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LECTURE 8
The Standard Model
Instructor: Shih-Chieh Hsu
Announcement
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ATLAS Virtual Visit (PAB B305)
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9:20am
LHC – CMS Masterclass (PAA A212)
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10:40am
Strongly suggest each of you bring your laptop
At least two people in a group
The CMS Masterclass
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Team-up with your seminar group
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At least one laptop per group (two laptops are better)
Make sure your browser can run iSpy software
https://catalyst.uw.edu/workspace/schsu/46264/338123
Read introduction and
analysis tips before lectures
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Let’s re-discovery W, Z and H
boson using CMS real data!
Lecture 8
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Particle Adventure
http://www.particleadventure.org
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New Quantum Theory
(First Quantization)
First Quantization
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The quantization of equation of motion of electron of atom is
induced by the boundary condition of wave function from
electric field of positive nucleus
Successfully describe the spectrum of atom.
Electron is moving in the speed v << c.
Quantum Mechanics + Newtonian Relativity
Dirac Equation
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Dirac tries to go beyond:
Quantum Mechanics + Special Relativity
-> Difficulty of negative
energy
-> Difficulty of develop
motion of equation for
photon
nd
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Quantization
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From Particle to Field
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Single-particle to Many-particle theory
Particle is an excitation (cluster of energy) of quantum field
Each type of particle has a corresponding Field
This leads to a complete quantum theory of electron and
light!!!
Timeline
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Timeline
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The Modern Atom Model
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If protons and neutrons
a centimeter in diameter
electrons and quarks
would be less than the
diameter of a hair
the entire atom's diameter
would be greater than the
length of thirty football
fields!
Scale of the atom
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We don't know exactly how small
quarks and electrons are; they are
definitely smaller than 10-18 meters,
and they might literally be points, but
we do not know.
It is also possible that quarks and
electrons are not fundamental after all!
What are we looking for?
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We have now discovered about
two hundred particles (most of
which aren't fundamental).
They are named with letters
from the Greek and Roman
alphabets.
Enrico Fermi said
“Young man, if I could remember the
names of these particles, I would have
been a botanist!"
The Standard Model
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6 quarks.
6 leptons. The best-known lepton is the electron. We will
talk about leptons in just a few pages.
Force carrier particles, like the photon. We will talk about
these particles later.
Quarks and Leptons
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Everything you saw is made from quarks and leptons.
Quarks behave differently than leptons, and for each kind of
matter particle there is a corresponding antimatter particle.
Matter and Antimatter
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Antiparticles look and behave just like their corresponding matter
particles, except they have opposite charges.
What is Antimatter
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evidence for antimatter in this early
bubble chamber photo.
The magnetic field in this chamber
makes negative particles curl left
and positive particles curl right.
the "up quark" u has an "up antiquark”,
pronounced u-bar.
The antielectron is called a positron
and is designated e+.)
Quarks
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Quarks have the unusual
characteristic of having a
fractional electric charge
Quarks also carry another
type of charge called color
charge,
Naming of Quarks
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1964, Murray Gell-Mann and George Zweig suggested
that hundreds of the particles known at the time could
be explained as combinations of just three fundamental
particles. Gell-Mann chose the name "quarks,"
pronounced "kworks,"
Gell-Mann
George Zweig
Quarks Naming
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There are six flavors of quarks. "Flavors" just means
different kinds. The two lightest are called up and
down.
The third quark is called strange. It was named after the
"strangely" long lifetime of the K particle, the first composite
particle found to contain this quark.
The fourth quark type, the charm quark, was named
on a whim. It was discovered in 1974 almost
simultaneously at both the Stanford Linear Accelerator
Center (SLAC) and at Brookhaven National Laboratory.
Heavy Quarks
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The bottom quark was first discovered at Fermi
National Lab (Fermilab) in 1977, in a composite particle
called Upsilon ().
The top quark was discovered last, also at Fermilab,
in 1995. It is the most massive quark. It had been
predicted for a long time but had never been observed
successfully until then.
Hadrons: Baryons and Mesons
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Like social elephants, quarks only exist in
groups with other quarks and are never
found alone. Composite particles made of
quarks are called
(uud), protons
Pion , which is made of an up quark and
a down anitiquark.
Leptons
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"Lepton" comes from the Greek for "small mass,"
However, the tau lepton is more than 3000 times as massive as
the electron.
Quarks are sociable and only exist in composite particles with other quarks,
whereas leptons are solitary particles.
Lepton Decays
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the muon and the tau, are not found in ordinary matter at
all. This is because when they are produced they very
quickly decay, or transform, into lighter leptons.
