Matterantimatter asymmetry in the Universe

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Matter-antimatter asymmetry in the Universe
Professor Michael G Green Royal Holloway
University of London June 1999
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Where the hell ?
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What is matter?
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The concept of elements
In Aristotles philosophy there were four
elements
Dalton (1808) listed, with weights, many elements
we recognize today
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The periodic table
Mendeleev (1869) introduced the periodic table
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The structure of atoms
Rutherford (1912) showed that atoms contain a
central nucleus
Electrons orbit nucleus with well-defined energy
and ill-defined positions
10-10 m
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The structure of nuclei
Nucleus contains protons with charge e and
uncharged neutrons
10-14 m
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The structure of nucleons
Neutrons and protons contain quarks
10-15 m
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The structure of quarks?
?
There is no evidence for further structure
lt10-18 m
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The constituents of matter
Protons contain uud - charge e Neutrons
contain udd - charge 0
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Prediction of antimatter
P A M Dirac predicted existence of the positron
in 1928
Diracs equation implies positron mass
electron mass positron charge e
The only equation in Westminster Abbey?
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Discovery of antimatter
Anderson (1932) discovered the positron predicted
by Dirac
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What is antimatter?
Electrons and positrons annihilate to produce
g-rays (energy)
E mc2
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Production of ee- pairs
Inverse process also occurs, with g-rays becoming
an electron-positron pair
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The neutrino
Invented by Pauli (1928), named by Fermi
(1933) Discovered by Reines Cowan (1956)
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The muon
Discovered in cosmic rays by Neddermeyer and
Anderson (1936) Appears identical to electron
but is 200 times as heavy Decays within 2.2ms
Who ordered that? - I I Rabi
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Strange particles
In 1947 Rochester and Butler discovered yet more
new objects, now known to contain a third quark
By the early 1960s beautiful patterns of
particles containing three quarks or a quark and
an antiquark were seen which were predictive
(recall Mendeleev)
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The fundamental particles (1963)
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The zoo grows larger
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CERN
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The LEP accelerator
Energy of electrons and positrons is about 100GeV
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LEP
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Inside the LEP tunnel
LEP is 27km in circumference Four bunches of
electrons and positrons circulate inside
the vacuum pipe 88ms for a complete
circuit About one electron-positron collision
per second
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Electron-positron collisions
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The ALEPH detector
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Collisions in ALEPH
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Three neutrinos ...
Number of different neutrinos 2.994 0.011
s measures rate at which ee- collisions occur
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The story so far
The everyday world contains two quarks and the
electron.
Additional quarks and leptons have been observed
with six of each in total most decay very
rapidly.
All particles have an antiparticle.
Quarks are always in groups of three or as
quark-antiquark.
When energy turns to mass equal numbers of
particles and antiparticles are produced.
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Evolution of the Universe
The Universe began with a Big Bang about 15
billion years ago
Big Bang
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Evolution with matter-antimatter symmetry
Eventually such a universe contains only photons
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CP violation in K0 decays
Phases of the amplitudes for the two processes
are not equal CP violation (1964) Occurs only
because there are three families of quarks
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The Sakharov conditions
Antimatter can turn into matter if (a) proton
decay occurs (b) there is a matter-antimatter
asymmetry (CP violation) (c) there is a period of
thermal non-equilibrium
Sakharov (1964)
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A universe with asymmetry
Perhaps one in every 109 antiquarks turned into a
quark very early in the life of the
Universe After the matter-antimatter
annihilation a small amount of matter will be
left (about one proton for 109 photons)
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Current problems
1. We have never observed proton decay 2.
Precise measurements of CP violation in K0 decay
are difficult and there are uncertain theoretical
corrections 3. Cosmological models do not easily
explain the ratio of 109 photons for each proton
in the Universe
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CP violation in B0 decays
Similar effect expected in B0
First measurements starting 1999, Stanford,
California
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The Stanford Linear Accelerator
Animate
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The BaBar detector