Transcript eXtremely Fast Tr

What’s the Matter with AntiMatter?
Richard E. Hughes
BABAR; p.1
Paul Dirac Predicts AntiMatter!
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In 1929, Theoretical Physicist Paul Dirac
combined
 Special Relativity
 Quantum Mechanics
to try to describe the behavior of the
electron
One problem with his equation: it had two
solutions
This is not always bad. For example, the
equation
x2=25
also has two solutions: +5 and -5
In Dirac’s case, his equation had two
solutions:
 An electron with positive energy
 An electron with negative energy
Usually, negative energy solutions are NOT
GOOD! Energy must always be positive.
But Dirac was pretty smart. He realized
that in his case, the negative energy
electrons could be INTERPRETED as antielectrons
Richard E. Hughes
Neils Bohr: Of all physicists,
Dirac has the purest soul.
BABAR; p.2
The Dirac Equation in Comic Book Form
Richard E. Hughes
BABAR; p.3
The Discovery of Cosmic Rays
 At the beginning of the 20th
century, scientists thought there
was too much radioactivity than
could be accounted for naturally.
Where was it coming from?
 Victor Hess decided to test the
idea that the additional radiation
came from outer space. In 1912,
one way to do this was by
BALLOON!
 He got to about 18,000 feet
(without oxygen) He noticed
that the radiation steadily
increased.
 COSMIC RAYS!
Richard E. Hughes
BABAR; p.4
Beaten to the Punch!
 Actually, a Jesuit Priest named
Theodor Wulf beat Hess by 2
years, noting that the
radioactivity at the top of the
Eiffel Tower was higher than at
the base.
 But alas, no Nobel Prize. This
went to Hess in 1936.
Richard E. Hughes
BABAR; p.5
The Discovery of Antimatter!
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In 1932 Carl Anderson studied
cosmic rays using a “cloud chamber”.
Charged particles produced in cosmic
rays would enter the chamber and
leave “tracks”. The tracks would
bend in circles because the chamber
was placed in a strong magnetic field
 Positive particles bend one way
 Negative particles bend the other
way
He found equal numbers of positive
and negative particles
 Maybe the negative particles were
electrons? (YES!)
 Maybe the positive particles were
protons? (NO!)
By studying how much energy the
positive particles lost, he figured out
that they had the same mass as the
electrons!
 Positive electrons!
 Antimatter!
 Nobel Prize!
Richard E. Hughes
BABAR; p.6
More Antimatter: Search for AntiProton
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The search for antiprotons heated up in the 1940s and 1950s, as laboratory
experiments reached ever higher energies...
In 1930, Ernest Lawrence (Nobel Prizewinner in 1939) had invented the
cyclotron, a machine that eventually could accelerate a particle like a proton up
to an energy of a few tens of MeV. Initially driven by the effort to discover the
antiproton, the accelerator era had begun, and with it the new science of "High
Energy Physics" was born.
It was Lawrence that, in 1954, built the Bevatron at Berkeley, California (BeV,
at the time, was what we now call GeV). The Bevatron could collide two protons
together at an energy of 6.2 GeV, expected to be the optimum for producing
antiprotons. Meanwhile a team of physicists, headed by Emilio Segre', designed
and built a special detector to see the antiprotons.
In October 1955 the big news hit the front page of the New York Times: "New
Atom Particle Found; Termed a Negative Proton". With the discovery of the
antiproton, Segre' and his group of collaborators (O. Chamberlain, C. Wiegand
and T. Ypsilantis) had succeeded in a further proof of the essential symmetry of
nature, between matter and antimatter.
Segre' and Chamberlain were awarded the Nobel Prize in 1959. Only a year
later, a second team working at the Bevatron (B. Cork, O. Piccione, W. Wenzel
and G. Lambertson) announced the discovery of the antineutron.
Richard E. Hughes
BABAR; p.7
But Can We Make and Anti-Nuclei? YES!
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By now, all three particles that make up atoms
(electrons, protons and neutrons) were know to
each have an antiparticle. So if particles, bound
together in atoms, are the basic units of
matter, it is natural to think that antiparticles,
bound together in antiatoms, are the basic
units of antimatter.
