The Mystery of the Missing Antimatter

The Mystery of the Missing Antimatter

Helen R. Quinn
Yossi Nir
Illustrations by Rutu Modan
Copyright Date: 2008
Pages: 292
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  • Book Info
    The Mystery of the Missing Antimatter
    Book Description:

    In the first fractions of a second after the Big Bang lingers a question at the heart of our very existence: why does the universe contain matter but almost no antimatter? The laws of physics tell us that equal amounts of matter and antimatter were produced in the early universe--but then, something odd happened. Matter won out over antimatter; had it not, the universe today would be dark and barren.

    But how and when did this occur? Helen Quinn and Yossi Nir guide readers into the very heart of this mystery--and along the way offer an exhilarating grand tour of cutting-edge physics. They explain both the history of antimatter and recent advances in particle physics and cosmology. And they discuss the enormous, high-precision experiments that particle physicists are undertaking to test the laws of physics at their most fundamental levels--and how their results reveal tantalizing new possibilities for solving this puzzle at the heart of the cosmos.

    The Mystery of the Missing Antimatteris at once a history of ideas and an exploration of modern science and the frontiers of human knowledge. This exciting, accessible book reveals how the interplay of theory and experimentation advances our understanding and redefines the questions we ask about our universe.

    eISBN: 978-1-4008-3571-3
    Subjects: Physics, Astronomy

Table of Contents

  1. Front Matter
    (pp. i-vi)
  2. Table of Contents
    (pp. vii-x)
    (pp. xi-xiv)
    (pp. 1-6)

    In the beginning—what was the beginning? Every culture asks this question. Traditionally each finds some answer, a creation myth, a cosmology. These stories satisfy an innate human longing to know about our origins. Only recently has our scientific understanding of the history of the Universe progressed to the point that we can begin to formulate a scientifically based answer—a scientific cosmology. We know that the Universe is evolving and we understand many facets of its history. We know its age, about fourteen billion years! We can ask, and often even answer, detailed questions about the very earliest times,...

    (pp. 7-20)

    You cannot begin to do physics, or any form of science for that matter, without making one fundamental assumption. All science is based on it. We must assume that there are underlying laws of nature that are the same whenever and wherever we look at the world. Without this assumption there would be no predictive power to science. Whatever we might decipher of the laws of nature today in our laboratory would not be useful to predict what would happen tomorrow or in any other place. If the physical laws are independent of place and time they are universal; the...

    (pp. 21-35)

    Let us set our clock to zero at the time of the Big Bang and follow the story of the Universe, this time running our clock forward and watching some tiny region of the Universe as it expands. But before we get into details about matter and antimatter, we need the overall picture so, to start with, we will run the clock forward focusing on what happens to radiation and particles, but not distinguishing between matter-particles and antimatter-particles for now. We will summarize the evidence that the Universe is indeed expanding and that Hubble and those who followed him got...

    (pp. 36-50)

    To talk about the physics of matter and antimatter, and even dark matter, we need first to define these words as physicists use them. We physicists have a tricky habit of redefining words as we learn more about the world. Like Humpty Dumpty, we assume that the ability to redefine the meaning of a word depends who is the master. (Many of us even forget we have redefined a word, expressing surprise when others fail to grasp the concepts described so precisely by our specialized and restricted physical or mathematical meaning of a word.) In everyday usage words typically have...

    (pp. 51-56)

    Wolfgang Pauli (1900–1958; Nobel Prize 1945) was not a man to be indefinite about things. Born in Austria and educated in Germany, he spent most of his working years in Zurich, Switzerland. He was a man who held strong opinions, and had no time for fuzzy thinking. The sight of him in the front row of the audience for a physics talk, shaking his head and muttering “ganz falsch” (completely wrong), caused many a speaker some nervous moments. Pauli had played an important role in many aspects of the development of quantum mechanics. He was a very active participant...

