1 overview of particle physics -- the path to the standard model
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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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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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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
ns
Ferm
ions
Particles of Standard ModelParticles of Standard Model
t t t
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1 22
q qF k
r
q2
q1
Electron
Proton
Photon
Electromagnetic interactionElectromagnetic interaction
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Weak interaction Beta decay
d u
d
Neutron
Proton
u d
u
e
Electron e
Anti-electron Neutrino
W
Mean lifetime of a free neutron ~ 10.3 minutes Mean lifetime of a free proton > 1031 years!
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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 Quark
1977 – 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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What do we actually “see”What do we actually “see”
Jet-1
Jet-2
Muon
Electron
Missing energy
jetsett _
jetsett _
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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 …………………