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What IS Fundamental???

Many new particles were discovered
with the advent of particle accelerators
…are they ALL fundamental???

Baryons: particles with lifetimes ~ 10-10 seconds,
ultimately decaying into protons


Mesons: particles with lifetimes ~ 10-8 seconds, typically
lighter than the proton and never decaying into protons


Λ0, Σ+, Σ-, Σ0, Ξ-, Ξ0
Κ-, Κ0, Κ+, π-, π0, π+
Antimatter: An antiparticle is simply a particle with opposing
quantum numbers
Too Many Particles
Murray Gell-Mann
1969 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.
3 + 3 = 6 Quarks
Quark
Mass
Date Where
[GeV/c2]
up,
down
-
~0.005,
~0.010
Constituents of hadrons,
most prominently, proton and
neutrons.
-
~0.2
discovered in cosmic rays
~1.5
Discovered simultaneously in
both pp and e+e- collisions.
~4.5
Discovered in collisions of
protons on nuclei
~175
Discovered in pp collisions
-
strange 1947
charm
1974
bottom 1977
top
1995
SLAC/
BNL
Fermilab
Fermilab
Comment
Three Families of Quarks
Generations
Increasing mass
Charge =
-1/3
Charge =
+2/3
I
II
III
d
s
b
(down)
(strange)
(bottom)
u
c
t
(up)
(charm)
(top)
Also, each quark has a corresponding antiquark.
The antiquarks have opposite charge to the quarks
How the Quark Model Works
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
2/3 + 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 !
Hadrons
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).
Mesons are also in the hadron family.
They are formed when a quark and an anti-quark “bind”
together.
Next Big Question


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.
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)
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 (μ- and μ+ )
discovered in cosmic rays.
M(μ) ~ 200*M(e)
 The muon behaves very
similarly to the electron (i.e., it’s
a lepton).
Neutrinos
1934: To account for the “unseen”
momentum in the reaction (decay):
n  p + e- + X
p
n
X
e
Fermi proposed that the unseen
momentum (X) was carried off by a
particle dubbed the neutrino (n).
Nobel Laureate: Enrico Fermi
(means “little neutral one”)
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
Q=0
1
e-
ne
e+
ne
2
m-
nm
m+
nm
Three happy families…
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 τ-lepton.
In 2000, first evidence of the τ’s partner, the tau-neutrino (ντ)
was announced at Fermi National Accelerator Lab.
Family
1
2
3
Leptons
Q = -1
Q=0
ene
mnm
tnt
Antileptons
Q = +1
Q=0
e+
ne
m+
nm
t+
nt
3 families, just like the quarks… interesting !!!
The Standard Model
Search for the Higgs!
What are Force Carriers?
Now the question is, how are these matter particles held together??
-- by the basic forces in nature!
There are four basic forces in nature. These are:
•Gravitational interaction which makes
apples fall on certain peoples heads. It is also
this which pulls together the Earth and the
Moon. Newton’s Apple story!
•Electromagnetic interaction which assures
the cohesion of our bodies and governs all
chemistry. It is this which pulls together the
electron and the atomic nucleus like earth
around the sun!
Fundamental Interactions
•Strong interaction which unites quarks together
and thus the nuclei of atoms i.e. world is not
broken apart!
•Weak interaction which is responsible for
beta radioactivity, which gives us the
conception of antimatter!
We were talking about forces, but why interactions?
Before quantum theory, forces were transmitted by virtue of a mysterious
force field emitted by particles.
According to quantum theory, forces are not exerted between two fermions
unless there is an exchange of a mediator particle, called a boson. Now the
heavier the boson, the shorter will be the range of the interaction!
Exchange Particles/Force Carriers
For electromagnetic interaction the exchange particle is γ.
For strong interaction the exchange particles are
gluons.
For weak interaction the exchange particles are W 
and Z0 bosons.
Gravitational interaction has the weakest intensity in particle physics scale!
Unification?
Up to this we have found the 12 (6 quarks + 6 leptons) fundamental particles as
well as four basic forces in nature and also the mediator particles of interactions
respectively.
What will happen if we try to bring it all together ?
----This synthesis of current knowledge, without any doubt is known as ---“The Standard Model ”
A glimpse into the Big Bang!
It is clear from the figure that all
4 forces were created from a
super force during the Big-Bang!
In reverse way, we are trying to
unify these forces to reach the
super force, aren’t we?
Heisenberg’s Folly?
In the 1950s, it was rumored that Heisenberg had done it, and just the
details remained to be sketched in. But nothing ever emerged from
Heisenberg. So Wolfgang Pauli responded with the following:
“Below is the proof that I am as great an artist as Rembrandt; the details
remain to be sketched in.”
Why Do We Need the LHC?
The Standard Model and Beyond.
What is Mass?
The Higg’s boson
What are dark matter and dark energy?
Supersymmetric particles
Why is there more matter then antimatter?
Symmetry breaking
What was it like just after the Big Bang?
Quark-gluon plasma
What about Gravity?
Extra dimensions, string theory
String Theory