Sunday, 20 December 2015

Friday wrap-up: diphoton excess, no diboson, no gluinos...

What a week! We have already seen some 40-odd papers submitted to hep-ph in the last few days on the "recent observed diphoton resonance" [1]. Well I certainly wouldn't go that far but ATLAS and CMS have each seen an excess of events in the diphoton spectrum at around 750 GeV, which is amazing since apparently they weren't even searching for it [2], and anyway beside the point because they also discovered a gluino [3]. Sloppy science writing aside, what do we know?...

  • The CMS and ATLAS Run II physics results presentations can be found here. Of course, all results presented are preliminary. The result that has hep-ph buzzing, though, is a little bump atop the falling diphoton invariant mass background (conference notes here and here). [See Jester, Motl, Strassler (here and here), PhysicsMatt, or Eilam Gross for some physicist perspectives. Else in popular media I thought the NY Times article was fairly balanced, but then I am a phenomenologist]. You can eyeball the bumps in question below (credit to Strassler for this image):


    But what about the numbers? The rumours were as accurate as one could reasonably ask: assuming a narrow width resonance, CMS observed a 2.6σ local (1.2σ global) excess at 760 GeV [increases to 3.0σ local (1.7σ global) at 750 GeV when combined with the 8 TeV data], and; ATLAS observed 3.6σ local (2.0σ global) at 750 GeV [have not yet combined with 8 TeV, but if they did it appears the significance would fall]. Allowing the width to float to larger values, the CMS result goes down to 2.0σ local, whereas ATLAS observes a best fit 45 GeV (6%) width at 3.9σ local (2.3σ with multivariate look-elsewhere). The relevant slides are below:


    It is a tantalizing excess. Sensibly, what one would like to know is the global significance of the fully combined (CMS+ATLAS 8+13 TeV) datasets. It is non-trivial to get an exact number (see here or here), but one can at least make a good bet that it's greater than about $\sim \sqrt{1.7^2+2.0^2}\approx 2.6\sigma$, perhaps in the vicinity of $\sim 3\sigma$. [I would imagine the demand for a joint analysis is high enough to be a priority for the collaborations (or they might try to avoid feeding the hep-ph sharks?), so maybe we will have that number by Moriond]. This being a (very rough!) ~1/300 chance then, and given the hundreds of plots CMS and ATLAS produce, it is very possible that this is just a statistical fluctuation. Nonetheless, this excess is being taken fairly seriously, and will be exercising our scrolling finger on hep-ph for the foreseeable future while we grapple with the sensible question: if it is real, then what could it be and what does it imply? The answer to this question may have implications for the experimental program of the LHC over the next few years (at least), and so phenomenologists are already relentlessly hard at work...

    So let's try to answer that question: what could it be? Well, there is no evidence for any extra activity in the excess events, so it appears consistent with a simple $gg\to X\to \gamma\gamma$ resonance. If taken as a resonance, the events translate to a cross-section $\sigma(pp\to X)\times Br(X\to \gamma\gamma)$ of $\sim 2$/fb ($\sim 6$/fb) in the narrow (wide) width scenario. Let us try to build a model with these properties. The simplest thing is to add a scalar singlet to the standard model. To couple it to gluons and photons let's borrow the Higgs' trick and couple it to some coloured/charged fermion(s) which then induce the couplings via a loop. Let's try Yukawa coupling it to a vector-like up-type quark first, write down the effective couplings, and calculate the Yukawa necessary; we find that it has to be huge ($\sim 5$ or so). And there's a potential problem, since the singlet will want to decay most of the time to the up-type quark. That's okay! We will just make it heavy enough (> 375 GeV) so that it's not allowed. Now we're done, and this solution is "already well-known" [4]. We can add more vector-like fermions to quell the large Yukawa(s) somewhat and/or dial the $gg$ and $\gamma\gamma$ couplings independently. If we take the large width seriously, we still have to add extra decay channels, and then dial up the production and/or branching to photons to compensate. The obvious options are a dark sector or some other standard model states, which we have to hide from previous searches. We could also try constraining ourselves inside some more predictive (restrictive) model.

