What IS Fundamental??? Many new particles were discovered with the advent of particle accelerators...

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 Λ0, Σ+, Σ-, Σ0, Ξ-, Ξ0

Mesons: particles with lifetimes ~ 10-8 seconds, typically lighter than the proton and never decaying into protons 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 Date WhereMass

[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 1974SLAC/

BNL~1.5

Discovered simultaneously in both pp and e+e- collisions.

bottom 1977Fermi-

lab~4.5

Discovered in collisions of protons on nuclei

top 1995Fermi-

lab~175 Discovered in pp collisions

Three Families of QuarksGenerations

I II III

Charge =

-1/3d

(down)

s(strange)

b (bottom)

Charge =

+2/3u

(up)

c (charm)

t(top)

Also, each quark has a corresponding antiquark.The antiquarks have opposite charge to the quarks

Increasing mass

How the Quark Model Works

To make a proton:We bind 2 up quarks of Q = +2/3and 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= -1/3with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 !

HadronsThe 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.

As far as we can tell, electrons appear to be indivisible.

LeptonsElectrons 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’sa lepton).

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’sa lepton).

Neutrinos

Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino ().

1934: To account for the “unseen” momentum in the reaction (decay):

Nobel Laureate: Enrico Fermi

n

p

eX

n p + e- + X

(means “little neutral one”)

Lepton Picture continues…

Family Leptons Antileptons

Q = -1 Q = 0 Q = +1 Q = 0

1 e- e e+ e

2

1962: An experiment at Brookhaven National Lab showed that there were in fact at least 2 types of neutrinos.

Three happy families… In 1975, researchers at the Stanford Linear Accelerator discovered

a third charged lepton, with a mass about 3500 times that of theelectron. 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 Leptons Antileptons

Q = -1 Q = 0 Q = +1 Q = 0

1 e- e e+ e

3 families, just like the quarks… interesting !!!

The Standard Model

Search for the Higgs!

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!

What are Force Carriers?

•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!

Fundamental Interactions

For electromagnetic interaction the exchange particle is γ.

For strong interaction the exchange particles are gluons.

For weak interaction the exchange particles are and Z0 bosons.

W

Gravitational interaction has the weakest intensity in particle physics scale!

Exchange Particles/Force Carriers

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?

Unification?

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.

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