Magic Monday Journal Club

27th October 2014

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Was cool and nice. And even nice and cool as several member of the audience naturally came to discuss different articles they read or even write (yes, yes, sometimes we also work around there..)



Of Contact Interactions and Colliders

by S. Davidson, S. Descotes-Genon, P. Verdier


Mmmm... Ok, ok, Sebastien (Descotes-Genon) is the new head of our Institute. He decides who goes or not in mission. Who we can invite or not. Which kind of fellowship we can be afford. Gives his opinion to the CNRS administration about our scientific work. And you ask why I begin by his article? You are stange.. Here the description of his work, by his own words:

Unless one wants to go for a specific model of New Physics, one generally describes the deviation from the Standard Model using contact interaction -- this is in particular done to extract bounds on NP from ATLAS and CMS data. This works fine for particles exchanged in the s-channel, but it is generally a rather poor approximation for particles exchanged in the t-channel. Starting from leptoquark exchanges in pp->l+l-, one can imagine a more general but simple parametrisation that proves accurate in a much larger range of energy, covers a wide span of models with new particles in the t-channel, and boils down to the contact interaction in a particular limit. A naive statistical analysis of CMS data shows that this general parametrisation can be converted into fairly stringent constraints for specific models, in a more efficient manner than contact interactions. It remains to be seen if the versatility and efficiency of this general parametrisation still hold in an actual experimental analysis.


Ultraviolet freeze in

by F. Elahi, C. Kolda, and J. Unwin


The article discusses the implementation of the freeze-in mechanism for dark matter production in the case the latter belongs to a hidden sector interacting with the SM only through higher-dimensional, not-renormalizable operators. The general expression of the DM yield, originated by scatterings with more than two final state particles, is provided. The most relevant point is that in the case on non renormalizable interactions most DM production occurs at the earliest stages of the history of the Universe and then its yield depends on the prymordial reheating temperature. In the second part of the paper two concrete examples of UV-freeze are briefly discussed, i.e. a heavy Z' portal and a axion portal.




Last results from the FERMI symposium in Nagoya

by FERMI LAT


For the first time, the Fermi collaboration speaks up on the gamma-ray excess from the center of galaxy. They fit the observed gamma-ray emission with point sources and a pulsar population, however residues remain. They note that the fit becomes better if one adds a new spherically symmetric component following the NFW profile, as may be  expected if dark matter annihilation contributes to the emission. However Fermi does not specify how significant and how robust the excess is, so it is still far from clear whether this could be a serious dark matter signal.



Potential solar axion signatures in X-ray observations with the XMM–Newton observatory

by G.W. Fraser,1A.M. Read,S. Sembay, J.A. Carter and E. Schyns


Ok, ok... Was not easy to know if one should or not discuss this article on the MMJC. I would say that this kind of work is (unfortunately) characteristic of the 2.0 era, where news of pseudo-discoveries, spread on the web, then on the eyes of journalists, avid of sensational articles for their review. It was presented by Scientific American et al. as a pure Dark Matter discovery from the sun. Just crazy. Adding to the fact that Prof. Fraser died two days after the submission, transforms all this bull into  a disgusting post-mortem something. The authors in the article even almost not mention dark matter in their paper. What they did? Just observed a modulation of X-rays coming from the sun, season-dependant, which «could» be translated as an axionic signal, the modulation being generated by the different strength of the magnetic field depending on the position of the earth relative to the sun. Can be useful to understand specific axion-like phenomenon like the Primakoff effect (that I didn’t really study before reading this article). It should help to understand the production mechanism of axion-like particles in the sun. Nothing more. The rest is just not credible at all. You can find an annotated version of the paper here and a nice figure of the effect there.



Superheavy Dark Matter in Light of Dark Radiation

by J.-C. Park and S.-C. Park


Well... When a Park, Kim or Lee are writing an article, even if one of them is a collaborator with who you used to work, you still wonder, «which Lee?» or «which Park?»  is this one. In this case, I may know one of this Park, bot not sure in fact. In this paper, they made a decent work looking for the effect of a superheavy DM decaying into hidden photon through loops (see their Fig.1). Supposing that the intermediate states running in the loop are quite heavy, the lifetime can be sufficiently long to render the particle quasi-stable, at least up to the second, minute or year scales and affect the dark radiation measured in the CMB. Fig.2 is a good summary, and as a conclusion, yes, constraints on effective relativistic degrees of freedom restrict the parameter space of such model.. But.. Someone was asking in the room «who cares». To tell the truth. I do not really have the answer.... You can find an annotated version of the paper here.





Observing Ultra-High Energy Cosmic Rays with Smartphones

by D. Whiteson, M. Mulhearn, C. Shimmin,  K. Brodie,  and D. Burns


At least. A fun article to read. The idea? Quite simple, transforming your cell phone into a Ultra High Cosmic Ray detector. How it works? Nothing to do, just downloading an application, you let the cell phone switch on on your desk while your sleep, then it measures the events, and send it to a local server for future analyse. With a much larger region than Auger, and using the correlation between each cell phone, the software can reconstruct the energy of the flux and its nature. Sounds maybe crazy, but joyful and original. A quick look at their Figure 10 shows that one needs  1 millions of devices to observe 100 events per year, more than the actual sensitivity of Auger telescope. So let’s do it? The only problem, if something is discovered, would be to write the list of collaborators in the experiments for sure.. You can find an annotated version of the paper around there.