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Where has all the antimatter gone? Persis Drell, of Stanford Linear Accelerator Center, explains how answering questions about antimatter may shed light on what happened in the early universe.
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Experiments teach us that for every fundamental
particle there exists an antiparticle. The big
bang and its aftermath almost certainly produced
particles and antiparticles in equal numbers.
However, for as far out in the universe as we can
probe, our observations indicate that we live in a
universe of matter, not antimatter. What happened
to the antimatter? A tiny imbalance between particles
and antiparticles must have developed early in the
evolution of the universe, or it all would have
annihilated, leaving only photons and neutrinos. Subtle
asymmetries between matter and antimatter, some of which
we have observed experimentally in the laboratory, must
be responsible for this imbalance. But our current
knowledge of these asymmetries is incomplete, insufficient
to account for the observed matter domination.
There must be some other undiscovered phenomenon that
makes matter and antimatter behave differently. We may
discover it in quarks—or in neutrinos. Its source may
lie in the properties of the Higgs boson, in supersymmetry
or even in extra dimensions.
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THE EARLY UNIVERSE
When matter and antimatter annihilated at the big bang, some tiny asymmetry...
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TODAY'S UNIVERSE
...produced our universe, made entirely of matter.
Credit: Htoshi Murayama
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The Standard Model can accommodate the phenomenology of CP
violation in quarks, because there are at least three generations
of quarks; and because there is mixing between the quark flavors
when they interact via the weak interaction. The CP violation
measured in the B mesons at BaBar and Belle, along with a wealth
of studies on quark flavor mixing over the past 20 years, are all
consistent with this phenomenology. However current knowledge of
CP violation is incomplete and insufficient by many orders of
magnitude to account for the primordial matter-antimatter asymmetry
of the universe. Present and planned accelerator experiments are
aimed at discovering other sources of CP violation that make matter
and antimatter behave differently. It may appear in quarks-or in
neutrinos. Its source may lie in the properties of the Higgs boson,
in supersymmetry or even in extra dimensions.
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NuMI
Horn for focusing particles at the NuMI experiment.
Credit: Fermilab
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CP violation in reactions that change the flavor of quarks is being
measured with strange quarks (K meson decays) and bottom quarks
(B meson decays). Ongoing and planned experiments include K0P10
at BNL (K decay); BaBar at SLAC, Belle at KEK (Bd decay), BTeV
at Fermilab and LHC-b at CERN (Bd and Bs decay). Pinning down
precisely the role of CP violation in the quarks is a critical step
in solving the puzzle of the fate of primordial antimatter.
Experiments have so far shown that, by itself, CP violation in quarks
from flavor mixing in the Standard Model is probably not the sole
source of the matter-antimatter asymmetry observed in the universe.
Current and future B physics experiments will be sensitive to sources
of CP violation beyond the Standard Model.
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| Particle physics research is a journey of exploration into the mystery and beauty of the universe... read more |
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The discovery that neutrinos have mass opens up the
search for CP violation in lepton reactions. Neutrino
mass can, in principle, turn matter into antimatter
and back, and can change the balance between them.
Experiments are required to discover the role of neutrinos
in the antimatter question. The MINOS experiment
at Fermilab and reactor-based neutrino oscillation
experiments will measure the parameters of neutrino
oscillation. If the oscillation parameters are favorable, a
neutrino superbeam facility with a large underground
experiment will detect CP violation in neutrinos.
Such a large detector, if sufficiently far underground,
for example at a potential Deep Underground Science
and Engineering Laboratory, could also serve as a next-
generation proton-decay experiment
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