    Of course there are several papers on just the above, the implication being that you need more than just the singlet scalar, which is obviously quite interesting. The immediate implications for the LHC are: look for anything at 750 GeV in $jj, Z\gamma, ZZ$ (in roughly descending order of promise) as soon as is possible.

    But this is just a minimal model. It could also be a scalar/pseudoscalar/bound state connected to compositeness/extended gauge group/extra dimensions/hidden valley/SUSY/dark matter/naturalness, and you can be sure there are already arXiv submissions on all of these. On that, it seems to me that arXiv isn't quite the ideal platform for all this. It would be nice instead to have all the various proposals in the same place, with the same formatting, in no-nonsense form, all grouped by some general properties. Then the interested phenomenologist/experimentalist could go and browse a list of, for example: (1) candidate; (2) production; (3) couplings; (4) decays; (5) associated activity; (6) additional particles; (7) additional predictions. Of course this will inevitably be done anyway by some authors in a review, but it seems like the same could be achieved much more efficiently with a community-run wiki or similar, as long as there were some moderators willing to dedicate their time to such a project... any thoughts on this from readers?

    In my book there's not much more to say except we need more data, to tell (1) if this is real, or (2) what it is. Looking forward to more excellent work from our experimental colleagues in the new year.

  • Now onto other matters from the presentation. First the diboson excess from Run I. Before the meeting a couple of useful papers appeared on the arXiv: a third-party CMS+ATLAS statistical combination, and; a thorough summary and literature survey. Now we know both CMS and ATLAS see nothing significant in Run II data (although they do not have sensitivity to conclusively probe the parameter space of interest):

  • Also in Run II data, the on-Z excess is not seen by CMS, but still persists at ATLAS...

  • As well, lots of gluino searches in different final states but nothing seen, and limits improve to roughly 1.2--1.8 TeV in the simplified models considered (but of course there are always compressed places to hide!).
  • CMS have not unblinded any of their Higgs analyses, but ATLAS reported results in γγ and ZZ: they were expecting 3.4σ observation and saw instead 1.4σ. Obviously the Higgs has packed up, moved to 750 GeV, and remembered its earlier proclivity for photons (this hypothesis will be robustly tested in upcoming LHC analyses).
  • Moving on to other news, LUX has released new limits on spin-independent dark matter nucleon scattering. See the press release and/or this blog post from Sally Shaw for a summary. They're almost observing solar neutrinos!

  • "NuPhys2015: Prospects in Neutrino Physics" was on this week (indico).
  • Links without thinks:
    • Strumia's insta-paper archive.
    • Quanta: "A Fight For the Soul of Science," on the recent meeting at the intersection of the philosophy of science and theoretical physics.
    • Quanta: "Landmark Algorithm Breaks 30-Year Impasse."
  • In audio/video media:

[1] arXiv: "The recent observed diphoton resonance around 750 GeV at the LHC..."
[2] Nature News: "... the 750 GeV boson is not one of the particles that LHC physicists have been searching for..."
[3] Tech Times: "Physicists Have Discovered Evidence Of A Gluino Particle, The Cousin Of The Higgs Boson."
[4] arXiv: "It is already well-known that a real singlet scalar ϕ with Yukawa couplings ϕXX to vector-like fermions X with mass mX>mϕ/2 can easily explain the observed signal, provided X carries both SM color and electric charge."


Friday, 11 December 2015

Friday wrap-up: diphoton excess?...

Things feel like they're starting to wrap up for the year down here, but the holiday season might not be so laid back for us theorists...

  • ATLAS and CMS are giving a joint presentation on 15th December: "ATLAS and CMS physics results from Run 2". The discipline is abuzz-with-rumours-circling online and in particle physics offices around the world suggesting there will be something very interesting presented indeed. The suggestion is a ~750 GeV diphoton excess at >3σ, seen by both ATLAS and CMS. If true, it seems very hard to accommodate with the absence of such a bump in Run 1, which makes it all the more interesting...


    I was going to make some speculations here, but let us just wait until next week... I'm sure by then there'll even be a bunch of papers on the arXiv which already have the answer.

