FERMI does not confirm the rise in positron fraction with energy...:(

Today I saw a paper by W. de Boer titled "Indirect Dark Matter Searches in the Light of ATIC, FERMI, EGRET and PAMELA" (http://arxiv.org/abs/0910.2601). It is related to the Invited talk at SUSY09, the 17th International Conference on Supersymmetry and the Unification of Fundamental Interactions, Boston, 2009. The author gives a good comparison between the results from various experiments related to cosmic ray positron excess. I must say after looking at the FERMI results in this paper I am not that much enthusiastic about working on the dark matter interpretation of positron excess as I was when I first saw PAMELA results. The basic difference between latest FERMI data is that it does not confirm the peak in positron fraction at several hundreds GeV. The FERMI spectrum is more or less flat. The plot shown in the paper is as follows

The author has mentioned all the attempts so far for the positron excess explanation and commented that all the explanations seem correct and nobody can rule out any one of them. He has considered contribution of all such effects in the paper. I was particularly interested in dark matter interpretation of this excess which was quite interesting as well as challenging, since you need to make your particle physics model such that the proposed dark matter candidate unlike neutralinos annihilate primarily into leptons and not hadrons. Anyway as the author says we should wait for the future FERMI data which might focus more on possible dark matter link.

Positron excess in cosmic rays is not being possible to explain with existing numerial packages...:(

I was looking for numerical packages available for calculating dark matter relic density like micrOMEGA, DarkSUSY etc. It seems to me that these packages won't be useful for calculating positron fraction which can fit with the observed PAMELA, FERMI data. One main reason is that the cross section of dark matter annihilation should be very high (than allowed from the thermal dark matter relic density argument). We need a boost of the order of ~100 so as to fit the positron excess with the observed data. Thus having a particle physics model where the proposed dark matter particle annihilates only to leptons in the tree level is not enough, we also need to look for some boost factor. I don't think the existing packages include such boost factor at all. What I am thinking of doing is that calculate the relic density (approximate) from the standard formula, find out the cross section of dark matter annihilation to all possible particles at tree level and then find out the positron fraction coming from it including the boost factor. I have no idea whether this approach will be good or not but have no other option than to do it.

Its surprising to see PAMELA data explanation within the framework of NMSSM...

After PAMELA (A Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) data have come out its very common to find at least one paper on arXiv which carries an explanation for the observed positron excess observed by PAMELA. These positrons carry much more energy than usual ones coming from pulsars or other astrophysical sources. Thus the most promising explanation for their source is cold dark matter annihilation in the galactic halo. This however became a problem for particle physicists to explain in the context of most popular dark matter models like supersymmetric extension of standard model. In these models generally the Lightest Super-particle (LSP) protected by R-parity is the most favourite dark matter candidate. The trouble was that the LSP is generally the neutralino (combination of neutral gauginos and higgsinos) which is a majorana fermion and hence its annihilation to fermions will be helicity suppressed. Thus we expect dominance of heavier quarks as the final product after their annihilation rather than positrons as observed by PAMELA. That's why people started talking about various new models containing some hidden sector, dark force etc with a light gauge boson (~GeV) whose decay to heavy quarks is kinematically not favoured. I wont go on talking about them now. The standard reference is the paper by Arkani-Hmaed et al.(http://arxiv.org/abs/0810.0713).
Today morning I was however surprised to see a paper by Bai, Carena and Lykken (Fermilab) (http://arxiv.org/abs/0905.2964) where they have come out with an explanation of PAMELA data within the NMSSM(Next to MSSM) framework. This looks more natural to me than those papers which talk about dark forces , hidden sectors etc. If we have an explanation within our own sector there is no point in bringing more complications by talking about one more force. In this paper they have proposed that the neutralinos(which is the LSP) annihilate most dominantly to the lightest CP-even scalar h_1 plus the lightest CP-odd scalar a_1. The annihilation takes place via resonance CP-odd scalar a_2. For LSP mass less than top quark mass a_2 will dominantly go to a_1 and h_1 and a_1 being light (~GeV) will go to \tau^+ \tau^- or \mu^+ \mu^- depending on its exact mass. Their parameter space is also in agreement with the null searches for light bosons like a_1. This paper is damn long. So far I am pretty convinced with their arguments and results. The good thing for me was the idea of resonance production. This is the first time I have seen this idea being implemented like this. If I get enough resources and time(and help from advisor as well as colleagues), will definitely try to use it in the context of other models as well.