Matter and Antimatter:
Not Quite a Mirror Image
(and some recent experimental results from the BABAR experiment
at the Stanford Linear Accelerator Center)
Patricia Burchat, Physics Department
Stanford University
Classes without Quizzes
October 17, 2003
The “Big Picture”
Why are we here?
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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Matter and Antimatter
How do we define matter and antimatter?
Are there any differences in the “static” properties
of matter and antimatter?
Are there any known differences in the way matter
and antimatter evolve in time?
Why would we care whether matter and antimatter
evolve differently?
What is the BABAR experiment and how are we using it
to explore differences in the evolution of matter and
antimatter?
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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How are matter and antimatter defined?
The elementary particles that make up our everyday world (and
the heavier particles made up of them) are defined to be
matter, and their antiparticles are defined to be antimatter.
The choice of which particles we call “matter” and which we
call “antimatter” is arbitrary.
Examples of matter: electrons (e-), quarks (up, down, strange,
charm, bottom, top)
Examples of antimatter: positrons (e+), antiquarks (u, d, s, c,
b, t )
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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Matter and antimatter… how are they
the same? how are they different?
Antiparticles have the same mass and lifetime as
their corresponding particles, but they have
opposite values of electric charge as well as
some other not-so-familiar properties.
Some particles are their own antiparticle:
e.g., the photon (the particle of light).
Particles of light are the same whether they are in
a “universe” or an “antiuniverse”.
October 22, 2002
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So how much matter is there
in the Universe?
Not a lot…
October 22, 2002
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Standard Big-Bang cosmology tells us
that the universe initially contained equal
amounts of matter and antimatter.
Most particles & antiparticles
annihilated each other while the universe
was still very dense, to form photons (→).
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
We are left with a
Universe with a lot of
cosmic microwave
photons and a tiny bit
of matter: only one
neutron or proton for
every 10 billion
microwave photons!
Somewhere along the
way, particles &
antiparticles evolved
slightly differently!
7
Are the laws of physics the same in a
universe made of matter and a universe
made of antimatter?
Until the early 1960’s, it was believed that the answer to the above
question was YES: there is no way to distinguish between a universe
made of matter and a universe made of antimatter.
In 1964, it was discovered that matter and antimatter evolve
differently in time. This phenomenon is called “CP violation”. It was
found that a particular heavy unstable particle, which is its own
antiparticle, decays slightly more often to positrons (e+) than to
electrons (e-).
By comparing the charge of the particle that is produced slightly more
copiously in this decay to the charge of the particles circulating the
nucleus of our atoms, we can tell whether we live in a universe made of
matter (electrons in the atom) or antimatter (positrons in the atom).
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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The New York Ti
CP Violation Saves Civilization!
People around the world are grateful to
physicists today as a doomed visit from the
Planet-X delegation was called off at the last
minute. “I never thought this stuff was useful”,
one physicst was overheard saying...
X or X?
??
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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Can we explain the excess of matter over
antimatter QUANTITATIVELY?
In the Standard Model of particle physics, we have a
way of accommodating a difference between the
evolution of matter and antimatter, but it falls short
of explaining the net excess of matter in the
universe by about 10 orders of magnitude ! !
With the BABAR experiment at SLAC and a similar
experiment in Japan (called Belle), we are testing the
Standard Model predictions for differences between
the time evolution of matter and antimatter (CP
violation).
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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Two new “Asymmetric-energy B Factories” started
accumulating data ~June 1999
The BABAR experiment
at the PEP-II storage ring
at the Stanford Linear
Accelerator Center
The Belle experiment
at the KEKB storage ring
at the KEK Laboratory
in Japan
What is a B anyway? It is a particle made up of a heavy
quark called the “bottom” quark and an ordinary light quark
(“up” or “down”).
Asymmetric-energy e+e- storage rings
⇒ B mesons are moving in the laboratory frame of reference.
October 22, 2002
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The Asymmetric-Energy
B Factories
ϒ(4S)
e-
B0 / B0
e+
B0 / B0
Δz
October 22, 2002
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The BABAR Author List
Large International
Collaborations:
BABAR has ~500
collaborators from
~70 institutions;
Belle has ~270
collaborators from
~45 institutions.
October 22, 2002
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The Asymmetric-Energy B Factory at
the Stanford Linear Accelerator Center
The BABAR Detector
October 22, 2002
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How many B’s does a B
Factory produce anyway?