    We've seen anomalies come (and some go) during Run 1 (Higgs diphoton, Higgs LFV, CMS kinematic edge, on-Z, diboson, B to KμμWH, etc.), but the buzz around this one seems different somehow. Perhaps because it is such a clean channel, because it is hard to explain on its own, because it is in a quite unexpected place for a first signature of genuinely new physics, and because it has shown up so damn early in the new high energy data. There seems to be a strong hope that this is only part of the story, and we're excited to wake up to a reality which might be rich and complex and full of new puzzles, when, in all honesty, many were expecting a desert (I know I was). Maybe this is just the tip of the TeV-scale.

    Certainly the presentation is one to watch. To be continued...
  • First new physics searches from Run 2 have hit the arXiv: dijet searches from ATLAS and CMS.
  • There was a workshop this week called, "Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics." The workshop aims to tackle questions such as: "Can a high degree of trust in an empirically unconfirmed or inconclusively confirmed theory be scientifically justified? Does the extent to which empirically unconfirmed theories are trusted today constitute a substantial change of the character of scientific reasoning? Might some important theories of contemporary fundamental physics be empirically untestable in principle?"

    Sounds very interesting, and has speakers such as Gross, Dawid, Kane, Silk, Polchinski, Dvali, et al. Some notes were also taken at this blog. Unfortunately I have not had time yet to peruse the talks, but hope to get some time this week.
  • Some evidence against the 3.5 keV line appeared on the arXiv.
  • Sean Carroll has finished preparation of a new book, "The Big Picture: On the Origins of Life, Meaning, and the Universe Itself." He published the Contents on his blog.
  • Links without thinks:
    • Quanta: "Math Quartet Joins Forces on Unified Theory."
    • Paul Davies via Guardian: "100 years on, is this Einstein’s greatest gift to human understanding?"
    • Nature: "How to build a better PhD."
    • Guardian: "Chris Hadfield meets Randall Munroe: Are we alone in the universe?"
    • Quantum Frontiers: "BTZ Black Holes."
    • Information Processing: "The cult of genius?"

Friday, 27 November 2015

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."

Saturday, 14 November 2015

Friday wrap-up: 4/fb, Fermi...

Another two week break, but plenty happened this week!

  • The LHC has finished collecting proton-proton collision data for the year. ATLAS ended up with ~4.0/fb, and CMS with ~3.6/fb.
  • Fermi Collaboration have released their own analysis of the galactic centre excess. I have not had time to read the paper in detail (not that I could comment with much authority even if I had), but it is interesting that they seem to see a residual excess after subtracting known sources, which can be somewhat accounted for by a peaking template (dark matter?). Perhaps we will know more by next week...


    There's a layperson article from The Economist about it which says, "There are still a few die-hards who do not believe in hooperons"! Well, call me a die-hard.
  • The XENON1T direct detection experiment at Gran Sasso was inaugurated on Wednesday. Read the press release here; they write, "Once fully operational, XENON1T will be the most sensitive dark matter experiment in the world. The detector installation has been completed just a few days ago and the first tests of its performance have already been started. The first science results are expected early 2016, as only one week of good data is sufficient to yet again take the lead in the field." And some cool pictures:


  • The $3mil Breakthrough Prize in Fundamental Physics was awarded for the experiments which established neutrino oscillations (the decision was made before the Nobel Prizes were known). The prize was shared equally among the Daya Bay, K2K/T2K, KamLAND, SNO, and Super K collaborations, with two-thirds of each share to the leaders and one-third to the remaining collaboration members: 1370 physicists in all! It is nice to see all of these scientists recognised.

    The Symposium and Panel Discussion are on YouTube. The Fundamental Physics talks are on the future of particle physics and feature Nima Arkani-Hamed, Lawrence Hall, Beate Heinemann, Gabriel Orebi Gann, and Tom Shutt.
  • Worth reading are two articles, one a piece by Ed Witten, "What every physicist should know about string theory," and another an interview with Abhay Ashtekar broken into four parts: 1. Getting started on gravity and cosmology; 2. Learning from Chandra; 3. Challenges in loop quantum gravity; 4. Arrogance in string theory.

