Presentation is loading. Please wait.

Presentation is loading. Please wait.

Particle Physics True or False?

Similar presentations

Presentation on theme: "Particle Physics True or False?"— Presentation transcript:

1 Particle Physics True or False? The fundamental particles of nature are protons, neutrons, and electrons.

2 What IS Matter ? Matter is all the “stuff” around you!
Here’s the picture we’re going to uncover (not all today though) Matter Hadrons Leptons Forces Baryons Mesons Charged Neutrinos Gravity Strong Weak EM Quarks Anti-Quarks

3 What are we made of ? 0.0002” We’re made of cells which contain DNA. - Different cells serve different functions in your body. Cells contain a nucleus, which holds your DNA ! And the DNA is simply a complex chain of molecules which contains your genetic code! And what are molecules made of ?

4 The Elements Molecules are complex structures of the elements

5 Elements are made of…? Electrons Nucleus 0.0000000002 m (2 x 10-10 m)
Note that the electrons are ~40,000 nuclear radii away from the nucleus! (2x10-10/5x10-15 = 40,000) Sort of resembles a miniature solar system… The nucleus we talk about here is different from the nucleus of a cell. The nucleus of a cell contains complex molecules. This nucleus contains protons and neutrons. m (2 x m)

6 What’s in the Nucleus? Protons Neutrons
Protons are positively charged and that amount of charge is exactly equal (and opposite) to the charge of the electron Neutrons are similar to protons (ie., similar mass), but have a net charge of zero. A “fm” stands for femtometer. The prefix “femto” means 10 to the power –15, or, 1 fm = meters ! Recall: 1 [fm] = [m]

7 Atomic Model 30 football fields!
To give you another perspective on the size of the atom. If the nucleons (protons/neutrons) were one cm in diameter… what is the diameter of the nucleus? 10 cm what is the diameter of the electron? less than the diameter of your hair What is the diameter of the entire atom? 30 football fields!

8 Are protons and neutrons fundamental?
(By fundamental, I mean are they indivisible?) The answer is NO ! Protons and neutrons are made of smaller objects called quarks! (1.6 x m) 1x m (at most) Protons 2 “up” quarks 1 “down” quark Neutrons 1 “up” quark 2 “down” quarks

9 Three Families of Quarks
Generations I II III Charge = -1/3 d (down) s (strange) b (bottom) +2/3 u (up) c (charm) t (top) Increasing mass Also, each quark has a corresponding antiquark. The antiquarks have opposite charge to the quarks

10 The 6 Quarks, when & where…
Date Where Mass [GeV/c2] Comment up, down - ~0.005, ~0.010 Constituents of hadrons, most prominently, proton and neutrons. strange 1947 ~0.2 discovered in cosmic rays charm 1974 SLAC/ BNL ~1.5 Discovered simultaneously in both pp and e+e- collisions. bottom 1977 Fermi- lab ~4.5 Discovered in collisions of protons on nuclei top 1995 Fermi-lab ~175 Discovered in pp collisions

11 How do we know any of this?
High energy particles provide a way to probe, or “see,” matter at the very smallest sizes. Today, high energy accelerators produce energetic beams which allow us to probe matter at its most fundamental level. As we go to higher energy particle collisions: 1) Wavelength probe is smaller  see finer detail 2) Can produce more massive objects, via E=mc2

12 Major High Energy Physics Labs
Fermilab DESY SLAC CERN KEK SLAC == Stanford Linear Accelerator in California. It’s an electron-positron collider. Most recently, the accelerator collides operates at a collision energy of about 10 GeV. Fermilab = Fermi National Accelerator laboratory, in Batavia, IL. Highest energy proton antiproton Collider in the world collides protons and antiprotons at a collision energy of 2000 GeV. CESR = Cornell Electron Storage Ring. Collides electrons and positrons. Collides electrons and positrons at about 10 GeV. BNL = Brookhaven National Lab. Collides protons and nuclei on each other at various energies. CERN = European Research Lab, on the border of France and Switzerland… Not a bad place to do some good science! Until 2000, it was a high energy electron-positron collider. The electron and positron beams typically had an energy around GeV. DESY: Laboratory in Germany. Accelerator creates beams of electrons at about 30 GeV and collides them into a beam of protons which have an energy of about 820 GeV. KEK: Laboratory in Japan which collides electrons and protons at about 10 GeV collision energy. CESR BNL

13 Fermilab Accelerator (30 miles from Chicago)
Main Injector Tevatron Experimental areas Top Quark discovered here at FNAL in 1995. FNAL = Fermi National Accelerator Laboratory.

