Announcements

1
Announcements

• Recap of important material today Monday.
• Review sheet will be handed out on Monday (4/22)
• Video on Wednesday (4/24), with questions to
  answer.
• Friday (4/26) is a pure QA session.
• HW solutions due last Wednesday (4/17) will be
  postedtoday.
• Next Wednesdays HW will be posted that evening.
• HWs handed in late will be greatly reduced in
  credit, or not accepted.

2
Putting it all together
3
What IS Matter ?

• Matter is all the stuff around you !

• Heres the picture weve uncovered

Matter
Hadrons
Leptons
Forces
Baryons
Mesons
Charged
Neutrinos
Strong
Gravity
Weak
EM
Quarks Anti-Quarks
4
The Quarks

• Each quark has a corresponding antiquark.
• Antiquarks have opposite charge to their quark.
• Huge variation in the masses, from 5 MeV/c2 to
  175,000 MeV/c2.

5
The Leptons

• Each lepton has a corresponding anti-lepton.
• Antileptons have opposite charge to their
  lepton.
• Huge variation in the masses, from 0.5 MeV/c2
  to 1,780 MeV/c2.

6
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
  toparticles 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
7
Particles Forces
quarks
Charged leptons(e,m,t)
Neutral leptons(n)
ColorCharge ?
Y
N
N
EMCharge ?
N
Y
Y
WeakCharge ?
Y
Y
Y

• Quarks can participate in Strong, EM Weak
  Interactions !
• All quarks all leptons carry weak charge

8
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 onlyparticipate in the weak interaction ?
  they can pass through the earth like it wasnt
  even there !

9
Why should we believe that forces are the result
of force carriers?

• The Standard Model (SM) which I have describe to
  you is just that, its a model, or better
  yet, a theory.
• All forces are described by exchange of force
  carriers, period !
• Its is an extremely successful theory.
• It explains all subatomic phenomenon to
  extraordinary precision!

One example is in a quantity referred to as the
electrons g-factor
g from experiment 2.0023193043768 g
from theory (SM) 2.0023193043070
They agree to better than 1 part in 10 billion !
Coincidence ?
10
Particle or Wave?
Two benchmark experiments established the
foundation for theparticle nature of light
1. Photoelectric Effect2. Compton Effect
Both experiments indicated that light was acting
like a particlewith energy and momentum given by
Uses c ln
E hn hc / l
p E / c (hc / l) / c h / l

• This light particle has energy and momentum, but
  no mass !!!
• Its energy momentum are inversely
  proportional to the wavelength

11
Photoelectric Effect
Classical Method
Increase energy by increasing amplitude
electrons emitted ?
No electrons were emitted until the frequency of
the light exceeded a critical frequency, at
which point electrons were emitted from the
surface! (Recall small l ? large n)
12
The Electromagnetic Spectrum
Shortest wavelengths (Most energetic photons)
E hn hc/l
h 6.6x10-34 Jsec(Plancks constant)
Longest wavelengths (Least energetic photons)
13
The EM force and the Photon

• The photon is the carrier of the EM force.
• It can only talk-to particles which have
  electric charge.
• A photon does NOT have electric charge, and
  therefore it cannot interact with other
  photons
• While the photon is massless, it does carry both
  momentum energy given by p h / l
  E pc hc / l hn
• When charged particles exchange photons, they
  are exchangingthis momentum. One particle emits
  the photon the other absorbs it !
• Can also have particle-antiparticle annihilation
  into a photon.

14
Electromagnetic Force
Quark PairProduction
Detectablehadrons,such asp, p-, p0,p, n, etc
q
e
g
e-
15
Electron Proton Collision !
u
Proton
u
d
16
Actual ee- Collision at Cornells Collider
E 5 GeV forthe e and e-
Hadrons which are chargedand are bentby a
magneticfield
Eventis notbalanced
Side view ofDetector
n ?
Probably a n in this interaction
17
Another e e- Collision at CERN
E 103 GeV forthe e and e-
LOTSMOREHADRONS !!!
18
How much energy is needed to produce a t t pair
via an ee- Collision ?
t
e
Me 0.5 MeV/c2
Mt 175 GeV/c2
e-
What minimum energy is needed by each incoming
particle to produce the top and antitop quark?
A) 175 MeV B) 350 GeV
C) 175 GeV D) 350 MeV
19
What maximum mass particle canbe produced?
particle
q
Ee 115 GeV (each)
antiparticle
What maximum mass particle can be created in this
collision ? A) 115 MeV/c2 B) 230
GeV/c2 C) 115 GeV/c2 D) 230
MeV/c2
20
Strong Force and the Gluon

