universe is a system with broken symmetries.
In the very early universe, the strong, weak, and electromagnetic
forces were indistinguishable. At
some point as things cooled off, the symmetry between these forces broke and
the three forces went their separate ways to become the three very different
forces that, along with gravity, operate in our universe.
today, at the microscopic scale standard symmetries are usually present
between antimatter and matter (charge-conjugation invariance or
“C-symmetry”) and between the two directions of time (time-reversal
invariance or “T-symmetry”). Matter
and antimatter interactions are subject to the same forces and look the same.
Fundamental interactions look the same when run forward or backwards in
we know that in the early universe some unknown processes broke the C-symmetry
and slightly favored the production of matter over antimatter, leading to an
excess of matter over antimatter of about one part per billion.
As the universe evolved and cooled and after almost all of the
matter-antimatter annihilation was over, we were left with the surviving
matter residue: lots of protons and electrons and almost no antiprotons and
positrons. That broken C-symmetry
of the early universe has made possible our matter-based world, and indeed our
despite the time-reversal invariance or T-symmetry of most of the fundamental
interactions at the microscopic scale, our universe presents us with a
built-in “arrow of time” that is quite obvious but has unknown origins.
Think about a movie that can be run either forward or backwards showing
some event. If the movie shows the
collision and interactions of fundamental particles, in almost all cases (see
below) there are no clues as to whether the movie was running forwards or
backwards. But think of a movie
showing some macroscopic event, an egg hitting the floor or a high dive into a
swimming pool. The
backward-running version would be quite obvious, and would seem unphysical and
contrary to experience. Eggs do
not gather their liquid parts, assemble a shell around them, and leap upward.
Water waves do not converge in a swimming pool to propel a diver up in
the air. The arrow of time, at the
psychological level, is also obvious. We
can remember the past but not the future. We
can take actions that can change the future but not the past.
The broken T-symmetry of the macroscopic world also makes our existence
possible. Evolution cannot happen
in a time-symmetric world.
addition to these symmetries applying to charge and time, there is a third
symmetry, the symmetry of space. Just
as T-symmetry is concerned with the reversal of the time direction, parity
invariance or “P-symmetry” is concerned with phenomena that may change or
appear different when the three space coordinate axes are reversed.
When you view an object in a mirror, the image you see has a
reversed coordinate axis in the direction perpendicular to the plane of the
mirror. This is roughly
equivalent to reversing all three spatial directions.
In both cases the letters on a page are reversed, clockwise rotations
become counterclockwise, and right-handed screw threads become left-handed
the 1950s, all physicists assumed that parity was a good symmetry, that all
physical processes looked the same in mirror-image as they did when viewed
directly. Then the first blow to
symmetry preservation arrived. It
was discovered that for the weak interaction, the physical force that can
change neutrons to protons or vice versa in the radioactive beta-decay
process, there was a massive violation of P-symmetry or parity invariance.
Spin-oriented nuclei emitted electrons in a preferred direction.
Neutrinos are always emitted with a left-handed (clockwise) spin if
viewed from the front. One could
watch a movie of a beta decay process and tell whether or not the images had
been mirror-reversed. For the weak
force, nature had an intrinsic “handedness”.
was noted in studying violations of P-symmetry, however, that this lack of
mirror symmetry was reversed for beta-decaying systems involving the emission
of antimatter positrons instead of matter electrons.
Antineutrinos are always emitted with a right-handed (counterclockwise)
spin if viewed from the front. Therefore,
it was assumed that even if P-symmetry was violated, CP-symmetry, involving
simultaneously reversing the space axes and converting matter to antimatter,
was preserved. There is a general
theorem in theoretical physics that
second blow to symmetry preservation arrived in 1964, when Val Fitch and Jim
Cronin discovered a violation of CP-symmetry in the decays of neutral K mesons
(which are quark-antiquark combinations involving a strange quark) into pi
mesons. This is equivalent to
finding a preferred time direction in the microscopic world.
The movie of a K meson decay process would have an observable change in
if it was running backwards instead of forward. Recent
studies of processes involving B mesons (quark-antiquark combinations
involving a bottom quark) have shown similar CP-symmetry violations.
