Magic Monday Journal Club

24th February 2014

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A Journal Club  dominated by a keV something somewhere in the sky (or not) but with other nice curiosities (except the haircut of A.)


by E. Bulbul, M. Markevitch, A.Foster, R. K. Smith M. Loewenstein and S. W. Randall

This analysis of x-ray spectrum of many galactic clusters claims the presence of a monochromatic 3.5 keV photon line that cannot be assigned to known atomic transitions. One possible interpretation of the line is that it's a signal of a 7 keV sterile neutrino dark matter decaying into a photon and a neutrino. This may be yet another false positive coming from  astrophysics, or may be not... For more informations, have a look on this blog.

An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster

by  A. Boyarsky1, O. Ruchayskiy, D. Iakubovskyi  and J. Franse1

This paper appeared shortly after 1402.2301 and makes a similar  claim about the 7 keV photon line.  Their claim is based on a smaller subset of data (only the Andromeda galaxy and the Perseus cluster) and a less elaborate analysis but the significance of the x-ray line is also large.  One added value is that  the angular distribution of the signal seems consistent with the dark matter origin.  

Dark Matter Production in Late Time Reheating

by K. Harigaya, M. Kawasaki, K. Mukaida and M. Yamada

A nice work where the authors summarized all the possibilities to thermalize a bath with low reheating temperatures (from MeV to GeV scale) and at the same time to produce (non-thermally) the relic abundance measured by PLANCK. The main originality of the work is the inclusion of the inelastic scattering process, taking into account the  Landau-Pomeranchuk-Migdal effect. They show that the inelastic scattering is the more efficient one when particles are produced from the inflaton decay at an energy Ei>> Treheating.

They wrote a similar study (without the dark matter treatment) here.To read especially if you want to understand the main thermalization process in the primordial plasma. For the hurry guy, just look at the figure 3 it is a nice summary of the allowed parameter space.  You can find an annotated version of the paper here.

Naturalness and the Weak Gravity Conjecture

by C. Cheung and G. N. Remmen

Another idea to circumvent the hierarchy problem. The authors argue that large quadratically divergent corrections to scalar masses may be in some circumstances inconsistent with the so-called weak gravity conjecture - the bound on the mass-to-charge ratio of particles that follows from plausible consistency requirements in the full quantum gravity theory.

Indirect and direct signatures of Higgs portal decaying vector dark matter for positron excess in cosmic rays

by S. Baek, P. Ko, W. Park and Y. Tang

A paper treating about the Higgs portal in the case of a vectorial dark matter. Lebedev et al in this paper showed that direct detection constraints combined with PLANCK data and Higgs searches from its invisible decay, exclude the main part of the parameter space. The study was valid up to 1 TeV. Below 1 TeV, you can find part of the parameter space still allowed. Then, adding a second scalar mixing with the SM Higgs (justified to give mass to the vectorial DM) you can play with the paramter space and try to fit PAMELA/AMS-02 excess (even if I do not really understand what for). The authors showed that for a decaying vectorial DM (through higher dimensional operator (see their page 15) into mu+ mu- they can fit the positron excess. Hourra.  You can find an annotated version of the paper here.

Monojet versus rest of the world: t-channel Models

by M. Papucci, A. Vichi and K. Zurek

Very serious work (as often with these authors) which will be divided in two parts (the s-channel one will surely appear on arXiv. Even if in the title, it seems the main point concern the importance of looking for multijets because if monojets are produced from partons annihilation, we have to suppose the presence of a SU(3) charged mediator (a squark for instance in the case of t-channel process) and so its decay in multijets. Look at their Fig.1 and 2 to understand their point. More important is the influence of a finite width compared to the classical effective field theory (EFT) approach commonly used by the LHC studies. Their Fig.6 summarize perfectly their point and shows how the effective approach is not valid if you want to work in a microscopic construction. You can find an annotated version of the paper here.

On the gravitational field of a mass point according to Einstein’s theory

by K. Schwarzschild


1915. Karl  Schwarzschild is in the Russian front, volunteer at 42 years old in the artillery company. He found an elegant way to find an exact solution to the Einstein equation (with special spherical coordinates) recovering the exact result of the perihelion of Mercury found by Einstein (who used an approximate solution in 1915). The article will be published uner the name of Scharzschild by Einstein before the former died in the Russian front 6 months later. Notice that he NEVER evoked the singularity and the possible existence of a black hole from its metric (14.)