But are matter and antimatter exactly equal
and opposite, or symmetric, as Dirac had
implied? The next important step was to test
this symmetry . Physicists wanted to know: how
do subatomic antiparticles behave when they
come together? Would an antiproton and an
antineutron stick together to form an
antinucleus, just as protons and neutrons stick
together to form an atom's nucleus?
The answer to the antinuclei question was
found in 1965 with the observation of the
antideuteron, a nucleus of antimatter made out
of an antiproton plus an antineutron (while a
deuteron, the nucleus of the deuterium atom, is
made of a proton plus a neutron). The goal was
simultaneously achieved by two teams of
physicists, one led by Antonino Zichichi, using
the Proton Synchrotron at CERN, and the other
led by Leon Lederman, using the Alternating
Gradient Synchrotron (AGS) accelerator at the
Brookhaven National Laboratory, New York.
Richard E. Hughes
BABAR; p.8
What about anti-atoms?
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At this point, a natural question to ask:
Can we form antiatoms?
The necessary ingredients: an antiproton
and an antielectron.
But typically when these things are made,
an accelerator is used, and the antiparticles are moving too fast. So we
need to slow them down.
Low Energy Antiproton Ring (LEAR).
This was done at the European
Laboratory CERN, using the Low Energy
Antiproton Ring (LEAR).
In 1995, scientists at LEAR succeed in
making the first anti-atoms (about 9 of
them).
So anti-atoms exist, and a natural
question to ask is: Are there anti-worlds
out there? Anti-galaxies?
Before answering this question, lets first
try to ask what practical use antiparticles have in our world.
Can anyone think of any?
Richard E. Hughes
BABAR; p.9
PET
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"PET“ stands for Positron Emission
Tomography. Positron Emission
Tomography uses positrons to look
at the brain.
 Radioactive nuclei in a fluid are
injected into the subject.
 The radioactive nuclei then emit
positrons at low velocities and these
then annihilate with nearby
electrons.
 The positrons and electrons are
moving slowly and don't have the
energy required to create a new pair
of particle and antiparticle. Instead,
2 gamma rays are emitted and these
are used to actively scan the brain.
The gamma rays leave the patient’s
body and are detected by the PET
scanner.
The information is then fed into a
computer to be converted into a
complex picture of the patient’s
working brain.
Richard E. Hughes
BABAR; p.10
Anti-Matter SpaceCraft!?
 NASA's Marshall Space Flight
Center, Pennsylvania State
University are studying using
annihilation of matter and
antimatter to fuel spacecraft.
 Matter and antimatter provides
the highest energy density of
any known propellant.
 it would require only a gram of
antimatter to put the shuttle
into orbit.
 about ten billion times more
energy than the
hydrogen/oxygen mixture that
powers the shuttle
 300 times more than the fusion
reactions at the Sun's core.
Richard E. Hughes
But… costs $62.5 trillion per
gram.
Might be able to bring this down
to $5 billion per gram
BABAR; p.11
 How do we really know that the universe is not matterantimatter symmetric?
 We have landed on the moon,so we know the moon is made of
matter.
 Cosmic rays from the sun are matter not antimatter.
 The other planets are matter (Mars Rovers are still taking
data!)
 The Milky Way: Cosmic rays sample material from the entire
galaxy. In cosmic rays, protons outnumber antiprotons 104
to 1.
 The Universe at large: This is tougher. If there were
antimatter galaxies then we should see gamma emissions
from annihilation.
Richard E. Hughes
BABAR; p.12
Colliding Galaxies
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The image shows the collision of two
galaxies from the Hubble Space
Telescope 63 million light years
away.
Such collisions would occur in other
places in the universe as well.
If there were anti-matter galaxies,
then such collisions would result in a
very specific signature of gamma
rays (like what we see in the PET
scanner).
No such signal is seen.
Also, by looking at cosmic rays,
there is some antimatter, but this
can be accounted for by radioactive
decays or by nuclear reactions
involving ordinary matter.
