## Friday, 1 May 2015

### Friday wrap-up: DM self-interactions, on-Z excess, AMS...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

• This arXiv preprint answers my hanging question from a couple of weeks ago as to how the recent Abell 3827 (four-)galaxy cluster measurement is $10^4$ times more sensitive to dark matter self-interactions than the larger scale Bullet Cluster type measurement that hit the news in late March. The answer according to the authors: it isn't! The system is composed of four galaxies, each with a dark matter subhalo, infalling into a larger dark matter halo. The subhaloes are observed to have lagged behind after a long infall period, with the possible interpretation that the dark matter is experiencing some DM-DM drag force that the stars are not. The claim in this new preprint is that the Massey et al paper made the assumption that the stars and the associated DM subhalo develop completely independently. But clearly they are gravitationally bound! And this matters. When taken into account, it is clear that a much stronger dark matter self-interaction is necessary to explain the offsets of the subhaloes from the stars. They find a strength $\sigma/m_{DM}\sim 3 \text{ cm}^2\text{g}^{-1}$, in tension with the limit from the larger clusters (also from Massey et al...).
• On the ATLAS on-Z excess, I count already six articles dedicated to discussing/explaining it. In particular, this one points out that an explanation in terms of the simplified General Gauge Mediation model taken as a benchmark in the ATLAS paper is inconsistent with other measurements. As shown below, the white band preferred by the on-Z excess is disfavoured by a collection of other measurements.

There are at least a-few-papers which claim that a decay chain more like $\tilde g \to q\bar{q}\tilde \chi_2^0 \to q\bar{q} Z \chi_1^0$ with a somewhat compressed spectra can go some way to explaining the excess while remaining consistent with other observations.
• On the AMS antiproton-proton "excess" there have been a few more preprints showing up on the arXiv. I took a quick look at this one, which does the sensible thing: notes that propagation models can fit the data fairly well, and that there is no unambiguous excess (though there are always some that see things another way...), nevertheless we can use these models along with the observations to bound the dark matter annihilation contribution at high energies -- which is the interesting physics after all!

Above are the limits they derive on the annihilation cross-section into $b\bar{b}$ obtained assuming two different propagation models. What's interesting is that they compete with the Fermi dwarf spheroidal bounds for $m_{DM}\lesssim 100$ GeV, which is the region of interest for the galactic centre excess.
• The Stawell Underground Physics Laboratory project aiming at setting up the southern hemisphere's first dark matter direct detection experiment looks like it's really coming along (It even has a Wiki page now)! There was a stakeholder event in Stawell on Tuesday and some buzz from the CAASTRO group on twitter...

• Ellis, Gaillard, and Nanopoulos have uploaded "An Updated Historical Profile of the Higgs Boson" to the arXiv.
• Another nice-couple of articles at Quanta Magazine on quantum phenomena.