Introduction to Cosmology

Introduction to particle physics

A brief history of the discovery of the structure of matter

Cormac ORaifeartaigh PhD

Waterford Institute of Technology

Prologue3

I The atomic theoryThe Greek atom, the chemistry of the elements, Kinetic theory, Brownian motion 4

II Early particles Cathode rays and the electron, canal rays and the proton

III The nuclear atom5The plum pudding atom, Rutherfords nuclear atom IV Nuclear physics Transmutation, the neutron, radioactivity, nuclear fission and fusion

Interlude: quantum theory and particle physics

V Cosmic rays, the weak force and the strong force8 The neutrino, the pion and the muon

VI Accelerators and the particle zoo9 Accelerators, strange particles, resonances

Interlude: the forces of nature

VII The quark model of particle physics12The eightfold way, the search for quarks, leptons and quarks

VIII The standard model 14The electro-weak interaction, quantum chromodynamics

IX Beyond the standard modelGrand unified theory, unified field theory, string theorySupersymmetry, supergravity and superstrings15

Epilogue16 Unified field theory and the Big Bang

I The atomic theory

1.The Greek atom

Thales (585 BC): (i) all substances can be classified as solid, liqid or gas

(ii) water exists in all 3 forms

is all matter made up of water?

Thales followers: matter made up of 4 fundamental elements

earth, fire, air and water

Democritus (~350 BC): matter made up from small, indivisible particles

atoms

Example:

What happens if a piece of metal is cut into smaller and smaller pieces?

Ans: if matter is continuous, piece is infinitely divisible

Democritus: at some stage reach immutable atoms (indivisible)

Epicurus of Samos (342-270BC): expanded idea of atomism

Snag: atomism disputed by Plato and Aristotle

matter continuous, made up of four elementary principles

hotness, coldness, dryness and wetness

2. The chemistry of the elements

Lavoisier (1734-1794): observations on combustion suggested that matter comprised discrete elements, and that matter was conserved in chemical reactions

the chemical elements hydrogen, oxygen, carbon, sodium etc

J.L.Proust (1799): study of chemical reactions

variety of substances could be formed by combining different

quantities of the chemical elements

Law of definite proportions: in every sample of a compound substance, the proportions by weight of the constituent elements are always the same

John Dalton (1804): concept of atomic weight

importance of the relative weights of atoms in obtaining

the composition of other substances

Law of multiple proportions: if substance A combines with substance B in two or more ways forming substances C and D, then if mass A is held constant, the masses of B in the various products will be related in proportions that are the ratios of small integers

Conclude: when elementary substances combine, they do so as discrete entities or atoms

Daltons atomic theory of matter

every element composed of atoms that are physically and

chemically identical - atoms of different elements differ

Gay Lussac (1808): if gas A combines with gas B to form C,

the ratios of the volumes of A,B and C will be in integers

again implies that substances participate in reactions in discrete or corpuscular amounts

Avogadro (1811): correlated work of Dalton and Gay-Lussac

Postulated the existence of elementary molecules as the smallest particles that can make up compounds

Postulated Avogadros Law: at equal temp and press, equal volumes of

gases contain equal numbers of molecules

Snag (1850s): atomic theory under threat due to inconsistent masses

Cannizzaro (1858): inconsistent results for atomic masses due to confusion of atomic and molecular masses

views accepted at international conference on atomic masses (Karlsruhe, 1860)

fundamentals of modern chemistry laid

relative atomic weights could be calculated (Avogadros law)

Dimitri Mendeleev (1869):Periodic Table of the Elements

Listing the chemical elements from the lightest (hydrogen) to the heaviest (uranium) caused elements with similar chemical properties to recur at regular intervals

gaps - unknown elements with predicted properties

- these elements soon discovered

Implications of Periodic Table for atomic theory

elements not truly independent

relation between atoms of different elements?

atoms not fundamental?

inner atomic structure?

