Friday wrap-up: diphoton uncertainties, dark matter uncertainties...

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...

• There's quite a bit of discussion over at Résonaances (see also the comments) surrounding the Davis-Fairbairn-Heal-Tunney paper proposing an underestimated systematic in the background parameterisation used in the ATLAS diphoton analysis. This (and related) discussion looks to have aided (according to the acknowledgments) the preparation of another paper from Bradley Kavanagh, which seems to clarify the issue. In that paper it is written:
  
  Davis et al. introduce a different possible parametrisation for the background (which was also validated by a Monte Carlo study) and find that the significance of the excess is further reduced with respect to the k = 1, fixed-N case. However, the empty bins at high mγγ were not included in that analysis, leading to a background fit which overestimates the high mγγ event rate. Indeed, using the Davis et al. background parametrisation (with free normalisation) in this analysis gives a local significance of 3.8σ for a free-width resonance. This does not discount the possibility that exploring a wider range of possible background functions may impact the significance of the 750 GeV excess, but the correct constraints from the entire range of mγγ should be taken into account.

• A few-interesting-papers appeared concerning baryonic effects on the local dark matter velocity distribution, of interest for interpreting direct detection experiments (see Matthew Buckley's blog for a write-up of one of them). Each of the papers takes a number of simulated Milky Way-like galaxies and looks at the dark matter distribution at Solar radius. Naturally, due to the small number of simulated galaxies, the papers reach slightly different conclusions. What is clear, though, is that there are significant uncertainties in both the local density and the local velocity distribution, which means that the usual direct detection limits you see drawn on e.g. σSI versus mχ space should be taken with a small grain of salt, since they assume the standard halo model. Also of note is that these effects alone cannot ameliorate tension with the DAMA/CoGeNT events. Further work in this area will be interesting to follow as additional (and more detailed) simulations become available.

• Links without thinks
• .Mic: "With One Hashtag, Female Astronomers Share Their Heartbreaking Stories of Harassment"
• Nicolas Gisin via IQOQI: "Thought police – on arXiv?"
• BackReaction: "Does the arXiv censor submissions?"
• nature: "Hawking’s latest black-hole paper splits physicists"
• Ars Technica: "The search for dark matter heats up"

• In audio/video media:
• IQIM Caltech: Anyone Can Quantum, a quantum chess match between Paul Rudd and Stephen Hawking. [11:50]
• CERN: CERN's new microcosm exhibition is now open. [0:41]
• Fermilab: Quantum Field Theory. [5:29]
• Fermilab: Why I Love Neutrinos - Professor Naba Mondal. [0:59]

• A sad day for Comic Sans enthusiasts everywhere (nowhere?) -- apparently no more from Fabiola...
  
  

  Historical moment announced at @CERN during DG Fabiola Gianotti's new year address: The End of the #ComicSans Era! pic.twitter.com/nhw6TabNbZ

  — Steven Goldfarb (@stevengoldfarb) January 18, 2016

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, 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!
  
  
  
  
  

Friday wrap-up: 3/fb, ATLAS on Higgs LFV decays...

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...

FYI, I've posted a list of recommended (active) high energy physics news and blog links in the sidebar, also here for those using a reader: A Perfectly Formed Puddle, A Quantum Diaries Survivor, ATLAS Blog, ATLAS Briefings, ATLAS News, Backreaction, CERN Press Releases, CERN Updates, CMS Blog: Cylindrical Onion, CMS Physics News, Collider Blog, Ellipsix, Interactions.org, Life and Physics, Life on the Lattice, Looking Inside the SM, Nautilus: Particle Physics, Neutrino Blog, Not Even Wrong, Of Particular Significance, PhysicsMatt, Preposterous Universe, Quanta Magazine: Physics, Quantum Diaries, RWTH Aachen, Resonaances, Tim Head, Transcyberphysix, symmetry magazine, the reference frame.