14 “Typical” Particle Detector

15 A bit more on quarks 6 different kinds of quarks.
Matter is composed mainly of up quarks and down quarks bound in the nuclei of atoms. The masses vary dramatically (from ~0.005 to 175 [GeV/c2]) Why is matter made only of up & down quarks ? This will be answered later! Mass [GeV/c2] Gold atom Silver atom Proton Recall that the unit [GeV/c2] is a unit of mass. E= m c2

16 Anti-particles too ! We also know that every particle has a corresponding antiparticle! That is, there are also 6 anti-quarks, they have opposite charge to the quarks. So, the full slate of quarks are: Quarks Q= +2/3 Q= -1/3 Particle Q= -2/3 Q= +1/3 Anti- Particle

17 Protons & Neutrons To make a proton: We bind 2 up quarks of Q = +2/3 and 1 down quark of Q = -1/3. The total charge is /3 + 2/3 + (-1/3) = +1 ! To make a neutron: We bind 2 down quarks of Q= -1/3 with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 !

18 Wow, I’m somebody… I’m a Baryon!
BARYONS The forces which hold the protons and neutrons together in the nucleus are VERY strong. Protons and neutrons are among a class of particles called “hadrons” (Greek for strong). Hadrons interact very strongly! Baryons are hadrons which contain 3 quarks (no anti-quarks). Anti-baryons are hadrons which contain 3 anti-quarks (no quarks). Wow, I’m somebody… I’m a Baryon! Me too, me too…

19 Are there baryons other than protons and neutrons?
Good question, my dear Watson… The answer is a resounding YES ! Other quarks can combine to form other baryons. For example: u s d This combination is called a Lambda baryon, or L0 for short What is the charge of this object? u This combination is called a Delta baryon, or D++ for short What’s this one’s charge?

20 Let’s make baryons! Quark up down strange Charge Q +2/3 -1/3 -1/3 Mass
~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] u u u d d d s s s Note: The neutron can be turned into a proton by simply replacing a “d” quark by a “u” quark! Proton Neutron u u d d u d In all cases, you will see that I have given the quarks inside baryons the colors red, green, and blue. This is because quarks also have an intrinsic “color charge”, or simply “color” for short. We will get into this in more detail later when we discuss the strong interaction. For now, assume that all baryons must have 1 RED, 1 GREEN and 1 BLUE quark. Taken together, the RED, GREEN, and BLUE produce an object which has no color (ie., it’s colorless). This is the same idea as the visible light be composed of the full spectrum of colors in the rainbow. Q = +1 M=938 MeV/c2 Q = 0 M=940 MeV/c2

21 Let’s make some more baryons !
u d s Q Mass +2/3 -1/3 Quark up down strange ~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] Lambda (L) Sigma (S+) Sigma (S0) Sigma (S-) u u u d d u d d s Here, Q means “the value of the electric charge” Note that the Lambda and Sigma_0 have the same quark content, but have different masses. How can this be? The answer is beyond the scope of this course. If you would like a deeper explanation, I encourage you to talk to the instructor. s s s Q = 0 M=1116 MeV/c2 Q = +1 M=1189 MeV/c2 Q = 0 M=1192 MeV/c2 Q = -1 M=1197 MeV/c2

22 Color I have been showing quarks as being either RED, GREEN or BLUE.
It turns out that quarks have a property called “COLOR”. This property is as intrinsic to the quarks as “electric charge”. This intrinsic property is not really a visible color, but it helps to visualize the property, which can have 3 values. We will discuss this in greater detail when we start talking about the strong force. Leptons do not have color.

23 Color (cont) For now, assume that quarks come in 3 colors:
RED, GREEN, BLUE Anti-quarks have “anti-color” ANTIRED, ANTIGREEN, ANTIBLUE Baryons always have 1 of each color. Antibaryons have one of each anti-color.

