In September 2016 a new result was announced that bears on the dominance of matter over antimatter in the universe. But before discussing the discovery, I want to review what particle physics has to say about matter and antimatter. There are three types of fundamental particles in the universe: leptons, quarks, and force-mediating bosons. The first two of these are fractional-spin fermions that can have the form of either matter or antimatter.
Leptons include three uncharged neutrino types and three electrically charged leptons. The lightest charged lepton is the electron. The two heavier charged leptons, the mu and the tau, ultimately decay to neutrinos and an electron.
Quarks come in six "flavors" and cannot exist as free fundamental particles. Quarks always exist in our universe either as two-quark mesons or three-quark baryons. Mesons are a combination of a matter quark and an antimatter quark held together by the strong force. The lightest charged meson; the pi-plus (or p+) meson is a combination of an up quark and an anti-down quark. The p+ is unstable, with a mean lifetime of 26 nanoseconds.
Baryons are composite particles, a combination of three matter quarks. The lightest charged baryon, the proton is a combination of two up quarks and one down quark held together by the strong force. The proton is stable, but its neutral baryon cousin, the neutron is not. In free space the neutron, a combination of two down quarks and one up quark, decays to a proton, an electron, and an antineutrino with a mean lifetime of 881.5 seconds. However, when bound in a nucleus the neutron can be stable. The antimatter equivalent of the electron is the positron.
The antimatter equivalent of the proton is the antiproton, made of two anti-up quarks and one anti-down quark. Protons and electrons are stable and together represent 5% of the mass-energy of the universe. On the other hand, the only antiprotons and positrons that we observe are found in cosmic rays or are produced in high energy particle collisions. The fundamental interactions that govern particle collisions are very even-handed about the creation of matter and antimatter. This creates a puzzle: why is matter in the form of protons and electrons dominant in the universe, when the interactions acting in the hot dense early stages of the Big Bang should have produced equal quantities of matter and antimatter? What process made all of the matter around us? This is one of the major unanswered questions of contemporary physics and cosmology.
Suppose that early interactions of the Big Bang did produce equal amounts of matter and antimatter. Later, in the hot dense "soup" of the early universe during the later stages of expansion, most of the protons and antiprotons (and most of the electrons and positrons) should have found each other, combined, and annihilated, leaving behind a matter-less universe that would be only thinly populated by photons, with perhaps a few neutrinos and antineutrinos and a few electrons and positrons.
Instead, our universe contains vast clusters of galaxies containing stars, planets, and us, all composed of 100% matter and 0% antimatter. How and why did this dominance of matter over antimatter occur? What broke the expected overall matter-antimatter symmetry?
Until recently, the only exception to the almost universal matter-antimatter symmetry present in particle physics was found in the weak-interaction decays of a few peculiar mesons. One of these is the K0 meson, which is made of a down quark and an antimatter strange quark. Its antimatter twin, the anti-K0 or K0-bar, is made of a strange quark and an anti-down quark. Both the K0 and the K0-bar have the same zero electrical charge, zero spin, negative parity (spatial mirror image symmetry), and both have the same mass (about half a proton mass). Therefore, on the basis of all the observables they cannot be told apart.
Quantum mechanics tells us that when two quantum states cannot be distinguished, a peculiar thing happens: they "mix". The two indistinguishable states mix to form two new states that are distinguishable. In the case of neutral kaons, the K0 and K0-bar combine in two different ways to make the KS particle (K-short), which decays in about 10-10 seconds, and the KL particle (K-long), which decays 581 times more slowly.
The decay of the KL meson shows what is called a "CP violation", a preference for matter over antimatter together with a preference for one space-symmetry or "handedness" over the other. Val Fitch and James Cronin won the Nobel Prize in Physics in 1980 for discovering this unexpected property of K mesons. The CP-violation of the KL means that systems composed of matter and of antimatter do not behave in precisely the same way. Its decay can favor matter over antimatter. It suggests a way in which the universe's preference for matter over antimatter might have come about.
The CP-violation in the K0 meson is related to the fact that it contains a second-generation strange quark as one of its component parts. The third-generation cousin of the strange quark is the bottom quark. Similar CP violations have been observed in three bottom-containing mesons, the B0 (down + anti-bottom), B+ (up + anti-bottom), and Bs0 (strange + anti-bottom) mesons. However, none of these CP-violating decays could have caused the universe's matter excess because the CP violations observed in these systems are too weak in magnitude and therefore too improbable to account for the amount of matter that is present in the universe.