So we believe most of the universe
(>99.99%) is made of matter.
Richard E. Hughes
The Antennae Galaxies
BABAR; p.13
Any Other Evidence for Antimatter in the
Universe?
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NASA's orbiting Compton Gamma Ray Observatory (CGRO) spacecraft spotted
unexpected clouds of antimatter in the Milky Way Galaxy. The clouds suggest a hot
fountain of gas filled with antimatter electrons is rising from the region around the
center of the our galaxy. Antimatter electrons also are known as positrons.
The nature of the furious activity producing the hot antimatter-filled fountain is unclear,
but could be related to massive numbers of stars being born near the large black hole at
the center of our galaxy. Other possibilities include winds from giant stars or black hole
antimatter factories.
Richard E. Hughes
BABAR; p.14
Ok. The Universe is Only Matter. So What?
 The fact that there is only matter present is a problem
 All models of how the universe started in the Big Bang indicate that there
should be as much matter as antimatter, initially.
 Every reaction we know of which makes a quark, also gives us an anti-quark
 This problem is referred to as the “Baryon Asymmetry” problem, where
“Baryon” is a general name for things like protons and neutrons.
 Biology Asymmetry: aminoacids only righthanded chains
 So there must be some mechanism which prefers matter to antimatter
 Since there is so much matter (in terms of baryons) you might think
that the mechanism must be very obvious – that is that it is a very large
effect
 But there are about 10 billion photons for every baryon in the universe
 Where did these photons come from?
 Baryon+anitbaryon -> two photons
 So an asymmetry which leaves 1 baryon leftover for every 10 billion baryons
would work fine
Richard E. Hughes
BABAR; p.15
Sakharov Conditions
 In 1967 Andrei Sakharov (father of the Soviet Bomb and later
dissident) proposed three conditions that – if satisfied – would account
for the propenderence of matter over antimatter
 1: Baryon number violation: There must be a way of making(or destroying)
baryons that differs from making (or destroying) antibaryons
 Possible in the early universe
 2: Must be a process which favors matter over anti-matter
 CP violation.
 CP is something called a transformation
 To understand CP, we need to understand first the processes called C and P
 The first two conditions can generate both baryons and an asymmetry of
baryons over antibaryons. But we need another condition to “freeze” this
situation in place to have what we observe today.
 3: The universe must fall out of thermal equilibrium, at the precise moment
when baryon number switches from being efficiently violated, to being
almost exactly conserved.
Richard E. Hughes
BABAR; p.16
Charge Conjugation C and Time Reversal T
 Charge Conjugation, C
 Charge conjugation turns a particle into its anti-particle
 e+  eK-  K+
gg
+
 Time Reversal, T
 Changes, for example, the direction of motion of
particles
 t  -t
Richard E. Hughes
BABAR; p.17
Parity Transformation
 Parity, P
 Vectors change sign
 The parity transformation changes a right-handed coordinate system into a
left-handed one or vice versa.
 Two applications of the parity transformation restores the coordinate
system to its original state.
Richard E. Hughes
BABAR; p.18
C and P Symmetry Can be Violated
 You can apply the Charge conjugation transformation to a particle
 Apply it to the electron: get a position: this exists
 Apply it to a neutrino:
 complication: there are only left-handed neutrinos or right handed
anti-neutrinos
 So C applied to a left-handed neutrino gives you a left-handed antineutrino.
 But this particle does not exist
 You can apply the Parity transformation to a particle.
 Applying P to a “left-handed neutrino” generates a “right-handed
neutrino”
 But this particle does not exist!
 As a result, it is said that the weak force (the only force that a neutrino
feels) is not symmetric under the parity transformation
 Turns out that the transformation CP does work for neutrinoes
 CP(left handed neutrino) = right handed antineutrino
Richard E. Hughes
BABAR; p.19
CP Symmetry
 The CP symmetry appears to work in the weak force
 The CP symmetry does work in both strong and
electromagnetic forces
 But to help explain matter antimatter asymmetry, we need
CP violation
 It turns out that CP symmetry is actually violated in some
weak force cases… at a very low rate
Richard E. Hughes
BABAR; p.20
CP can be violated
 There is a particle called the KL (read K-long)
 It has a well defined mass (and lifetime)
 No other particle has such a mass
 Therefore, the KL is it OWN anti-particle!