24 Spin There’s another intrinsic property which Quarks have known as “spin”. For convenience, you can think of it as a spinning top, but this is just a conceptual aid… Quarks have spin S = ½. Also, another class of particles, called leptons, also have spin. We’ll talk about leptons in the next lecture…

25 Mesons Mesons are also in the hadron family.
They are formed when a quark and an anti-quark “bind” together. Because they are hadrons, they must be colorless. So, the quark has color, and the antiquark has “anticolor” c d s d u d It is difficult to show here, but the two quarks inside mesons must have opposite color. The quark can be either RED, GREEN, or BLUE. Choose one.. If we choose GREEN, then the anti-quark’s color is “ANTI-GREEN”. How do we draw anti-green? Sorry, I’m not sure how, but try and keep in mind that the quark and antiquark in a a meson are one of (or some combination) of these colors combinations: Quark Antiquark RED ANTIRED BLUE ANTIBLUE GREEN + ANTIGREEN What’s the charge of this particle? What’s the charge of this particle? What’s the charge of this particle? Q=+1, and it’s called a p+ Q= -1, and this charm meson is called a D- Q= 0, this strange meson is called a K0

26 Summary Up & down quarks make up protons & neutrons
Quarks have an intrinsic property known as color, of which there are 3 varieties: red, green or blue. Quarks also have a property known as Spin, and have Spin = 1/2. Hadrons refer to strongly interacting particles: Baryons & Mesons Baryons contain 3 quarks: 1 red + 1 green + 1 blue  colorless They may have spin 1/2 or spin 3/2. Mesons contain 1 quark & 1 antiquark: rr, gg, or bb  colorless They may be spin 0, or spin 1

27 What IS Matter ? Matter is all the “stuff” around you!
Here’s the picture we’re going to uncover (not all today though) Matter Hadrons Baryons Mesons Quarks Anti-Quarks Leptons Forces Charged Neutrinos Gravity Strong Weak EM

28 The Quarks – a Recap Proton u d Quarks Antiquarks Q = +2/3 Q = -1/3
b Quarks can have 3 color values: red, green & blue Quarks have total spin S = ½ (SZ = -½ or +½) Anti-quarks have the same mass as their quark does. Hadrons = Baryons + Mesons Baryons (antibaryons) contain 3 quarks (3 antiquarks) Mesons contain a quark and an antiquark

29 Murray Gell-Mann 1969 Nobel Prize in Physics
Why quarks? Murray Gell-Mann Nobel Prize in Physics Why should nature be this complicated? To simplify the picture, and still account for this plethora of particles which were observed, Murray Gell-Mann proposed all these particles were composed of just 3 smaller constituents, called quarks.

30 As far as we can tell, electrons appear to be indivisible.
If neutrons & protons are not fundamental, what about electrons? Are they made up of smaller constituents also ? As far as we can tell, electrons appear to be indivisible.

31 Leptons Electrons belong to a general class of particles, called “Leptons” As far as we can tell, the leptons are “fundamental”. Each charged lepton has an uncharged partner called the “neutrino” The leptons behave quite differently than the quarks - They don’t form hadrons (no binding between leptons)

32 Are there other types of charged leptons (like the electron) ?
1932: Discovery of the positron, the “anti-particle” of the electron. Anti-particles really exist !!!!! 1937 – Muons (m- and m+ ) discovered in cosmic rays. M(m) ~ 200*M(e) The muon behaves very similarly to the electron (i.e., it’s a lepton). Cosmic Rays. Cosmic rays are energetic particles that are found in space and filter through our atmosphere. Cosmic rays have interested scientists for many different reasons. They come from all directions in space, and the origination of many of these cosmic rays is unknown. Cosmic rays were originally discovered because of the ionozation they produce in our atmosphere. In the past, we have often referred to cosmic rays as "galactic cosmic rays", because we did not know where they originated. Now scientists have determined that the sun discharges a significant amount of these high-energy particles. "Solar cosmic rays" (cosmic rays from the sun) originate in the sun's chromosphere. Most solar cosmic ray events correlate relatively well with solar flares. From: To learn more about the history of neutrino discovery, see

33 Neutrinos 1934: To account for the “unseen” momentum in the reaction (decay): n  p + e- + X n p e X Nobel Laureate: Enrico Fermi Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino (n ). (means “little neutral one”)

34 Lepton Picture continues…
1962: An experiment at Brookhaven National Lab showed that there were in fact at least 2 types of neutrinos. Family Leptons Antileptons Q = -1 Q = 0 Q = +1 1 e- ne e+ 2 m- nm m+ Q = electric charge

35 Three happy families… Family Leptons Antileptons Q = -1 Q = 0 Q = +1 1
In 1975, researchers at the Stanford Linear Accelerator discovered a third charged lepton, with a mass about 3500 times that of the electron. It was named the t-lepton. In 2000, first evidence of the t’s partner, the tau-neutrino (nt) was announced at Fermi National Accelerator Lab. Family Leptons Antileptons Q = -1 Q = 0 Q = +1 1 e- ne e+ 2 m- nm m+ 3 t- nt t+ 3 families, just like the quarks… interesting !!!