    Of course these are bat-signals for Woit and/or Motl, so click their names to read their reactions/reflections too.
  • The people's choice voting is open for photographs from the InterActions Physics Photowalk, featuring shots of CERN, DESY, FermiLab, INFN, KEK, SLAC, SUPL, and TRIUMF.
  • Links without thinks:
    • There's some buzz about a quasipolynomial time algorithm for the graph isomorphism problem. See this blog for a (research-level) detailed description. Else for the layperson see Science, New Scientist, or Motherboard.
    • Nautilus: "Will Quantum Mechanics Swallow Relativity? The contest between gravity and quantum physics takes a new turn."
    • Dan Waddell via Medium: "Saving Tim Hunt," a long read on the Tim Hunt story.
    • Thesis Whisperer: "Academic assholes and the circle of niceness."
    • Nature: "Networking: Hello, stranger," on conference etiquette and career development.
    • Existential Comics: "Trolley Madness."
  • In audio/video media:
    • 2016 Breakthrough Prize Symposium in Fundamental Physics, with: 1. Nima Arkani-Hamed - Motivations for 100km Circular Colliders; 2. Lawrence Hall - New Searches for Dark Matter in Particle Collisions; 3. Beate Heinemann - The LHC and beyond: what can colliders teach us?; 4. Gabriel Orebi Gann - Unravelling the Secrets of the Universe with Neutrinos; 5. Tom Shutt - The Hunt for Dark Matter. [1:36:49]
    • CBS 60 Minutes: "The Collider," on the LHC. [11:54]
    • Catalyst: "Einstein's Extraordinary Universe", featuring visits to LHC, Gran Sasso, and LIGO. [28:17]
    • WHYY RadioTimes: with Lisa Randall on dark matter and the dinosaurs. [48:57]
    • MinutePhysics: "How to go to space," Thing Explainer edition. [2:57]
    • Numberphile: "The Uncracked Problem with 33". [8:28]

Saturday, 31 October 2015

Friday wrap-up: Chinese collider...

Not too much happening lately, or perhaps I have been keeping too busy with other things? Either way, we had a week away; so here's some news from the last two weeks.

  • A number of articles appeared yesterday saying that state-run Chinese media is reporting (as has been brewing for a while now) China will begin building the next supercollider in 2020. Here's hoping they carry through with it! As well this week, Phase II (prototyping) of the HL-LHC project has begun.
  • Ending with some visual delights, here is François Moncarey’s projection mapping work which opened TEDxCERN:

  • A few days ago the Cassini spacecraft performed its deepest ever dive through the southern plumes of Saturn's moon Enceladus (and returned some incredible images).

Friday, 16 October 2015

Friday wrap-up: Marcy...

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...

  • Geoff Marcy resigned from the Berkeley astronomy department after it was found he sexually harassed students (and after the majority of the department signed this letter); see the NY Times article here (who themselves were accused in an open letter of taking an empathetic stance in an earlier article). See also Ethan Siegel and links therein.
  • CosPA 2015 (indico) was on this week.
  • APS gave out some prizes: a new award, the APS Medal for Exceptional Achievement in Research went to Ed Witten, and the J.J. Sakurai Prize for Theoretical Particle Physics went to Peter Lepage.
  • NASA released high resolution Apollo 11 photographs on flickr not too long ago. Planetary Society made a great video out of them:

Saturday, 10 October 2015

Friday wrap-up: neutrino Nobel Prize...

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 Nobel Prize in Physics 2015 was awarded jointly to Takaaki Kajita and Arthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass." See the plethora of articles already online: Nobel Prize site, APS, CERN, symmetry, ConversationNew Yorker, Forbes...


  • TEDxCERN (rulebreakers and visionaries) was on Friday; find the videos here.

Saturday, 3 October 2015

Friday wrap-up: 1/fb, various links...

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...

  • A huge amount of data was accumulated this week. ATLAS surpassed 1/fb of integrated luminosity, and at the time of this writing is sitting at >1.4/fb!