• The XXVII International Symposium on Lepton Photon Interactions at High Energies (Lepton Photon 2015) has been going this week (indico/twitter). We heard from Mike Lamont about LHC performance; multiple commissioning issues (electron cloud, UFOs, ULO, ...), none of which are expected to be long term, mean that predicted integrated luminosity for ATLAS/CMS in 2015 is now at ~3/fb. [See also a brief story at New Scientist].

• Following up the CMS 2.4σ excess from February, ATLAS on Monday placed their search for LFV Higgs decays in the $\mu\tau_{had}$ channel on the arXiv. Their result is consistent with zero, but also consistent with CMS. Their best fit is a $\mathcal{B}=(0.77\pm 0.62)\%$, compared to $\mathcal{B}=(0.84^{+0.39}_{-0.30})\%$ from CMS. One can see that the CMS search is more sensitive; this is likely driven by the fact that CMS also included the $\mu\tau_e$ channel. Do ATLAS have plans to look at this channel soon as well?

• A few weeks ago we mentioned that LHCb announced preliminary results in a search for displaced light scalar bosons. The preprint is on the arXiv now, which allowed me to scrape their data points and reinterpret their branching limits for the real singlet scalar portal. For interest, the result is below in orange, quite similar to the approximate plot from that previous blog post (more information there). Anyway, LHCb have done a great job excluding parameter space!
  
  
  
  
  

• The Dark Energy Survey (DES) has discovered eight new dwarf galaxy candidates (arXiv/press release), to add to the nine they discovered earlier this year. The sky is filling with satellites...
  
  
  
  
  

  Now taking bets on which one has an excess of gamma rays consistent with dark matter annihilation... 

• On that note, the first paper pointing out the gamma ray excess in Reticulum II (on the day of the first DES dwarf galaxy candidates announcement) was published in Physical Review Letters this week. Tracy Slatyer has a Viewpoint here.

• The case for complex dark matter, at Quanta with James Bullock (of University of California, Irvine).

• Superfluid dark matter, at Backreaction.

• Mary K. Gaillard has a book out: A Singularly Unfeminine Profession: One Woman's Journey in Physics. There's a review on nature.com from Val Gibson.

• In video/audio media:
• US LHC: How to make a discovery. [3 minutes]
• Fermilab: Pentaquarks. [9 minutes]
• CERN: The music of physics. [3 minutes]

• For your enjoyment, here's a Pluto flyby...
  
  

Friday wrap-up: diboson excess, EPS-HEP, XENON100...

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...

I am back from a six week tour of Europe (Greece for Planck conference, UK for seminar talks, Italy for ICTP Summer School and talk in Rome) followed by a proper two week holiday (Hawai'i for lava and turtles)... hence the inactivity here. In my absence, the diboson excess has been hot, the first 13 TeV results have been already trickling out, and in other good news it is 92% probable we are even doing something "worthwhile" -- hey, that's almost 2σ!

Let me attempt an incomplete summary of the last month...

• It's been almost two months now since the ATLAS diboson excess hit the arXiv (see Resonaances for a description), and many theorists/phenomenologists have now had the time to digest and interpret the result. The paper has been cited 37 times, and I count 31 dedicated studies. Let's take a stroll through them in the hopes of learning (in some Bayesian sense -- of course you will have to unweight for selection bias) what is a likely explanation if the signal persists... [This is only a quick survey and probably not completely accurate; send me a message or leave a comment if you believe I've done any of these papers a grave injustice...]
  
