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Neutrino, Right-Handed (and Some SM Problems, 2021 Edition)


Contents

See-saw Mechanism
Leptogenesis
Ultra-High-Energy Cosmic Rays (UHECR)
Dark Matter, Wimpzilla as
GUT in SM Extension


Since the discovery of parity violation in weak interaction, It has become a doctrine that only left-handed neutrinos exist in this world. The believe is then codified in the Standard Model of Elementary Particles (SM). Nevertheless, the spectre of right-handed neutrino keeps raising up now and then as an instrument to resolve some of the puzzles in elementary particles as well in cosmology. A few examples below explain why is it necessary and why it has not been detected ?

See-saw Mechanism

It is necessary to trace the various steps in understanding the nature of neutrino before explaining the see-saw mechanism :

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Leptogenesis

    There are four different theories trying to explain the matter-antimatter asymmetry:

  1. CP violation - The standard model of elementary particle suggests that when the universe was less than 10-12 sec old, the condition was ripe for the production of more matter than antimatter with CP violation to provide the mechanism for different reaction rate (to produce matter and antimatter). The explanation sounds reasonable, however, theoretical calculation as well as experimental measurement shows an excess far too small to account for the observed degree of asymmetry.

  2. Supersymmetry - Supersymmetry is one of the most promising extensions of the standard model of elementary particle. It encompasses many as-yet-unknown particles and perhaps new kind of interactions. It is suggested that new interaction outside the standard model might act differently on quarks and antiquarks, and produced the excess of quarks in our universe. Although there are inconclusive claims of discovery, so far (as of 2021) there is no hard evidence for supersymmetry from experiments.

  3. Electric dipole moment - The presence of an electric dipole moment (EDM) in elementary particles would allow matter and antimatter to decay at different rates leading to a possible matter-antimatter asymmetry. But all experiments including the one by ACME in 2014 have failed to detect such EDM. See "Axion" for an attempt to explain the absence or near zero of the EDM.

  4. Leptogenesis - This explanation also requires new physics beyond the standard model. It assumes the existence of a new type of very heavy neutrino in very early universe. According to the leptogenesis scenario, the heavy neutrinos would decay into either neutrinos or antineutrinos. Then the standard model predicts that certain reactions could occur in the very high-temperature conditions to convert antineutrinos into matter particles (and thus the lepton number is not conserved, e.g., from 0 to 2), eventually producing neutrons and protons leaving the universe devoid of antimatter (see Neutrinoless Double-Beta decay). So far there is only one controversial claim in 2001 to have observed such reaction.
The disagreement with observation is now supported by three years of B meson decay data. It seems that the Standard model alone is not able to explain the phenomenon of matter-antimatter asymmetry. A new theory called leptogenesis (Figure 04, also known as
Leptogenesis baryogenesis) suggests that an exceptionally heavy but unstable breed of Majorana neutrino existed in the very early universe. Their subsequent decay generated more anti-leptons than leptons. A mechanism called "Sphaleron" then converted 1/3 of the excess anti-leptons into baryons leading to the imbalance between matter and antimatter at the dawn of time. Theoretical analysis shows that leptogenesis works best when the neutrino masses are in the range 0.1 ev - 1 mev; the mass of the Majorana neutrino and the reheating temperature must be larger than 109 Gev.

Figure 04 Leptogenesis
[view large image]

Such theory is the most favored mechanism to explain the matter anti-matter asymmetry because there is good circumstantial evidence for existence of various ingredients (except the requirement of heavy Majorana neutrino at reheating when particles were massless according to SM). See detail :

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Ultra-High-Energy Cosmic Rays (UHECR)

Cosmic rays are energetic particles come from outer space traveling near the speed of light. Before the development of particle accelerators, cosmic rays provided physicists with their only sources of high-energy particles to study. Although low-energy cosmic rays are emitted from the Sun, the origin of the highest energy cosmic rays is one of the outstanding puzzles in astrophysics. These particles (mostly protons, but including heavier atomic nuclei and gamma rays) have energies of up to 1011 GeV. The sources of high-energy cosmic rays could be supernovae, blackholes, or defects in space-time (Figure 11). The UHECRs (ultra-high-energy cosmic rays) events are rare, but they should not be here at all; because within a few hundred million years at most, they should be slowed down by successive collisions with the omnipresent photons of the cosmic microwave background. They could be produced nearby, e.g., within some 150 million light-years such as from the active galaxy M87, which is about 60 million, or from the Milky Way.
Cosmic Ray Spectrum Cosmic Rays However, this kind of source seems to be unlikely since UHECRs are isotropic, and not affected by magnetic fields. The yellow line in Figure 10 is the observed cosmic ray energy spectrum. The three other lines show how suspected sources possibly contribute to the overall signature of cosmic rays. It is not known what causes the kinks at the "knee" and "ankle". Normally, cosmic rays bombard the Earth at a rate of about 1 particle/km2-sec. At ultra-high energies, above 1010 Gev, the rate falls to less than 1 particle/km2-year.

Figure 10 C-Rays Spectrum


Figure 11 Cosmic Rays, Origins of [view large image]


Figure 11 lists some of the sources such as Supernovae for High-Energy (up to 106 Gev), Black-holes for Medium Energy (up to 109 Gev). The UHECR (up to 1011 Gev) is the one difficult to explain. The following is an attemp to link such events to the right handed neutrinos in the very early universe.

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Dark Matter, Wimpzilla as

    By observations over the years, the dark matter should have the characteristics as listed below (Figure 13) :

    Dark Matter Propeties
  1. It interacts with other matter by gravity. It should be cold (i.e., speed << velocity of light), otherwise it could not clumped together to form structure such as the halo of galaxy. It would be even better if it is warm since it doesn't aggregate into smaller object like planet.
  2. As it does not absorb or emit electromagnetic radiation, it must be electrically neutral.
  3. It also does not interact with strong interaction as high energy cosmic rays fail to produce anything from the dark matter.
  4. Since there is no known mechanism to replenish dark matter, it must be stable on cosmic timescales. Very long life time is OK, the decay products may reveal some information.
  5. Figure 13 Dark Matter Properties [view large image]

  6. There should be enough mass and quantity of whatever species to account for the 27% cosmic matter density of present-day dark matter.

It turns out that there are not too many objects satisfying these requirements :

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GUT in SM Extension

Cosmic Evolution Although the Standard Model has been very successful in accounting for all experimental phenomena, it is not expected to be the ultimate theory because of the many problems (including neutrino mass now) it leaves unanswered. These objections suggest that there may be deeper symmetries underlying the standard model, leading perhaps to the unification of the strong and electroweak interactions into a single "Grand Unified theory (GUT)" at the dawn of time (see Figure 17). Such scheme is indeed possible if the internal rotation group is further generalized to SO(10).
BTW, the SU(5) scheme is dead because its prediction of proton decay fails to materialized after more than 20 years of futile searches.

Figure 17 Cosmic Evolution [view large image]