Particle Physics
 Literature:
 “Introduction to Elementary Particles” D. Griffiths
 “Quarks & Leptons” F. Halzen & A. Martin
 “Quarks, Leptons & Gauge Fields” K. Huang
 “Collider Physics” V. D. barger & R. J. N. Phillips
 “Introduction to High Energy Physics” D.H. Perkins
 “The Review of Particle Physics” http://pdg.lbl.gov
2008 1st semester
Physics Department, Hanyang Univ.
ByungGu Cheon
Ch. 1. Historical Introduction to
the Elementary Particles
Mendeleev: periodic system of elements
Chaos
 order
 better understanding
 predictions (new elements)
 new insights
1.1 The Classical Era (1897-1932)
Thompson (1897): electron
E
BE
E,B0  v=Ec/B
B0  R=vmc/qB
No deflection in EB configuration:




Ec
 
 v 

0  F  q E  B  v  
B
c
 Measured q/m much larger than for
 (with electron charge) me31026 g
Circle with radius R with only B0:
mc
q vc
R v
 
qB
m RB
1H-atom
“Plum”-model
of the atom
atom
Joseph Thomson (1856-1940)
Nobel Prize 1906
In recognition of the great
merits of his theoretical and
experimental investigations
on the conduction of
electricity by gases
1909  13: Rutherford’s scattering experiments
Discovery of the atomic nucleus
observation:
unexpected a few number of alpha particles deflected
over large angles!  all positive charge at center!
Atom consists of a positively charged nucleus surrounded
by a cloud of electrons
R+<10-12 cm “Solar system”-model
nucleus
of the atom
note:
compare shooting bullets at bag of sand

Cross-section of Rutherford’s Scattering
Note : b( ) 
Z
1
2E tan / 2
density ; velocity v
flux: v [#/cm2/s]
Effective transverse cross-sectional area = 2bb
2
2


N
1
Z   cos
1
 Z 

  v 2 
 v 2bb  v 2 
 


t
 2 E  tan  / 2   tan  / 2  
 2 E  4sin 4 / 2 
Z 
1
 d  
d  2 E  4sin 4 / 2 
2
Earnest Rutherford (1871-1937)
Nobel Prize 1908 (Chemistry!)
For his investigations into the disintegration
of the elements and the chemistry of
radioactive substances
He discovered alpha and beta rays, set forth the laws of
radioactive decay, and identified alpha particles as helium nuclei.
Atomic Model:
Atom consists of a positively charged nucleus
surrounded by a cloud of electrons
Nuclear radius  1013 cm  105 x atomic radius
Mass of the nucleus  mass of the atom
Bohr (1914): energy levels in atoms
Experiment showed emission (absorption) of specific, element dependent, wavelengths!
Example:
Balmer series in hydrogen
 1
1 
1

 RH  2  2 
n  3,4,5,...



n 
2
410
434
486
Discreteness of energy levels hard to
reconcile with the classical atomic model
Bohr’s Atomic model
v
p+
e
r
Hydrogen: 1 proton with 1 electron
Electron angular momentum quantized!
Discrete lines: transitions between states
1
L  mvr  nh  E  2
n
n
656 nm
Niels Bohr (1885-1962)
Nobel prize 1922
For his services in the
investigation of the structure
of atoms and of the radiation
emanating from them"
First (wrong) ideas about nuclear structure
(before 1932)
Observations
 Mass values of light nuclei  multiples of proton mass (to few %)
(proton  nucleus of the hydrogen atom)
 b decay: spontaneous emission of electrons by some radioactive nuclei
Hypothesis: the atomic nucleus is a system of protons and electrons
strongly bound together
Nucleus of the atom with atomic number Z and mass number A:
a bound system of A protons and (A – Z) electrons
Total electric charge of the nucleus = [A – (A – Z)]e = Z e
Problem with this model: the “Nitrogen anomaly”
Spin of the Nitrogen nucleus = 1
Spin: intrinsic angular momentum of a particle (or system of particles)
In Quantum Mechanics only integer or half-integer multiples of ħ  (h  2)
are possible:
 integer values for orbital angular momentum
 both integer and half-integer values for spin
Electron, proton spin = ½ħ (measured)
Nitrogen nucleus (A = 14, Z = 7): 14 protons + 7 electrons = 21 spin ½ particles
TOTAL SPIN MUST HAVE HALF-INTEGER VALUE Measured spin = 1
Chadwick (1932): the neutron discovery
Chadwick’s experiments: a 210Po radioactive source
(5 MeV  – particles ) mixed with Beryllium powder  emission of electrically
neutral radiation capable of traversing several centimetres of Pb:
4 He + 9 Be  12 C + 1 neutrons
2
4
6
1
2
2 1
2
2
2
2
2 1

