Friday wrap-up: diphoton, self-interacting dark matter direct detection...

Back from the end of year break and getting stuck into new projects! Here is the first Friday wrap-up of 2016...

• Fabiola Gianotti is now CERN's Director-General.

• The 750 GeV diphoton monsoon which hit the arXiv on 16th December has not yet abated. There are 150-odd papers now up on the arXiv. See ReSonaances here and here, Tommaso Dorigo, and recent posts on the reference frame.
  
  I personally think that it is a good exercise for the hep-ph community to ask the question, if it is real, then what could it be? At least for the scientifically motivated reason that extra predictions are generally made which might be tested, and these predictions could in principle serve as a guide to tell experimentalists where to probe nature next (in the case that this turns out to be real). It is also sensible to collectively gather ideas which might help to fit the thing into a bigger picture. Unfortunately these good scientific motivations are confounded by citation-chasing, repetition, ill-motivated "Hail Mary" models, repetition, repetition, etc. We must also be aware of our (unscientific) cognitive bias toward fluctuations from the mean: given the statistical significance of the signal, is all this work sufficiently scientifically motivated? This is an interesting question, if rather academic... it is naive to think that scientists are (or even should be) motivated by purely scientific considerations.
  
  Anyway, the time should come for we as a community to sit back and take stock. The problem then is, among the noise, how to reduce the growing theory-space to a set of distinct generic predictions. I am considering pursuing this in the form of a wiki (or similar) as an experiment in large-scale collaboration; the idea would be to produce a summary document which represents a balanced cross-section of hep-ph ideas on this thing (with no cap on author count). The difficulties include the administrative one of keeping such a project economic and efficient, but also keeping a fair balance and controlling the (possibly inevitable) politics involved. If you have ideas or would like to get involved in such a project, please leave a comment or send me an email, so that I may gauge the interest in such a thing...
  
  There is not too much more to say except that there are myriad explanations for this possible signal, and I think it is sensible to be ready if it does turn out to be real. That being said, it would take a brave person to claim that the odds are in its favour...

• Before Christmas we finished up on a fun project: "Plasma dark matter direct detection." The paper concerns what is a rather under-appreciated and somewhat generic point about self-interacting dark matter models and direct detection experiments. The logic goes like this:
  
  (1) If dark matter is self-interacting and capable of giving a direct detection signal, then some amount will be captured within the Earth. (2) The annually varying dark matter wind will interact with this captured dark matter in a highly non-trivial way. (3) This will result in a complex space- and time-varying dark matter near-Earth environment. (4) The dark matter detector moves through this environment throughout the day/year, and the rate it measures will be a time-average of the local rate along its path through space.
  
  In the well studied WIMP dark matter scenario, there is no spatial dependence of the dark matter distribution near the Earth, and so it doesn't matter where your detector is in space. Our scenario is quite different. Both the dark matter wind speed and the detector's daily path annually modulate due to the Earth's motion around the Sun. These modulations have different phases (155 days vs 115 days). So now you have two sources of annual modulation which, due to the complex dark matter environment, give an annually modulating rate which does not necessarily resemble a sinusoid. The following animations should help to visualise this picture:
  
  
  
  
  
  These are two simplified captured dark matter scenarios (fully absorbing/reflective) which we considered. The dark matter wind comes in from the left and its speed annually modulates. The direction of the Earth's rotation axis with respect to the wind also annually modulates, and therefore so do the detectors' daily paths: the black, green, orange, red bars represent the location of detectors in Gran Sasso, Kamioka, China Jin-Ping, and Stawell, respectively. Clearly, due to the complex environment, they will measure very different things! This is the qualitative picture; to make quantitative predictions is very difficult. This is why multiple experiments at multiple latitudes will be important for probing this scenario, especially experiments in the Southern Hemisphere (such as Stawell) which inhabit a unique location behind the Earth with respect to the wind.
  
