LECTURE 7 The Standard Model
Instructor: Shih-Chieh Hsu Announcement
2 ¨ ATLAS Virtual Visit (PAB A110) ¤ Sep 7 – Vidyo connection will start from 9:20am ¤ At least one question for CERN host from each group http://atlas-live-virtual-visit.web.cern.ch/atlas-live-virtual-visit/2016/20160907-Seattle.html The CMS Masterclass
3 ¨ LHC – CMS Masterclass (PAB A110) ¤ Sep 8 and Sep 9 ¨ Team-up with your symposium group ¤ At least one laptop per group (two laptops are better) ¤ Make sure your browser can run iSpy and CIMA software https://canvas.uw.edu/courses/1089028/pages/cms-masterclass-2016 ¨ Read introduction and analysis tips before lectures ¤ Let’s re-discovery W, Z and possibly H boson using CMS real data! Lecture 7
¨ The Particle Adventure http://www.particleadventure.org 5
¨ 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
2 2 ∂ − ∇ Ψ(r ,t) + U (r )Ψ(r ,t) = i Ψ(r ,t) 2m ∂t Schrödinger equation Dirac Equation
Dirac equation 2nd Quantization
8 From Particle to Field
¨ 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 I
9 Timeline II
H The Modern Atom Model
the entire atom's diameter would be greater than the length of thirty football fields! Scale of the atom
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?
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
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
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
16 Antiparticles look and behave just like their corresponding matter particles, except they have opposite charges. What is Antimatter
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
Quarks have the unusual characteristic of having a fractional electric charge
Quarks also carry another type of charge called color charge, Naming of Quarks
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
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
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
22 Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called
23 "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
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
25 We use the terms "electron number," "muon number," and "tau number" to refer to the lepton family of a particle.
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
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
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
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
30 How does matter interact?
How do two magnets "feel" each other's presence and attract or repel accordingly? How does the sun attract the earth?
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
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
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
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?
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
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
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
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
39 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
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
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
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
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
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
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
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
Which interactions act on neutrinos? Weak and Gravity
Which interaction has heavy Weak (W+, W-, and Z) carriers?
Which interactions act on the All of them. protons in you?
Which force carriers cannot be Gluons, because they carry color isolated? Why? charge themselves.
Which force carriers have not been Gravitons (Gluons have been observed? observed indirectly.) Interactions
49 Quantum Mechanics
"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
• Spin is a bizarre but important physical quantity.
• 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
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
53 Fermions and Bosons: Explained
The predicted graviton has a spin of 2. A Lot To Remember
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
56 Big Theory Chart
57 The Higgs Boson
58 Its discovery helps confirm the mechanism by which fundamental particles get mass. The Higgs Boson
59 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
60 Interaction with the Higgs Field. The Mechanism giving mass to the Boson
61 How does the Higgs Boson get mass? How to detect the Higgs Boson?
Detecting invisible What are debris?
64 ¨ Life time is longer enough to fly through the detector ¤ The tracker radius is about 1m ¤ The lifetime of particle is longer than http://pdg.lbl.gov Particles and Detectors
65 How to measure charged & momentum?
What happen for a neutral particle passing through magnet? Particles and Detectors
67 Detector Design
Generic Design Cylinders wrapped around the beam pipe From inner to outer . . . Tracking Electromagnetic calorimeter Hadronic calorimeter Magnet* Muon chamber
* location of magnet depends on specific detector design Particle Detection
e-, e+ muon+, muon- Quiz2
quark-antiquark quark-antiquark+ gluon (?)