How elementary are the elementary particles?

The science of particle physics

Halina Abramowicz (Tel Aviv University)

May 2000

� What is the owrld made of?� What holds it together?

A continuing series of experiments has resulted in a modelof the fundamental particles and forces of matter, theStandard Model.

1

The Big Bang

2

History

Structure of matter

Aristotle (384-322 BC) - matter in the universe wascomposed of 4 basic elements: air, earth, fire and water.

Matter could be divided into smaller parts without anylimit. These elements were acted on by two forces:gravity, the tendency for earth and water to sink, andlevity, the tendency for air and fire to rise. The division ofthe universe into matter and forces is used today.

Democritus (c. 455-370 BC) - matter was inherently grainyand everything was made up of large numbers ofdifferent atoms.

Dalton (1803) - the fact that chemical compounds alwayscombined in certain proportions could be due to atomsgrouping into molecules.

W. Roentgen (1986) - discovery of strange rays of unknownnature, X rays.

3

H. Becquerel (1896) - discovery of radioactivity on filmaccidentally exposed to uranium.

J. J. Thompson (1897) - existence of electron, first elementaryparticle discovered before the 20th century.

4

E. Rutherford (1911) - internal structure of an atomconsisting of a tiny positively charged nucleus at thecenter, with nearly all the mass of the atom) andelectrons going around the nucleus in orbits.

Experiment performed by H. Geiger and E. Marsdendesign expectation

observation explanation

E. Rutherford (1919) - produced protons from nucleibombarded by � particles, predicts existence ofneutrons.

W. Pauli (1930) - invents the neutrino to explain whyenergy is not conserved in � decays.AZX !AZ+1Y + e� + ��e

5

J. Chadwick (1932) - discovery of the neutron, emittedin the reaction 42He +94Be !162 C + n, from energyconservation.

C. D. Anderson (1932) - discovery of the positron, predictedby Dirac. The existence of anti-matter established.

Summary 1932 - 5 particles thought to be elementaryp, n, e�, , �

6

Forces - what holds it together

I. Newton (1680) - gravity

J. C. Maxwell (1860) - unification of electric and magneticforces, electromagnetism.

M. Planck (1900) - radiation is quantized, quanta.

A. Einstein (1905) - light is quantized into particles,photon, explains photo-electric effect.

N. Bohr (1913) - explains orbital structure of atoms

A. Compton (1923) - from scattering of light on electrons,convincing evidence that light has particles as well aswave properties.

W. Heinsenberg (1925) - formulation of quantum mechanicsand the uncertainty principle.

E. Schroedinger (1925) - formulation of quantum mechanicsin wave mechanics and relativistic wave equation.

P. Dirac (1928) - Dirac equation predicting the existence ofthe positron.

7

H. Yukawa (1934) - invents the pion to be the photon ofstrong interactions binding the nucleus. Three pions�+; �0; �� with masses 140 MeV were predicted.

E. Fermi (1934) - formulation of weak interactions, aspoint-like interactions, to explain the � decay.

Summary 1934 - theoretical foundation for field theory

The concept of a carrier of forces is borned, as well as theconcept of a virtual particle. The interaction between twoparticles is mediated by the exchange of a virtual carrier.

Gravity Weak Electro- Strong

forces magnetism forces10�39 10�5 1=137 1.

8

Particles discovered until 1964

M. Gell-Mann, G. Zweig (1964) put forth the idea of quarks.

quark up down strangename u d scharge

23 �13 �13

d-- dd

d

proton neutron

u

u

π+ −0π

d u-

π

uu

dd

9

Controlled Experiments

10

The world of units

Object typical scale in [m]

DNA 10�7 0.1 micron [�m]

Molecule 10�9 1 nanometer [nm]

Atom 10�10 1 A

Nulceus 10�14Proton 10�15 1 fermi

Quark < 1��181[J℄ = 1[kg℄1[m℄101[eV℄ = 1[electron charge℄1[V℄1[eV℄ = 1610�18[J℄units of eV name103 keV106 MeV109 GeV1012 TeV

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Principle of experiments

What is the shape of the unseen target?

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Answer

13

Accelerators and Detectors

Accelerator types

Linear Circular

Beam configuration

Fixed target Colliding beams

Detector types

Fixed target Colliding beams

14

Typical modern detector

15

Stanford Linear Accelerator, (1968-69) - repeat Rutherford’sexperiment on proton target.

scattering

angle

E ’

beam

electron

rails

target

Θ

������������������������

���������������������������������������������������������������������������������

�������������������������������������������������������������������������������������������������������

detector

electron

E

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Schematic representation of the interaction

beam

electron scattered

photon

target

proton

electron

Denote by Q the momentum of the exchanged (virtual)photon. It’s resolving power is, from the uncertaintyprinciple Q � r � ~ :If the proton consists of a ’pudding’ of evenly distributedcharge, with decreasing r the cross section shouldbecome smaller and smaller, contrary to data - point-likeconstituents.