Physicists have observed that some types of lepton decays are
possible and some are not. In order to explain this,
three lepton families: the electron and its neutrino, the muon and
its neutrino, and the tau and its neutrino.
The number of members in each family must remain constant in a
decay.
Lepton Type Conservation
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We use the terms "electron number," "muon number," and "tau
number" to refer to the lepton family of a particle.
Electrons and their neutrinos have electron number +1,
positrons and their antineutrinos have electron number -1,
Quiz
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Which lepton decays are possible?
Why or why not?
Yes! Charge, tau number, electron
number, and energy are all conserved.
No! Muon number is not conserved. A muon has a muon number of
1, and thus the right side of the decay equation has muon number 1
No! energy is not conserved. A muon has a lot more mass than an electron,
Neutrinos
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it was through a careful study of radioactive decays that
physicists hypothesized the neutrino's existence.
Because neutrinos were produced in great abundance in the early
universe and rarely interact with matter, there are a lot of them in
the Universe. Their tiny mass but huge numbers may contribute to
total mass of the universe and affect its expansion.
Quiz
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What are protons made of?
Protons are made of two up quarks and one down quark, expressed as
uud.
What are electrons made of?
As far as we know, electrons aren't composed of smaller particles, they
are fundamental!
Which of the following are made of quarks?
Baryons? Yes, they are made of three quarks put together.
Mesons? Yes, they are made of one quark and one antiquark.
Barons?
Yes, the English nobility are also made of quarks.
The Four Interactions
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What holds things together?
What's the difference between
a force and an interaction?
a force is the effect on a particle
due to the presence of other
particles.
The interactions of a particle include
all the forces that affect it, but also
include decays and annihilations that
the particle might go through.
the particles which carry the
interactions force carrier particles.
Elementary Particles
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How does matter interact?
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How do two magnets "feel" each
other's presence and attract or repel
accordingly? How does the sun attract
the earth?
We know the answers to these
questions are "magnetism" and
"gravity," but what are these forces?
At a fundamental level, a force isn't
just something that happens to
particles. It is a thing which is passed
between two particles.
The Unseen effect
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You can think about forces as being
analogous to the following situation:
all interactions which affect matter particles are due to an
exchange of force carrier particles
Electromagnetism
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The carrier particle of the
electromagnetic force is the photon
Photons have zero mass, as far as we
know, and always travel at the "speed
of light", c, which is about 300,000,000
meters per second, or 186,000 miles
per second, in a vacuum.
Residual EM force
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Atoms usually have the same numbers
of protons and electrons. They are
electrically neutral,
Since they are neutral, what causes
them to stick together to form stable
molecules?
the charged parts of one atom can
interact with the charged parts of
another atom. This allows different
atoms to bind together, an effect called
the residual electromagnetic force.
What about the nucleus?
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What binds the nucleus together?
why doesn't the nucleus blow
apart? Since neutrons have no charge
and the positively-charged protons
repel one another,
So how can we account for this
dilemma?
Strong and Color Charge
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Quarks have an altogether different
kind of charge called color charge
The force between color-charged
particles is very strong, so this force is
"creatively" called “Strong”
The force carrier is called “Gluon”
composite particles made out of
quarks have no net color charge
(they are color neutral).
Color Charge
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Gluons carry two colors
"Color charge" has nothing to do with
the visible colors, it is just a convenient
naming convention for a mathematical
system physicists developed to explain
their observations about quarks in
hadrons.
Quark Confinrment
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Color-charged particles cannot be
found individually. For this reason, the
color-charged quarks are confined in
groups (hadrons) with other quarks.
These composites are color neutral.
only baryons (three different colors)
and mesons (color and anticolor) are
color-neutral.
ud or uddd that cannot be combined
into color-neutral states are never
observed.
Gluons and Quarks
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The quarks in a given hadron madly exchange gluons. For
this reason, physicists talk about the color-force field which
consists of the gluons holding the bunch of quarks together.
Quarks cannot exist individually because the color
force increases as they are pulled apart.
Color exchange
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When a quark emits or absorbs a gluon, that quark's color must change
in order to conserve color charge.
For example, suppose a red quark changes into a blue quark and emits
a gluon. What is the color of the gluon?
red/antiblue gluon (the image below illustrates antiblue as
yellow). The net color is still red.
Residual strong force
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the strong force binds quarks together because quarks have color
charge.
What holds the nucleus together? since positive protons repel each other
with electromagnetic force, and protons and neutrons are color-neutral.
