Yaakov (J) Stein Chief Scientist RAD Data Communications Particle Physics October 2008

Yaakov (J) Stein

Chief Scientist

RAD Data Communications

ParticlePhysics

October 2008

particles Oct .2008 Slide 2

Physics ?

Physics is the search for simplicity

Aristotelian physics held that there were 4 terrestrial elements1. earth2. fire3. air4. water

All materials under the sky are combination of several elements

Aristotle (and Democritus and Epicurus) further believed that matter is not infinitely indivisible

i.e. that there smallest units of matter (atoms)

All Aristotelian physics was derived from pure thought(it is commonly held that Galileo invented the idea of experiments)


Atoms

From the quantitative study of chemistry (Lavoisier)

Dalton concluded that matter is made of atoms

For example - carbon and oxygen can combine in two waysIn one the mass ratio was 3:4 in the other 3:8From this he concluded that • the 2 combinations were 1:1 and 2:1 in terms of atoms• an oxygen atom is 1 1/3 times heavier than a carbon one

By careful measurement he made a list of atomic weights A(e.g. C has atomic weight 12 and O has atomic weight 16)

But how many different atoms were there ?


Chemistry

By comparing chemical characteristics of different elements

Mendeleev came up with the periodic table

Here each element has a atomic number Z (serial number)

For example• H has Z=1 A=1• C has Z=6 A=12• O has Z=8 A=16• Cu has Z=29 A=64


Complexity - not simplicity

So we have a nice picture of elements made up of atomsAnd all materials made up of elements and thus of atoms

But there are many many different kinds of atomsThis is too complex ! Physics is the search for simplicity !

Perhaps the atoms themselves are made up of simpler units ?

Unfortunately, the table is monotonic in atomic weight Abut not linear in A

so the atoms are not made up of Z smaller particles


Electrons and protons

The first elementary particle discovered was the electron via cathode rays (Thomson), oil drops (Millikan),

and the photoelectric effect (Hertz)

What was the connection between electrons and atoms ?

After a series of scattering experiments Rutherford came up with the planetary atomic model

• the atom was mostly empty• at the center was a very small nucleus• electrons circulate around the nucleus• since electrons are negative and the atom neutral

the nucleus must be positive

In later experiments Rutherford proved that the nucleus was made up of protons (nuclei of H atoms)


Scattering experiments

In a scattering experiment • particles are used as projectiles• other particles are targets

Low energy scattering is good to measure the cross-sectional area of the target

For example, Rutherford bombarded thin gold foil with alpha particles

most particles go through without deflection, so nucleii are very small

High energy scattering can break up the target

Very high energy scattering can create new particles


Sensors

Weak collisions are observed by using detectors

To observe new particles created in strong collisions

we need a new tool

In 1911 Wilson invented the cloud chamber (supercooled gas)

While looking into a glass of beer in 1952 Glaser came up with the bubble chamber (superheated liquid)

In both, tracks are left by all charged particles

By using a magnetic field one can determine charge and mass

Today there are many sophisticated sensors and many Israeli specialists in this space


Bubble chamber tracks


Nuclei

Isotopes are the same element (same Z)

but different atomic weightsSo there must be something in the nucleus other than the proton

This also helped understand what kept the nucleus togetherso Rutherford invented the neutronwhich was found experimentally by Chadwick in 1932

Neutrons and protons experience a strong force when they are very close

that overcomes the electric repulsion of the protons

Beta decay changes Z without changing Aand the beta particles turn out to be electrons

So a neutron can change into a proton by ejecting an electronand the force responsible is called the weak force

e - r / d

r2


Forces

Let's take a short rest from matter and look into forces

4 different types of forces were known to classical physics1. contact2. gravity3. electric4. magnetic

Then Maxwell unified the electric and magnetic fieldsSince a changing E field builds a changing B field and vice versa

the field can build itself and travel far from sourcesthe speed turns out to be the speed of light !

So the field is more fundamental than the action at a distance

action at a distance


Interactions

Today we speak of interactions between particles

There are four known interactions (in order of decreasing strength)

• strong (hadrons are particles that feel the strong interaction) • electromagnetic (charged particles feel it)

• weak (hadrons and leptons feel it)

• gravitation (all particles feel it)

Theories that further unify these are called unified field theoriesEveryone wants a Theory of Everything (ToE) that explains all 4

In quantum theory all interactions are mediated by bosons


Antiparticles

In 1932, three particles were known• electron (negative, light)• proton (positive, heavy)• neutron (neutral, heavy)

In 1928, Dirac's came up with the first relativistic quantum theoryIt predicted an antiparticle for each particle

In 1933 Anderson discovered a positron (antielectron) in a bubble chamber picture

So we need to add• positron• antiproton• antineutron

This is a nice simple picture !