  9. 6 MESONS
    (pp. 57-62)

    Before we can describe a very important ingredient that was missing in our attempt to follow the cosmological history of matter and antimatter, we need to get to know a few more of the particle participants in this story, beyond the particles that we have encountered so far (protons, neutrons, electrons, and neutrinos and their antiparticles).

    It did not take long after the idea of antimatter and its experimental verification for the particle story to become even more complicated. In 1935, a remarkable Japanese physicist, Hideki Yukawa (1907–1981; Nobel Prize 1949), predicted an entirely new type of particle which...

    (pp. 63-72)

    We have described many of the important particles that play a role in the story of matter and antimatter. Now we need to review some other aspects of particle theories that are important in this story. Remember that our history of the Universe contained some critical assumptions. We assumed that the early Universe had equal numbers of particles and antiparticles. We took into account that matching particle–antiparticle pairs can annihilate into pure radiation at any temperature but can be produced from pure radiation only when the temperature is high enough. We found that this theory said that the present...

    (pp. 73-79)

    Weak interactions violate the symmetry called parity. But what does this have to do with the symmetry between matter and antimatter? Let us consider that case next.

    Imagine another day at the races, with even more bizarre race cars: one of them is made of matter (and driven by a driver) and the other is made of antimatter (and, of course, needs an antidriver). We also make them mirror images of each other. We do so because an antimatter particle, in the sense of the Dirac equation, not only carries opposite charge to that of the matter particle, but would...

    (pp. 80-90)

    Wolfgang Pauli’s letter to the “Radioactive Gentlemen” suggested the neutrino. Now another letter written by Wolfgang Pauli has a place in our story. He was, as far as we know, the first to discuss the fact that the observation of the positron raised a deep new question. Why is the world populated with electrons but not positrons? This cosmological issue is the central mystery of our book—why are we surrounded by matter but no antimatter? Pauli raised it in a remarkable June 1933 letter to Werner Heisenberg. He said: “ . . . I do not believe in ....

    (pp. 91-110)

    Particle physics took a great leap forward in the 1960s with the idea that many so-called elementary particles—protons, neutrons, mesons, indeed, all particles that are subject to the strong nuclear force, collectively known ashadrons—might not be so elementary after all. By that time over a hundred types of hadrons had been observed in accelerator experiments; physicists had found many additional baryons beyond the proton and neutron, and many mesons in addition to the pion and kaon that we have so far introduced. The notion that all these different types of particles were distinct elementary building blocks of...

    (pp. 111-120)

    The mathematical language used in modern particle physics to understand the fundamental interactions and to write the Standard Model is that ofquantum field theory. This is a general mathematical formalism, within which one can write many possible physical theories. But only one of them will describe the world we live in. Our job as particle physicists is to decipher what that one might be. The field theory formalism supersedes but builds on the Dirac equation. In this chapter, we will explain how physicists think about energy, and what they mean by the word fields. We will try to give...

    (pp. 121-131)

    Like the child who refuses to accept the parent’s answer “because I say so,” so the particle physicists continue to seek deeper reasons for the rules they have discovered. A major role in these answers in modern theories of particle physics is played by symmetries. Remember that, in the physicists’ language, the termsymmetryrefers to an invariance of the equations that describe a physical system. The fact that a symmetry and an invariance are related concepts is obvious enough—a smooth ball has spherical symmetry and its appearance is invariant under rotation.

    As described above, in the mathematical language...

    (pp. 132-139)

    So now we see that the basic building blocks of the Standard Model are its symmetries. Remember that when physicists say symmetries they mean the invariances of the energy function under certain redefinitions of the fields or their coordinates. These symmetries were first noticed through empirical discovery of properties such as conservation laws.

    Let us begin again with the familiar case of electromagnetism. Here there are three crucial observational facts: Any electric charge (of a particle in isolation) could be quantified in integer (or whole number) multiples of the charge of one electron. There is a universal force law between...

