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Bubbles of Broken Symmetry

by John G. Cramer

Alternate View Column AV-153
Keywords: RHIC, collider, STAR, collaboration, heavy, ion, collisions, bubbles, broken, symmetry, parity, violation, strong, interaction
Published in the July-August-2010 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 3
/2/2010 and is copyrighted ©2010 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

Our 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.

Even 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 time.

However, 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 very existence.

Further, 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.

In 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 screw threads.

Until 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”.

It 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 CPT -symmetry, invariance under simultaneous reversal of the space and time directions and the swapping of matter and antimatter, must always be preserved for very fundamental reasons.  Therefore, breaking CP-symmetry is equivalent to breaking time reversal invariance or T-symmetry.  For a time it was believed that this symmetry, at least, was preserved at the fundamental level.

The 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.

Fortunately, 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 STAR detector is one of the two major detectors at RHIC.  Since it’s inception in the early 1990s I have been a member of the STAR Collaboration, the group that built and operates the detector.  STAR is a large time-projection chamber detector placed inside a 0.5 tesla magnetic solenoid located at the 6-oclock position in the RHIC collider ring.

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 environment.

A new analysis of STAR data may provide a needed clue into the mysteries of fundamental symmetries and their breaking in the early universe.  There are experimental and theoretical reasons for expecting that any “global” or overall breaking of P-symmetry in strong interactions should be extremely small, less than one part on 1010, at all energies.  However, several theorists have suggested that in small regions of space-time in dense systems at high temperatures, the fields produced by gluons can create “local” violations of the P, PC, and T symmetries.  The theory suggests that in these localized regions, particles of the same electric charge should be preferentially emitted in the direction of the local magnetic field and either parallel or anti-parallel to it, thereby producing a symmetry violation.

STAR 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.

Because 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.

As 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?

Perhaps.  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 breaking.

We 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.


STAR Parity-Violation Results:

R. I. Abelev, et al, “Observation of charge-dependent azimuthal correlations and possible strong parity violations in heavy ion collisions”, arXiv preprint 0909.1717v1 [nucl-ex].

(See also

Theory of Local Symmetry breaking:

D. E. Kharzeev “Parity violation in hot QCD: why it can happen, and how to look for it”, , Physics Letters B633, 260-264 (2006),  arXiv preprint 0406125 [hep-ph].

D. 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  [hep-ph].  

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