Photon

In 1923 Einstein predicted that electromagnetic fields were made up of photons

Later relativistic quantum theories showed him to be correct

The photon was the first boson discovered

Photons have no mass, and thus travel at the speed of light

Photons have no charge and are their own antiparticles

But photons do have energy

The frequency of EM radiation is related to the photon energythrough the fundamental relation E = h


Quantum numbers

According to quantum theoryall elementary particles have certain characteristics

These include its mass, charge, and spin

Later new quantum numbers needed to be added

In interactions, characteristics are ruled by conservation laws

Table of particles we know so far :


Fermions and Bosons

Classical particles obey Maxwell-Boltzmann statisticsbut quantum particles are indistinguishable

In quantum mechanics particles are described by a field The probability of finding a particle is ||2

Indistinguishability means ||2 = | |2

which can either mean = Bose-Einstein statistics (bosons) = - Fermi-Dirac statistics (fermions)

Note that two Fermions can't be in the same state (Pauli principle)

Spin-statistics theorem - fermions have half integral spin bosons have integral spin


Neutrinos

Enrico Fermi observed that in beta decaynot all the expected energy was in the emitted electron

It was later more directly observed

He concluded that some other particle took some of the energyand called it the neutrino (small neutral particle)

The neutrino is almost masslessand only reacts via the weak interaction

And we also need an antineutrino !

Later it was discovered that there are different types of neutrino


Muons

While observing byproducts of cosmic radiation in 1936Anderson observed a very heavy electron (mass about 100 MeV)

Since its mass was between • the light electron (lepton = light) and • the proton (baryon = heavy)

he called it a meson

But today that name is used for other particlesand we call this negatively charge particle the muonor more precisely the mu minusand the muon is known to be a lepton not a meson

Its antiparticle is the mu plus


Pions

Yukawa's theory of the strong force predicts a boson with intermediate mass - the meson

At first the muon was thought to be that particlebut it turned out to be a fermion and not to participate in the strong force

In 1947 the pi meson (or simply pion) was discoveredwith mass about 140 MeV

There are three types - pi zero, pi plus, and pi minus

Later other mesons were predicted and discovered - K and eta


So what Fermions do we have ?

Leptons :• electron, positron• electron neutrino, electron antineutrino• mu minus, mu plus• muon neutrino, muon antineutrino• tau minus, tau plus• tau neutrino, tau antineutrino

Mesons :• pi zero, pi plus, pi minus• kay zero, antikay zero, kay plus, kay minus• eta

Baryons :• proton, antiproton• neutron, antineutron• lambda, antilambda• sigma zero, sigma plus, sigma minus and their three anti-s• xi zero, antixi zero, xi minus, antixi plus• omega minus, antiomega plus


So what Bosons do we have ?

Gauge bosons :• photon (charge 0) - electromagnetic interaction

• gluon (g) (charge 0) - strong interaction

• W (charge -1) and antiW (charge +1) - weak interaction

• Z (charge 0) - weak interaction

• graviton (?) - gravity

Higgs boson - in electroweak theory creates mass

And many more are unconfirmed as yet …• X • Y • W-prime, Z-prime, …

grand unified theories


The eight-fold way

The Fermion picture is no longer simple

In the early 1960s, Gellmann and Neeman (independently)

observed new symmetries that connected baryons/mesons


Quarks

This observation led to a new picture, called the standard model

In the standard model, baryons and mesons are composite

Quarks com in 6 flavors - up, down, charm, strange, top, and bottom

There are thus 6 particles and 6 antiparticles (all are spin ½)

Due to color confinement, quarks never exist as free particlesInstead, they form hadrons - particles that feel the strong interaction• baryons are made up of 3 quarks• mesons are made of one quark and one antiquark


Color confinement

Quarks can be either red, green, or blue

Antiquarks can be either antired, antigreen, or antiblue

Only combinations with resulting color white attract

Hadrons are made up of quarkssuch that the resulting color is zeroand the resulting charge is always an integer

The model explains all the properties of the baryons and mesonsFor example, • proton = u u d (charge +1)• neutron = u d d (charge 0)• lambda = u d s (charge 0)• pi-plus = u anti-d (charge +1)• kay zero = d anti-s (charge 0)


A simple picture again !

6 quark types (u d c s t b)

6 lepton type (e e-neutrino mu mu-neutrino tau tau-neutrino)

4 gauge boson types (photon gluon Z W)

and maybe one Higgs !

Detector from the LHC (Geneva)

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Yaakov (J) Stein Chief Scientist RAD Data Communications Particle Physics October 2008