The CP violations that have been observed in these systems are, however, too weak to explain matter dominance. While hinting at a preference for matter over antimatter, they are not strong enough to have produced the part per billion dominance of matter over antimatter in the early universe. The nature of the forces that produced that matter dominance remains as one of the major unsolved mysteries of physics.
we now have a way of re-creating the conditions of the early universe in the
laboratory, using the Relativistic Heavy Ion Collider (RHIC) facility at
Brookhaven National Laboratory. The
RHIC facility brings gold (and lighter) nuclei into collision at energies of
up to 200 GeV per nucleon, producing a relativistic fireball that replicates
conditions in the early universe at about one microsecond after the Big Bang.
The temperatures reached in RHIC collisions are several trillion
degrees Celsius, about 250,000 times hotter than the central temperature of
our Sun. At such temperatures, a
strongly-interacting phase of nuclear matter, a quark-gluon plasma, is
expected. Further, the highly
charged nuclei passing each other in RHIC collisions with a slight offset can
produce extremely intense magnetic fields that can reach strengths of up to
about 1015 tesla. These
conditions make it possible to look for possible symmetry breaking in strong
interactions operating in collisions in this new and unprecedented
new analysis of
has studied collisions between gold nuclei and between copper nuclei at
collision energies of 200 GeV per nucleon.
At this collision energy, the two nuclei are heading toward each other
at 99.9957% of the speed of light or only 4.32 parts in 100,000 below light
speed. No all such collisions are
head-on, but one can distinguish the offset or “centrality” of the
colliding systems by counting the number of neutrons that were
non-participants and went straight ahead after the collision.
In this way, the collisions can be broken up into eight centrality
groups ranging from head-on collisions to near misses.
In offset collisions there is a tendency for there to be more particles
produced in the “reaction plane”, which includes the beam and the
collision offset, than in the direction perpendicular to it.
Since thousands of particles are produced in a typical RHIC collision,
finding the preferred emission plane gives a good estimate of the reaction
plane of each collision, and each particle can be characterized in terms of
the angle perpendicular to the beam that it makes with the reaction plane.
collision events have randomly oriented reaction planes and magnetic field
directions, most of the potentially observable effects of a hypothetical local
parity violation are averaged out. However,
the STAR Collaboration has looked for an event-by-event signal in the form of
two-particle correlations between the particle emission angles with respect to
the reaction plane of particles of the same sign of electric charge.
To eliminate issues of how accurately the reaction plane was
determined, they have moved to three-particle correlations that replace the
reaction plane angle by the emission angle of all the other particles observed
in the collision.
The results show an unambiguous correlation in the emission of pairs of particles of the same electric charge. There is no similar correlation between pairs of particles with opposite electric charge. The collisions studied prefer to emit same-charge particles in the same direction, which is a strong indication of a local violation of P-symmetry or parity. The effect is present in both gold-gold and copper-copper collisions but stronger in the latter, and it is strongest when the collision offset is about half a nuclear diameter. Theoretical collision calculations that do not include any expectation of local parity violations predict only weak correlations having the opposite sign from those observed, and predict no difference in the correlations of same-charge and opposite charge particle pairs. Thus, there is good evidence that local parity violations occur in RHIC collisions.
mentioned above, the theory that stimulated the STAR investigation of parity
violations also suggested that there should be local violations of CP symmetry
created by the high temperature gluon fields in the environment of RHIC
collisions and in the conditions of the early universe.
Can this be the missing key to understanding the dominance of matter
over antimatter in our universe?
The sign of the possible local CP violations at STAR appears to be in
the wrong direction and cannot, if taken at face value, explain the
matter-dominance of the universe. However,
there are many questions raised by the initial observation that remain to be
answered, and these should provide new insights into how such local symmetry
violations occur, and into their implications for the universe as a whole.
It is expected that the STAR results will checked by other experiments,
will be extended to lower energies at RHIC and to higher energies at the LHC,
and will trigger more theoretical activity on the issues of local symmetry
may be on the verge of answering one of the major questions about the nature
of our universe: why is there more matter than antimatter?
Watch this column for further results.
Electronic reprints of about 150 "The Alternate View" columns by
John G. Cramer, previously published in
I. Abelev, et al, “Observation of charge-dependent azimuthal correlations
and possible strong parity violations in heavy ion collisions”, arXiv
(See also http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=1073
E. Kharzeev “Parity violation in hot QCD: why it can happen, and how
to look for it”, , Physics Letters B633,
260-264 (2006), arXiv
E. Kharzeev, L. D. McLerran, and H. J. Warringa, “The effects of topological
charge change in heavy ion collisions: ‘Event by event P and CP
violation’”, Nucl.Phys.A803, 227-253 (2008), arXiv preprint 0711.0950
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