  • A rumour emerged last weekend of gravitational wave discovery at LIGO; there's a nature column claiming it is unlikely and/or just a drill.
  • The Quark Matter 2015 conference (indico; hashtag) was on this week.
  • The CMS Experiment has a new outreach initiative: CMS Voices on twitter, with a new CMS physicist every month.
  • Links without thinks:
    • Wall Street Journal: "China's Great Scientific Leap Forward," an opinion piece from Gross and Witten [behind a paywall but for me is accessible via a Google search].
    • Life and Physics by Jon Butterworth: "Fermilab's giant magnet begins its journey into the quantum badlands," on the muon $g-2$ experiment which began this week at Fermilab.
    • Nautilus: "The Trouble with Theories of Everything," from Lawrence Krauss.
    • Backreaction has been busy: "No, Loop Quantum Gravity has not been shown to violate the Holographic Principle," and "When string theorists are out of luck, will Loop Quantum Gravity come to rescue?"
    • Terence Tao has submitted a solution to the Erdős discrepancy problem, motivated by a suggestive comment on his blog; see articles at nature news, and Quanta.
  • In audio/video media:
  • Myriad awesome images emerged from NASA this week of "recurring slope lineae" on Mars, hypothesised to be formed by the flow of briny liquid water. See Emily Lakdawalla's blog for a scientist's take.


Friday, 25 September 2015

Friday wrap-up: Nima, weakly coupled high-scale physics...

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 is an article at Quanta Magazine constructed around a profile of Nima Arkani-Hamed that is well worth a read. It includes his (and others') visions of and predictions for the future of high-energy physics, and the important role the Chinese might play in constructing a 100 TeV collider.
  • A few things wrapped up for me this week...

    (1) Uploaded to the arXiv v2 of a paper on displaced Higgs decays (see blog from back in June). In particular, the new version has the plots updated and include some recent results. Besides the scientific content, at the very least they are pleasing to the eye (well at least to mine)! I find this kind of phenomenology very interesting, and there is certainly more to be said in conversation between phenomenologists and experimentalists on where to search and how to present results for displaced physics.


    (2) Uploaded to the arXiv a conference proceedings (PLANCK) summarising two recent papers: "How to avoid unnatural hierarchical thermal leptogenesis." If you'd like to know why explaining baryogenesis and neutrino masses with the minimal three-flavour Type I seesaw and hierarchical leptogenesis is necessarily unnatural, and the various ways around it, this document should serve as a good summary. Or see the blog post from May for an even shorter summary. The second part of the proceedings describes a two-Higgs-doublet model with right-handed neutrinos (ν2HDM) which can achieve hierarchical leptogenesis and realise the neutrino masses without introducing a naturalness problem. This model serves as the basis for the following...

    (3) Uploaded an arXiv preprint titled: "νDFSZ: a technically natural non-supersymmetric model of neutrino masses, baryogenesis, the strong CP problem, and dark matter." It is a rather short paper which contains an existence proof that weakly coupled high-scale physics can explain phenomenological shortcomings of the SM without introducing a naturalness problem. The model adds only three right-handed neutrinos, a scalar doublet, and a scalar singlet to the SM. It contains a hierarchy of scales up to $\sim 10^{11}\text{ GeV}$. Nevertheless, corrections to the Higgs mass (and other mass scales) can be calculated, and it is shown that a technically natural decoupling limit of the model can protect all scales from large quantum corrections. If this is surprising in any way for you, since it is (or at least appears to be) a widely held misconception that high-scale physics implies a naturalness problem, then I suggest you read our preprint, or this earlier blog post! Let's be clear here: the model does not solve the big hierarchy problem; we don't explain where the hierarchy of scales comes from, we just show that the hierarchy we introduce is not fine-tuned (that is the real worry), i.e. it is a radiatively stable hierarchy, or, it is "technically natural".