  

  Paper	Authors	Candidate	Comment
  1507.07406	Faraggi, Guzzi	$Z', W'$	String inspired GUTs
  1507.07102	Lane, Prichett	$\rho, a_1$	Vector or axial triplet in composite Higgs
  1507.06499	Fritzsch	$Z^*, W^*$	Excited states of composite weak bosons
  1507.06312	Kim et al.	-	EFT study
  1507.06018	Bian et al.	$\rho$	Vector triplet in composite Higgs
  1507.05299	Anchordoqui et al.	$Z'$	Leptophobic, string inspired
  1507.05310	Chao	$H$	2HDM
  1507.05028	Omura et al.	$H$	2HDM
  1507.04431	Chen, Nomura	$H, H^\pm$	2HDM
  1507.03553	Sanz	Exotic glueballs	Perhaps within composite Higgs framework
  1507.03428	Fukano et al.	Dilaton	e.g. scale-invariant generic heavy vector triplet model
  1507.03098	Cacciapaglia et al.	Pseudoscalar	Weak singlet with Wess-Zumino-Witten (effective) couplings
  1507.02483	Chiang et al.	Composite Spin-0	Hidden confining gauge theory coupled to SM via D5 operators
  1507.01923	Dobrescu, Liu	$W'$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model
  1507.01638	Allanach et al.	$Z', W'$	(motivated by EFT) within $SU(2)_L$ or $SU(2)_R$ vector triplet
  1507.01914	Carmona et al.	Vector resonances	Composite Higgs (non-custodial)
  1507.01681	Abe et al.	Vector resonances	Partially composite [G221 model with one dynamical SU(2)]
  1507.01584	Heeck, Patra	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$
  1507.01185	Abe et al.	$Z', W'$	G(221) 'three site moose model' e.g. KK excitations of weak bosons
  1507.00900	Cacciapaglia, Frandsen	-	Unitarity study
  1507.00268	Cao et al.	$Z', W'$	In G221 and G331 models
  1507.00013	Brehmer et al.	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)'$
  1506.08688	Thamm et al.	Composite $Z', W'$	Within vector triplet
  1506.07511	Gao et al.	$W_R$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$
  1506.06767	Alves et al.	$Z'$	$U(1)_{d-u}$
  1506.06739	Aguilar-Saavedra	$(VVX)$	Triboson final state mimicking a VV resonance
  1506.06736	Dobrescu, Liu	$W'$	$SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model
  1506.06064	Cheung et al.	$W'$	$SU(2)_L\times SU(2)_R\times U(1)'$
  1506.04392	Franzosi et al.	Composite $Z', W'$	Within vector triplet
  1506.03931	Hisano et al.	$Z'$	Leptophobic
  1506.03751	Fukano et al.	Technirho	Vector triplet within walking technicolour (composite) model

  
  Looks like the most popular explanation is a $W'$ within an extra vector triplet, either arising from an extended gauge sector (minimally a G221 model) or as a low-lying composite state. Less popular, but still well represented, are explanations via a leptophobic $Z'$ or a heavy Higgs in a 2HDM with the second Higgs doublet coupling strongly to the first generation quarks. A notable absence is any (minimal) SUSY explanation.
  
  Many (but certainly not all) of these models tend to predict observable $WZ$ and $WW$ resonances ($ZZ$ is difficult for a spin-1 due to Landau-Yang), usually in conjunction with $Wh$ (just by naive equivalence theorem). These are channels to keep an eye on during Run II.

• The first 13 TeV results are already being released! E.g. check out all-these ATLAS notes which have appeared in the last couple of weeks (just in time for EPS-HEP). For the record, CMS had the first as far as I know (charged hadron pseudorapidity distributions).
  
  In particular, ATLAS released a plot (below) which beautifully agrees with the standard model as per usual: top quark pairs at 13 TeV just where they're supposed to be!
  
  
  
  
  
• The EPS-HEP conference ran this week from 22-29 July. The slides are available on Indico here. I was impressed by the live webcast of plenary sessions, the daily newsletters, and the well-used hashtag which almost made it possible to attend the whole conference online. Some highlights for me...

• LHCb presented preliminary results in their search for long-lived light scalars (see this talk [pdf] from Andrea Mauri) in $B\to K^* s \to K^*(\mu^+\mu^-)_{displaced}$ decays; they see no significant signal above background. Last year I gave a talk to the LHCb rare decays group motivating such a search, so it is very exciting to now see results! Below are the limits they set on the $B$ meson branching fraction for different lifetimes.
  
