fermi incorporated Pauli’s idea into his theory of
nuclear decays. He called the missing particles neutrinos (“little
neutral ones”).
 Major assumption: neutrinos almost never interact with ordinary
matter, except in decays.
E. Fermi
NobelPrize.org
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Discovery of neutrinos (1950s)
 By introducing neutrinos (symbol n) to radioactive decay,
conservation of energy was restored. Decay reactions started to
look like this:
 By 1950, there was compelling theoretical evidence for neutrinos,
but no neutrino had ever been experimentally isolated.
 Finally, in the mid-1950s, C. Cowan and F. Reines came up with a
method to directly detect neutrinos using “inverse”
b-decay:
 A difficult experiment: Cowan and Reines set up a large water tank
outside a commercial nuclear reactor, expecting to see evidence
of the above reaction only 2 to 3 times per hour (which they did).
Conclusion: (anti) neutrinos (n’s) exist.
C. Cowan and F. Reines
Image: CUA
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Antineutrinos?
 Because all particles have anti-particles, physicists assumed that
neutrinos must have corresponding anti-neutrinos.
 But does anything distinguish a neutrino from an anti-neutrino?
 From the results of Cowan and Reines, the reaction below must
occur:
 If anti-neutrinos are the same as neutrinos, the anti-neutrino
version of this reaction must also occur:
 In fact, in the late1950s, R. Davis and D.S. Harmer found that the
anti-neutrino reaction does not occur. Therefore, something is
different about the anti-neutrino that forbids the process. But
what?
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Discovery of strange particles (1947)
 By 1947, the catalog of elementary particles consisted of the p, n, p, m, e,
and the n (and the anti-particles). The overall scheme seemed pretty
simple.
 However, at the end of that year, a new neutral particle was discovered:
the K0 (“kaon”):
 In 1949, a charged kaon was found:
 The K’s behaved somewhat like heavy p’s, so they were classified as
mesons (“mass roughly between the proton and electron mass”).
 Over the next two decades, many more mesons were discovered: the
h, the f, the w, the r’s, etc.
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




“Strange” Behavior: The new mesons and baryons discovered during the 1950s all had the
following properties:
1) They are produced on short timescales (10-23s)
2) But they decay relatively slowly (10-10s)
This suggests the force causing their production (strong force) differs from the force causing
their decay (weak force).
In 1953, M. Gell-Mann and K. Nishijima introduced a new quantum number, strangeness (S),
to explain this behavior.
According to this scheme, strangeness is conserved in strong interactions, but not conserved
(violated) in weak decays.
IMPORTANT POINT: In addition, particles with non-zero S are always produced in pairs –no
interaction produces just one strange particle.
The quark model (1964)
In 1964, Gell-Mann and G. Zweig proposed an
explanation for the structure in the hadron multiplets:
all hadrons are composed of even more fundamental
constituents, called quarks.
According to their quark scheme, quarks came in three types, or “flavors”:
up (u), down (d), and strange (s).
To get the right
hadronic properties,
Gell-Mann gave his
quarks fractional
electric charge
Until the mid-1970s, most physicists did not accept
quarks as real particles.
Then, in 1974, two experimental groups discovered a
neutral, extremely heavy meson called the J/y.
The J/y had a lifetime about 1000 times longer than
other hadrons in its mass range.
A simple way to explain its properties uses the quark
model. A new quark, called charm (c), was
introduced; and the J/y was shown to be a bound
state of a charm-anticharm pair (sometimes called
“charmonium”).
We have since discovered the bottom (beauty) quark,
in 1977, and the top (truth) quark, in 1995.
http://www.symmetrymagazine.org/breaking/2012/01/11/belle-experiment-makes-exotic-discovery/
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Standard Model (1978-present)
The Standard Model Now