Saturday, 12 September 2015

Friday wrap-up: XMASS, multi-component dark matter...

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...

  • The XIV International Conference on Topics in Astroparticle and Underground Physics (TAUP 2015) conference has been happening this week (hashtag here). The plenary talks are available but unfortunately a very many interesting parallel sessions are inaccessible...
  • One of those parallel sessions included a preliminary new result of the search for an annual modulation signal at XMASS. A summary and some plots can be found in this document [pdf]. They see "a weak modulation effect" which they say can be explained by a modest fluctuation background fluctuation, i.e., not significant results. Such are the difficulties in searching for annual modulation in only ~1.5yrs of data. No quote of the phase, but the fit for the modulation in their Figure 1 (below) has a negative amplitude, which might suggest that the best fit phase is ~6 months displaced from the standard halo model maximum in June... anyone have more information?


  • Robert Foot here in Melbourne maintains that it is still possible that dark matter could be the explanation for annual modulation signals seen by DAMA/LIBRA, CoGeNT, and recently by XENON100 (and now perhaps XMASS?). He posted to the arXiv last week outlining a scenario...

    The possible explanation is predicated on a dark matter halo made up of a pressure supported multi-component self-interacting plasma. Considering the mirror dark matter model for definiteness, the halo is mostly made up of dark electrons and dark He ions. There is a (massless) dark photon which mixes with the SM photon, imbuing the dark matter with dark charge and SM nanocharge. Far from the Earth the plasma is in thermal equilibrium; turns out this naively implies a ~100 times larger flux of dark electrons incident on the Earth than dark He. However, dark matter will be captured within the Earth, and by contradiction one can argue that dark electromagnetic fields must arise to equilibrate the (charge weighted) flux of dark electrons and dark He. The flux of the dark electrons on the Earth's surface, which can be possibly detected in direct detection experiments via single electron scattering, then depends on the details of these dark fields, which are assumed to arise from bulk movement of the charged dark matter on/near the surface of the captured dark matter sphere. Since the flux annually modulates due to the motion of the Earth relative to the halo, then so will these dark fields, and so will the electron flux incident on the Earth's surface. Needless to say, determining the flux is a very thorny dynamical problem... the preprint presents a "somewhat primitive" analysis to show in principal that such physics can give a large annual modulation fraction (which is a function of latitude). The "smoking gun" (and the make-or-break) for this scenario is a large diurnal (daily) modulation.

    This just goes to highlight the obvious fact that direct detection results are not as simple as comparing exclusion curves in spin-independent nucleon scattering cross section versus mass.
  • Further on the direct detection front, Lateral Mag have a story on the dark matter direct detection project getting underway here in Australia, in the Stawell Underground Physics Laboratory (SUPL). Funding for the lab has been obtained, and construction should start early next year!
  • On this blog:
    • I have updated my thoughts on the hierarchy/naturalness problem from a month ago. I wanted to distinguish between a hierarchy problem and a naturalness problem; it is my opinion that these terms are used too loosely in modern hep parlance (and perhaps people have different definitions anyway), and this causes confusion (especially from the point of view of an impressionable PhD student). So...

      At least to me, the following definitions make sense: a hierarchy problem is an unexplained hierarchy of scales within a model, and; a naturalness problem (for a mass parameter) arises when a scale receives very large and physically meaningful quantum corrections. The SM+gravity suffers a hierarchy problem by definition, but it is not clear to me that this implies a naturalness problem for the electroweak scale. That is what I blogged about a month ago. Actually, taken this way, minimal supersymmetry alone doesn't solve the hierarchy problem (i.e. it has a mu problem). Nevertheless (and if it arises at the TeV scale) supersymmetry ensures that the electroweak scale does not have a naturalness problem whatever the theory of gravity, and whatever scales are introduced in between (such as a GUT scale), which is in my opinion a very nice property and an admirable achievement for such models.
    • Playing with google charts recently I added a geomap and new/returning pageview charts using google analytics tracking, the google analytics superproxy, and a little javascript withquerying. They're a little messy right now but the information is there; the blog is getting >500 views a week now, so thanks for reading!
  • News from space...
    • A detailed image of the bright spot on Ceres...


    • ... and incredible new images of Pluto and Charon!


No comments:

Post a Comment