DM candidate anapole Majorana fermion

[marcus]

http://www.eurekalert.org/pub_releases/2013-06/vu-stm061013.php
http://arxiv.org/abs/1211.0503
Anapole Dark Matter
Chiu Man Ho, Robert J. Scherrer
(Submitted on 2 Nov 2012)
We consider dark matter (DM) that interacts with ordinary matter exclusively through an electromagnetic anapole, which is the only allowed electromagnetic form factor for Majorana fermions. We show that unlike DM particles with an electric or magnetic dipole moment, anapole dark matter particles annihilate exclusively into fermions via purely p-wave interactions, while tree-level annihilations into photons are forbidden. We calculate the anapole moment needed to produce a thermal relic abundance in agreement with cosmological observations, and show that it is consistent with current XENON100 detection limits on the DM-nucleus cross-section for all masses, while lying just below the detection threshold for a mass ~ 30-40 GeV.
7 pages, 5 figures, published Phys. Letters B (24 May 2013)

 

[marcus]

the Greek prefix "ana-" can have several meanings one of which is "again" or "back-on-itself".

Anapole moment is different from Dipole moment.

With electric or magnetic dipole moment you have a N and a S (or a + and a -) sticking out in opposite directions and the field lines go out into the world at large and then come around back to the other end.

Like with a conductor wire coiled in a solenoid, the magnetic field lines go out the S end and venture thru the outside world and come back to dive down the N end.

But suppose you took a solenoid and curved it around and joined it. So it is like a donut shaped tunnel that doesn't go anywhere. It comes back on itself, so the N and S ends no longer exist.

Make it out of ideal superconduct wire so that once started the current never stops, then the magnetic field lines are all inside the donut shaped tunnel and never get out into the outside world.

So such a thing wouldn't be very INTERACTIVE electromagnetismwise. I guess by Lorentz force law it could interact with charged particles that were moving by it very fast, or if it was going very fast. But without some relative motion in the picture it would not be very interactive.

So they think that there could be a FERMION that does not have any dipole moment but that does have anapole moment. They suggest that DM consists of that and that DM used to be interactive in ancient times when everything was very hot and the ordinary fermions and the anapole fermions were moving fast relative to each other.

But as U expanded and all this matter cooled the DM became more and more transparent and less and less interactive.

Interesting idea.

The top one of the three pictures here http://www.eurekalert.org/pub_releases/2013-06/vu-stm061013.php shows the anapole situation. The solenoid current is blue and it wraps around and around the donut. The magnetic field lines are in red, and the circle endlessly in the tunnel that comes back on itself.

 

[Mordred]

It is an interesting idea. The Majorona particle is a quasi particle. Also it is its own anti particle.

Here is a technical paper on the fermion.

http://arxiv.org/abs/1204.2792

 

[marcus]

Thanks for link, Mordy! However the Majorana we are talking about in this thread is NOT a quasi-particle. Notice that the paper you link makes the distinction between an actual elementary particle and a quasi, in the first paragraph of the intro.
http://arxiv.org/abs/1204.2792
"They may appear naturally as elementary particles, or emerge as charge-neutral and zero-energy quasi-particles in a superconductor (2, 3)..."

 

[marcus]

Ho and Scherrer have an interesting follow-up on this!
http://inspirehep.net/record/1206314?ln=en
Phys. Review D (2013)
already has 7 cites.
They deal with the Planck report low N_eff
http://arxiv.org/abs/1212.1689
Sterile Neutrinos and Light Dark Matter Save Each Other
Chiu Man Ho, Robert J. Scherrer
(Submitted on 7 Dec 2012 (v1), last revised 13 Mar 2013 (this version, v3))
Short baseline neutrino experiments such as LSND and MiniBooNE seem to suggest the existence of light sterile neutrinos. Meanwhile, current cosmic microwave background (CMB) and big bang nucleosynthesis (BBN) measurements place an upper bound on the effective number of light neutrinos, N_eff and the PLANCK satellite will measure N_eff to a much higher accuracy and further constrain the number of sterile neutrinos allowed. We demonstrate that if an MeV dark matter particle couples more strongly to electrons and/or photons than to neutrinos, then p-wave annihilation after neutrino decoupling can reduce the value of N_eff inferred from BBN and PLANCK. This mechanism can accommodate two eV sterile neutrinos even if PLANCK observes N_eff as low as the standard model theoretical value of 3.046, and a large neutrino asymmetry is not needed to obtain the correct primordial element abundances. The dark matter annihilation also weakens the cosmological upper bounds on the neutrino masses, and we derive a relationship between the change in these bounds and the corresponding change in N_eff. Dark matter with an electric dipole moment or anapole moment is a natural candidate that exhibits the desired properties for this mechanism. Coincidentally, a dark matter particle with these properties and lighter than 3 MeV is precisely one that can explain the 511 keV gamma-ray line observed by INTEGRAL. We show that the addition of two eV sterile neutrinos allows this kind of dark matter to be lighter than 3 MeV, which is otherwise ruled out by the CMB bound on N_eff if only active neutrinos are considered.
21 pages, 1 figure, to appear in PRD

 

[Mordred]

Yeah I missed that part somehow.