m
v

m
c
m
v

m
c

m
c
Energy Conservation :
N N
N  mnvn  mnc
 

B
2
2
2
m v  mnvn  mN vN
Momentum Conservation:
mn=938  1.8 MeV
Chadwick postulated the existing of a neutral particle inside
the atomic nucleus: neutron!
James Chadwick (1891 – 1974)
Nobel Prize 1935
For the discovery of the neutron
After Chadwick discovered the neutron, the proton, electron and
neutron accounted for all the atoms of all the elements in the Universe.
Thompson
Rutherford
Chadwick/Bohr
e-
1H
atom
nucleus
p+
nucleus
e-
4He
“Plum”-model
of the atom
14N
“Solar system”-model
of the atom
nucleus:
14 protons + 7 electrons
experiment: spin 1
spin ½
“modern”-model
of the atomic nucleus
nucleus:
7 neutrons
7 protons + 7 electrons
experiment: spin 1
2 p+
2n
14N
spin 1
1.2 The photon (1900-1924) as a particle
Einstein/Millikan
• Photoelectric effect:
observation:
electron emission stops abruptly as soon as
wavelength exceeds a certain (material
dependent) value.
explanation: Ee  h-W
• Compton Scattering
observation:
deflected photon wavelength shifted from incident photon
wavelength according to: f= i + (1-cos) h/mc
•Blackbody radiation spectrum
Planck
Raleigh
-Jeans
Planck:
 ( , T ) 
8
4
kT
Raleigh - Jeans 

tot
   (  ,T ) d 
1
 (,T )  8hc

5 exp hc

KT  1

lim  ( ,T )  0 lim  ( ,T )  Planck  RJ
 0
 
0
Max Planck (1858-1947)
Nobel prize 1918
In recognition of the services
he rendered to the
advancement of Physics by his
discovery of energy quanta
E  nh
In 1916 Millikan stated on the poto-electric effect:
“Einstein’s photo electric equation … appears in every case to predict exactly the
observed results…. Yet the semi-corpuscular theory by which Einstein arrived at
this equation seems at present wholly untenable”
Albert Einstein (1879-1955)
Nobel prize 1921
For his services to Theoretical
Physics, and especially for his
discovery of the law of the
photoelectric effect
Robert Andres Millikan (1868-1953)
Nobel price 1923
For his work on the
elementary charge of
electricity and on the
photo-electric effect
Arthur Holly Compton (1892-1962)
Charles Thomson Rees Wilson (1969-1959)
Nobel prize 1927
"for his discovery of
the effect named
after him"
"for his method of making
the paths of electrically
charged particles visible
by condensation of
vapour"