The strong force between the quarks in one proton and the quarks in
another proton is strong enough to overwhelm the repulsive
electromagnetic force.
Weak Interactions
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Weak interactions are responsible for
the decay of massive quarks and
leptons into lighter quarks and leptons.
When a quark or lepton changes type
(a muon changing to an electron, for
instance) it is said to change flavor.
All flavor changes are due to the
weak interaction.
the weak interactions are the W+, W-,
and the Z
Electroweak
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In the Standard Model the weak and the
electromagnetic interactions have been combined into
a unified electroweak theory.
Physicists had long believed that weak forces were
closely related to electromagnetic force
At very short distances (about 10-18 meters) the
strength of the weak interaction is comparable to
that of the electromagnetic.
at thirty times that distance (3x10-17 m) the strength of the weak
interaction is 1/10,000th than that of the electromagnetic interaction.
At distances typical for quarks in a proton or neutron (10-15 m) the
force is even tinier.
Force Carrier Comparison
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the weak and electromagnetic forces
have essentially equal strengths.
the strength of the interaction depends strongly on both the
mass of the force carrier and the distance of the interaction.
The difference between their observed strengths is due to the huge
difference in mass between the W and Z particles, which are very
massive, and the photon, which has no mass as far as we know.
Gravity
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the gravity force carrier particle has not
been found. Such a particle, however, is
predicted to exist and may someday be
found: the graviton.
Why does the SM work without explaining
Gravity?
the effects of gravity are extremely tiny
in most particle physics situations
compared to the other three
interactions, so theory and experiment
can be compared without including
gravity in the calculations.
Interaction Summary
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Quiz
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Which fundamental interaction is responsible for:
Friction?
residual electromagnetic interactions
between the atoms of the two materials.
Nuclear Binding?
residual strong interactions between
the various parts of the nucleus.
Planetary orbits?
the gravity that attracts them to the
sun!
Quiz2
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Which interactions act on neutrinos?
Weak and Gravity
Which interaction has heavy
carriers?
Weak (W+, W-, and Z)
Which interactions act on the
protons in you?
All of them.
Which force carriers cannot be
isolated? Why?
Gluons, because they carry color
charge themselves.
Which force carriers have not been
observed?
Gravitons (Gluons have been
observed indirectly.)
Interactions
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Quantum Mechanics
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"quantum," which means "broken into
increments or parcels,” is used to describe
the physics of very small particles
A few of the important quantum numbers of particles are:
Electric charge. Quarks may have 2/3 or 1/3 electron charges, but
they only form composite particles with integer electric charge.
Color charge. A quark carries one of three color charges and a gluon
carries one of eight color-anticolor charges. All other particles are color
neutral.
Flavor. Flavor distinguishes quarks (and leptons) from one another.
Spin
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• Spin is a bizarre but important physical quantity.
• Large objects like planets or marbles may have angular momentum and
a magnetic field because they spin.
• Since particles also to appear to have their own angular momentum and
tiny magnetic moments, physicists called this particle property spin.
• This is a misleading term since particles are not actually "spinning." Spin
is quantized to units of 0, 1/2, 1, 3/2 (times Planck's Constant, ) and so
on.
Pauli Exclusion Principle
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Pauli Exclusion Principle,
no two particles in the same quantum state could exist in the same
place at the same time.
But it has been since discovered that a certain group of particles do not
obey this principle. Particles that do obey the Pauli Exclusion Principle
are called fermions, and those that do not are called bosons.
Fermions & Bosons Behavior
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Fermions and Bosons: Explained
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The predicted graviton has a spin of 2.
A Lot To Remember
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We have answered the questions,
"What is the world made of?" and
"What holds it together?"
The world is made of six quarks and
six leptons. Everything we see is a
conglomeration of quarks and leptons.
There are four fundamental forces and
there are force carrier particles
associated with each force.
We have also discussed how a particle's state (set of quantum numbers)
may affect how it interacts with other particles.
These are the essential aspects of the Standard Model. It is the most
complete explanation of the fundamental particles and interactions to
date.
Elementary Particles
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Big Theory Chart
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The Higgs Boson
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Its discovery helps confirm the mechanism by which fundamental
particles get mass.
The Higgs Boson
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In 1964, six theoretical physicists hypothesized a new field (like an
electromagnetic field) that would permeate all of space and solve a
critical problem for our understanding of the universe.
Photo of Francois Englert and Peter Higgs - © CERN
The Mechanism giving mass to Particle
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Interaction with the Higgs Field.
The Mechanism giving mass to the Boson
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How does the Higgs Boson get mass?
How to detect the Higgs Boson?
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