Q2[GeV2]

σ/σ M

OT

T

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J. Bjorken, R. Feynman (1968-69) - introduce the notionof partons, point-like constituents of the proton - lateridentified with quarks of Gell-Mann.

Why the quarks are not observed as free particles?What keeps them together in the proton?

O. W. Greeneberg, M. Y. Han and Y. Nambu (1965) - introducecolor charge for the quarks. Hadrons are assumed to becolor neutral.

H. Fritsch, M. Gell-Mann (1973) - formulate the theory ofstrong interactions QCD. Gluons play the role of thephoton and couple to color charge.

D. Politzer, D. Gross and F. Wilczek (1973) - show thatQCD has the property of asymptotic freedom - twoquarks close to each other behave like free particles.When far apart, the force is very large - quarks areconfined.

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The new picture of the nucleus

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Particles discovered 1964 - present

S. Weinberg, A. Salam (1967) - propose unified theoryof electromagnetic and weak interactions into theelectroweak interaction. The Fermi theory is explainedin terms of heavy, charged intermediate bosons W�.It predicts the existence of a neutral boson Z0 and anadditional massive boson, Higgs.

W

u

d

u

-

protonneutron

e-

-

ν

e

d

u

d

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Discovery of neutral currents (1973) - expected to bemediated by the Z0.

Discovery of J= (1974) , by the groups of B. Richter(e+e� collisions in SLAC) and S. Ting (pp atBrokkhaven), thought to be the bound state of � quarks,predicted by S. Glashow, J. Iliopoulos and L. Maiani in1970. This completes the second generation of quarksand leptons.

Discovery of the � lepton by M. Perl and collaborators atSLAC. Starts the third generation of quarks and leptons.

Discovery of � (1977) by L. Ledermann and collaboratorsat Fermilab (pp collisions), thought to the the bound stateof b�b quarks.

Strong evidence for gluons (1978) found at PETRA e+e�collider at DESY.

e +

q

q-

-e e +

g

q-

q

-e

21

Discovery of W� and Z0 (1983) in p�p collisions at CERN,using techniques developed by C. Rubia and S. Van derMeer.

-

-

-

u

d

proton

u

+

-

0

anti-proton

e

e

Z

u

d

u

Summary of known forces at this point

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Strong evidence for 3 generations of quarks and leptons (1989),at CERN and SLAC, by measuring the width of theZ0 decay in e+e� collisions, consistent only with theexistence of exactly three very light neutrinos.

γ

Zo

e+

e-

ν

ν γ

W

e+

e-

ν

ν

γW

e+

e-

ν

ν

0

50

100

150

10 20 30 40 50 60 70 80 90 100

Nν=2.57 ± 0.19fW=1.22 ± 0.17

Nν=3fW=1

OPAL 189 GeV

Eγ (GeV)

Eve

nts/

(4 G

eV)

23

Discovery of the top quark (1995) by CDF and D0 experimentsat the Fermilab p�p Tevatron collider, with mass 175 GeV,compared to the mass of the previous heaviest b quarkof 4.5 GeV ???

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How elementary is the electron

OPAL experiment at LEP (CERN) with the participation ofTAU

10 2

10 3

10 4

10 5

60 80 100 120 140 160 180 200

-2.5%

0

+2.5%

-5%

0

+5%

-5%

0

+5%

120 140 160 180

√s / GeV

Cro

ss-s

ecti

on /

pb

OPALe+e-→e+e-

|cosθ|<0.96;θacol<10˚

|cosθ|<0.9;θacol<170˚

|cosθe-|<0.7;θacol<10˚

(a)

(b)

(c)

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How elementary are the quarks

HERA experiments at DESY (Hamburg) with theparticipation of TAU in ZEUS

0

2

4

6

8

10

12

14

16

1 10 102

103

104

105

x=0.65

x=0.40

x=0.25

x=0.18

x=0.13

x=0.08

x=0.05

x=0.032

x=0.02

x=0.013

x=0.008

x=0.005

x=0.0032

x=0.002

x=0.0013

x=0.0008

x=0.0005

x=0.00032

x=0.0002

x=0.00013

x=0.00008

x=0.00005

x=0.000032

(i=1)

(i=10)

(i=20)

Q2 /GeV2

F2+

c i(x)

NMC BCDMSSLAC

H1 94-97 e+p

ZEUS 94 e+p

H1 96-97 preliminary

NLO QCD Fit

ci(x)= 0.6 • (i(x)-0.4)

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Summary

27

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Ultimate goal - unification

Future projects� LHC (CERN) pp at 14 TeV� Linear Collider (DESY? SLAC? KEK?) e+e� 1 TeV� Muon Collider (CERN? Fermilab?)

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