-


1.4 Anti-Particles (1930-1956)
1927: Dirac equation with two energy solutions:
2
2 2
2 4
E p c m c
E   p 2c 2  m 2c 4
E   p 2c 2  m 2c 4
How do you avoid that all particles tumble into the negative energy levels?
Simple: assume that all negative energy levels are
filled (possible thanks to Pauli exclusion principle!)
E=0
-
Excitation of an electron with negative energy to
one with positive energy yields:
- a real electron with positive energy
- “hole” in the sea i.e. presence of a + charge
with positive energy!
1940-1950: Feynman Stuckelberg interpretation: negative energy
solutions correspond to positive energy solutions of an
other particle: the anti-particle!
e
n
p



e
n
p
Erwin Schrodinger (1887 – 1961)
Paul Dirac (1902 – 1984)
Nobel Prize 1933
For the discovery of
new productive forms
of atomic theory
P. Dirac
E. Schrödinger
m v 2 p2
E

2
2m
Non-relativistic kinetic energy:
Relativistic kinetic energy (Einstein):
E 2  m 2c 4  p 2c 2
E   m 2c 4  p 2c 2
“negative” solution is related to the existence of antimatter
C. D. Anderson (1905 – 1991) : positron
Nobel Prize 1936
For his discovery of the positron
Co-winner: V. F. Hess
For his discovery of cosmic rays
Photon conversions e+ e in a bubble chamber
C. Anderson (1905-1991)
e+
e-
V. F. Hess (1883-1964)
Antiproton were identified in 1955 by
Emilio Segre and Owen Chamberlain
Nobel Prize 1959
For their discovery of the antiproton
O. Chamberlain
E. Segre
Proton Synchrotron
at U.C. Berkeley
p
Copper
target
p  6.3 GeV
p
p p  p p p p
p
p
p
p
Sin-Itiro Tomonaga (1906 – 1979)
Julian S. Schwinger
(1918 – 1994)
Richard Feynman (1918 – 1988)
Nobel prize 1965
For their fundamental work in quantum
electrodynamics, with deepploughing consequences for the
physics of elementary particles
Quantum-Electro-Dynamics (QED)
Tomonaga
Schwinger
Feynman
Discovery of muon ()

Phys. Rev. 51 (1937) 884.
In 1937, Discovery of muon () S. Neddermeyer, C. Anderson
 penetrating cosmic ray tracks with unit charge but mass in between
electron and proton
 muons were proven not to have any nuclear interactions and to be just
heavier versions of electrons

 decays to electron and two invisible neutrinos via weak interactions
(b decay): - →  e- e
 first encounter of the generation problem
Particle
Electric charge
(x 1.6 10-19 C)
e
1

1
p
n
1
0

0

Mass
(GeV=x 1.86 10-27 kg)
0.0005
0.106
0.938
0.940
0
70 years later we still don’t have a good answer
I.I Rabi,
Nobel 1944
Discovery of pion meson ()


Yukawa Hideki
Prediction of pion existence :Yukawa 1935
 Nucleons (protons and neutrons) are held together by stronger
force than electrostatic repulsion of protons
 In 1935 Yukawa predicted existence of a mediator of the strong
interactions. Estimated its mass to be around 0.1 GeV.
 Nobel Prize in 1949
Discovery of pions :Cecil Powell 1947 (Nature 159 (1947) 186.)
 detected in cosmic rays captured in photographic emulsion
 Unlike muons they do interact with nuclei
 charged pions eventually decay to muons:  → 
 view of the particle world seemed complete for entire two months...
 Nobel Prize in 1950
Particle
Electric charge
(x 1.6 10-19 C)
C.F. Powell
Mass
(GeV=x 1.86 10-27 kg)
e
1

1

0
p
0.938
n
1
0

110
0.14
0.0005
0.106
}
Leptons: no strong interactions
}
Hadrons: feel strong interactions
0
0.940
Neutrinos ()
existence of the neutrino postulated by Pauli (1930)


n  p  e  e
not this
but this
 # events
 # events
n pe
mn-mp-me  17 keV
 mn2  m 2p  me2  2
c
Ee  