3. Kinetic theory of gases

Boyle, Charles: pV = nRT (Ideal gas law)

(empirical, macroscopic)

Can this law be derived by assuming a gas comprise large numbers of molecules in constant random motion?

Maxwell, Boltzmann, Gibbs (1850-1900):

mechanics of molecular motion in gases (kinetic theory)

Boltzmann: root mean square speed of molecules

Maxwell: distribution of molecular speeds

Gibbs: mean free path of molecule

Result: ideal gas law can be derived from atomic theory

gases comprise large numbers of atoms in constant motion?

Experimental clue

Robert Brown (1827): random motion of pollen grains in water

motion due to collisions with water molecules?

molecules in gases and liquids in constant motion?

4. Brownian motion

Albert Einstein (PhD, 1902-05): suspensions in liquids

applied kinetic theory to particles in liquids

derived expression for diffusion constant D

used expression to estimate size of water molecule

used expression to estimate Avogadros number N

Good agreement with estimates of N by other means

Albert Einstein (1905): Brownian motion paper

derived expression for mean displacement of particle

relation between , D and t

simplified calculation to 1dimension

(x)2 = 2Dt

Clear prediction that could be tested experimentally

Jean Perrin (1908): experiments on Brownian motion

- gamboge particles in water

- large enough to be seen with microscope

- small enough to be influenced by molecular collision

- uniform size and mass

Results

mean free path proportional to

predicted N in agreement with other estimates

Support for atomic hypothesis!

small particles suspended in a liquid move about as

predicted by the kinetic theory of molecules

Measure displacement of particle in 2 D in a given time interval

Number of diffused grains as function of

(Number of diffused grains ~ mean displacement)

5. Other evidence for atomic hypothesis

Estimates of N by a variety of means

N

Viscosity of gases 60 x 1022

Brownian motion (displacement) 72 x 1022

Brownian motion (rotation) 65 x 1022

Diffusion of solutes (40-90) x 1022

Mobility of ions in solution (60-150) x 1022

Brightness of blue in the sky (30-150) x 1022

Measurement of atomic charge (60-90) x 1022

Emission of alpha particles (70) x 1022

Black body radiation (60-80) x 1022

Covergence: suggests real phenomeonon

Atomic hypothesis accepted

Most probable value for N (71) x 1022

Most probable value for molecular magnitude (2.8) x 10-8 cm

IIEarly particles

1.Cathode rays and the electron

(http://boomeria.org/physicslectures/secondsemester/nuclear/nuclear1/nuclear1.html)

Study of the passage of electricity through gases

discharge tubes

electrodes at opposite ends of sealed tube

pressure reduced by pump system

E-field established between electrodes

rays travel great distances, tube glows green

William Crookes (1879): Crookess discharge tube

Observed: rays emitted at cathode (see shadow exps)

attracted by +ve charges, repelled by ve

deflected by magnetic field

Deduced: cathode rays are negatively charged

Jean Perrin (1895): Paddle wheel discharge tube

cathode rays push wheels

rays have mass and velocity

must be particles

negatively charged

named electrons

J.J. Thompson (1897): ratio of charge to mass of electron

deflect electron beam using E- field

yE =

estimate q/m of electron if vx known

Using B-field to balance E-field

(since )

calculate q/m = 1.76 x 1011 C/g

R.A.Milikan (1909-11): measured charge of electron

1.charge oil drop by rubbing against nozzle of atomizer

2. experiences upward force qE due to applied E-field

3. balance against gravity

vg: terminal velocity of gravity fall (measure by timing drop)

vE: terminal velocity of rise (depends on q: measure series of vE)

experiment with many different charges on drop

set of values for vE , q

all integer multiples of one value

qe = 1.6 x 10-19 C

mass of the electron

since q/me = 1.76 x 1011 C/kg (Thomson)

and qe = 1.6 x 10-19 C(Millikan)

deduce me = 9.1 x 10-31 C/kg 2.Canal rays and the proton

Thomson (~1890):

does anode produce +ve rays?

+ve rays detected when slit put in cathode

measure q/m ratio o