Section 7: Matter and Antimatter
In his speech accepting the 1933 Nobel Prize for predicting the positron, Paul Dirac speculated on the existence of anti-worlds in which everything consisted of antimatter. More than three-quarters of a century later, we have experimentally observed that every particle has a corresponding antiparticle with the opposite quantum properties. Particle physicists have collided electrons with positrons, as well as protons with anti-protons, to produce new kinds of particle-antiparticle pairs. This was how scientists at Fermilab's Tevatron collider discovered the top quark.
There are a few possible exceptions to the general rule that an antiparticle exists for every particle. As we saw in the last section, the neutrino may be its own antiparticle. But this remains an open question that experimenters will try to answer in the next decade.
Figure 21: Matter and antimatter: An imperfect mirror.
Source: © Fermilab. More info
However, astronomers have not detected the smoking gun for anti-worlds: energetic forms of high-frequency radiation known as gamma rays that would be produced when anti-hydrogen and hydrogen gas annihilate each other along the boundary region between clumps of matter and antimatter. The lack of any signal suggests that Dirac's anti-worlds do not exist in our universe. But the biggest problem for physicists today is not the absence of antimatter. Rather, it is how to explain why the universe contains any matter at all. To understand this, we need to go back to the beginning.
Astrophysicists have strong circumstantial evidence that the universe started with a Big Bang, an explosion assumed to have produced matter and antimatter. Conservation principles require that matter and antimatter pairs appear together. But if every particle created in the Big Bang had its own antiparticle, why did they not eventually annihilate, leaving an empty universe filled only with radiation? Today, ordinary matter accounts for just about 4 percent of the universe's total energy budget. (Dark matter and dark energy make up the rest, as we shall see in later units.) Five percent does not seem like much, but the Standard Model cannot explain how even this much matter remained after the fiery particle soup of the early universe cooled and expanded to form the galaxies, stars, and planets we see today.
Pondering the question of how any matter could have survived, Russian physicist and dissident Andrei Sakharov concluded that our world could have come about only if there exists an asymmetry between matter and antimatter known as "CP violation." CP is the acronym for charge conjugation, C, and parity, P. Charge conjugation is an operation that changes a matter particle to its corresponding antiparticle. Parity creates a mirror image of a particle or system, reversing left and right. Both charge and parity must be flipped to change matter to antimatter with the correct particle "helicity," the term that indicates left- or right-handedness.
Differences in behavior
Broken CP symmetry would imply that matter and antimatter behave differently. It would mean, for example, that if we were to discover intelligent life in a distant part of the universe, we could ask their physicists about particle reactions they had observed and from their answers tell if their world consisted of matter or antimatter. That would be a good thing to know before embarking on a visit, even if we are quite sure the universe does not contain a lot of antimatter.
Figure 22: Neutral kaon oscillation.
Source: © Wikimedia Commons, GNU Free Documentation License, Version 1.2. Author: Bambaiah, 22 June 2005. More info
Physicists know that particle reactions involving the electromagnetic and strong forces are symmetric with respect to C, P, and their product, CP. In other words, they conserve CP. But it turns out that weak interactions, such as beta decay, are not symmetric with respect to CP. Princeton University physicists James Cronin and Val Fitch first demonstrated that in 1964 in an experiment involving neutral kaons. These mesons can oscillate between matter and antimatter states: The combination of a strange quark and an anti-down quark changes into an anti-strange quark and a down quark.
This oscillation, or mixing, is analogous to that observed more than three decades later between neutrino flavors. But Cronin and Fitch found that the oscillation rate was not exactly the same in both directions—a clear violation of the expected symmetry between matter and antimatter.
More recently, physicists have measured CP violation with very high precision in B mesons. These differ from kaons by substituting a bottom quark for the strange quark. They are produced in copious quantities in machines called B factories. These contain particle colliders to produce the B mesons and detectors that identify the particles produced when the mesons decay. By producing literally hundreds of millions of the mesons each year, they give scientists a picture of the processes at work in the early universe—and enough unusual decays to provide some understanding of that environment.
Figure 23: Detector under construction at SLAC's B factory.
Source: © SLAC National Accelerator Laboratory. More info
In the 1990s, engineers at SLAC and in Japan built B factories for precision studies of CP violation in B decay. Those studies, they hoped, would provide a window into physics beyond the Standard Model. That's because, although CP violation is necessary to create a matter-dominated universe, the amount of CP violation in the Standard Model falls orders of magnitude too short to account for the makeup of our world. Yet, despite successful runs that have produced hundreds of millions of mesons, the B factories have observed no detectable difference from the predictions of the Standard Model.
This is why physicists have expressed so much interest in the possibility of CP violation in neutrinos. The early universe was flooded with neutrinos. Perhaps, the speculation goes, they could have caused the tiny asymmetry between matter and antimatter that eventually allowed roughly one in 10 billion matter particles to escape annihilation—producing enough excess matter to create the universe, including our little blue orb circling around a modest star on the outskirts of an ordinary galaxy that we call "the Milky Way."