2mn
 Ee


 Ee
experiment to demonstrate neutrino’s existence
was done by Clyde Cowan & Frederick Reines from 1953-1956
e pne
followed by


e e  

n-capture

n
e
e+

e+ e
annihilation

1947, Discovery of strange meson (kaon) Rochester, Butler



using a cloud chamber saw something unusual. Two tracks appeared
from a single point beneath a lead plate, as if from nowhere.
cosmic ray particles with masses in between pions and protons
which were just like pions except for strangely long lifetime (decay
to pions or a muon and neutrino)
Mass ~ 0.5 GeV (Nature 160 (1947) 855)
Butler
K0
→+
-
0 →p+ -
Rochester
Production of particles with a very long lifetime!
Typically in pairs  production mechanism  decay mechanism
(strong interaction) (weak interaction)
The peculiar properties led to the new quantum number, strangeness.

In 1950’s, Discovery of entire particle Zoo

thanks to the rapid progress in particle accelerator technology
• new particles either pion-like (mesons) or proton-like (baryons)
• either type can be strange or non-strange
• mesons and baryons (hadrons) feel strong interactions contrary to leptons (e,,)

Periodic tables of particles
S - Strangeness
Q - Electric Charge
Q= -1
S=+1
S= 0 
S= -1 K
Q= 0
K0
0 
Q=+1
K

K0
Q= -1 Q= 0 Q=+1
n
p
S= 0
S= -1 
0  
S= -2 
0
S=
S=
S=
S=
0
-1
-2
-3
Q=-1




Spin 0 Meson Octet
Q= 0
0
0
0
Q=+1


Spin 1/2 Baryon Octet
Q=+2

Spin 3/2 Baryon Decuplet
Quark model of hadrons : Gell-Man

Nobel prize 1969: Murray Gell-Mann
For his fundamental contributions to our
knowledge of mesons and baryons and
their interactions
Also for having developed new algebraic
methods which have led to a far-reaching
classification of these particles according
to their symmetry properties. The methods
introduced are among the most powerful
tools for further research in particle
physics.









0

0


1232 MeV

1385 MeV


ddd ddu
sdd sud

1533 MeV


1680 MeV
duu
ssd
sss
ssu
suu
uuu
Fundamental particles:
u-, d- & s-quarks!
THE QUARK MODEL
1964 (Gell-Mann, Zweig): Hadron classification into
“families”; observation that all hadrons could be
built from three spin ½ “building blocks” (named
“quarks” by Gell-Mann):
Gell-Mann
G. Zweig
s
S=1
Q=-1/3
Q=2/3
d
u
S=0
u
d
Q=-2/3
s
S=-1
Q=-1/3
Eightfold Way
Baryon Octect: three quarks bound together
33 3=(6 3) 3= 6  3 +3 3= 8+10+8+1
proton  uud ; neutron  udd
p
n
S=0
  suu ; 0  sud ;   sdd
S=-1
Σ-
S=-2
Σ+
Σ0 ; Λ
Q=1
Ξ-
Ξ0
Q=-1
Q=0
0  ssu ;   ssd
Meson Octet: quark – antiquark pairs
33=8+1
Examples of non-strange mesons:
  ud ;   u d ; 0  (dd  uu ) / 2
Examples of strange mesons:
S=1
K   su ; K 0  sd ; K   s u ; K 0  s d
K0
K+
3
π-
S= 0
S= 1
π+
π0 ; η
Q=1
K-
K0
Q=0
Prediction and discovery of the – particle
A success of the static quark model
3
The “decuplet” of spin 2 baryons
Mass (MeVc 2 )
Strangeness
0
–1
–2
–3
N*++
uuu
N*+
uud
*+
suu
N*–
ddd
N*°
udd
*–
sdd
*°
sud
*–
ssd
*°
ssu
–
sss
1232
1384
1533
1672 (predicted)
–: the bound state of three s – quarks with the lowest mass
with total angular momentum = 3 2 
Pauli’s exclusion principle requires that the three quarks
cannot be identical
The first – event (observed in the 2 m liquid hydrogen bubble chamber at BNL
using a 5 GeV/c K– beam from the 30 GeV AGS, 1964)
V.E. Barnes et al. PRL 12 (1964) 204.
Chain of events in the picture:
K– + p   – + K+ + K°
(strangeness conserving)
 –  ° +  –
(S = 1 weak decay)
°  ° + 
(S = 1 weak decay)
 – +p
(S = 1 weak decay)
°   +  (electromagnetic decay)
with both  – rays converting to an e+e – in liquid hydrogen
(very lucky event, because the mean free path for   e+e – in liquid hydrogen is ~10 cm)
– mass measured from this event = 1686 ± 12 MeVc2
In 1975, Discovery of Tau Lepton ()
• In 1973, at the electron-positron storage ring SPEAR was
installed to search for the reaction mechanism for production
of new leptons viz., e- + e+ => X+ + X-.