  Lastly, the generic and distinctive prediction of these models is a possibly strong and non-trivial modulation as a function of time of sidereal day (diurnal modulation). A sidereal day is an "astronomical day" slightly shorter than a 24 hour day; there are approximately 366 sidereal days in a year. It is hard to imagine any background process which would modulate with period of one sidereal day. It therefore seems like a very sensible dark matter search to perform in addition to an annual modulation search.
  
• Already in a few previous posts I mentioned the recent XMASS annual modulation search and its possible hint of a modulation signal with opposite sign to that of DAMA. Out of interest, last week I got around to scraping their central values from the data in the backup slides of their TAUP talk [pdf]. Below I present their measurement of rate as a function of time for energy bins summed from 0.5--2.0 keV57Co.
  
  
  
  
  

  The error bars are statistical only (though they dominate the systematic error) and have been estimated assuming equally spaced bins (which is not exactly correct); these errors are therefore only there to guide the eye and the actual ones would be if anything slightly larger. For interest the sinusoid of best fit, with a phase of 129 (or 311) days, is also plotted.
  
  Their result is clearly intriguing. It looks convincing to me, though one would need another year of data to tell for sure, and it will be interesting to see whether this effect continues in their fiducial volume (this analysis is full volume). What's going on here? It is consistent with a seasonal effect, but with amplitude opposite to that of DAMA. Though possible, if the modulation is due to an environmental effect then at least qualitatively this seems strange, since each of XMASS/DAMA are in the Northern Hemisphere (XMASS at Kamioka 36°N, DAMA at 43°N). The results of the annual modulation experiments sure are puzzling: there are four published now each seeing an effect at some level (though apart from DAMA are statistically weak)...
  
  Time might tell, but a speculative observation: if the XMASS effect is due to a non-trivial dark matter distribution, then the small change in latitude suggests that their signal will almost certainly be accompanied by large diurnal variation. So if XMASS see annual modulation in their fiducial volume, I would be very interested to see their search for a diurnal signal.

• The XXII The Cracow Epiphany Conference on Run II LHC Physics (indico) is currently on.

• Links without thinks:
• The Hawking/Perry/Strominger Black Hole information loss problem paper hit the arXiv; see BackReaction for more.
• nature: "CERN’s next director-general on the LHC and her hopes for international particle physics"
• nature: "Hint of new boson at LHC sparks flood of papers"
• nature: "Open journals that piggyback on arXiv gather momentum"
• Frank Wilczek via Quanta: "Time's (Almost) Reversible Arrow"
• JSTOR: "An Amateur Critique of String Theory"
• Lee Smolin via Perimeter Institute: "Einstein, Storyteller"

• In audio/video media:
• In Particular: Things That Go Bump In The Light, on the diphoton excess. [21:47]
• omega tau: String Theory. [2:43:07]
• CBC radio: Similes and Science, on the Big Bang, string theory, black holes. [53:58]

• Lastly, in case you missed it, the fixion:
  
  
  
  
  

Friday wrap-up: XMASS again, Fermi...

Feels like a lot of links and not much thinks this week, but we have been madly working to finish a paper before I go home for Christmas in a couple of weeks. I think we'll get there, but in the mean time...

• XMASS placed on the arXiv the annual modulation analysis we first saw presented at TAUP a couple of months back, showing some preference for negative annual modulation (opposite to the DAMA/LIBRA claim). They write, "The result of a simple modulation analysis, without assuming any specific dark matter model, showed a slight negative amplitude. As the p-values are 6.1 or 17% in our two independent analyses, these results are consistent with fluctuations." However, staring at their Figure 3, I somehow just can't seem to believe it's consistent with fluctuations...
  
  
  
  
  
  Sure, the fits in each bin are within 2σ of zero modulation, but there are a very large number of them below 3 keVee going the same way. If indeed the bins are largely independent (larger than resolution) and there is no correlated (annually modulating?) systematic, then at least naively I would expect that to be very improbable. So, an honest question, why is this not reflected in the p-value? An interesting sentence from the paper is, "Note that the energy bin width in Fig. 3 is one fifth of DAMA/LIBRA’s so that our limits would even get stricter with DAMA/LIBRA’s bin width." Is this suggesting that the result becomes more significant when all of those negative bins are collected into a larger bin? Comments are welcome.
  
  This is yet another annual modulation measurement with an intriguing result to add to the pile, and perhaps we're seeing a conservative downplaying, especially given the history of such measurements.

• There's a nice article at Scientific American on Fermi's recent contribution to the galactic centre excess saga. The short story is that they confirm the excess above known backgrounds, which when fit with an diffuse NFW source is in broad agreement with previous works, as shown below (one notes the significant uncertainty in the tail as evidenced by the fits assuming different models of the background).
  
  
  
  
  
  We're left now with an official analysis which confirms what we have heard for a while now: that there is definitely something unknown there. So, what is it? The leading standard astrophysical explanation is some population of unresolved point sources (such as millisecond pulsars), which Slatyer now claims are favoured by the data. Still, the dark matter hypothesis is alive, albeit grappling with limits from dwarf spheroidals. And perhaps it can be settled soon; one thing I learned from the SA article is that the pulsar hypothesis might be probed in the near future:
  
  The good news is that if pulsars are behind the excess, more powerful, telescopes in the future should be able to spot the too-faint spinning stars directly. Pulsars would be prime targets for next-generation radio telescopes... “Should we fail to find them in the next five or ten years, a dark matter explanation becomes more likely again,” Weniger says. “This is pretty much a win–win situation. But we have to be patient.”

• Tommaso Dorigo has found a publisher for his book on the Tevatron (and in particular CDF), "Anomaly! - Scientific Discoveries and the Quest for the Unknown." Should be out end of 2016; very much looking forward to it.

• Links without thinks...
• Starts With a Bang: "Strange But True: Dark Matter Grows 'Hair' Around Stars And Planets."
• SLAC: "Q&A: SLAC Theorist Lance Dixon Explains Quantum Gravity."
• New Yorker: "The Space Doctor's Big Idea," the Special Theory of Relativity explained in the 1000 most used words in the English language, by xkcd artist Randall Munroe, who also has a related book out. 
• New Yorker: "The Doomsday Invention," a long read on artificial intelligence; do yourself a favour and get your hands on Nick Bostrom's very interesting book!
• Nature: "Einstein was no lone genius."
• Nature: "The quantum source of space-time."
• Preposterous Universe: "Thanksgiving [for Riemannian Geometry]"
• Wired: "Physicists Are Desperate to Be Wrong About the Higgs Boson."

• In audio/video media...
• Claire Lee via TEDx: Our universe in a box, particle physics with Lego. [15:46]
• Fermilab: What good is particle physics? [6:48]
• Fermilab: Physics Slam 2015. [1:29:47]
• WaitWait: Lisa Randall Interview. [22:36]
• Einstein 100: Theory of General Relativity, a short narrated by David Tennant. [3:04]
• SixtySymbols: Einstein's Famous Blunder. [18:47]
• SmarterEveryday: Hovering a Helicopter is Hilariously Hard. [9:44]
• Veritasium: How Long Will You Live? [6:25]
• Numberphile: Tic-Tac-Toe (with Xs only). [7:19]

Friday wrap-up: diboson update, XMASS...

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...

• It's the season for conferences! This week we have...
• 8th International Workshop on Top Quark Physics (TOP2015: indico; twitter). One of the interesting new results includes evidence for (>3σ) single top quark production in the s-channel with the 8 TeV dataset. There's an entertaining review of the first two days here from James Howarth.
• Particle Astrophysics and Cosmology Including Fundamental InteraCtions (PACIFIC 2015: agenda).
• Corfu Summer Institute: 15th Hellenic School and Workshops on Elementary Particle Physics and Gravity (programme).