 The KL decays in the following way
K L   + e - e
K L   - e + e
sometimes
sometimes,but slightlyless often
 It decays both to and to , but slightly more often to the latter mode.
Therefore, it violates both C and CP. So CP can be violated.
 But the violation is really rare: like waving to yourself in a mirror one
thousand times, and once your reflection waves back with the other
hand!
 Interesting aside: Say there was an alien, and you wanted to meet them
 Are they made of matter or antimatter? How could you tell?
 Hint: Ask them to look at how the KL decays….
Richard E. Hughes
BABAR; p.21
BaBar
 Why BaBar?
 Bottom AntiBottom Assymetric
Ring
 The detector is designed to
study Bottom-mesons
 A meson is a combination of a
quark and an antiquark
 Bottom mesons contain one
bottom quark
 Why study bottom quarks?
 CP violation is expected just like
for KL decays
 But it could (should) be much
larger
 Is the amount needed to explain
matter-antimater assymetry in
the universe?
Richard E. Hughes
BABAR; p.22
Stanford Linear Accelerator Center (SLAC)
The 3-km long linear
accelerator in
Stanford, California
uses electromagnetic
fields to accelerate
electrons and
positrons to close to
the speed of light:
Richard E. Hughes
BABAR; p.23
PEP-II Rings
 The electrons and positrons are
then guided into the two PEP-II
storage rings (PEP stands for
Positron Electron Project). The
rings are located one on top of
the other. Electrons go clockwise
round the lower ring (which is an
upgrade of the older PEP storage
ring, which came into operation in
1980). Positrons go anticlockwise
round the newly built upper ring.
Richard E. Hughes
BABAR; p.24
A BaBar “Event”
The B mesons live for about a billionth of a second, in which time they travel less
than a millimetre. The BaBar detector observes the particles to which the Bs
decay. From the decay products, the physicists can deduce which was the B and
which was the anti-B. They can also measure how far and fast the Bs travel before
decaying, and hence they can calculate their lifetimes.
Richard E. Hughes
BABAR; p.25
The Unitarity Triangle
The physicists are interested in the difference in the decay times of the
and the . By observing millions of decays, they can build up a distribution of
the differences in the decay times. It is predicted that the actual
distribution will be different from that which you would get if there were
complete symmetry between matter and antimatter.
 Amazingly enough, studying CP
violation in Bottom mesons can be
reduced to measuring the sides
and angles of a special triangle
B  l , rl ,...
 The Unitarity triangle
 BaBar is mostly interested in
measuring the angle called 
 Actually measures sin(2)
 Result is 0.74 +/- 0.07
Richard E. Hughes
a
B   , r ,...
V*
Bd+-,r± ,...
V*
ubVud
(0,0)
B
(r,h)
g
D±K
Bd  Bd
tbVtd

(1,0)
V*cbVcd BdJ/y
±D,..
Ks,D*
B  Dl , D*l ,...
B  D ,...
(rescale sides by 1/|V*cbVcd| and choose V*cbVcd r
BABAR; p.26
A possible problem with all of this!
 The first is that the CP violation of the Standard Model is
far, far too weak to explain the matter-antimatter
asymmetry.
 There must be extra physics which introduces new CP
violation. There are some strong limits on such new CP
violation, which generally require it to occur via interactions
which will be very hard to measure in future particle physics
experiments.
 In any case, the CP violation must involve new physics we
don't know about.
Richard E. Hughes
BABAR; p.27
 Evidence that the laws of nature are not completely
symmetric with respect to matter and antimatter first
emerged in 1964, when a violation of the so-called chargeparity (CP) symmetry was observed in ephemeral particles
known as K mesons, or kaons. Researchers discovered a tiny
discrepancy between kaons and anti-kaons in the way they
decay.
Richard E. Hughes
BABAR; p.28
Richard E. Hughes
BABAR; p.29