• The gluon is the carrier of the strong force.
• Unlike the EM force, it gets stronger as quarks
  separate !
• It can only talk-to particles which have color
  charge (quarks).
• Since gluons do have color charge, they can
  interact with other gluons !
• The gluon is also massless.
• When quarks exchange gluons, they are exchanging
  color charge. One quark emits the gluon the
  other absorbs it !
• Quarks and antiquarks can annihilate into a
  gluon!

21
Color (or Color charge)

• Like electric charge, quarks have an internal
  property which allows gluons to interact with
  them (i.e., couple to them).
• This property is called color. Quarks can have
  one of three colorsred, green, or blue.
• Antiquarks have anticolor antired, antigreen
  or antiblue.
• Gluons also carry color (r b, b g, g r, etc
  ), and therefore caninteract among themselves
  !!! This is the most striking differencebetween
  gluons photons!
• FYI, it is the fact that gluons have color which
  leads to confinement

22
Hadrons

• Because of the strong force, quarks are bound
  into hadrons.
• Hadrons are simply particles which interact via
  the strong force.
• Our inability to directly observe the color of
  hadrons have lead us to believe that all hadrons
  are colorless
• There are two types of hadrons Baryons
  bound state of any 3 quarks (except the top
  quark) ( 1 red 1 green 1 blue
  colorless ) Mesons bound state of a quark
  and antiquark (except t) one
  color one anticolor ( r r, g g, or b b )
  colorless )
• Antibaryons contain 3 antiquarks

23
Proton-Proton Collision
The up downquarks have exchangeda gluon, and
henceunderwent an interaction!
24
Quark-Quark Interaction
tohadrons
tohadrons
u
u
g
d
d
tohadrons
tohadrons
25
Quark-Gluon Interaction
tohadrons
tohadrons
g
g
g
tohadrons
d
d
tohadrons
26
Hadronization
In this way, you can see that quarksare always
confined inside hadrons (thats CONFINEMENT) !
27
p p ? t t from Fermilab
Jet sprayof particleswhen a
quarkundergoeshadronization4 jets ? 4
quarksemerging from the interaction.
28
Putting it all together
29
The Carriers of the Weak Force

• Three force carriers for the weak force W, W-
  and Z0
• The W and W- are the ones I have emphasized,
  and their rolein the decay of heavy quarks to
  lighter quarks.
• The W and W- carry both electrical and weak
  charge.
• They connect the 2/3 charge quarks with the
  1/3 chargequarks (a change in charge of 1
  unit).
• These range of the weak force is very short !!
  Its about 10-18 m,which is about 10,000 times
  smaller than the range of the strong force

30
Particles Forces
Quarks carry strong, weak EM charge !!!!!
31
Weak Force

• They W and Z particles can onlytalk-to
  particles which have weak charge (the leptons
  and the quarks !).
• Heavy quark decay to lighter quarks via emission
  of a W or W-.
• The weak force is also responsible for neutron
  decay.
• Because the weak force is sooo weak, neutrinos
  can passthrough matter (like the earth) as if it
  wasnt there !
• Quarks and leptons can interact by exchanging a
  W or Zforce carrier

32
Neutron Decay (cont)
33
What about the decay of a b-quark?b ? c m- nm
34
b-quark decay at the hadron level
Decay of a B- Meson
Could end up asB- ? D0 p-B- ? D0 p-p0B- ?
D0 p- p p-etc
to hadrons
B-
D0

• Additional particles are created when the strong
  force produces morequark-antiquark pairs. They
  then combine to form hadrons!
• Notice that the charge of the particles other
  than the D0 add up to the charge of the W- (Q
  -1), as they must!

