Should have written a paper on it already ;-)

Like most of the experimental anomalies and surprises, the Fermi 130 GeV line has also come under dark shadows recently (http://www.newscientist.com/article/dn22466-doubt-cast-on-fermis-dark-matter-smoking-gun.html). It's not that I believed the dark matter interpretation of this line too much, but I still thought this would survive at least for a year ;) This so called game of town in particle astrophysics was initiated by Weniger in April 2012 with a detailed analysis of the publicly available Fermi-LAT data. This was followed by a series of papers from theorists with many well motivated models explaining the origin of this 130 GeV monochromatic line which apparently does not seem to have any obvious astrophysical origin. As the above link to the article in New Scientist says, the Fermi collaboration hasn't denied the presence of this peak in their data, but the dark matter origin of this line is in doubt as the same line is present even when the Fermi detector is pointed towards earth rather than the galactic center (where dark matter is more abundant). Hope Fermi collaboration would soon come up with an official publication telling us more about it. It won't be too much of a surprise if this goes away like B meson anomalies at DO/CDF went away in the wake of LHC results or the phantom of OPERA disappeared after the loose cable connection was found. Nevertheless, one obvious and immediate advantage of these anomalies is that theorists get a chance to write couple of more papers trying to fit their favorite theory or model with the data. Anyways, as someone pointed out correctly at the BENE workshop at ICTP, Trieste two months back, models don't get killed, they sometimes die out of starvation ;-)  So even if future Fermi-LAT analysis kills the dark matter interpretation of this interesting gamma ray line, all the models used to explain it already are still in safe heaven. Vive Le Model-Building! 

PS: For more information on this doubt cast on Fermi gamma ray line, see RESONANCE blog post!

Looking for a common theme of work!

Pursuing research in an academic institute or university in India is not easy with so much teaching load as well as other administrative works. And probably that's the reason why most of the good research in India are carried out in special research institutes or laboratories where the teaching load is almost zero. This is quite a different picture compared to universities/institutes in other countries (developed ones, in particular) where most of the research output comes from universities. I don't know whom to blame for the poor research conditions of Indian universities, but hopefully the situation will change slowly with the central government authorities seem to be taking a series of initiatives. Anyways, without going further into the blame game, the best way is to make maximum use of the existing system and produce maximum possible quality output. In the physics department of Tezpur University (http://www.tezu.ernet.in/dphy/) where I work, there are around 16 faculty members working in different areas like nano-physics, astrophysics, high energy physics etc. The experimental people are comparatively more active in research as they have a large number of research students under their supervision as well as good number of equipments. Whereas, the members of the theory people do not have many students under them and so is the number of publications. Recently, we have decided to choose a common theme for the theory people so that the theory people can work together on it with a long run perspective. That should hopefully take some load of an individual faculty who do not have the "eligibility" yet to take students under him/her. Here "eligibility" doesn't refer to intellectual eligibility of course, but the "eligibility" criteria defined by the system here which I neither try to justify nor ridicule. But, choosing a common theme of work is not as easy as it may sound, since almost all of us had worked on quite different things during doctoral research. But looking at the fact that the present era is the era of inter-disciplinary research with people explaining condensed matter problems using theory of gravity (just an example), the possible options for such a theme should not be a scarcity.