  
  
  
  
  The simplest model which can give this phenomenology is the standard model plus a real singlet scalar (Higgs portal), as described in an earlier post here. The pertinent free parameters of that model are the light scalar mass and a mixing parameter, and this new result will constrain that parameter space. To get a feel for how much, I picked off the limit lines (sans the statistical fluctuations which can be scraped from the vector plot once the preprint is out) and translated them. [Here I am taking data from an unpublished plot presented at a conference... have I learned nothing from BICEP?] Anyway, the exclusion result is shown in orange in the following figure (the grey shaded regions indicate lifetimes of 0.1mm, 1mm, 1cm,... for more details on the plot see here):
  
  
  
  
  Interestingly, LHCb competes with the BaBar exclusion curve (grey) even for very low masses. It was not obvious at all that LHCb would be able to do this, since for these low masses the long-lived light scalars are very boosted and many will escape their detector. Looking forward to reading the preprint when it is out!

• Two months ago we mentioned the new LHCb result on $R(D^*)=Br(B\to D^*\tau\nu)/Br(B\to D^*\mu\nu)$. The heavy flavour averaging group (HFAG) have now released their combination average; it's 3.9σ from the SM. (See talk from Marta Calvi [pdf]).

• LHCb published in Nature Physics their exclusive measurement of $|V_{ub}|$ in $\Lambda_b$ decays, an important result in resolving the $V_{ub}$ puzzle. You can read the LHCb release here. It has been on the arXiv since April, so it's not a "hot off the press" result, nevertheless it is now for some reason being picked up by various news sources as a blow for supersymmetry (see-these-four-examples). Good to know that if we see nothing in LHC Run II there is at least one way to sell the null result to the media... even though as a scientist such a result would be extremely interesting!

• LHCb have claimed the discovery of pentaquarks (paper here and EPS-HEP slides from Sheldon Stone here [pdf]), a $J/\psi p$ resonance in $\Lambda_b\to J/\psi p K$ decays.
  
  
  
  
  
  This comes 12 years after SPring-8 first announced (the later ruled out) evidence for such states. One cool thing about the LHCb result is that you can even see it by eye in the Dalitz plot (below as line in $m^2_{J/\psi p}$); the LHCb team cannot account for it with any known $\Lambda^*$ resonance or interference. The best fit is in fact found by including two new $uudc\bar{c}$ pentaquark states.
  
  
  
  
  There's a good Quantum Diaries post from Adam Davis about it here (see also nature news, symmetry, Jon Butterworth, and Tommaso Dorigo + comments).

• This week the XENON Collaboration released an arXiv paper, "Search for Event Rate Modulation in XENON100 Electronic Recoil Data". They see a 2.8σ annual modulation signal in low energy single scatterings with a phase consistent with DAMA/LIBRA (!) ... and then pour a serious amount of cold water on the measurement. In order of decreasing temperature, here are the buckets they use: (1) There is no globally significant modulation in the data. (2) The phase of the annual modulation signal deviates from that expected for a standard dark matter halo by 2.5σ. (3) The amplitude is much lower than that expected if DAMA/LIBRA was correct. (4) A 2.5σ annual modulation signal is seen in low energy multiple scatterings as well.
  
  Some comments now... Bucket (1) is lukewarm; we should only be interested in annual modulation for a dark matter hypothesis and there is no look-elsewhere effect. For buckets (2) and (3) let's look first at their Figure 4.
  
  
  
  
  Bucket (2) is room temperature. The phase of an annual modulation hypothesis is found to be inconsistent from the standard stationary halo expectation by 2.5σ. However, it is plain to see that it is consistent with the DAMA/LIBRA phase. If there is some bulk rotation/movement in the halo, perhaps this can be explained? Bucket (3) is certainly chilly, but there are two things to keep in mind. The amplitude is calculated for a particular model (WIMP-electron scattering with axial vector coupling), and the two experiments have very different targets (NaI crystal versus Xenon). Unfortunately we cannot compare apples with apples here and a conversion must take place, for which there is more information in a second XENON paper. For the last bucket let's look at their Figure 3.
  