One question that might be asked in the context of CP violations is: Why do the processes of the early Big Bang seem to have produced roughly equal numbers of protons and electrons? Were there two matched CP violations going on, one producing the excess of protons over antiprotons and the other producing the excess of electrons over positrons?
The answer to this question is: No, we do not need two separate CP violations. The CP violation that produced the proton excess made an equal number of some other electrically negative particles that were not antiprotons. At the culmination of the decay of these negative particles, an electron was left over. In other words, the electrons we see around us and use in our electronic devices are the dead husks, the left-over electric charges, of particles that might have been antiprotons if the universe had been even-handed about matter and antimatter production.
In addition to mesons, a large quantity of baryons was also created in the early stages of the Big Bang. The proton is the only stable baryon, but there are many heavier unstable baryons that decay into lighter particles. Up to now, no CP violation has ever been observed in any of these baryon decays, but there has been a strong theoretical suspicion that the secret of the universe's preference for matter over antimatter may lie in some CP-violating decay of a heavy baryon.
Now, from the CERN experiment LHCb, there is a preliminary report of the first observation of a CP violation in a baryon system. This new result promises to provide fresh insights on the dominance of matter over antimatter in the universe. The LHCb experiment is located at one of the collision intersection points of the LHC collider and is a collider detector specifically dedicated to studying the physics of bottom quarks. In September of 2016 in the journal Nature Physics the LHCb Collaboration reported the discovery of a strong CP violation in one of the decay modes of the Lambda-b-zero or Lb0 baryon.
The Lb0 is a composite baryon composed of an up, a down, and a bottom quark. (Note that in some physics circles, the bottom quark is called the "beauty" quark, as it is in the LHCb paper referenced below.) It is about 10 times heavier than the K0, with a mass of about 5.62 GeV, and it has a half-life of about 1.47 picoseconds. When it decays, a small fraction of the time breaks up in a 4-particle process through the weak interaction, producing one proton (p+) and three p mesons, p+p-p+p- . Its antimatter twin, the Lb0-bar is composed of anti-up, anti-down, and anti-bottom quarks and similarly decays into p-p+p-p+, where p- is an antiproton.
The LHCb collaboration has studied about 6,646 of these Lb0 baryon decays. They have focused on certain "CP-odd" variables constructed from the relative momentum directions of the proton and two of the three p mesons produced in the decay. If such a CP-odd variable is significantly greater than zero, that implies the observation of a CP violation in the decay. The LHCb collaboration has observed such a violation. They find that the CP-odd variable has a value greater than zero by 3.3 standard deviations. At that level of statistics, the data is taken to be "significant evidence" for a CP violation in the Lb0 baryon decay rather than a definite observation.
The conventional standard in particle physics is that a definite observation of a CP violation would require five or more standard deviations. Therefore, the LHCb result is preliminary and there is a non-zero possibility that it could go away when better statistical precision is achieved. As the LHC continues to operate, the LHCb collaboration will accumulate more statistics toward the goal of establishing unambiguously that a strong CP violation is present in the decay of the Lb0 baryon.
How does the LHCb experimental evidence for a baryon-decay CP violation help in explaining the dominance of matter over antimatter in the universe? The answer to that question is not yet clear. We previously knew that a few mesons containing strange and bottom quarks showed CP violations, but this is the first observation of a CP violation in the decay of a baryon. However, we need a detailed theoretical analysis to know whether the strength of the observed CP violation is compatible with the Standard Model, or whether it goes beyond the Standard Model and represents new physics.
Further, LHCb analysis is based on what is called a CP-odd variable, a quantity that is related to the momentum geometry of the decay products. The LHCb analysis does not directly deal with the question of whether the decay of Lb0 baryons and their antimatter twins leads to an excess of protons over antiprotons in their decay products, as would be needed to explain the universe's matter-antimatter asymmetry. The LHCb result needs to be incorporated in some cosmological models to determine whether the observed Lb0 baryon CP violation is large enough to explain that asymmetry. We also note that, given the existence of a CP violation in the Lb0 decay, it is very likely that other baryons containing bottom quarks could also have CP violations and could also contribute to the excess production of protons over antiprotons in the early universe.
Thus, the LHCb results represent only a first step, but it's a very interesting one. See this column for future developments.
Baryon CP Violation:
"Measurement of matter-antimatter differences in beauty baryon decays", The LHCb Collaboration, Nature Physics; doi: 10.1038/NPHYS4021; preprint arXiv: 1609.05216 [hep-ex].
John Cramer's new book: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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