The expertise the theory people here have are mainly neutrino physics, hadron physics, cosmology, astrophysics/astronomy and plasma physics. As a first trial, we are thinking of considering the problem of dark matter as a common theme (yet to be decided though). Since this is a huge research area, the general feeling is that it should touch upon the field of expertise of each one of us to some extent at least. To go ahead with this theme, it was necessary to highlight some of the interesting and recent things happening in this area to the members since not all of us are actively working on it. I gave an overview seminar on dark matter starting with astrophysical hints like galaxy rotation curve, bullet cluster, structure formation and then going into some details of particle characteristics of dark matter, direct/indirect experimental evidence highlighting many recent anomalies (130 GeV Fermi-LAT line for example) which indirectly point towards the existence of dark matter. Of course, in my ways of looking at this problem, the knowledge of particle physics would be very important to find out the dark matter abundance, its cross section with nuclei in direct detection experiments, the cross section for producing gamma rays or other stuff which we treat as indirect evidence of dark matter etc. One of my colleague here (who specializes in UV astronomy, high energy astrophysics) gave a follow up seminar bringing up an astrophysics problem which could be linked to dark matter in some ways which I do not understand yet. It's about the Missing Baryon problem. This is about the fraction of baryons predicted by big bang nucleosynthesis (BBN) which remain unobserved (about 30% if I am not wrong). Initially I thought this is similar to the Missing Satellite problem which is generic in cold dark matter scenarios which predict more number of satellite galaxies in the clusters which are not observed. Missing Satellite problem can be fixed in scenarios of warm dark matter or non-thermal production of dark matter scenarios well studied in the literature. But this missing baryon problem seems to be different as my colleague says. Its independent of cold or warm dark matter scenarios. Its hard to understand from theory point of view, where those baryons could have gone. Certainly they cant decay into something else, may be BBN scenario has to be modified, but that's also tricky as BBN is one of the very successful predictions of big bang cosmology. I got a Nature news today (http://www.nature.com/nature/journal/v490/n7418/full/490008b.html?WT.ec_id=NATURE-20121004) which talks about a paper claiming that the missing baryons have been spotted in the X-ray data. If confirmed, it would imply that the missing baryons are not exactly missing but just were not being detected somehow. Anyways, it might be too early to settle this mystery and more future data analysis like this group would be needed.

Dark Matter Workshop

The Dark Matter Workshop at SINP, Kolkata is going good so far. Today is the fourth day of the workshop and most of the areas related to dark matter theory, phenomenology and experiments have been covered so far by various invited speakers. There were talks related to model building using SUSY, GUT, Extra U(1) as well some string theory inspired models by speakers like Mambrini, Ibarra, Covi, X. Tata and many others. At the same time there were talks on experiments like DAMA (by F. Capella) , PAMELA, Fermi-LAT and other experiments as well as the possible collider signatures. It was indeed good to listen to talks covering such a wide varieties but of course crucial as far as dark matter is concerned. It seemed somehow to me that people are taking direct detection experiments and possible LHC signatures of Dark matter more seriously than indirect detection experiments like PAMELA, Fermi-LAT etc. It might be because of the existing astrophysical explanations of various cosmic ray anomalies. But as Ibarra pointed out, the Gamma ray line in the sky should be taken more seriously compared to positron excess, since it has got no astrophysical explanations. There was an interesting model where people put a chern-simons term mixing the photon with a hidden gauge boson. In that case dark matter annihilation(for relic density) and dark matter going to gamma rays involve the same vertex (rather than loop suppressed in case of neutralinos in MSSM) and hence can give rise to a TeV gamma ray line observed. Anyway it seems constraints from CDMS, XENON, signal from DAMA, Gamma ray line observed by Fermi and future collider signatures are going to play the key role in the business of dark matter!

CDM vs WDM

I have been a fan of CDM(Cold Dark Matter) since I learned about dark matter and and it's particle interpretation. By "Cold" it simply means non-relativistic at very early stage of the Universe ( = when it decoupled from the thermal equilibrium and got frozen whereas other standard model particles were still in equilibrium). Their mass can vary from a few GeV to TeV. People who love supersymmetry, would always bet for CDM since supersymmetric models naturally provide a CDM candidate without any ad-hoc assumptions. R-parity, a discrete symmetry which is ad-hoc in Minimal Supersymmetric Standard Model (MSSM), but naturally arises in gauge theories where B-L is a part of the gauge symmetry (Left-Right Symmetric Models for example) makes the CDM perfectly stable. Thus it can satisfy both cosmological relic density constraints as well as structure formation bounds.