Experiment at the Stanford Linear Accelerator Center in 1975 by M. Perl et al using the Stanford
positron-electron asymmetric ring (SPEAR).
Centre of mass energies of order 4GeV
24 events out of 35,000 interaction events: (Phys. Rev. Lett. 35, 1489 (1975))
m~1.8 GeV
Martin Perl (1927)
Frederick Reines (1918 – 1998)
(Cowan had died)
Nobel Prize 1995
For pioneering experimental
contributions to lepton physics:
for the discovery of the tau lepton
for the detection of the neutrino
Lepton Family
1962: Experiment shows that there exists something like “conservation of lepton number”
Particles count as “+1”
Anti-particles count as “1”
Lepton lepton # electron# muon #
e
1
1
0
e
1
1
0
1
0
1


1
0
1
()
()
 e  n p  e 
 e  n p  e 
  e 
Yes
No
No
   p     n Yes
  p e   n

No
Lepton lepton # electron# muon #
e
1
1
0
e
1
1
0
1
0
1


1
0
1
()
()
Later:
We will see that these particles can be
organized in doublets; much alike e.g.
the electron spin states:
Spin-up: 
Spin-down: 
Lederman, Schwartz, Steinberger
And many many more particles ………
Leon M. Lederman (1922)
Melvin Schwartz (1932)
Jack Steinberger (1921)
Nobel Prize 1988
For the neutrino beam method
and the demonstration of the
doublet structure of the leptons
through the discovery of the
muon neutrino
In 1974, Discovery of Charm Quark
• S. C. C. Ting et al.:
p Be → J (→e+ e-) X at Brookhaven
AGS proton synchrotron
Phys. Rev. Lett. 33 (1974) 1404
m =3.1 GeV
• B. Richter et al.: e+ e- → ψ(1s) →e+ e- at SLAC
SPEAR e+ e- collider with Mark-I detector
m= 3.105  0.003 GeV
Phys Rev. Lett 33 (1974) 1406
mc~1.5 GeV
Charmed particles (1974)
SLAC:
excess events @ s  3.1 GeV
ee  hadrons
Brookhaven:
excess events @ Mee  3.1 GeV
p+Be  ee
Burt Richter
Sam Ting
interpretation:
new quark: ee  cc  hadrons
interpretation:
new bound state: cc  ee
Burton Richter (1931)
Samuel Ting (1936)
Nobel Prize 1976
For their pioneering work
in the discovery of a heavy
elementary particle of
a new kind
quark baryon # u / d # c / s #
1
u
3
1
0
( d)
c
( s)
3
1
0
3
1
0
1
3
0
1
1
1
Later:
We will see that these particles can be
organized in doublets; much alike e.g.
the electron spin states:
Spin-up: 
Spin-down: 
In 1977, Discovery of Bottom quark ( Fermilab)
Leon Lederman : Upsilon (Y(1s) →+-)
"Observation of a Dimuon Resonance at 9.5 GeV in 400 GeV ProtonNucleus Collisions," Physical Review Letters 39, p. 252, (1977).
p+ nucleus (target) →Υ(1s)X → +- X
mb~4.7 GeV
1995, Discovery of Top quark (Fermilab)
• The experiment was carried out at Fermi National Accelerator Laboratory's Tevatron, by the
CDF and the D0 collaborations.
• The CDF found 37 top candidate events as against an expected background of 12 events. The
D0 collaboration found 17 top candidate events and estimated a background of about 4 events
• CDF reports a mass of 176 GeV (statistical uncertainty of 8 GeV and systematic uncertainty
of 10 GeV). D0 reports 199 GeV (statistical uncertainty of 20 GeV and systematic uncertainty
of 22 GeV)
Double b-tagged dilepton event @ CDF
CDF
D0
The t-quark: Tevatron collider
pp  Xtt
tt  Wb Wb
W  e  e or    (clean)
W  qq (difficult )
What are the fundamental building blocks?
The fundamental particles are split up into two families, namely the quarks and the
leptons. Both of these families consist of six particles, split into three generations,
with the first generation being the lightest, and the third the heaviest.
Summary of Matter Particles
Present Atomic Model
• Particle Physicists study the fundamental
particles that make up all of matter, and
how they interact with each other.
What holds it together?
There are four fundamental interactions between fundamental
particles, and all forces in the world can be attributed to these
four interactions!
What holds it together?
There are four fundamental interactions between fundamental
particles, and all forces in the world can be attributed to these
four interactions!
What's the difference
between a force and an
interaction?
