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Overview of Particle Physics Overview of Particle Physics -- the path to the Standard -- the path to the Standard

ModelModel

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TopicsTopics

historical flashback over development of the fieldo “prehistory” 19th centuryo electron, radioactivity, nucleuso cosmic rayso spectroscopy erao collider era

standard model of particle physics

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Electron – 1897

J.J Thomson

Top quark1995

A Century of Particle PhysicsA Century of Particle Physics

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Sizes and Sizes and distance scalesdistance scales

visible light: wavelength

≈5∙10-7m virus 10-7m molecule 10-9m atom 10-10m nucleus 10-14m nucleon 10-15m quark <10-18m

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The Building Blocks of a Dew DropThe Building Blocks of a Dew Drop dew drop: 1021

molecules of water. Each molecule = one

oxygen atom and two hydrogen atoms (H2O).

Atom: nucleus surrounded by electrons.

Electrons bound to the nucleus by photons

nucleus of a hydrogen atom = single proton.

Proton: three quarks, held together by gluons just as photons hold the electron to the nucleus in the atom   

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Very early era (19Very early era (19thth century) century) chemistry, electromagnetism discharge tubes, “canal rays”, “cathode rays” photoelectric effect (Hertz, 1887) radioactivity (Becquerel, 1895) X-rays (Röntgen, 1895)

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Atoms, NucleusAtoms, Nucleus electron: first hint that atom not indivisible natural radioactivity understanding of

composition of atom, nucleus atom = nucleus surrounded by electrons

(Geiger, Marsden, Rutherford, 1906 -1911) hydrogen nucleus = proton, is component of all

nuclei (1920) neutron (Bothe, Becker, Joliot-Curie, Chadwick,

1930 – 1932)

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Cosmic raysCosmic rays Discovered by Victor Hess (1912) Observations on mountains and in balloon: intensity of cosmic

radiation increases with height above surface of Earth – must come from “outer space”

Much of cosmic radiation from sun (rather low energy protons) Very high energy radiation from outside solar system, but

probably from within galaxy

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Cosmic rays -- “elementary” Cosmic rays -- “elementary” particlesparticles

new detectors (cloud chambers, emulsions) exposed to cosmic rays discovery of many new particlespositron (anti-electron) : predicted by Dirac

(1928), discovered by Anderson 1932 muon (μ): 1937 Nedermeyer pion (π) predicted by Yukawa (1935),

observed 1947 (Lattes, Occhialini, Powell) strange particles (K, Λ, Σ,…..

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Particle ZooParticle Zoo 1940’s to 1960’s :

Plethora of new particles discovered (mainly in cosmic rays):

e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , Ξ,…. question:

Can nature be so messy? are all these particles really

intrinsically different? or can we recognize patterns or

symmetries in their nature (charge, mass, flavor) or the way they behave (decays)?

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The Particle Zoo!The Particle Zoo!

±, 0, e, ±,K±, K0

S, K0L,

0, p, n, +, 0, , , …

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Particle spectroscopy eraParticle spectroscopy era 1950’s – 1960’s: accelerators, better detectors even more new particles are found, many of them

extremely short-lived (decay after 10-21 sec) 1962: “eightfold way”, “flavor SU(3)” symmetry

(Gell-Mann, Ne’eman) allows classification of particles into “multiplets” Mass formula relating masses of particles in

same multiplet quark model – three different kinds of quarks

(u, d, s) Allows prediction of new particle Ω- , with all of its

properties (mass, spin, expected decay modes,..) subsequent observation of Ω- with expected

properties at BNL (1964)

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ΩΩ--

BNLBNL19641964

eight-fold way quark model – particles made up of three different “quarks” – u, d, s

p = uud, n = udd,… Ω- = sss refinement of these ideas, more quarks,

“color”, gauge field theory Standard Model

http://www.bnl.gov/bnlweb/history/Omega-minus.asp

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A theoretical model of interactions of elementary particles, based on quantum field theory

Symmetry: SU(3) x SU(2) x U(1)

“Matter particles” Quarks: up, down,

charm,strange, top, bottom Leptons: electron, muon, tau,

neutrinos “Force particles”

Gauge Bosons (electromagnetic force)o W, Z (weak, electromagnetic)o g gluons (strong force)

Higgs boson spontaneous symmetry

breaking of SU(2) mass

Standard Model

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Contemporary Contemporary Physics Physics

Education Education Project Project

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u u u d d d e

b b b

c c c s s s

g g g g g g g g

Z W±

e

Quarks Leptons+2/3 -1/3 -1 0

I

II

III

Boso

nsFe

rmio

ns

Particles of Standard ModelParticles of Standard Model

t t t

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“every-day” matterProton

d ud

e

Neutron

e

u du

Electron Electron Neutrino

Photon

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1 22

q qF kr

q2

q1

Electron

Proton

Photon

Electromagnetic interactionElectromagnetic interaction

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Weak interaction Beta decay

d u

d

NeutronProton

u d

u

e

Electron e

Anti-electron NeutrinoW

Mean lifetime of a free neutron ~ 10.3 minutes Mean lifetime of a free proton > 1031 years!

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The Strong ForceThe Strong Force

u

u d

d

g

Strong force caused bythe exchange of gluons

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Forces (interactions)Forces (interactions)

Strong interaction 1 Binds protons and neutrons to form

nuclei Electromagnetic interaction 10-2

Binds electrons and nuclei to form atoms

Binds atoms to form molecules etc. Weak interaction 10-10

Causes radioactivity Gravitational interaction 10-39

Binds matter on large scales

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What holds the world together?What holds the world together?

interaction

participants

relative strength

field quantum(boson)

strong

quarks

1

ggluon

electro-magnetic

charged particles

10-2

photon

weak

all particles

10-10

W± Z0

gravity

all particles

10-39

Ggraviton

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The Discovery of Top QuarkThe Discovery of Top Quark1977 – 1992

Many null results

1992 – 1993 A few interesting

events show up

1994, DØmt > 131 GeV/c2

1994, CDFFirst evidencemt ~ 170 GeV/c2

1995 – CDF, DØDiscovery!

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e u c

e d s

e u c

e d s

P Pt2 3/

t 2 3/

W b1 3/

W

b 1 3/

Creating Top Anti-Top Quark pairsCreating Top Anti-Top Quark pairs

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Artist’s impression of a top eventArtist’s impression of a top event

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What do we actually “see”What do we actually “see”

Jet-1

Jet-2

Muon

Electron

Missing energy

jetsett _

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““event display” of a event display” of a DØ top eventDØ top event

jetse tt -

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ΩΩb b (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html(http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html

2008 DØ experiment at Fermilab: discover brother

of Ω- , the Ωb

Ω- = sss, Ωb = ssb,

theory predicts properties, decay modes, ..

confirmed by experiment

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SummarySummary we’ve come a long way ……Standard Model (theory of particle interactions)

works embarrassingly well!Has been tested by many hundreds of

precision measurements over last three decades – very few measurements differ by more than 1 or 2 standard deviations

Even some amount of frustration – always hope to see experimental result in disagreement with theory

But there are some open questions …………………