    (pp. 140-158)

    There is a big puzzle in the Standard Model. The W- and Z-bosons have spin-1. Spin-1 force-mediating bosons require that we build a gauge theory—a theory with a gauge (or local) symmetry, which we described in chapter 12. We know no other way to get a well-behaved theory for spin-1 particles. But we also know that gauge theories predict massless spin-1 bosons and universal coupling. TheW±andZ0are massive and the W-boson does not seem to have universal couplings with the various quark flavors.

    Furthermore, in order to reproduce the observed parity violations of weak interactions, the...

    (pp. 159-167)

    Armed with modern particle physics or, more concretely, with the Standard Model, to enrich our understanding, how does the history of matter and antimatter look now? In our previous run through the history of the Universe, we suggested a possibility known as baryogenesis. Does the story work if we use the Standard Model to set the physics parameters?

    Baryogenesis starts from equal amounts of matter and antimatter. The imbalance between them develops as a result of CP-violating interactions. The small difference in the physics of matter and that of antimatter determines how much matter would be there in the present...

    (pp. 168-179)

    The history of particle physics we have described throughout this book is based, to a large extent, on the invention of accelerators as tools to probe matter structure at subnuclear scales. In this chapter we describe what these machines are like, particularly the modern versions that are now testing the Standard Model picture of CP violation in great detail.

    You can think of an accelerator as a gigantic microscope, allowing physicists to view phenomena at ever smaller scales as the energy of the accelerated particles is increased. To understand this point you need to know two facts, namely, that particles...

    (pp. 180-193)

    The observed baryon asymmetry of the Universe suggests that the Standard Model does not provide the full picture of CP violation. So we look for new interactions that violate CP. To make real progress, we need experiments that can discover these new sources of CP violation. How can we attempt to do that? There are two different approaches to try to find new sources of CP violation, effects not included in the current Standard Model. First, we can measure processes where the Standard Model predicts that there are no CP violating effects or that these effects are very, very small....

    (pp. 194-205)

    You might think that physicists would be celebrating the successes of the Standard Model. Indeed we do, but we also long to find places where it is wrong and focus much of our efforts on that. Why do that? Partly it is a matter of experience. In the past there have been times when the understanding of physics seemed almost complete, yet the explanation of what seemed a small discrepancy opened up entire new vistas.

    Furthermore, there are certain features of the Standard Model that seem to be achieved very artificially. These features can, however, appear naturally, as side effects...

    (pp. 206-221)

    To return to more mundane extensions of the Standard Model—we already know that neutrinos have mass, and that alone requires us to extend our original Standard Model theory. Indeed, this fact leads us to another, quite different attempt to solve the mystery of the missing antimatter, a scenario that involves neutrinos. It took physics detectives with three different types of expertise to collect the clues that led us to suspect that neutrinos might be to blame for the baryon asymmetry: astrophysicists, particle physicists, and cosmologists. Let us begin with the astrophysics side of the story, and its interplay with...

    (pp. 222-230)

    Could the asymmetry between matter and antimatter in the Universe, observed through its baryon content, have its origins in an asymmetry in the lepton content of the universe? The answer is—maybe. This possibility, first suggested by two Japanese physicists, Masataka Fukugita and Tsutomu Yanagida in 1986, has been namedleptogenesis. At present it is the prime suspect for solving our matter–antimatter mystery, the most plausible scenario to explain the puzzle of the missing antimatter. How does it work? Why did it become an attractive scenario only recently? What can we learn (and, no less important, what can we...

  24. 21 FINALE
    (pp. 231-232)

    So we come to the end of our book, but not to the end of the tale. We have explored some grand themes: the Universe; matter and antimatter; energy; symmetry and universality. These themes are interwoven together in the fabric of physics, developed and explored by experiments in our particle laboratories and in space. Anything we learn about one affects our understanding of the others. Of course we must rely on these experiments and observations to test and refine out theories, guided by whether our ideas can describe the world around us. Carefully designed probes must be made, alert for...

    (pp. 233-272)
  26. INDEX
    (pp. 273-278)