Transcript Ernest Orlando Lawrence Berkeley National Laboratory

What’s the Matter With Antimatter?
Or, The Leftover Universe
Dr. Natalie A. Roe
Lawrence Berkeley National Laboratory
Outline
• What is Antimatter? (What is Matter? )
• What is the Matter with Antimatter
• New Result on CP Violation from the
Asymmetric B Factory
• Why Does it Matter?
N. Roe What’s the Matter with Antimatter?
August 14, 2001
What is Matter?
• What are the fundamental constituents?
 Greeks: Earth, Air, Fire, Water
 1800’s: Periodic table of the elements
 1897: Thomson discovers the electron
 1919: Rutherford discovers the proton
 1932: Chadwick discovers the neutron
 1967: Kendall, Friedman and Taylor
discover quarks in electron-nucleon
scattering experiments at SLAC.
• Quarks
• are fractionally charged
• always occur in pairs or
triplets, never singly
• come in 6 different kinds,
arranged in 3 generations
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Why Three Generations?
• Only the first generation of particles is
needed to make our world
 proton = (uud) and neutron = (udd)
 protons and neutrons form nuclei,
electrons to form neutral atoms
 3 fundamental particles to make all of
the Universe!
+2/3
-1/3
• Nature has 2 additional generations
of heavier, unstable particles
 Why?
 What determines the quark and
lepton masses?
 Central questions for particle
physicists
N. Roe What’s the Matter with Antimatter?
0
-1
August 14, 2001
The Prediction of Antimatter
• Nature is full of symmetries; mathematics is the
language we use to describe symmetry
• Dirac’s equation for motion of a relativistic electron
(1928) had two solutions
one was the electron
the other was an electron with
“negative energy”
Dirac originally attributed this
solution to the proton (only known
positively charged particle in 1928!)
Later, Dirac predicted the existence of the positron
and, by symmetry, that of the anti-proton, or p
N. Roe What’s the Matter with Antimatter?
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Matter and Energy
• Einstein first realized the
equivalence of matter and energy
• When matter and antimatter meet,
they annihilate into energy
e-
e+
g
g
• Energy can also materialize as
particle-antiparticle pairs
e-
g
e+
N. Roe What’s the Matter with Antimatter?
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The Discovery of Antimatter
• The positron was discovered in 1932
in cosmic rays by Carl Anderson at
Caltech
 The photograph shows how positrons
were first identified in cosmic rays using
a cloud chamber, magnetic field and lead
plate
N. Roe What’s the Matter with Antimatter?
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More About Antimatter
• All charged leptons and quarks have
anti-matter partners
 n may be its own anti-particle
• Anti-proton was discovered
in 1955 at Berkeley
 billions are now produced
every year at accelerator labs
• Quark-antiquark combinations
called mesons also exist
 p+ = ud
K0 = ds
 B0 = bd
B0 = bd
N. Roe What’s the Matter with Antimatter?
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Imaging With Antimatter
• PET scans use positrons created in radioactive
beta decay, by detecting the photons created in
the subsequent e+ e- annihilation
photon
detector
photon
detector
N. Roe What’s the Matter with
Antimatter?
August 14, 2001
How to create and store anti-particles
1. create p p pairs;
separate out the p
2. store p with magnetic fields;
accelerate with RF cavities
N. Roe What’s the Matter with Antimatter?
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Colliding protons and anti-protons
• At Fermilab, located near
Chicago, protons and
antiprotons collide with the
highest energy available in
the world
• The discovery of the top
quark was made in 1995 at
the Tevatron Collider
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Colliding electrons and positrons
Stanford Linear Accelerator Center,
Stanford, California
• At Stanford, electron-positron collisions have been
used to discover the charm quark, the tau lepton - and
to study the matter-antimatter asymmetry.
N. Roe What’s the Matter with Antimatter?
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What is the Matter with Antimatter?
• In the “Big Bang” particle-antiparticle pairs were
created from pure energy in a spontaneous explosion
• Today we cannot detect significant amounts of
antimatter in the universe - why not?
• Matter and antimatter then annihilated into photons;
but a small amount of matter survived - but how?
• In 1967, Sakharov stated
three conditions necessary
to create an excess of
matter in the early
Universe - one of which
is CP Violation in particle
interactions.
N. Roe What’s the Matter with Antimatter?
August 14, 2001
What is CP Violation?
C: Charge conjugation:
particle antiparticle
P: Parity (mirror reflection):
x  -x
e-
e+
+
CP Mirror
C and P together change matter to antimatter. If the world
were CP symmetric, an antimatter world would be
indistinguishable from our world. Any difference between
matter and antimatter is evidence for CP Violation.Until 1964,
most physicists assumed that CP symmetry was obeyed in
all particle interactions.
N. Roe What’s the Matter with Antimatter?
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CP Violation Was Discovered In 1964
1980 NOBEL PRIZE
V. Fitch
J.Cronin
Cronin and Fitch discovered the violation of CP in the
decay of the long-lived, CP-odd neutral K meson into a CPeven final state:
Br(KL -> p+p- ) ~ 0.2%
There is a difference between matter and antimatter!
“We are hopeful… that at some epoch, perhaps distant, this
cryptic message from nature will be deciphered.” J. Cronin
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The Motivation for Asymmetric B Factories
• CP violation in K0 (= sd) meson decays has been
exhaustively studied, but the effect is small and
theoretical uncertainties are large.
• In B0 (= bd) meson decays, the Standard Model
predictions for CP violation are theoretically clean.
