<<

LECTURE 7 The

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 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 using CMS real data! Lecture 7

4

New Theory (First ) First Quantization

6

¨ The quantization of equation of motion of of is induced by the boundary condition of wave function from electric 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

7 ¨ Dirac tries to go beyond: +

-> Difficulty of negative -> Difficulty of develop motion of equation for

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 !!! Timeline I

9 Timeline II

10

H The Modern Atom Model

11

If and a centimeter in diameter

and 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

12

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?

13

We have now discovered about two hundred (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

14

6 quarks. 6 . The best-known is the electron. We will talk about leptons in just a few pages. carrier particles, like the photon. We will talk about these particles later. Quarks and Leptons

15

Everything you saw is made from quarks and leptons.

Quarks behave differently than leptons, and for each kind of particle there is a corresponding antimatter particle. Matter and Antimatter

16 look and behave just like their corresponding matter particles, except they have opposite charges. What is Antimatter

17

evidence for antimatter in this early bubble chamber photo.

The in this chamber makes negative particles curl left and positive particles curl right.

the "up " u has an "up antiquark”, pronounced u-bar.

The antielectron is called a and is designated e+.) Quarks

18

Quarks have the unusual characteristic of having a fractional electric

Quarks also carry another type of charge called , Naming of Quarks

19

1964, Murray Gell-Mann and 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

20

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 , 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

21

The was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called Upsilon .

The 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. : and

22 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 , which is made of an and a down anitiquark. Leptons

23 "Lepton" comes from the Greek for "small mass," However, the 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

24

the 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 , 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.

Electrons and their have electron number +1, and their antineutrinos have electron number -1, Quiz

26

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

27

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 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

28

What are protons made of? Protons are made of two up quarks and one , 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

29

What holds things together?

What's the difference between a force and an ?

a force is the effect on a particle due to the presence of other particles. The interactions of a particle include all the that affect it, but also include decays and that the particle might go through. the particles which carry the interactions particles. Elementary Particles

30 How does matter interact?

31

How do two "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 "" and "," 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

32

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

33

The carrier particle of the electromagnetic force is the photon

Photons have zero mass, as far as we know, and always travel at the "", c, which is about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum. Residual EM force

34

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 ? the charged parts of one atom can interact with the charged parts of another atom. This allows different to bind together, an effect called the residual electromagnetic force. What about the nucleus?

35

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

36

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 “

composite particles made out of quarks have no net color charge (they are color neutral). Color Charge

37

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

38

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. and Quarks

39 The quarks in a given 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

40

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

41

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 and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force. Weak Interactions

42

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 .

the weak interactions are the W+, W-, and the Z Electroweak

43

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 (10-15 m) the force is even tinier. Force Carrier Comparison

44

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

45

the gravity force carrier particle has not been found. Such a particle, however, is predicted to exist and may someday be found: the .

Why does the SM work without explaining Gravity? the effects of gravity are extremely tiny in most situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Interaction Summary

46 Quiz

47

Which 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 ? the gravity that attracts them to the sun! Quiz2

48

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 (Gluons have been observed? observed indirectly.) Interactions

49 Quantum Mechanics

50

"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 . 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.

51

• Spin is a bizarre but important physical quantity.

• Large objects like or marbles may have angular and a magnetic field because they spin.

• Since particles also to appear to have their own 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

52

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 , and those that do not are called . Fermions & Bosons Behavior

53 Fermions and Bosons: Explained

54

The predicted graviton has a spin of 2. A Lot To Remember

55

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. 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

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 ) that would permeate all of space and solve a critical problem for our understanding of the universe.

Photo of Francois Englert and - © 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?

62 63

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?

66

Charge Momentum

What happen for a neutral particle passing through ? 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

69 Quiz1

70

e-, e+ muon+, muon- Quiz2

71

quark-antiquark quark-antiquark+ gluon (?)