  
  
  
  Bucket (4) is potentially large and freezing; a dark matter explanation should not induce an annual modulation in low energy multiple scatterings, and it appears to at 2.5σ. However, I can find nowhere in the paper where they quote the phase of this modulation! If indeed the phase is consistent with the single scattering phase, then this would be evidence for a background origin. Note that XENON100 is in Gran Sasso, as is DAMA/LIBRA, thus such a measurement would have implications for the DAMA/LIBRA result. So, XENON, what is the phase of the annual modulation in low energy multiple scatterings?

• Those following this blog will know we have been documenting somewhat the status of the galactic central excess of gamma rays seen in the Fermi data. The excess (over standard astrophysical backgrounds) is undeniably there, but the question of course to be answered is its origin: dark matter, or some (not yet fully understood) baryonic astrophysics? The most popular explanation in the latter set is by some population of millisecond pulsars (i.e. point sources) [see Sabine Hossenfelder's post here]. Recently, some-studies have analysed the Fermi data to see if the excess prefers a diffuse (e.g. dark matter) or point source origin. Both of the studies find a preference for a point source origin...
  
  This morning I stumbled upon a (days old) CERN Seminar from Tracy Slatyer, who may be in a unique position to comment on the issue, being an author of one of those new studies, as well as an author on one of the well cited papers arguing a dark matter interpretation. Below is the conclusion slide from the Slatyer talk, where it is interesting to see that the game has changed, with preference now for a point source origin over dark matter.
  
  
  
  
  

  This is science in action; it sounds like some very interesting new astrophysics will be revealed by the time the book is closed on this excess, and this should be celebrated.

• The PASCOS conference happened at ICTP at the end of last month; a very many interesting plenary talks (~30 mins each) are available as videos and worth a peruse.

• There's a public kaggle competition to design machine learning algorithms to best discover $\tau\to 3\mu$ decays at LHCb.

• Frank Wilczek's new book on beauty in nature is out. See Peter Woit's blog for a good summary and further links.

• Over the coming weeks, Stephen Hawking will be answering (some) submitted questions on artificial intelligence in a reddit AMA.

• In audio/video media:
• BBC documentary on dark matter, Dancing in the Dark. [50 mins]
• Pentaquarks on Sixty Symbols. [8 mins]
• A 20 minute short on SUSY and the LHC from Motherboard.
• A Brief History of Everything featuring Neil deGrasse Tyson at MinutePhysics. [8 mins]
• Physics Girl on New Horizons. [5 mins]
• Two bike-physics videos at MinutePhysics.
• How many chess games are possible? on Numberphile. [12 mins]
• Frank Wilczek talks on his new book at Google [40 mins], at the LSE [>1hr], or on radio [30 mins].

• The Kepler mission has discovered the first ~Earth-sized planet within the habitable zone of a Sun-like star [see xkcd]. There has been significant hype; for a no-nonsense take see Bad Astronomy. You can read the actual paper [pdf] here; they state, "The likelihood that this planet has a rocky composition lies between 49% and 62%."

• And while I was away, New Horizons flew by Pluto! In the tradition of ending each post with stunning shots of space, this probably takes the cake: the money shot in natural colour, a surface shot, and the farewell. Truly magnificent. (For more information, Nat Geo has a good story).
  
  
  
  

Friday wrap-up: IBL, SABRE...

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...

• ATLAS News had a wrap-up of the 900 GeV collisions performed last week. It was cool to see an event display with the insertable B-layer in action (the fourth and most inner layer of the pixel layers on the right insert):
  
  
  
  
  

• The Stawell Underground Physics Laboratory project here in Australia has received $1.75 mil from the federal government to match the State's contribution from earlier this year. The funding will go a significant way to constructing the clean room to host SABRE, the first southern hemisphere dark matter direct-detection experiment, the advantages of which I've mentioned earlier.

