A new calculation, reported in the January 25th issue of Physical Review Letters,
confirms the six-quark theory of particle-anti-particle asymmetry. This is the first
complete calculation of this phenomena to employ a highly-accurate description of the
quarks which adds a fifth dimension beyond those of space and time. This result allows
recent experiments studying the decays of bottom quarks to be compared with earlier,
strange quark experiments. This comparison agrees with the predictions of the Standard
Model of particle physics and implies that the particle-anti-particle asymmetry
(technically known as "CP-symmetry violation") seen in these two different decay
processes have a common origin. This research was carried by physicists from the
Brookhaven National Laboratory, Columbia University, the University of Connecticut,
Edinburgh University, Southampton University and the RIKEN BNL Research Center using
powerful, massively parallel supercomputers specially constructed to perform these
calculations and capable of tens of trillions of arithmetic operations per second.
The work was funded by the Particle Physics and Astronomy Research Council in the
UK (now the Science & Technology Facilities Council), the RIKEN Laboratory in Japan
and the U.S. Department of Energy.
The Earth, our solar system, our galaxy and likely the entire visible Universe are made of
matter, not anti-matter. While it is easy to create anti-matter in the particle collisions
which take place in large particle accelerators, the produced anti-particles immediately
annihilate with matching particles in the surrounding normal material, disappearing into a
burst of light and other particles whose energy quickly dissipates. Could we distinguish a
mirror universe made of anti-matter instead of matter, in which research workers made of
anti-matter could create fleeting examples of matter in their anti-matter accelerators? If
there were a perfect symmetry between our "matter" universe and one composed of
anti-matter, what determines which type of universe we occupy? In a Nobel Prize-winning
experiment performed in 1964 at the Brookhaven National Laboratory, small differences were
discovered between the laws obeyed by particles and anti-particles. These clearly
distinguish a matter universe from an anti-matter universe.
Accommodating such a matter-anti-matter asymmetry into a fundamental theory is not easy. A
theory made from only two pairs of fundamental quarks, the so-called up-down pair and the
charmed-strange pair cannot easily allow such matter-anti-matter differences. It is only
when a third pair of quarks, the top-bottom pair, are included does the theory support
this particle anti-particle asymmetry. This connection between the number of types of
quarks and matter-anti-matter asymmetry was discovered by Kobayashi and Masakawa who made
a compelling case for the existence of the then undiscovered top quark on this basis. The
resulting theory, with the now known three quark pairs, offers a very beautiful
explanation for matter-anti-matter asymmetry. Is it correct?
Since the six-quark theory supports only a single type of matter-anti-matter asymmetry,
every signature for such an asymmetry must be connected to every other. Thus, the
asymmetry seen in the decays of mesons containing bottom quarks can be related to the
matter-anti-matter asymmetry seen in the decay of mesons containing strange quarks.
However, to accurately link the recent bottom quark experiments carried out at the
B-Factories at SLAC (Stanford, CA) and at KEK (Japan) with the 1964 Brookhaven experiment,
one must have a quantitative understanding of the quarks which appear in tightly bound
combinations which make up the particles being studied. Fortunately, the calculation of
such decays is now possible using very powerful supercomputers and a computational
treatment of the quarks known as lattice QCD. (Here QCD stands for quantum chromodynamics,
the theory of the strong interaction between the quarks and the gluons, only indirectly
seen particles which act to hold the quarks together. The term lattice refers to the grid
of points in space-time used as a framework for the numerical calculation.)
The actual phenomena being calculated is a surprising oscillation between a K meson (made
up of a strange quark and an anti-down quark) and its anti-particle which occurs roughly 5
billion times per second. Unfortunately, the aspect of this oscillation which
distinguishes particles and anti-particles is very subtle and would not be present at all
if the mass of the down quark, which is already very small, were zero. Since there are
many other, much larger possible sources of such oscillations, a very delicate calculation
is required which treats the light down quark accurately and can distinguish between the
correct and the incorrect oscillation mechanisms.
A formulation, sufficiently accurate to perform this calculation correctly, was invented
in the early nineties. In this approachnormal spin-1/2 particles, like electrons and
quarks, are allowed to move in four, not three, spatial dimensions so that space-time
acquires a fifth spatial dimension. In contrast to the standard space-time dimensions
which are presumed to extend to infinity, this fifth dimension is bounded, ending in two
four-dimensional boundaries, similar to the two-dimensional top and bottom of a three
dimensional cardboard box. When spin-1/2 particles travel into this fifth dimension, they
behave as very massive particles---so heavy that they would not have been seen in
experiments to date. However, when they move along the two boundaries, these particles
appear physical. The particles on one boundary, spin in a right-handed sense and those on
the other carry a left-handed spin. The spin-1/2 particles realized using this new
approach are called domain wall fermions and this method provides just the accurate
description of the quarks that is needed for the oscillation calculation described above
to be reliably carried out.
The calculation reported here builds upon earlier, pioneering, lattice QCD studies of this
phenomena carried out over the past 20 years in Europe, Japan, the U.S. and the UK.
However, it is only with this 5-dimensional domain wall fermion method, made practical by
the current massively parallel supercomputers, that it has become possible to accurately
incorporate the subtleties implied by the very small quark masses and to reach a level of
accuracy comparable to the current experiments.
The result suggests that the six-quark theory correctly describes the matter-anti-matter
asymmetry seen in the decays of both the bottom and strange mesons. If the strange and
bottom experiments are combined with the less demanding theory for the bottom meson
system, the oscillation amplitude for the strange mesons (a quantity call BK) can be
determined to be 0.78 with an error of 0.09. The result of this new calculation gives an
entirely consistent value of 0.72 for this same quantity with an error of 0.04.
Of course, there are still many important questions that need to be answered. Calculations
of even greater numerical accuracy can reduce the errors and permit an even more
challenging comparison between theory and experiment. Of great interest is the possibility
that this difference between matter and anti-matter explains why we are made of matter
instead of anti-matter. Perhaps as our universe cooled after the Big Bang, this difference
between particles and anti-particles pushed the cooling Universe in the "matter
direction" causing us to ultimately be made of matter not anti-matter? Unfortunately,
even though the six-quark theory explains all experiments carried out to date, its
intrinsic asymmetry between quarks and anti-quarks appears to be too weak to account for
the matter-anti-matter asymmetry implied by our observable Universe. Entirely new ideas
and phenomena may well be required before this puzzle is understood.
This site last modified June 29, 2010.