Although CDM has been taken most seriously among other Dark Matter interpretations in the last 2-3 decades, there is no experimental evidence yet which can confirm CDM over others. And this slightly discouraging fact demands us to be a little open minded. I recently read a paper by Manfred Lindner et al (Physical Review D 2010) where they have talked about Warm Dark Matter in Left-Right Models (LRSM). Warm means slightly lighter than CDM (mass of the order of say keV) and hence they could be relativistic for a long time. And the most natural candidate is sterile neutrino which naturally arise in LRSM as a part of the Right handed doublet. But since, they are considering non-SUSY models, the stability of sterile neutrino is not guaranteed and hence they have to fit the parameters in such a way that the life-time of the lightest sterile neutrino exceeds the age of the Universe. Although this does not look elegant like in SUSY models, but this is a good alternative and it does not cost you too much. You have far less free parameters compared to SUSY models and it will be easier to rule out or confirm such models in the experiments. In this paper, the authors are focusing more on how the keV scale sterile neutrino DM can satisfy the cosmological bounds as well as neutrino oscillation data. And they have shown it does, although with some undesirable features like one active neutrino becomes too light ( 9 orders of magnitude smaller than 1 eV, but still allowed from neutrino oscillation data).

Apart from cosmological and neutrino oscillation bounds which are of course the most crucial test for a dark matter candidate, there should be a way to actually observe it. The ongoing direct detection experimental results, I don't think we can fit with a keV sterile neutrino. The indirect detection experiments like Positron excess will be even harder to fit with such WDM candidate. But in any case these indirect detection experimental results have other astrophysical explanations and the various direct detection experiments don't agree with each other. Even if LHC gives clue about such keV sterile neutrino, we still wont be able to say if it is the true dark matter candidate or not. We still have to rely on direct detection experiments, provided all of them agree with each other. Anyway as G. Bertone commented in one of his recent review (to be published in Nature), we can not keep on proposing more and more experiments to search for dark matter endlessly, if we do not get some positive signal in coming few years, may be there is a need of paradigm shift and we need to look for alternative scenarios. The worst such alternative to me would the MOND theories. They look so ugly to me that I wont ever be able to believe Nature surrendered herself to them!

A Light Talk on Dark Matter

I gave a semi-popular talk on Dark Matter on Monday afternoon in the department. This talk was organized by Research Scholars Association (RSA), Physics Department and all the research scholars were invited. Although there are more than fifty research scholars in the department, only 15-20 turned up which was however not surprising. There is severe lack of enthusiasm among the people here and some might have been busy with some other important things. Anyway, it was good that not too many people turned up because we could not have the talk in the seminar room (which was booked for a class), but in a small classroom where there were not more than 25 seats. Coming to the main point, the talk I gave was semi popular since people from various working areas were supposed to come and I tried my best to make it as simple as possible. This was my first talk at a popular level and hence was very tough for me. Even after the talk, I was not much satisfied worrying that people might not have got any feeling of such a crucial problem of dark matter in the Universe. My talk went like this: (i) Summary of Standard Cosmology, (ii) Theoretical as well as experimental motivations for Dark Matter, (iii) Role of Dark Matter in the evolution of Large Scale Structure, (iv) Indirect Dark Matter detection experiments and (v) Direct Detection. Most of my slides had diagrams and words only and no equations ;-) Anyway it was a good experience for me to speak at a popular level. I am hopeful of preparing more such talks in future and keep this activity going on.

Micromega 2.4 is awesome!

Recently the micromega developing group has come up with its latest version 2.4 where they have included some really useful subroutines to calculate various other things which were not there in the previous versions. To tell you what micromega is, its a publicly available numerical package based on fortran and C, which calculates the relic density of dark matter in any supersymmetric model with conserved R-parity. In version 2.2 which I used last year, I could calculate the relic density , as well as the branching ratios of dark matter self annihilations as well as co-annihilations into various standard model particles, and direct detection rates etc. The best thing about micromega I found was the option of incorporating any supersymmetric model. Last year I was trying to use it for supersymmetric left-right model. Last year I was dying to study some particle physics models from dark matter indirect detection experiments point of view, that is , I wanted to know whether a particular dark matter candidate in my model can explain the anomalous positron excess over antiprotons found in various indirect detection experiments like PAMELA, FERMI etc. But when I came to know that for that I need to use both micromega as well as another package called GALPROP which basically studies the cosmic ray propagation, I gave up. But last month the micromega group has come up with something is a combination of earlier micromega and the GALPROP. Its really exciting to me since now I can incorporate any model into it and study it from indirect detection experiments point of view. I run it yesterday for the MSSM, and the output I got contained relic density, branching fractions of Dark matter self as well as co-annihilations and more importantly flux of positrons, antiprotons and photons. Although the plots of positron did not give any kind of peak (which PAMELA observed), as its the case for usual dark matter candidates in MSSM, but may be some other models will give rise to a desired peak. This will anyway decrease the amount of work a theorist have to do, now we don't have to write long programmes for the calculations , just write the .mdl files for a particular model and put it inside micromega and test the model against the indirect detection experimental results. Kudos to micromega developers..keep it up!