The 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.
How do matter particles interact?
Magnetic force
Electric Forcr
All interactions which affect matter particles are due to an
exchange of force carrier particles, a different type of particle
altogether.
Electromagnetic Interaction
 The electromagnetic interaction acts between all particles
that have electric charge. It is attractive for oppositely charged
particles, and repulsive for particles of the same charge.
 The force carrier particle of the electromagnetic interaction is
the photon ().
So the electromagnetic interaction is what allows atoms to bond and
form molecules, allowing the world to stay together and create the
matter you interact with all of the time.
Strong Interaction
 The nucleus of an atom consists of a bunch of protons and
neutrons crammed together.
 We cannot account
for the nucleus staying
together with just
electromagnetic force.
 In addition to electric charge, quarks also contain something
called “colour charge”. The force between colour charged particles
is very powerful, thus it is called the "strong interaction".
• The strong interaction actually acts between
quarks, and it's the residual strong force that
causes nucleons to attract.
• The force carrier of strong interaction is the
gluons.
While quarks have color charge, composite particles made out of
quarks have no net color charge (they are color neutral).
What is the Color Charge ??
• Quarks and gluons are color-charged particles.
• Just as electrically-charged particles interact by exchanging photons
in electromagnetic interactions, color-charged particles exchange
gluons in strong interactions.
•When two quarks are close to one another, they
exchange gluons and create a very strong color
gluon
force field that binds the quarks together. The force
field gets stronger as the quarks get further apart.
• Quarks constantly change their color charges as
they exchange gluons with other quarks.
Quark Confinement
 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.
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.
 If one of the quarks in a given hadron is
pulled away from its neighbors, the colorforce field "stretches" between that
quark and its neighbors.
At some point, it is energetically cheaper
for the color-force field to "snap" into a
new quark-antiquark pair.
In 1979, Discovery of Gluon
 The TASSO experiment at the PETRA of the Deutsches Elektronen-Synchrotron
(DESY) shows three jets of particles produced in an electron-positron collision at
s= 27.4 GeV.
 Similar three-jet event topologies were announced by JADE, MARK J and
PLUTO, the other groups working at PETRA.
Three Jet Events in TASSO Collaboration
JADE Collaboration 1980 Phys. Lett.B91 142., MARK J Collaboration 1979 Phys. Rev. Lett.43 830.
PLUTO Collaboration 1979 Phys. Lett.B86 418., TASSO Collaboration 1979 Phys. Lett.B86 243.
Gluon discovery
q
e+
q
e-
q
e+
q
g
e-
The Weak Interaction
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 only matter around us that is stable is made up of the smallest
quarks and leptons, which cannot decay any further.
Neutron decay
The force carrier particles of the weak
interactions are the W+, W-, and the Z particles.
The W's are electrically charged and the Z is
neutral.
1983, Discovery of mediators of weak interaction (W±, Z0)
• The W and Z particles were first observed at CERN by the
UA1 and UA2 experiments.
• Both proton and antiproton were accelerated to 270 GeV and
brought into collision in two interaction regions at √s = 540 GeV.
• In April/May 1983 came the next run with 118 nb-1 of integrated
luminosity for UA1. This gave an additional sample of 54 W →
eν events, giving Mw = 80.3 + 0.4-1.3 GeV
• In UA1, four Z → e+e- events with no visible experimental
background in 55 nb-1 of data were observed. The first mass
determination gave Mz = 95.5 ± 2.5 GeV
C. Rubia and van der Meer
W decay to e  in UA1
Z decay to e+e- in UA1
e

