Saturday, 23 February 2008


At the last Cosmo Coffee, Celine Boehm was discussing the current status of the 511 keV gamma-ray line from the Milky Way center, in light of the recent results from the INTEGRAL satellite. Photons carrying 511 keV energy arise from annihilation of positrons and electrons that are more less at rest. It is not clear which mechanism is responsible for injecting enough positrons into the interstellar medium of the galactic bulge. One hypothesis is that the positrons are scattered remnants of the ILC (Interplanetary Linear Collider) - an unfinished project of a technologically advanced civilization from that region. More contrived explanations involve black holes, radioactive nuclei from supernovae, pulsars and other fluffy toys.

Yet another, quite exciting possibility is that the positrons come from annihilation of dark matter. At first sight this seems quite natural. We are pretty confident that a lot of dark matter is present in the galactic center and its distribution should be roughly spherical, something which agreed quite well with the earlier INTEGRAL observations. Furthermore, it is likely that dark matter can annihilate into ordinary matter so that its present abundance is a thermal relic. But a dark matter particle that could explain the INTEGRAL signal must be quite peculiar. In particular, it cannot be a WIMP: its mass should be in the MeV range. If it were heavier, annihilation would yield too energetic positrons and the 511 keV line would not stand so prominently over the continuum gamma-ray radiation.

On the microscopic level, MeV dark matter can be realized as a scalar particle (so as to avoid the Lee-Weinberg bound) annihilating via an exotic heavy fermion exchange, or via an exotic heavy gauge boson. See the artist's view above. In fact, both diagrams are needed to explain the INTEGRAL signal and, at the same time, derive the correct dark matter abundance from the thermal relic density computation. The first diagram leads to the cross section that goes to a constant at small velocities and this one is relevant for the annihilation of dark matter today. The second leads to the cross section that goes like $\sigma \sim b v^2$, and it's supposed to dominate in the early hot universe.

Recently, the INTEGRAL satellite announced new results which show an asymmetry of emission (by a factor of two) with respect to the central axis of the galaxy. Morevover, the asymmetry seems to be correlated with the distribution of low mass X-ray binaries (LMXB) - systems including a neutron star or a black hole that accretes matter from its companion. LMXBs have long been one of the suspects in the positron case. The general feeling is that the new results make the dark matter explanation unlikely; Julianne on CV readily flushed MeV dark matter down her toilet. Celine, on the other hand, is more reluctant to push the button (she is, in fact, responsible for much of the stuff being thrown into the toilet). She argued that: 1) We still don't know a conventional astrophysical explanation that could account for enough positron emission, 2) While part of the gamma-ray emission maybe due to boring astrophysics, there's still a large, roughly spherical component that could be due to MeV dark matter. In fact, if only a part of the emission is assigned to dark matter, the microscopic models can more readily satisfy constraints from other experiments, for example, from measurements of the electron anomalous magnetic moment.

I guess it's fair to say that, at the moment, a conventional astrophysical explanation seems far more likely. My faith in that signal is further diminished by the fact that astrophysics provides us with too many excesses (EGRET, for example), each one having its dark matter particle that's supposed to explain it. Finally, MeV dark matter is hard to accommodate in our favourite Beyond the Standard Model schemes. But this last argument should always be taken with a grain of salt, since all existing BSM models suck. It's better to keep our eyes open...


Matti Pitkänen said...

I have tried many times to communicate an explanation of this and some other anomalies in terms of what I call leptohadron hypothesis emerging naturally in TGD framework.

Already at seventies a resonant production of electron positron pairs in heavy ion collisions just above Coulomb wall was observed and the proposal was that they result in a decay of a pseudoscalar state which has mass slightly above 2m_e. There is also a well-known anomaly in positronium decay which could be explained in terms of coupling of leptopion to photon pair by a universal coupling dictated by axial anomaly considerations.

About year ago also similar production of muon antimuon pairs was observed. In the blog of Lubos there were comments about a new particle. The finding has been published (Phys. Rev. D74) and (Phys. Rev. Lett. 98). The mass of the new particle, which is either scalar or pseudoscalar, is 214.4 MeV whereas muon mass is 105.6 MeV. The mass is about 1.5 per cent higher than two times muon mass. The proposed interpretation is as light Higgs. I do not resonate with this interpretation although p-adically scaled up variants of also Higgs bosons live happily in the fractal Universe of TGD.

My explanation for all these findings is in terms of leptohadron hypothesis. One of the basic deviations of TGD from standard model is the prediction of colored excitations of quarks and leptons. The reason is that color is not spin like quantum number but partial wave in CP_2 degrees of freedom and thus angular momentum like. Accordingly, new scaled variants of QCD are predicted. As a matter fact, dark matter hierarchy and p-adic length scale hierarchy populate many-sheeted Universe with fractal variants of standard model physics. The colored excitations of leptons can form hadron like states: electropions, muopions, and taupions.

I have proposed that the decays of electropions residing at magnetic flux tubes of the strong galactic magnetic fields, and indeed dark matter - but in these sense of TGD- are responsible for the observed narrow lines from Milky Way Center.

For more details about the leptohadron hypothesis see the chapter The Recent Status of Lepto-Hadron Hypothesis.

Below a list of references about early evidence for pionlike states which people for some reason probably related to shortcomings of ancient theories;-) want to call axions.

1. A.T. Goshaw et al(1979), Phys. Rev. Lett. 43, 1065.

2.J.Schweppe et al(1983), Phys. Rev. Lett. 51, 2261.

3. M. Clemente et al (1984), Phys. Rev. Lett. 137B, 41.

4. P.V. Chliapnikov et al(1984), Phys. Lett. B 141, 276.

5. L. Kraus and M. Zeller (1986), Phys. Rev. D 34, 3385.

6. A. Chodos (1987), Comments Nucl. Part. Phys., Vol 17, No 4, pp. 211, 223.

7. W. Koenig et al(1987), Zeitschrift fur Physik A, 3288, 1297.

8. C. I. Westbrook ,D. W Kidley, R. S. Gidley, R. S. Conti and A. Rich (1987), Phys. Rev. Lett. 58 , 1328.

Anonymous said...

Whoa, dude.

I want some of what he's smoking...