35
Hadronization Producing hadrons!
In this way, you can see that quarksare always
confined inside hadrons (thats CONFINEMENT) !
36
Conservation Laws

• Conservation of Total Energy
• Conservation of Total Momentum
• Conservation of Electric Charge
• Conservation of Baryon Number
• Conservation of Lepton Number (Le, Lm, and Lt)

37
Energy Conservation (I)
A B ? C 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, andenergy
conservation allows you to compute the fourth!
38
Energy Conservation (II)
Decay Process A ? B D
If particle A has non-zero mass (mA gt 0), then
mB lt mA
mD lt mA
This is a consequence of energy conservation (see
lecture 24) !
This canthappen ifMBgtMA, orMDgtMA
39
Interaction Conversion of KE to Mass
Notice that the total mass of the particles after
the interaction islarger than the incoming
masses (2 proton masses) !This is OK, as long
as the incoming protons have enough
kineticenergy to produce all these particles
40
Then why cant this happen in decays?
41
Momentum Conservation (I)
(I) If this electron positron have equal
opposite velocity, whatcan be said about their
total momentum? A) Its twice as large
B) Its zero C) Its negative D) Its
positive
(II) What can be said about the total momentum of
all the particles which are produced in this
collision at top? A) Its positive B)
Its negative C) Its same as in (I) D)
Its 0.5 MeV/c
42
Momentum Conservation (II)
neutron at rest appears todecay to a proton
electron
p
n
mP
me
e
Since both the electron and proton are both
moving off to theright, their total momentum
cannot be zero.
? In other words, this reaction, as shown cannot
occur, since it would violate momentum
conservation.
43
Charge Conservation
Total charge on left has to equal total charge on
right in order forcharge to be conserved! The
process could still be forbidden to occur if it
violates some other conservation law !
44
Baryon Number Conservation
Rules of the game For each baryon,
assign B 1 For each antibaryon, assign B
-1Compare total Baryon number on left side to
right side
X
X
X
n ? p0 p0
X
X
45
Why cant the proton decay ?
p ? ? ? ?
Since baryon number must be conserved , there
MUST BE abaryon among the ? decay
products.But, the proton is the lowest mass
baryon (938 MeV/c2).So there is nowhere it
can go ! It CANNOT decay into something heavier,
as this would violateenergy conservation !
p ? n e ne
938 MeV/c2
46
Lepton Number Conservation
A B ? C D
At its heart, its just Total lepton number
on LHS Total lepton number on RHS All
leptons get assigned L 1 All
antileptons get assigned L -1But, its
more powerful than that ! It can be applied for
electron-type, muon-type and
tau-typeobjects separately!
47
Example
g ? e m-
Photon Conversion
Lepton
Antilepton

• If we dont distinguish between electron-type
  and muon-typeobjects, we would conclude that
  this process can occur, sincewe have a lepton
  and anti-lepton on the RHS !
• If we require both Le and Lm conserved
  separately, we see thatthis process violates
  both ? cannot occur ! And, in fact this process
  is never observed

g ? e m-
48
Example II
Energy(Checkmass)
940 MeV/c2
Charge
0
BaryonNumber
-1
Le
0
Lm
0
49
The Big Bang !Everything that could have
possibly existed, did exist !
And ???
50
Matter today

• Today, the universe is a relatively cold place
  (rememberthe 3o microwave background thats
  270 oC)
• Nearly all heavy quarks have decayed through the
  weak interaction into up down quarks. t ? b ?
  c ? s ? u ? d
• The up down quarks which are the lightest of
  the quarks are thelightest, and have combined to
  form protons neutrons
• The protons and neutrons have combined with
  electrons to formour atomic elementsand hence,
  US !
• The heavy quarks are produced in cosmic rays or
  at large acceleratorlaboratories, like Fermilab..

51
Space is mostly empty space
52
Atoms are gt 99.999 empty space
Electron
g
Nucleus
53
Protons Neutrons are gt 99.999 empty space
g
u
Proton
u
d
The quarksmake up a negligiblefraction of the
protons volume !!
54
The Universe
The universe and all the matter in it is almost
allempty space !(YIKES)
55
So why does matter appear tobe so rigid ?
Forces, forces, forces !!!! It is primarily the
strong and electromagnetic forces which give
matter its solid structure. Strong force ?
defines nuclear size Electromagnetic force ?
defines atomic sizes
56
So why is this stuff interesting/important?

• All matter, including us, takes on its shape and
  structurebecause of the way that quarks, leptons
  and force carriersbehave.
• Our bodies, and the whole universe is almost all
  empty space !
• By studying these particles and forces, were
  trying to getat the question which has plagued
  humans for millenia How did the universe
  start? And how did we emerge from it all?