e
e
The W and Z bosons: SppS collider
pp  WX with W  e e or W    
 
pp  ZX with Z  e e or Z   
Carlo Rubbia (1934)
Simon van der Meer (1925)
Nobel Prize 1984
For their decisive contributions
to the large project, which
led to the discovery of
the field particles W and Z,
communicators of weak
interaction
Sheldon Lee Glashow (1932)
Abdus Salam
(1926 – 1996)
Steven Weinberg
(1933)
Nobel Prize 1979
For their contributions to
the theory of the unified
weak and electromagnetic
interaction between
elementary particles,
including the prediction
of the weak neutral
current
Gerardus 't Hooft (1946)
Martinus Veltman (1931)
Nobel Prize 1999
For elucidating
the quantum structure of
electroweak interactions
in physics
The Gravity
 Gravity acts between all particles that have mass. Mass will attract
other mass with a force that gets weaker as the distance between them
gets larger.
 Gravity is responsible for the large scale structure of the universe.
Here's a pretty picture of a galaxy, which, of course, is held together by
gravity.
 Although gravity appears to be
a very powerful force, when it
comes to things on smaller scales,
like tiny particles, can be ignored
because of its weakness.
 The carrier of the gravitational
force is the graviton. Although it
has never been observed in
experiment, it is strongly believed
to exist.
Standard Model of Particle Physics
 Physicists have developed a theory known as the Standard Model that explains
our current understanding of the nature of matter -- what it's made of and how its
components interact.
 All the particles in the universe can be grouped into just three "families" of
particles: quarks, leptons, and force carrier particles.
Matter Particles
Fundamental Forces and Force Carrier Particles
The Standard Model
A quantum theory that successfully describes how all know fundamental particles
interact via the strong, weak and electromagnetic interactions
based on a gauge field theory with a symmetry group
G  SU (3)c  SU (2) L U (1)Y
Fermions
Symmetries
Three families,
with leptons
eL
eL
, R, eR,
1) Poincaré Group




