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Beauty and the B-Factory

John G. Cramer

Alternate View Column AV-67
Keywords: B mesons bottom quark beauty CP violation antimatter asymmetry
Published in the September-1994 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 2/9/94 and is copyrighted ©1994 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.


    The galaxies, stars, planets, and people of our universe are made of matter, not antimatter. This is a conclusion deduced from the convincing indirect evidence of cosmic ray studies and astronomical observations. There are a few antiprotons in cosmic rays but no sign of an antiproton solar wind from anti-stars or anti-galaxies, no gamma rays from nearby anti-planets or asteroids or from great annihilation interfaces where matter and antimatter come violently together in intergalactic space.

    Yet normal particle interactions produce matter and antimatter in equal amounts. If matter and antimatter in the early universe had been in perfect balance, they would long since have been annihilated out of existence, leaving behind a universe of photons and a few electrons. We would not be here to study such a universe, since our very existence depends on the gross excess of matter over antimatter now present. Where did the matter come from? What happened in the early stages of the Big Bang that produced the contemporary dominance of matter?

    The short answer to these questions is that we don't know. The Standard Model of particle physics can accommodate a matter preference (called a "charge + parity" or CP violation). However, in the Standard Model the CP violation is characterized by a single parameter, and the underlying mechanism is not understood. We do not even know if all aspects of CP violation that seems to be wired into our universe can be fitted into the Standard Model. The path to better understanding requires new experimental data that must be obtained by studying an exotic flavor of heavy quark, the "bottom" or "beauty" quark as it behaves in a very peculiar particle, the Bo meson.

    In 1968 James W. Cronin and Val L. Fitch of Princeton University and their group working at Brookhaven National Laboratory discovered a CP violation in the decay of KS (or K-short) mesons. This breakthrough experiment, which earned them the 1980 Nobel Prize in Physics, was one of the most surprising and unexpected discoveries of 20th century physics. Before discussing B mesons we will look a bit deeper into the CP violation as it was discovered in K mesons.

    At the quantum level, Nature seems intolerant of identical twins. Whenever twin "states" of a quantum system are so similar as to be indistinguishable, they mix together to produce two other states that are distinguishable. A well known particle physics example of this prejudice against twins is the quantum mixing of the Ko meson with its antimatter twin the Ko-bar.

    The Ko has a mass of 498 MeV in energy units, intermediate between that of an electron (0.511 MeV) and a proton (938 MeV) and might be considered the light-weight little brother of the Bo meson (5,279 MeV) which is describe below. At the quark level, the Ko is made of a "down" quark (matter) and an "anti-strange" quark (antimatter). Similarly, the Ko-bar is composed of an anti-down quark (antimatter) and a strange quark (matter). Both Kos have no electrical charge, zero intrinsic angular momentum (or spin), and precisely the same mass. Thus we have two particles with very different (and opposite) internal structures but with the same charge and mass. On the basis of these observables they are indistinguishable twins.

    Since Nature doesn't tolerate identical twins in quantum states, the two indistinguishable Ko states mix to form two new states that are distinguishable. The Ko and Ko-bar are scrambled together. What emerges are two new mesons, the KS meson (K-short) with the short decay half-life of about 10-10 seconds and the infinitesimally heavier KL meson (K-long) with a half-life of about 5.2 x 10-8 seconds, about 581 times that of the KS. The KL and KS masses differ by only about 3 one-millionths of an electron volt.

    Fitch and Cronin discovered that decay of the KS meson into lighter particles violated some of the most cherished symmetry principles of theoretical physics: parity (i.e., mirror-image symmetry), charge conjugation (matter-antimatter symmetry), and time-reversal invariance (the symmetry of time-running-forward vs. time-running-backwards). The KS decay showed a definite preference for "left-handedness" over "right-handedness". Later experiments indicated that the decay of a KL into p± mesons and electrons or positrons showed a tiny preference for p+ over p- (matter over antimatter) and for one direction of time over the other. If a movie were made of certain particle reactions involving the KL , one could in principle tell if the film were running forward or backwards through the projector.

    Cosmologists realized the implications of the preference for matter particles in the KL decay processes: if this kind of imbalance were present in some particle decay processes in the early universe, it could explain why more protons than antiprotons were created in the Big Bang. After most of the protons and antiprotons had annihilated each other, the leftover protons (about one per million) would account for the dominance of matter in the universe today.

    There is, however, a conspicuous flaw in this logic: the KL has a mass that is only about half that of protons and neutrons. From simple considerations of conservation of mass-energy, there is no scenario under which today's protons could have been created in KL processes in the early Big Bang.