36 This all looks Greek to me ?
electron muon-minus tau-minus electron neutrino muon neutrino tau neutrino Lepton (particle) positron muon-plus tau-plus electron anti-neutrino muon anti-neutrino tau anti-neutrino Anti-lepton (anti-particle)

37 So here’s the big picture
Quarks and leptons are the most fundamental particles of nature that we know about. Up & down quarks and electrons are the constituents of ordinary matter. The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators. Each particle has a corresponding antiparticle. Quarks and leptons differ significantly in the way that they interact with each other. That is, the processes which can occur when two quarks collide is quite different from the situation when two leptons collide. This brings us to the concept of “force carriers”. Force carriers are the particles which transmit a force from one particle to another. We will get into this in greater detail…

38 The Standard Model of Particle Physics

39 Interactions and Decays
Forces are responsible for: Interactions Decays Decays are really a type of interaction though…

40 What do I mean by an Interaction?
3 4 4 3 1 2 4 3 1 2 BAM 4 3 2 1 1 2

41 What do I mean by a Decay ? 3 4 4 3 4 3 4 3 BAM 1 1 1 1

42 Conserved Quantities A + B  C + D
Consider the process where two particles A and B collide, and produce particles C and D: A + B  C + D By conserved, we simply mean that the value on the left hand side must equal the value on the right hand side. Some of the most important examples are: Total energy is ALWAYS conserved – energy cannot be created nor destroyed, only transformed Total momentum is ALWAYS conserved Total charge is ALWAYS conserved - cannot simply create net charge

43 Energy Conservation (I)
A + B  C + D Energy conservation means: Energy of particle “A” + Energy of particle “B” = Energy of particle “C” + Energy of particle “D” If you knew any 3 of the energies, you could compute the fourth!  So, in such a reaction, you only need to measure 3 particles, and energy conservation allows you to compute the fourth! BUT, it really is not quite that easy!

44 Energy Conservation (II)
A  B + C Energy conservation “really” means: Rest Mass and Kinetic Energy of particle “A” ≥ = Rest Mass and Kinetic Energy of particle “B” + Rest Mass and Kinetic Energy of particle “C” What does all that craziness mean??? Decays easy…interactions not so much

45 Energy Conservation (III)
So, energy conservation helps in 2 ways: 1. It allows you to predict the energy of particles which you do not, or cannot measure It may tell you that a process cannot occur, since energy conservation cannot be violated!

46 Momentum Conservation (I)
Momentum conservation is used just as frequently as energy conservation The classic scenario is the case of neutron decay: p n mP me e neutron at rest decays to a proton + electron

47 Momentum Conservation
neutron at rest appears to decay to a proton + electron p n mP me e Total Momentum Before Decay = Total Momentum after Decay P(neutron) = P(proton) + P(electron) This is precisely what lead to the conjecture that there must be an undetected particle, called the neutrino! The symbol most often used to represent the momentum is “P”. You will recall from the previous lecture on kinematic quantities that the momentum is defined as: P = m*v Where m=mass of the particle v = velocity Here we use the notation: P(neutron) to mean “the momentum of the neutron”. P(proton) means ““the momentum of the proton”. P(electron) means ““the momentum of the electron”. Since the neutron is initially at rest, it’s velocity, v=0. Therefore, the momentum must also be zero. p = mv = m*0 = 0 ! = P(proton) + P(electron) We don’t know what the value is, but we know it is NOT zero. In other words the proton and electrons momentum cannot cancel, because they are in the same direction!

48 Charge Conservation (I)
Again, let’s consider the process: A + B  C + D Charge conservation implies that: Charge of particle “A” + Charge of particle “B” = Charge of particle “C” + Charge of particle “D” So, if you know the charge of any 3 of the particles, you can immediately say what the charge of the 4th MUST BE!

49 Charge Conservation (II)
A + B  C + D A B C D -1 +1 ? -2 -1 +2 Determine what the missing charges must be in this table. +1

50 Other conserved quantities
Baryon Number Conservation When we collide particles together, we find that the number of baryons is conserved. A + B  C + D For each baryon, we simply assign B = +1 (protons, neutrons, for example) For each anti-baryon ,we assign B = -1 (antiprotons, antineutrons, for example) Compute the total baryon number on each side and they must be equal!

