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.

## Friday, May 22, 2009

## Saturday, May 16, 2009

### Never thought about it...:(:-o

Two days back my advisor pointed out some mistakes in the draft I prepared on some supersymmetric model of particle physics. It was related to extra scalar fields needed to break some gauge symmetries in the model. I realised that I included too many Higgs fields which was not needed in fact. Although in non-supersymmetric case we have to include these extra Higgs fields, in the susy case that purpose can be achieved by giving vacuum expectation values(vev) to some neutral sfermion field also like the sneutrino. After I searched paper related to sneutrino vev, it became clear that we can break gauge symmetry with sneutrino vev. In left-right model a right handed sneutrino field vev can break the SU(2)_R symmetry thereby giving masses to right handed gauge bosons whereas the fermions masses arise by the left handed Higgs fields vev only. But I don't think we can construct a supersymmetric standard model (no right handed gauge symmetry) without any Higgs fields because although we can break gauge symmetry we wont be able to give masses to the standard model leptons and quarks.

It feels nice after correcting oneself. After searching the literature I could find many models based on this idea. Actually whether sneutrinos get vev or not comes out automatically from the minimization of the scalar potential constructed from the superpotential. It went unnoticed in my case because I did not minimize the potential with respect to all the fields. I just did the calculations based on the usual Higgs scalar fields. One more interesting thing I noticed is that if sneutrino gets vev then the neutrinos will couple to the neutralinos(neutral gauginos, Higgsinos) and hence appear in the neutralino mass matrix. So particle mixes with sparticles in this case. Will it gives rise to some kind of R-parity violation?? Because to prevent proton decay via sfermion exchange diagrams in susy theories we impose a discrete R-parity under which standard model particles have R=+1 whereas superparticles have R=-1. Thus if we have an eigenstate of the neutralino mass matrix which is a linear combination of sparticles and particles I think that may result in R-parity violation. Thus sneutrino vev may be constrained from the unobserved proton decay. Hope I will be able to raise these issues next time I meet my advisor..:):).

It feels nice after correcting oneself. After searching the literature I could find many models based on this idea. Actually whether sneutrinos get vev or not comes out automatically from the minimization of the scalar potential constructed from the superpotential. It went unnoticed in my case because I did not minimize the potential with respect to all the fields. I just did the calculations based on the usual Higgs scalar fields. One more interesting thing I noticed is that if sneutrino gets vev then the neutrinos will couple to the neutralinos(neutral gauginos, Higgsinos) and hence appear in the neutralino mass matrix. So particle mixes with sparticles in this case. Will it gives rise to some kind of R-parity violation?? Because to prevent proton decay via sfermion exchange diagrams in susy theories we impose a discrete R-parity under which standard model particles have R=+1 whereas superparticles have R=-1. Thus if we have an eigenstate of the neutralino mass matrix which is a linear combination of sparticles and particles I think that may result in R-parity violation. Thus sneutrino vev may be constrained from the unobserved proton decay. Hope I will be able to raise these issues next time I meet my advisor..:):).

## Thursday, May 14, 2009

### Do I have enough Reasons to quit PhD...??

Life has become worst in the last couple of days. Neither I am doing anything important nor my works so far has been checked by my supervisor so that I can at least finish it and move to a new problem. By the way he has not given me any new things to work on also. I am kinda jobless here, I really hate this kind of life where you got nothing to do. To add to my nightmares I got to know he is going to Canada on sabbatical leave for may be 6 months or one year. For the last one year he is not there in our home institute. Somehow I managed to come to IIT Gandhinagar (where he is working) for the spring semester. My last autumn semester was a total loss since we were in two different places. And the next autumn semester will be a perfect disaster for me since it is going to be just a repetition of the previous autumn semester: Advisor and Student in two different places. Now he is saying after the end of next autumn semester he is going to Canada...wow....Do I need more reasons to celebrate? I am really frustrated and feeling like I am on the Highway to Hell....I wish I could quit PhD at this stage and start everything anew....

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