    However, the lesson learned from KL decays is instructive. If there were a similar particle more massive than a proton that also exhibited a preference for matter similar to that shown by the KL decay, we might have the ancestor of all modern protons and neutrons in the universe. And, as it turns out, there probably is such a particle -- it's the Bo meson.

    The Bo has a mass of more than 5 proton masses and decays with a lifetime of about 10-12 seconds (about 100 times faster than the KS). There is preliminary evidence that the B0 meson and its antimatter twin, the Bo-bar meson, mix to form a B-long and a B-short, mixed particles similar to the KL and KS but much more massive. If the hypothetical BL and/or BS shows a preference for matter over antimatter similar to that found in the Ko system, this could explain what happened in the first stages of the Big Bang to create the present matter-dominated universe and would also give a new and independent look at the nature of CP violations.

    There have been attempts to study the Bo system using existing particle accelerators, but the very small probability of producing these exotic particles has limited what can be learned. What is needed is a dedicated high-current accelerator to produce B meson and some years of experimental measurements on Bo decay processes before data and answers are in hand. The Department of Energy has recently announced plans to build such a dedicated accelerator. It will collide electrons and positrons in a way that will produce enough Bos to address questions at the heart of the matter-antimatter imbalance.

    There was strong competition between the CESR (Cornell Electron Storage Ring) laboratory at Cornell University in upstate New York, supported by the National Science Foundation, and the SLAC (Stanford Linear Accelerator Center) laboratory operated by Stanford University in Palo Alto, California, supported by the Department of Energy, for approval to construct the new B-factory. Although Cornell's proposal was less expensive, a review committee decided that the SLAC proposal had the greatest scientific merit, and recommended construction of the B-factory at SLAC. When President Clinton visited California last Fall, he made a special stop at SLAC to announce that they had won the competition and had received the go-ahead to construct the new machine.

    How do you mass-produce Bos in a factory? The trick is to produce bottom (or as the Europeans say, beauty) quarks in matter-antimatter pairs that can then combine with more ordinary down quarks to make Bos and Bo-bars. The b-quarks were discovered by Leon Lederman and his group by producing the upsilon or Yo(1S) meson, a special particle state with a mass of 9,460 MeV in which a bottom and anti-bottom quark are locked together in a tight mutual orbit. However, the Yo(1S) meson cannot decay into a pair of Bos because it does not have enough mass-energy. Fortunately for B physics, there is another state corresponding to a higher orbit of the b-quarks, the Yo(4S) meson, which has a mass of 10,580 MeV, just 22 MeV more massive than a pair of Bos. About half the time a Yo(4S) meson decays to a Bo + Bo-bar pair, so that the production of Bos is very efficient.

    There is, however, another problem: the Bo mesons that are produced decay from a near standstill in about 10-12 seconds (perhaps a bit longer in the case of the BL), making it very difficult to study the decay products. The solution to this problem is to "boost" the whole decaying system in order to give the decaying B particles and their decay products a significant forward velocity. This is done by using an asymmetric particle collider, i.e., using two particle beams (electrons and positrons) moving in opposite directions and brought into collisions, but with one beam made considerably more energetic than the other. The SLAC design for the B-factory will modify the venerable PEP storage ring for asymmetric collisions, bringing 9 GeV electrons into collision with 3.1 GeV positrons with a luminosity (in effect, collision probability) of 3 x 1033 cm-2 sec-1, about 50 times greater than that of the old PEP storage ring. The B-Factory project was included in the new Department of Energy budget just presented to the 1994 Congress. It will cost a total of $273 million in construction funds to be spread over 5 years. It will go into operation in about the year 2000.

    What will be learned from building yet another particle accelerator? The Standard Model of particle physics (which has recently been called TONE, the Theory Of Nearly Everything) works very well but contains some 17 adjustable parameters with values that must be "put in by hand" rather than derived from more fundamental principles. To make any further progress in understanding the underlying structure of our universe, we need to somehow get beyond the Standard Model. There are currently three major attempts to do this: the RHIC project at Brookhaven which will attempt to break the Standard Model with the extremely high energy densities produced by colliding heavy nuclei, the LHC project at CERN (successor of the SSC) which will attempt to break the Standard Model in the "Higgs sector" by searching for H particles embodying the splitting of the strong and electro-weak forces, and the newly begun B-Factory which will test the most peculiar aspect of the Standard Model, its CP violating processes.

    In the coming decades, experimental physicists will be working very hard to test to destruction the theory that presently gives us the best description of our universe at the most fundamental level. It's going to be fun to watch, and even more fun to participate in.


Neutral K Mesons:
Robert Adair, Scientific American 258, #2 (February, 1988).

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