• If the dark matter distribution in M87 is spiked at the centre, then this arXiv preprint claims that thermal relic dark matter is ruled out for an unprecedented $m_{DM}\lesssim 100$ TeV! As well, an apparent excess at high energies can be explained by $\mathcal{O}(1-100)$ TeV dark matter. I wonder if this paper will become another galactic centre excess for hep-ph?

• The result has been on the arXiv for a while, but the CMS+LHCb $B_s (B^0)\to \mu^+\mu^-$ analysis was published in Nature, which I thought was interesting enough to note. As far as I can tell from a quick Inspire search, this is the first paper from the LHC Collaborations published in Nature. Note the 6 months from receipt to publication...

• A few very interesting articles this week:
• Nautilus: the story behind the OPERA superluminal neutrinos.
• Aeon: on the pervasiveness and apparent non-falsifiability of inflation.
• Scientific American: on physicists as philosophers.
• Quanta: ultra-high energy cosmic rays, the Oh-My-God particle, and an EeV+ hotspot in the sky.

• In video/audio media:
• New physics frontiers at the 13 TeV LHC from CERN. [3 minutes]
• A first video spot at Quanta Magazine: In Theory with David Kaplan (of Particle Fever fame) on what happens if you fall into a black hole. [2 minutes]
• And if you'd like to learn more about the man responsible for backing the foundation that supports Quanta Magazine in the first place (and for Chern-Simons forms, and for Renaissance Technologies), you should watch the very interesting interview with James Harris Simons at Numberphile. [19 minutes]
• Stephen Hawking on intelligence. [15 minute talk]
• Excellent video at SmarterEveryday on how the window shutters on the space station work. [8 minutes]

• Finally, updates on space missions: 
• New Horizons can now make out all of Pluto's known moons.
  
  
• The bright spots on Ceres we've been following now appear to be deposits of ice at the bottom of a crater.
  
  
  
  
  
  

Friday wrap-up: 6.5 TeV, AMS, young pulsars, 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...

• On this day last week we had the first 6.5 TeV beam fly around the LHC, and a day later we had both beams at once. You can read a little more at symmetry magazine.
  
  
  
  
• AMS have presented their results on the cosmic ray proton/helium/lithium flux and the antiproton-proton ratio at a CERN mini-conference. Recordings of the talks are here and slides here. The press release is here. The plot they are pushing is the following one:
  
  
  
  
  The claim in the press release is, "This behavior cannot be explained by secondary production of antiprotons from ordinary cosmic ray collisions," with the suggestion that a new primary source(s) may be needed. But Sam Ting made sure during his talk to emphasise that it cannot be explained by existing secondary production models, and when presenting this figure he noted that there are many secondary production models, but "this is the one we choose" -- I wonder why... (an aside: he also in the same talk referred to his $J/\psi$ particle first as $J$ and then as $\psi$ on the same slide!). The point is that these models are very uncertain and the claim in the press release is unfounded. Indeed, there was a preprint on hep-ph yesterday which had the following to say: "Our first and main result is that there is no unambiguous antiproton excess that can be identified in the first place, and thus, at this stage, no real need for primary sources of antiprotons. Within errors, secondary astrophysical production alone can account for the data." Their Figure 2:
  
  
  
  
  

  So, don't believe the hype.

• There was an arXiv preprint on Wednesday suggesting that young pulsars can explain the galactic centre excess. Their money plot is the following comparison of the expected dark matter spectrum with that from a prototypical young pulsar (Geminga).
  