• A couple of interesting conf-notes out this week as well. The first is the ATLAS/CMS combined Higgs results we saw presented a couple of weeks ago.

• The second is an ATLAS diboson resonance search which combines the results from the large-R dijet channel with the leptonic channels. The results are well summed up by the first Figure in the Appendix:
  
  
  
  
  
  In short, when interpreted as a $W'$ resonance decaying to $WZ$, they see a 3.4σ local excess in the boosted jet topology and absolutely nothing in the leptonic channels. As well, these leptonic channels were sensitive to the $W'$ interpretation of the dijet excess, so that the local significance when combined falls to 2.5σ. Taken at face value then, if the dijet excess is really new physics, it is unlikely to be as simple as $W'\to WZ$. [As an aside: I do wonder how the community's reaction would have differed if this were that paper that was published first?]. To mimic such a signal without the leptons you would need a heavy resonance decaying to two exotic particles with mass $\sim m_Z$, which then decay mostly to quarks... would be difficult to hide these low mass exotics. Or else it is something more complex that happens to pass the selection criteria for the fat jet analysis but produces very few isolated leptons. Anyway, there have been >30 extra citations to the original ATLAS paper since I made a quick literature survey seven weeks ago, and more every week. For me it seems sensible to just wait and see what the new data says (probably some time next year), happy to watch the ambulance in the distance, starting to speed up...

• The TAUP2015 parallel session slides are now up. Indeed, as speculated last week, XMASS has a best fit modulation that is opposite in phase to that seen by DAMA/LIBRA (see Slide 10 [pdf]). It is enough evidence to exclude much of the region where the DAMA signal can be interpreted as a standard WIMP with spin-independent nucleon scattering cross-section (though this is nothing new). Interesting to see what their results will be in the fiducial volume (analysis ongoing).
  
  
  
  
  
• 32 Australian institutions have signed up to the Science in Australia Gender Equity (SAGE) pilot: "Commencing in September 2015, the pilot requires participants to collect, analyse and present data on gender equity policies and practices in STEM departments, as well as identify gaps and opportunities for improvement."

• Links without thinks:
• Institute for Advanced Study: "Beyond the Higgs: From the LHC to China."
• Richard Dawid wrote a guest blog on the reference frame: "What confirms a physical theory?" This should be taken in the context of that Ellis/Silk nature comment article and the ensuing debate on post-empirical science.
• New Scientist: "Black holes may be brick walls that bounce information back out." On 't Hooft's new contribution to the black hole information paradox...
• ... and Sabine Hossenfelder's reaction at Starts With a Bang: "Black holes and academic walls."
• Also at Starts With a Bang: "Will The LHC Be The End Of Experimental Particle Physics?"

• In video/audio media:
• CERN: A superfast chip for particle detection. [3:34]
• MinutePhysics: Why It's Impossible to Tune a Piano. [4:19]
• Vi Hart: Infinite Trees Are Super Weird. [6:18]
• Numberphile: Langton's Ant. [4:28]

• Lastly, in images from space, it is hard to top these new images of Pluto!
  
  
  
  
  

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!

• Links without thinks:
• Backreaction: Macro dark matter.
• Lawrence Krauss via New Yorker: All scientists should be militant atheists.
• Legacy Project: Peter Jenni (former ATLAS spokesperson).
• Quanta: Einstein’s Parable of Quantum Insanity.
• Quanta: A Tricky Path to Quantum-Safe Encryption.

• In video/audio media:
• CERN: Innovative cooling system for silicon detectors. [3:35]
• Frank Wilczek via bigthink: How Einstein changed the way we see everything. [3:22]
• Sixty Symbols: Hydrogen alpha. [8:30]
• Numberphile: numbers and free will [15:02]; the infinitesimal monad [7:10], and; the opposite of infinity [15:04].
• Objectivity: Magnets and black holes. [5:45]

• News from space...

• A detailed image of the bright spot on Ceres...
  
  
  
  
  
• ... and incredible new images of Pluto and Charon!