Why is our universe dominated by matter?  Where is all the antimatter?  The microscopic world appears to be even-handed and symmetric.  In energetic collisions, matter and antimatter are made with equal probability, with no obvious preference for one or the other.  Particle reactions look the same if the "movie" is run backwards, or if viewed in a mirror, or if matter and antimatter are interchanged.  Physicists say that the situation represents symmetries called C, P, and T, where C means exchanging matter and antimatter, P means mirror reflection or reversing the x, y, and z coordinates, and T means reversing time.

Our macroscopic world is very different in terms of such symmetries.  Time clearly runs in a preferred direction, bolts and nuts prefer right handed threads, and our world contains far more protons and electrons than antiprotons and positrons, with matter everywhere and essentially no antimatter.  Somewhere along the path from the Big Bang to the present some symmetries must have been broken, producing a universe in which matter and antimatter are dramatically out of balance.

The explanation is that the Big Bang initially produced a slight excess of protons and electrons over their antimatter twins.  While the universe was still a hot dense plasma, all of the antimatter particles were annihilated by their matter twins, leaving the surviving excess behind as matter only.  If the matter/antimatter symmetry had not been broken, there would be no stars and galaxies, and we would not be here to worry about their absence.

How do symmetries become broken?  In 1956, T. D. Lee and C. N. Yang provided the first clue.  They proposed, based on suggestive particle-physics data, that the P symmetry (parity invariance) might be broken in the weak interaction, the fundamental force governing the process of beta decay.  This would create a preferred "handedness" in weak interactions, for example, in the reaction products that follow beta decay.  Experimentalists (including myself as a graduate student) quickly confirmed this prediction.

This manifest violation of parity invariance suggested that under the proper circumstances the other two symmetries, C (charge-conjugation invariance) and T (time-reversal invariance) might also be broken.  There are, however, good theoretical reasons for assuming that overall PCT symmetry will always remain unbroken.  In other words, if one changes matter to antimatter, reverses x, y, z coordinates, and makes time run backwards, the resulting system's behavior will be  identical to the original one.

The next symmetry violation clue emerged in 1963, when Val Fitch and Jim Cronin demonstrated a violation of CP symmetry in neutral meson decay.  Mesons have massed that are intermediate between the electron and proton mass. They are unstable, existing for only a few billionths of a second before undergoing strong-interaction decays into lighter particles. The standard model tells us that mesons are not fundamental particles but composites composed of a matter quark and an antimatter quark locked together by the strong interaction.

An example is the K⁰ meson, made of a matter down quark and an antimatter strange quark.  Its antimatter twin, the anti-K⁰, is made of a strange quark and an anti-down quark. Both of these K⁰s have zero electrical charge, zero spin, and the same mass (about half a proton mass). Therefore, on the basis of observables they are completely indistinguishable.

    When two quantum states cannot be distinguished, a peculiar thing happens. The two indistinguishable states mix to form two new quantum states that are distinguishable. In the case of neutral K mesons, the K⁰ and anti-K⁰ combine in two different ways to make the K_S meson (K-short), which decays to two pi mesons in about 10^-10 seconds, and the K_L meson (K-long), which decays into three pi mesons 581 times more slowly.  Fitch and Cronin discovered that in the decay of the K_L mesons, for each decay into three pi mesons there were about 0.002 "forbidden" decays into two pi mesons, indicating a CP violation.  This CP violation can be interpreted as a preference for matter over antimatter.  The K_L decay also implies that if a movie were made of certain particle reactions involving a K_L, the implicit T violation would allow one to tell if the film was running forward or backwards through the projector.

More recently, it has been discovered that the strong decay of neutral B⁰ mesons, made of  a down quark and a bottom quark in two matter-antimatter combinations, also shows a similar CP violation.  However, the K⁰ and B⁰ meson CP violations involving quarks and the strong interaction are rather small, not nearly large enough to explain the dominance of matter over antimatter in the universe.  Broken CP symmetry in some other sector must be responsible.  Therefore, experimental physicists have been looking for CP violations involving neutrinos.

Super-Kamiokande (Super-K) is a giant water-filled neutrino observatory located 1,000 m underground beneath Mount Ikeno near the city of Hida in the Gifu Prefecture of Japan.   Super-K consists of a cylindrical stainless steel tank about 40 m in height and diameter containing 50,000 tons of ultrapure water.  Mounted on an inside superstructure are about 13,000 large photomultiplier tubes that detect light from Cherenkov radiation.

Super-K is used as a detector by the neutrino oscillation experiment T2K, an international collaboration of about 500 members from 69 institutions in 12 countries.  T2K investigates how matter and antimatter mu neutrinos change from one flavor to another by the process of neutrino oscillation as they travel large distances.

An intense beam of mu neutrinos is generated at the J-PARC nuclear physics site on the East Coast of Japan and is directed 295 km across the country to the Super-K neutrino detector located in the mountains of western Japan . The beam is measured once before it leaves the J-PARC site, using near-detector ND280, and again at Super-K.  Any changes in the measured intensity and composition of the neutrino beam are recorded and analyzed.