• Large asymmetries between B and anti-B meson decay
rates into special CP eigenstates are expected (10- 50%),
but the decay rates are small => need a B meson “factory”
• To observe the CP asymmetry between B and anti-B
mesons, a special type of e+e- collider is required with
unequal beam energies - the Asymmetric B Factory,
proposed by Pier Oddone of Berkeley Lab in 1987
N. Roe What’s the Matter with Antimatter?
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Measuring CP Violation with B0s
B0
B0
fcp
B0
Not equal –
CP Violation!
fcp
B0
CP violation occurs in the interference between
mixing and decay to a CP eigenstate,
eg B0 -> p+ p -
N. Roe What’s the Matter with Antimatter?
August 14, 2001
PEP-II
Stanford Linear Accelerator Center,
Stanford, California
Approved as a Presidential
Initiative in 1993; completed
in 1999. Reached full design
luminosity in 2000.
Japanese B Factory has also
been built with similar design.
N. Roe What’s the Matter with Antimatter?
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The Asymmetric B Factory Concept
e+ e-->U(4S) ->BB
decay of B “tag”
9 GeV e-
B ->CP eigenstate
3 GeV e+
Dt  Dz
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The BaBar Detector
The BaBar Collaboration:
~500 physicists from 73
institutions and 9 countries
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How the BaBar Detector Works
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Silicon Vertex Tracker (SVT)
Uses five layers of silicon microstrip
detectors to measure B decay vertices
to better than 0.1 mm
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Tracking Charged Particles in the SVT
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Recipe for Measuring CP Violation in
B Meson Decays
•
•
•
•
•
Produce many B0 B0 pairs (several 10’s of millions)
Reconstruct one B in a special decay called a CP eigenstate
“Tag” the other B0 to make the matter/antimatter distinction
Determine the time between the two B0 decays, Dt, using SVT
Compare Dt distributions for
B0 and B0 tagged events;
B tagged
the difference measures
CP violation, the difference
B tagged
between matter and
antimatter
Dt (ps)
N. Roe What’s the Matter with Antimatter?
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The PEP-II asymmetric e+e- storage ring
E(e-) = 9.0 GeV,
E(e+) = 3.1 GeV
This result
(56 fb-1)
v  0.56 c
Design
Luminosity
(cm-2
s-1)
3x
Int. Lum / day (pb-1)
Int. Lum / month (fb-1)
1033
Achieved
4.5 x
135
303
3.3
6.3
Run2b
1033
2nd PRL (30 fb-1)
1st PRL (20 fb-1)
This result: 56 fb-1 on-resonance.
Run1
62 million BB events.
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Run2a
First Observation of CP Violation in
B Decays - Announced July 6, 2001
NYT: “Tiny Discovery May Answer a Question About the Big Bang”
Dt Distributions for B0
and B0 - tagged events
Difference between B0
and B0 tagged events vs
time between decays
-6 -5 -4-3 -2-1 0 1 2 3 4 5 6 7
Dt in trillionths of a second
CP Asymmetry Measuurement: sin2b=0.59±0.14
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Latest result:
Sin2b = 0.75  0.10
N. Roe What’s the Matter with Antimatter?
August 14, 2001
What does this result mean?
sin2b=0.75±0.10
• Maximum asymmetry => sin2b = 1
• Zero asymmetry => sin2b = 0
• Much larger CP asymmetry than in K0 decays
(75% vs 0.2%); and very small theoretical error
• Error is ~ 1/7 of the central value => very unlikely to
be a statistical fluctuation
• The result is consistent with the prediction of the
Standard Model, our best current particle theory
• But: Standard Model calculations of early Universe
do not produce a large enough matter/antimatter
asymmetry!
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Why Does it Matter?
• We believe there must be some new physics
beyond the Standard Model - this is only one of
several clues
• As we collect more data, we will measure CP
violation more precisely, and compare different
types of asymmetries
• These precision tests of the Standard Model may
show us an inconsistency which will point the
way to new physics
• This approach is complementary to the searches
for new particles at the energy frontier
accelerators, such as the Fermilab Tevatron and
the CERN Large Hadron Collider
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Summary
• Antimatter exists and can be created at
accelerators; but there is very little antimatter
naturally occurring in our Universe
• CP violation is required in any theory starting
from the Big Bang to explain the dominance of
matter over antimatter
• CP symmetry between matter and antimatter is
violated at the quark level
recent measurement of CP violation in B
mesons by the BaBar experiment
• We need more CP violation to produce our
Universe - our theory is not complete!
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Could Antiprotons Be Useful?
• Antimatter engines on Starship Enterprise were
powered by p + p annihilation...
• Creating, storing antiprotons requires a lot of energy,
and trapping them is also very inefficient
Penning Trap
• All the antiprotons created in one year at Fermilab
would only power a 100 watt bulb for 30 minutes, even
with 100% trapping and conversion efficiency!
N. Roe What’s the Matter with Antimatter?
August 14, 2001
Creating Anti-hydrogen
Anti-protrons are created and stored, then
passed through cold xenon gas. An e+epair is created as the antiproton passes by
the heavy xenon nucleus. The antiproton and
the positron will occasionally form a stable
bound state - an atom of anti-hydrogen!
Detecting Anti-hydrogen
Neutral atom is not bent in magnetic field
Antiproton is deflected
and detected
Positron annihilates with electron;
photons are detected
N. Roe What’s the Matter with Antimatter?
August 14, 2001