51 Baryon Number Conservation
A + B  C + D Assume the only particles we know about are: p, n, p, n, p+, p-, and p0. A B C D p- p ? n p+ p0 po π0 Determine what the missing particles must or can be in this table. p π0

52 Lepton Number Conservation (I)
Electron, Muon and Tau Lepton Number Lepton Conserved Quantity Lepton Number e- Le +1 ne m- Lm nm t- Lt nt Anti-Lepton Conserved Quantity Lepton Number e+ Le -1 ne m+ Lm nm t+ Lt nt We find that Le , Lm and Lt are each conserved quantities

53 Lepton Number Conservation (II)
Let’s look at how this works: p  m nm Lm -1 +1 m  e ne nm Lm Le -1 -1 +1 -1 The first two processes are not forbidden by lepton number conservation, while the third is. Note that we require that both electron number and muon number are conserved. If tau or tau neutrinos were involved, we would also check this as well. m  e g Lm Le -1 -1 X

54 Lepton & Baryon Number Conservation
More examples: g  e e- Le -1 +1 n  p e ne Le B +1 +1 +1 -1 The first two processes are not forbidden by lepton number conservation, while the third is. Note that we require that both electron number and muon number are conserved. If tau or tau neutrinos were involved, we would also check this as well. p  m ne Lm Le -1 -1 X

55 There is no such thing as “Meson Number CONSERVATION”

56 Summary of Conservation Laws
Conservation of Total Energy Conservation of Total Momentum Conservation of Baryon Number Conservation of Lepton Number These conservation laws always apply There are other conservation laws, but they only apply to certain forces.

57 What IS Matter ? Matter is all the “stuff” around you!
Here’s the picture we’re going to uncover (the last piece!) Matter Hadrons Baryons Mesons Quarks Anti-Quarks Leptons Forces Charged Neutrinos Gravity Strong Weak EM

58 The Four Fundamental Forces
Weaker Stronger Gravity Weak Force Electromagnetic force Strong Force All other forces you know about can be attributed to one of these!

59 Gravity Gravity is the weakest of the 4 forces. The gravitational force between two objects of masses m1 and m2, separated by a distance d is: F = Gm1m2/d2 G = gravitational constant = 6.7x10-11[N*m2/kg2] d = distance from center-to-center The units of each are: [Force] = [Newton] = [N] [mass] = [kg] [distance] = [meters] Gravity is only an attractive force

60 Electric Forces + - + + - - Opposites charges attract
Like charges repel + + - -

61 Strong Force The strong force is the strongest of the known forces.
It is responsible for the binding of quarks into baryons and mesons. Its residual effects also account for the binding of protons & neutrons in the nucleus. This force behaves more like a “spring”. That is, the the force actually gets stronger as quarks move apart! This in striking contrast to the EM & Grav. Force. Their forces decreases with separation (recall F a 1/d2)

62 Weak Force The weak force is the weakest of the known forces.
It is responsible for neutron decay, and decays of heavy quarks to the lighter quarks It’s interaction is very short range (as opposed to the long range interactions of the EM and gravitational force).

63 Forces Forces are the due to the exchange of force carriers.
For each fundamental force, there is a force carrier (or set of them). The force carriers only “talk-to” or “couple to” particles which carry the proper charge. Electromagnetic: the photon (g) Strong: the gluon (g) Weak: the W+, W- & Z0 Electric Charge (+, -) Color Charge (r,g,b) Weak Charge

64 Particles & Forces Y N N Y Y N Y Y Y quarks Charged leptons (e,m,t)
Neutral leptons (n) Color Charge ? Y N N EM Charge ? Y Y N Weak Charge ? Y Y Y Quarks can participate in Strong, EM & Weak Interactions ! All quarks & all leptons carry weak charge

65 In other words… Since quarks have color charge, EM charge & weak charge, they can engage in all 3 types of interactions ! Charged leptons (e,m,t) carry EM and weak charge, but no strong charge. Therefore, they can participate in the EM & weak interaction, but they cannot participate in the strong interaction. Neutrinos only carry weak charge, and therefore they only participate in the weak interaction  they can pass through the earth like it wasn’t even there !

66 Why should we believe that forces are the result of force carriers?
The Standard Model (SM) which I have described to you is just that, it’s a model, or better yet, a theory. All forces are described by exchange of force carriers, period ! It’s is an extremely successful theory. It explains all subatomic phenomenon to extraordinary precision! Don’t worry about what the electron’s g-factor is, just note the precision to which it is predicted by the theory, and how well it agrees with the experiment ! One example is in a quantity referred to as the electron’s “g-factor” “g” from experiment: “g” from theory (SM): They agree to better than 1 part in 10 billion ! Coincidence ?


Download ppt "Particle Physics True or False?"

Similar presentations

Ads by Google