BABAR and Belle each record about 5 to 10
BB “events” per second, ~24 hours a day, 7
days a week, for many months at a time.
So far, BABAR and Belle have each recorded
over 100 million BB pairs.
October 22, 2002
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Blind Analysis Techniques
BABAR and Belle both use “blind”
analysis strategies for the
extraction of the time-dependent
asymmetry in order to minimize
possible experimenters’ bias.
We use a technique that hides not only the result of
the fit, but also the visual CP asymmetry in the time
distribution.
The statistical error on the asymmetry is not hidden.
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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Since we have a “Factory”, we must
have a lot of signal events, right?
Wrong…
~200 million B pairs have been recorded and analysed by BABAR and Belle.
~100 million of these are neutral B pairs.
~one B in a thousand decays to the CP final states we need.
Of these, ~10% decay into final states we can reconstruct.
Of these, ~50% pass all the selection criteria.
We are left with about 5000 signal events.
October 22, 2002
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CP violation in decays of B mesons is expected to exhibit itself
as oscillations in the decay rate.
B0
Ratio of oscillation frequency to decay
rate:
Decay Rate
very
large
B0
~ few
~ 0.1
time (ps)
October 22, 2002
In B decays, the oscillation
frequency is small
compared to the decay
rate!
Patricia Burchat, Classes w/o Quizzes
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With symmetric beam energies, we cannot measure the difference in
decay times for the Btag and BCP.
e+
Btag BCP
5.3 GeV
5.3 GeV
Δz ≈ 20 µm
With asymmetric beam energies, we can measure the difference in decay
times by measuring the difference in decay positions.
Btag
BCP
e+
9.0 GeV
3.1 GeV
Βzγ ≈ 0.55
Δz ≈ 255 µm
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
Δz can be positive
or negative.
19
⇒ We measure the time between decays by measuring the
distance between the decays.
A time interval of ~1 picosecond is translated into a
distance of ~150 microns.
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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From the ideal world to “reality”…
Btag= B0
Btag= B0
Now add effect of imperfect
measurement of Δt.
Btag= B0
Btag= B0
First add effect of imperfect tagging.
Time-dependent CP asymmetry is diluted.
Btag= B0
October 22, 2002
Btag=
B0
Finally add background contribution.
Btag= B0
Patricia Burchat, Classes w/o Quizzes
Btag= B0
21
An Example of an “Event”
B→J/ψ K0S
J/ψ →e+eK0S → π+ π-
October 22, 2002
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BABAR
Ks modes
KL modes
sin2β = 0.74 ± 0.07 ± 0.03
October 22, 2002
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Belle
B0
B0
-8
0
Δt (ps)
+8
Red Curve (B0) minus
Blue Curve (B0)
sin2β = 0.82 ± 0.12 ± 0.05
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
sin2β is the amplitude
of this asymmetry.
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Imaginary axis
Constraints on upper vertex of Unitarity Triangle from
all measurements EXCEPT sin2β
October 22, 2002
Regions of >5% CL
With BABAR and Belle, we are
measuring directly one of the
angles of the green triangle
shown in the figure.
β
Real axis
Patricia Burchat, Classes w/o Quizzes
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Imaginary axis
World Average
sin2β = 0.73 ± 0.06
October 22, 2002
The Standard Model
wins again … at least at
the current level of
experimental precision,
in this decay mode.
Real axis
Patricia Burchat, Classes w/o Quizzes
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Future Prospects
The B Factories are working on “upgrades” to further increase the rate at
which B mesons are produced.
In addition, new measurements will come from proton accelerators
(Fermilab in the near term and the Large Hadron Collider at CERN in the
longer term).
This will open up opportunities for not only more precise
measurements of the angle β, but also measurements of
the other two angles in the “Unitarity” triangle, further
constraining the Standard Model and increasing our
sensitivity to physics beyond the Standard Model.
α
γ
β
IF we find an inconsistency between our measurements and the predictions
of the Standard Model, we may have a hint of the “Physics Beyond the
Standard Model” that is necessary to explain how we ended up with an
excess of matter over antimatter in the Universe.
October 22, 2002
Patricia Burchat, Classes w/o Quizzes
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The End
Acknowledgements: Graphics on pp. 7, 9, 18, 21
were borrowed from David Kirkby (UCI) with
permission.
October 22, 2002
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