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.

Dark Matter and Black Hole!!

A few days back, I was having an interesting discussion with one of my friend in a public forum known as Actaphysica http://www.actaphysica.com/ . He seemed to be an expert in Black Hole physics who opened various threads related to some interesting aspects related to black holes, hawking radiation(HR) as well as dark matter. He talked about the possibility of a black hole formed out of two unstable micro black holes. As we know the black body temperature associated with a black hole in inversely proportional to the mass squared. Thus smaller the mass is , higher is the temperature and sooner the black hole will evaporate. I had a doubt while discussing the possibility of an atom formed out of a black hole with charge -1 and mass $m_e$ which we call and eBH (electron black hole) and another with charge +1 and mass $m_p$ which we call a pBH (proton black hole). I raised the doubt saying that eBH being tiny will evaporate soon by emitting HR and hence there won't be any bound state forming between eBH and pBH. But my friend pointed out that only isolated black holes can radiate HR and finally evaporate. Thus within a bound state eBH and pBH won't be emitting HR. However if the radius of their bound state is smaller than the corresponding Schwarzchild Radius then the bound state itself can behave like a black hole and hence emit HR since the bound state is an isolated state by itself. This would be , according to him, a ground breaking result since so far people have no idea about internal substructure of black holes. One more interesting aspect of such bound state could be the relation with dark matter. If the bound state does not become a black hole then it would be a stable entity and hence can be studied from dark matter point of view. I do not know how much work is done in this direction but it seems like an interesting field to explore.

The Darker Black Hole...

Black holes as we know are the infinitely dense objects in the Universe which keeps sucking the matter around it. They are generally formed from the gravitational collapse of stars . They are dark in the sense that even light can't escape their gravitational field. But from quantum mechanical point of view they are not so dark as they keep radiating( so called Hawking Radiation). So far the observed black holes are formed from the collapse of ordinary matter we see around us. However its worth investigating the possibility of a black hole which cud have formed from the collapse of dark matter: the darker black hole so to say. I had a few words with Dr. Yajnik regarding this couple of days back. He was saying the number of such darker black holes may be very much constrained from the structure formation data. Since dark matter particles will decouple very early (compared to usual standard model particles) and hence will start forming potential wells....and finally collapsing on its own. If such formation takes a long time like billion years from the big bang then it won't be constrained from structure formation data. But early formation will be tightly constrained.
I am just wondering how such black holes can be formed. One obvious thought comes to mind which says it may be formed from the collapse of a dark star. But does such dark star exist? Very difficult to predict. I found a paper(http://arxiv.org/abs/0902.3662) by two fellows from SUNY, Buffalo where they assume neutralino as dark matter particle and show that neutralino star can't exist. I have no idea how reliable their calculation is but it sounds really interesting. So is there any other way by which such darker black holes can form? If yes how and how tightly it will be constrained from cosmology observations?
After few day I saw a news which indeed talked about such stuffs. According to some physicist in University College London very highly massive black holes can be formed from collapse of dark matter. They call it "Dark Gulping". Only future observations will tell how much truth is there in their model. But the topic is really interesting. In fact although in the above mentioned paper people have shown that dark stars can't exist, they are assuming that the dark matter particles only have weak interaction. But it may well be true and worth exploring that there is a dark sector where the dark matter particles may have strong self-interaction which can arise from some hidden sector gauge group. People have worked on such dark sector models but I have not found any paper where they talk about star or black hole formation from dark particles. I am really excited about this new field and would be looking forward to work on it as well...:)