The T2K mu neutrino beam is generated by directing an intense 30-GeV proton beam onto a graphite target. Charged pi mesons from proton-carbon collisions in the target are focused downstream using a "neutrino horn", a magnetic device that focuses pions of a selected charge that are diverging from a collision.   With a half-life of about 26 nanoseconds, pi mesons will decay nearly 100% of the time to a charged mu lepton and a mu neutrino.  For T2K, mu neutrinos with an average kinetic energy of about 0.6 GeV are created.

The decay of a positive pion (p⁺) will create a matter-type mu neutrino (n_m), while the decay of a negative pion (p^-) creates an antimatter mu neutrino (n_m-bar).  By reversing the magnetic field of the neutrino horn, the experimenters can select a beam of either n_m or n_m-bar to be directed to the nearby detector and to Super-K located 295 km across the island.  Because the interaction between neutrinos and the detectors is so weak, the T2K experiment collected data from 2009 to 2018, using about 3×10²¹ potential neutrino-generating proton-carbon collisions at J-PARC, but detecting just 90 neutrinos and 15 antineutrinos at Super-K.

Neutrinos come in three flavors: electron, mu and tau varieties, i.e., n_e, n_m, and n_t.  In the process of neutrino oscillation, a neutrino that starts with one of these flavors will oscillate into the other two as it travels through space.  One of the main goals addressed by the T2K experiment is to observe the n_m→n_e flavor-change of the initial mu neutrino beam as it traverses the 295 km flight path from J-PARC to Super-K, and to observe whether there is any difference in the flavor change of  the initial n_m vs. n_m-bar beam, which would indicate a violation of CP symmetry.

Neutrino oscillations will also produce tau neutrinos.  However, in Super-K, neutrinos are detected by observing the particles they produce when they interact. At neutrino energies of 0.6 GeV the dominant interaction process is the conversion of the neutrino (or antineutrino) into the charged lepton (or antilepton) of the same flavor (n_m→m and n _e→e).  T2K is thereby able to identify the incoming neutrino's flavor.   However, because the neutrinos used in the T2K experiment have an average kinetic energy of only about 0.6 GeV while the rest mass of the tau lepton is about 1.8 GeV, there is no possibility of detecting tau neutrinos by this process.  Therefore, the T2K experiment is focused on the n_m→n_e process with matter and antimatter neutrino types.

After a decade of running and analysis, the T2K Collaboration published their first results in the April 16, 2020 issue of the journal Nature.  They find that of the observed neutrino detection events, there are about 67 events for n_m→n _e  and 8 events for n_m-bar→n_e-bar.  This indicates a large violation of CP-symmetry at the 3s level or 99.73% confidence level.  Unfortunately, this result cannot be taken as a definite observation of a CP violation, because that would require a confidence-level of 5s.  In terms of the theory of weak interactions, this result implies a CP-violation phase d_CP of about -p/2, while ruling out d_CP values of 0 and p at the 95% level.

It is not yet clear whether this result can by itself explain the dominance of matter over antimatter in the universe, but it represents a definite step in that direction.  The T2K experiment will continue to take data, but it is unlikely to reach the 5s confidence level.  That will have to wait for three new neutrino oscillation experiments presently on the horizon.  One these will be T2HK, the new version of experiment T2K using Hyper-Kamiokande, an new version of Super-K that increases the detector's active water volume by a factor of 10.  It was just funded at $600 million by the Japanese Government and will begin operation in 2027.  Another experiment, scheduled for a 2025 start, is DUNE, which will produce mu neutrinos at Fermilab outside Chicago and send them 1,300 km to a large liquid-argon filled detector buried deep underground at the Sanford Underground Research Facility in Lead, South Dakota.   A third experiment that may shed light on CP violations in the neutrino sector is the Jiangmen Underground Neutrino Observatory (JUNO) detector, a 35.4 m diameter transparent acrylic glass sphere containing 20,000 tons of linear alkylbenzene liquid scintillator.  JUNO is located at Kaiping, Jiangmen in Southern China .  It will primarily detect electron anti-neutrinos from 10 reactor cores, and it is scheduled to begin operation in 2022.

So the bottom line is that there is a new clue on the reason for the dominance of matter over antimatter in the universe.  If the size of the neutrino-sector CP symmetry violation observed by T2K holds up, it will be about a thousand times stronger than the CP violation observed in the quark sector by Fitch and Cronin.  This could lead to identifying the proton-favoring process that occurred in the early stages of the Big Bang.  However, we still have some distance to go, both in the form of better experiments and of better theoretical and cosmological models.

Watch this column for new developments.

John G. Cramercs 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at:  http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer:  Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106 .  His new novel, Fermi's Question is coming soon from Baen Books.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

Reference:

CP Violation in Neutrino Oscillations:
"Constraints on the matter-antimatter symmetry violating phase in neutrino ocscillations", The T2k Collaboration, Nature 580, 339-344 (2020); arXiv:1910.03887 [hep-ex]

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