  
  
  
  

• Spectroscopic measurements of Reticulum II (here, here, and here) confirm it is an ultra-faint dwarf galaxy. Recall from the Hooper paper on the observed gamma ray excess, "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies... Reticulum II must contain a high density of dark matter, corresponding to $J \gtrsim 10^{19.6}\text{ GeV}^2/\text{cm}^{-5}$." One the new preprints has the J-factor measured at $10^{18.8\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.2 degs, and $10^{18.9\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs, and another has it at $10^{19.5^{+1.0(+1.6)}_{−0.6(−1.3)}}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs. So measurements seem to be disfavouring a dark matter interpretation. Also, it is of note that there is a radio source (likely a blazar) located 0.1 degs from the Ret II location which could be responsible for excess gamma rays...

• An arXiv preprint (press release here) has observed that, in a system of four colliding elliptical galaxies, "each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars." The abstract adds, "With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments – but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section $\sigma_{DM}/m \sim (1.7\pm0.7)\times10^{−4} \text{ cm}^2/\text{g}\times(t_{infall}/10^9\text{yrs})^{−2}$, where $t_{infall}$ is the infall duration."
  
  We should keep in mind that these limits are based on the assumption that the interaction is velocity-independent, which is not true of a low-mass mediator. I don't have anything illuminating to add, but I do find it interesting that the previous study of colliding galaxy clusters set an upper limit of $\sigma_{DM}/m < 0.47 \text{ cm}^2/\text{g}$, and this measurement is almost four orders of magnitude smaller! Are measurements of these kinds of systems that much more sensitive? Why doesn't this system set an even stronger upper limit?

• The "Evidence for dark matter in the inner Milky Way" saga appears to have reached an end. Both the comment and the reply to comment were updated this week, with the former reproducing a plot from a 1988 paper which is "in essence, identical to that of Iocco et al," and the latter writing, "In our letter we made a claim based on a specific technical point: that current data are constraining enough to make the claim robust against statistical and systematic errors. We believe we have made this point clear with our letter and two replies, and we shall not continue the discussion on the arXiv."

• The Dark Energy Survey has produced a dark matter map (arXiv here, nature article here) of part of our sky using graviational lensing. Below is a heat map showing the mass density along with locations of galaxy clusters superimposed as grey dots. The map supports the standard picture that dark matter drives large-scale structure formation.
  
  
  
• The EPS HEPP prizes were given out this week; the main prize went to James D. Bjorken “for his prediction of scaling behaviour in the structure of the proton that led to a new understanding of the strong interaction”, and to Guido Altarelli, Yuri L. Dokshitzer, Lev Lipatov, and Giorgio Parisi “for developing a probabilistic field theory framework for the dynamics of quarks and gluons, enabling a quantitative understanding of high-energy collisions involving hadrons”.

• Strassler has been tackling the issue of dark matter searches at the LHC this week. He has a blog post and a new article for the layman.

• Paul Jackson has written a blog post for the ATLAS blog on CoEPP and our conference in February.

• Our understanding of the particle zoo as a function of time at Scientific American.

• You can explore ATLAS, CMS, LHCb, ALICE and the LHC Tunnel on Google streetview.

• Frank Wilczek has a new book coming out in July, "A Beautiful Question: Finding Nature’s Deep Design."

• Lincoln Wolfenstein has passed away at the age of 92; read his obituary here.

• In video/audio media:
• CERN has a few uploads this week: Welcome to Season 2 [2 minutes], Higgs boson: what’s next? [3 minutes], and Driving the LHC [3 minutes].
• US LHC channel have put out a few new videos for LHC Season 2, on higher energy [1 minute], the Higgs [2 minutes], and other new physics [2 minutes].
• Erik Verlinde talk, "A new theory of gravity," (abstract) at Fysica 2015.
• Heavy metal Higgs boson; two plots from the Higgs discovery seminar at CERN, transformed into music. [1 minute]
• Astrokatie and others chatting about the Big Bang. [33 minutes]
• SpaceX attempt to land a rocket on a floating barge. [20 seconds]
• Tupper's Self-Referential Formula plots itself, at Numberphile. [7 minutes]

• Lastly, space images...

• The first colour image of Pluto (and Charon) from New Horizons:
  
  
  
• And here is our first view of Ceres from Dawn's new address: