About a year ago, Matt Strassler noted some recent reports of high energy photons from galactic source that have been pitched as possible direct evidence of dark matter:
For the moment let me just note that there are two completely different excesses —The 3.5 keV photons are at an energy level that would be on the right order of magnitude to be produced by the annihilation of warm dark matter of some sort. A paper describing of these two observations is here. Sterile neutrinos and axions are among dark matter candidates that have been proposed as sources of this signal.
* one in X-ray photons (specifically photons with energies of about 3500 eV) noticed by two groups of scientists in a number of different galaxies, and
* one in gamma ray photons (specifically photons with energies of 1 – 10 GeV [GeV = 1,000,000,000 eV]), extracted with care by one group of scientists from a complex set of astrophysical gamma ray sources, coming from a spherical region around, and extending well beyond, the center of our own galaxy.
These seem to the experts I’ve spoken with to be real excesses, signs of real phenomena — that is, they do not appear to be artifacts of measurement problems or to be pure statistical flukes. This is in contrast to yet another bright hint of dark matter — an excess of photons with energy of about 130 GeV measured by the Fermi satellite — which currently is suspected by some experts, though not all, to be due to a measurement problem.
The 1- 10 GeV photons observed by the Fermi satellite have been pitched as having a distribution and scale consistent with the annihilation of dark matter at the light end of the WIMP range (e.g. 25 Gev to 30 GeV) and have been dubbed by some as "hooperons", after one of the lead investigators. One of the earliest announcement of this excess from 2009 is here. More than four years later, a more firmly argued case for this as a signal of a dark matter candidate has been advanced.
Neither of the photon excesses have a source that is well known to astonomers, and the "hooperons" have an apparently cuspy halo distribution very much like that expected for Cold Dark Matter with that mass scale.
The debate about both possible sources is, so far as I know, ongoing.
Deep Theory
So far as we know, fundamental physics conserves the conjugate quantity CPT (charge-parity-time). In an equation where there is CP violation in one direction of time, the inverse amount of CP violation takes place in the other direction of time. Antiparticles, for example, have the opposite charge and parity of the corresponding particles.
In charged particles, there is a correspondence between charge and status as matter or antimatter. Positively charged up-like quarks, negatively charged down-like quarks, and negatively charged leptons are matter. Negatively charged up-like quarks, positively charged down-like quarks, and positively charged leptons are anti-matter. In each of these cases particles of matter can be both left handed and right handed, and particles of antimatter can be both left handed and right handed.
Baryon number and leptons number are both concepts tied to CPT conservation, because both concepts distinguish between particles and antiparticles. Quarks can be created so long as corresponding antiquarks are created. Leptons can be created so long as corresponding antileptons are created. The Standard Model places no limit on the number of particles in the universe, it just fixes the number of quarks minus antiquarks, and fixes the number of leptons minus antileptons.
The Standard Model also theoretically allows for sphaleron processes that don't separately conserve B or L number, but do conserve B-L, but no one has ever observed such a process in real life.
For neutrinos, which have no electric charge, parity corresponds to matter and antimatter status. Left handed neutrinos are matter and right handed neutrinos are anti-matter. This property is a central one to the concept of a neutrino that was pivotal in the hypothesis that they existed in the first place. A direct oscillation of a neutrino from a left handed parity to a right handed parity would be a lepton number violating event that would violate lepton number by an increment of two. Yet, lepton number violation is something never observed experimentally outside one set of neutrinoless double beta decay studies that have been discredited in efforts to replicate the results.
This analysis seems to rule out the possibility of right handed or Majorana neutrinos.
F Mesons
There are a variety of scalar mesons whose internal structure is not well understood. This section recaps some of the relevant raw data:
A meson with zero isospin (I), zero electric charge, quantum number G=+, and quantum numbers C and P = + and + is denoted with the symbol "f" and an integer number for total angular momentum with values 0, 1, 2, 4 and 6 observed.
The lightest of them, f0(500) is also known as a "sigma meson".
There have been ten different kinds of f mesons with J=0 that differ only in mass that scientists think that they have observed:
f0(500), f0(980), f0(1370), f0(1500), f0(1710), f0(2020), f0(2100), f0(2200), f0(2330) and f0(2510).
There are three that have been observed with J=1:
f1(1285), f1(1420), and f1(1510).
Eleven f mesons have been observed with J=2:
f2(1270), f2(1430), f'1(1525) (with J=2), f2(1565), f2(1640), f2(1810), f2(1910) f2(1950) f2(2010), f2(2150), f2(2340).
Two f mesons have been observed with J=4:
f4(2050) and f4(2300).
One f meson has been observed with J=6:
f6(2510).
There is also one f meson observation fJ(2200) that could be J=0, 2 or 4.
All ground state mesons have excited states with higher spin. But, there are no simple two quark combinations that produce the quantum numbers of the observed f mesons.
Vector Meson Dominance
One approach to doing QCD calculations that is quite old school is called "vector meson dominance." (full disclosure: I've contributed to this Wikipedia page). It is notable because it sometimes outperforms some better theoretically grounded QCD calculation methods, for example in [1] and [2].
[1] The COMPASS Collaboration, Measurement of radiative widths of a2(1320) and pi 2(1670) (March 2014) (citing J.L. Resner, Phys. Rev. D 23 (1981) using VDM as superior to three other alternatives). See also here.
[2] Susan Coito, Unquenched Meson Spectroscopy (December 2013).
The implication would be that modern models may be missing something important that VMD captured. VMD was dropped because it didn't work well with heavy mesons, but workarounds and fixes to those problems have been developed since those problems were identified.
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