What is the matter?

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What is the matter?
Where is the antimatter?
Professor Michael G Green Royal Holloway
University of London
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Where the hell ?
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What is matter?
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Where is the antimatter?
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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 plum pudding model
J J Thomson believed the electrons were embedded
in a positively charged matrix - plum pudding
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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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Evidence for substructure
Atom absorbs energy
Electron energy increases
Only certain energy levels (orbits) allowed
Later de-excites
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Evidence for substructure
Measure size of struck objects (Rutherford 1912)
1970 - substructure of protons and neutrons
discovered using electrons as projectiles
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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
Paul 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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How to produce antimatter
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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 200 times as heavy Decays within 2.2 msec
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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A particle accelerator
Energy of electrons is about 20kV
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The LEP accelerator
Energy of electrons and positrons is about 100GeV
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CERN
Europes research laboratory for particle physics
in Geneva.
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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 100ms for a complete
circuit About one electron-positron collision
per second
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Electron-positron collisions
E mc2
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The ALEPH detector
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Collisions in ALEPH
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ALEPH - a LEP particle detector
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Three neutrinos ...
Number of different neutrinos 2.984 0.008
s measures rate at which ee- collisions occur
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and no further substructure
Excited states produced if substructure exists
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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.
When energy turns to mass equal numbers of
particles and antiparticles are produced.
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Matter-antimatter asymmetry in the Universe
When energy turns to mass equal numbers of
particles and antiparticles are produced.
This observation creates problems for our
understanding of the present day Universe, which
appears to contain only matter and essentially no
antimatter
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Evolution of the Universe
The Universe began with a Big Bang about 15
billion years ago
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The Big Bang
What happened at times less than 10-9s is
uncertain
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Evolution with matter-antimatter symmetry
Eventually such a universe contains only
photons (almost true for our Universe - cosmic
microwave background)
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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 thermal
non- equilibrium
Sakharov (1964)
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Proton decay
Life on earth implies protons exist, on average,
for gt1023 seconds Searches for proton decay have
set limits gt1032 seconds
Proton decay converts quarks into leptons -
important only in early stages of the Big Bang
but a small effect will remain
However antiprotons will decay similarly
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Parity violation
Macroscopic systems obey the same physical laws
in a mirror system, e.g. planetary motion parity
conservation.
b-decay (weak interaction) does not conserve
parity. Discovered in 1956 in polarized 60Co
decay.
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P violation - CP conservation
Parity violation leads to an asymmetry for
neutrinos -only left-handed ones exist.
Changing particle to antiparticle (C) then
applying the parity operation (P) produces the
right-handed antineutrino, which exists CP
conservation
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Matter-antimatter asymmetry

• In 1964 it was discovered that the radioactive
  decay of antimatter differs by a tiny amount from
  the decay of matter.
• Since then progress in understanding has been
  very slow
• experiments are very difficult
• astronomy is an observational science, not
  experimental (cannot repeat the Big Bang).
• BUT we have learned that the matter-antimatter
  asymmetry can only occur if there are three pairs
  of quarks.

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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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CP violation
Leads to beautiful interference effects and
non-exponential decay distributions
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A universe with CP violation
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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Weak decay eigenstates
In the Standard Model the weak decay eigenstates
are
where d and s are a mixture of d and s of the
form d d cos?c s sin?c s -d sin?c s cos?c
We write this as
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CP violation parameters
Further, there are relations among the elements
of V such as Vud Vub Vcd Vcb Vtd Vtb
0 that can be represented by a triangle.
It is a condition for CP violation to occur that
b is non-zero.
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CP violation parameters
Prior to 1999 the shape of this triangle had been
approximately determined from measurement of
several parameters of V. However the angle had
not been measured directly.
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BaBar experiment at SLAC
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The process ee- ? B0B0
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B decays
The two particles decay
We identify them and measure the separation of
the decay points
The separation (typically 1mm) is translated to
a time difference (typically 1ps)
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Predicted distributions
We measure many examples of the process to
produce a distribution in Dt
Without CP violation the distribution of Dt is
exponential
(a) CP violation makes the distribution
asymmetric (b) experimental resolution modifies it
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Evidence for CP violation
The data show clear evidence of CP
violation The size of the effect is consistent
with the prediction of the Standard Model
of particle physics
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Values of sin 2b
Recently direct measurements have been
made (summer 2002 values) OPAL 3.21.8-2.00.5
CDF 0.790.41-0.44 ALEPH 0.840.84-1.05 Bel
le 0.72 0.07 0.04 BaBar 0.75 0.09
0.04
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Summary
The everyday world is made from up and down
quarks and the electron. Experiments tell us that
six quarks and six leptons exist. The extra
ones seem to be needed to explain why there is an
asymmetry between matter and antimatter and hence
why we exist. However it is likely to be a long
time before we have a good understanding of what
happened in the first fraction of a second of the
Universes existence
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What is the matter?
Where is the antimatter?
THE END
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Particle Physics Summary Sheets - the story so
far
GO
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Example of Poster No.1
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Example of Poster No.2
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Example of Poster No.3
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Example of Poster No.4
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Example of Poster No.5
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Example of Poster No.6
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Particle Physics Summary Sheets - the story so
far
Available as double sided A4 sheets for
individual student use, or as a set of 6 single
sided A3 wall posters, they are entitled
'Particle Physics - the story so far'.
For details of how to get the summary sheets and
posters call 01784 443448 or e-mail
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