2) Gauge Symmetries:
and quarks
uL
dL
, uR, dR.
c
t
s
b
Someone said, “Let there be mass !”.
And there was mass…
U(1)Y
SU(2)L
SU(3)c
Bosons, Interactions
 : QED, g’
Z, W : Weak, g
tan W 
g
g
(electroweak unification)
8 gluons : QCD, gs
And others saw that mass was bad,
because it broke the SU(2)L symmetry.
Only known solution: the Higgs Mechanism
Important Questions of Particle Physics
1. What is the origin of mass? Higgs?
2. The question of unification of interactions?
3.
Why matter/anti-matter balanced distorted? Dark
matter in universe?
4. Why 3 families?
5. Neutrino masses?
6. Gravity?
1. What is the origin of mass?
e   e-  
.
.
u d s
c
b
top quark
u- d- -s
-c
b-
anti-top quark
.
.
-e -  - e+  
leptons
 gluons
W+, W-
quarks
Z
Gauge bosons (force carriers)
(Mass proportional to area shown but all sizes still < 10-19 m)
The Higgs Boson
In the “Standard Model” the origin of mass is
addressed using a mechanism named after the
British physicist Peter Higgs.
This predicts a new particle: the Higgs boson.
What is the Higgs boson?
In 1993, the then UK Science
Minister, William Waldegrave,
issued a challenge to physicists
to answer the questions 'What is
the Higgs boson, and why do we
want to find it?' on one side of a
single sheet of paper. This
cartoon is based on David
Millar’s winning entry.
Does the Higgs particle exist proposed by P. Higgs (1964)?
The mass of the Higgs is a free parameter in the Standard Model.
Constraints : 114.4 GeV/c2 (exp.) < mH < ~ 1000 GeV/c2 (theo.)
 SM Higgs-boson mass now quite constrained:
114.4 < mH < 193 GeV at 95% C.L.
from beautiful precision measurements and
direct searches from the e+e- colliders
“This does not
necessarily mean that
this is the Higgs mass !”
“Particle physics know everything about this
particle, the only thing they don’t know
is whether it exists.”
LHC ??
Higgs discovered @ LEP?
signal: e e  ZH  qq bb
background: e e  ZZ  qq bb
2. The question of Unification
Is there a universal force, a common origin of the different interactions?
• Einstein tried to unify electromagnetism and gravity but failed.
• 1864, Unification of electricity and magnetism (J.C. Maxwell)
• 1962-1973: Glashow, Salam and Weinberg
• Unification of the electromagnetic and weak
Interactions  electroweak interaction
• Prediction of W- and Z-bosons
J.C. Maxwell
• Higgs mechanism as a cornerstone of the model
• The Standard Model fails to unify the strong and electroweak
forces.
• The Supersymmetry (SUSY) Model can unify
the strong and electroweak forces.
Strong
Weak
Electromagnetic
Need to find SUSY particles in LHC, LC, ..
What is Supersymmetry ?
There are two types of particles in nature: fermions and bosons.
Fermions have half units of spin, and tend to shy
away from each other, like people who always
stay in single rooms at the fermion motel.
Bosons have zero or integer units of spin, and
like to be with each other, like people who stay
in shared dormitories at the boson inn.
Supersymmetry says that for
every fermion in Nature there
must be a boson and vice-versa.
Supersymmetric particles have
not been observed (yet) so they
must be heavier - SUSY must be
broken by some mechanism
u c t
d
s b
 e   
e  
The Generations of Matter
SPIN 0
Sleptons Squarks
Leptons
Quarks
SPIN ½ FERMIONS
BOSONS
u c t
d s b
 e   
e  
The Generations of Smatter
BOSONS
FERMIONS
Gravitino
W  W  Z0
Photino
Gluino
Unifying gravity to the other three forces may possible by String theory.
String theory predicts extra hidden dimensions in space beyond the three
dimensional space we sense daily.
Inverse Strength
Gravitational
Force
G
EM/Hypercharge
Force
Weak Force
Strong Force
x5
MGUT
102
MPl
1016 1019 E [GeV]
3. What is the origin of the matter-antimatter asymmetry in the
universe? What is the origin of the CP-violation?
Accelerators
Create particles & antiparticles that
existed ~0.001 ns after Big Bang
Inflation
Big
Bang
particles
anti-particles
Where did all antimatter go?
Can we produce dark matter?
Accelerating Universe! Where does energy come from?
particles
Summary of open questions in SM
• Why do some particles have a mass, and others do not, and what
determines the value of a particles mass?  Higgs mechanism ??
Where is the Higgs?
• Are there just three generations of matter, and if so, why?
• Are quarks and leptons fundamental, or are they also composed
of something smaller?
• What is the origin of the matter-antimatter asymmetry in the
universe? What is the origin of the CP-violation?
• Dark Matter
• Dark Energy
• + others