    I find it extremely interesting that the major shortcomings of the standard model can be answered naturally in such a modest extension of the SM. Models like this with weakly coupled high-scale physics, in my opinion, deserve more attention.
    • The Taller de Altas Energías 2015 School is currently ongoing (programme here).
      • Links without (too many) thinks:
        • Life and Physics from Jon Butterworth: "How the Higgs boson is born and how it dies: the most precise picture so far."
        • ATLAS Blogs: Part 2 of James Howarth's TOP2015 review.
        • The Conversation: "How we plan to bring dark matter to light," with a little on SUPL and SABRE.
        • Cosmos: "Ghost traps: the hunt for dark matter," interesting to read if only to observe how the field's "dark matter = WIMP" prejudice leads to misleading (even incorrect) statements in lay articles...
      • In video/audio media:
        • In Particular Ep 3: Particle Zoo... what is your favourite particle? [35:15]
        • CERN: Timelapse video of the CERN Axion Solar Telescope (CAST) following the Sun [1:22], and a bit of noise rock in situ; Deerhoof vs. the Large Hadron Collider [9:05].
        • Waking Up with Sam Harris: The Multiverse & You (& You & You & You…), A Conversation with Max Tegmark. [1:26:42]
        • MinutePhysics: Why do we put telescopes in space? [2:20]
        • It's Okay to be Smart: Theory vs. Hypothesis vs. Law... Explained! [7:11]
        • Numberphile: Philosophy of Numbers. [9:40]

      Friday, 18 September 2015

      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: indicotwitter). 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).
      • 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?"
      • Lastly, in images from space, it is hard to top these new images of Pluto!


      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!


      Saturday, 5 September 2015

      Friday wrap-up: ATLAS+CMS Higgs combination, THE rankings...

      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...

      • Conferences of interest this week include The 3rd Annual Large Hadron Collider Physics Conference (LHCP2015; indico; twitter), and QCD@LHC 2015 (indico).
      • At LHCP, Marco Pieri presented the brand new ATLAS+CMS Higgs combination (talk here [pdf]). Slides 17 and 18 tell the story for the SM Higgs versus the null:


        The interesting things for me are: global signal strength fit is $\mu=1.09^{+0.11}_{-0.10}$, H→ττ and VBF production are (preliminarily) "discovered" at >5σ, and ttH has a mild (2.3σ) excess (already hinted at Moriond) to keep an eye on. Different parameterisations are also studied, finding, of course, everything consistent with a SM Higgs. Would imagine we can look forward to the arXiv paper soon.
      • The Times Higher Education World University Rankings have decided to exclude from their analysis all papers with more than 1000 authors for the 2015-16 rankings. This obviously has a big impact on those involved in the ATLAS/CMS collaborations. Interesting to read the comments below the post from John Ellis, James Stirling, and Andrew Hamilton, among others.
      • According to this nature news article, there are some concerns for the cooling pumps in the AMS-02 experiment. There were originally four cooling pumps. They write: "Only one pump is needed at any given time. One failed in February 2014 and at least one of the other three is showing possible signs of trouble." Also: "[Ting] exhibited little patience for questions about the cooling pumps. 'We have four pumps — we only need one,' he says. 'We expect to operate for the lifetime of the space station.'"
      • Some movement on the Hawking/Perry/Strominger proposal for the black hole information loss problem. There's now a short stake-claiming arXiv paper, and an hour-long talk from Malcolm Perry on YouTube. Sabine Hossenfelder reacts here.
      • From CERN: a summary on LHC Run 2 so far from Rolf Heuer (he comments on the CMS magnet: "... it’s clear that there are contaminants in the cold box that supplies the magnet with liquid helium, and this will therefore need a thorough clean.... All being well, CMS will be able to take data satisfactorily with field on until the end of the 2015 physics programme, postponing the cleaning operation until the winter stop in order to be ready for the start of 2016."), and a summary on recent scrubbing runs.
      • In video/audio media:
      • Lastly, here is an antineutrino global map: an experimentally informed model of Earth’s surface antineutrino flux over the 0 to 11 MeV energy spectrum.

      Friday, 28 August 2015

      Friday wrap-up: 3.5 keV line, SUSY2015, Hawking...

      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 23rd International Conference on Supersymmetry and Unification of Fundamental Interactions (SUSY2015; indico; hashtag) and 4th International Conference on New Frontiers in Physics (ICNFP2015; indico; hashtag) have been going this week.
      • Links without thinks:
        • Strangely Familiar: dark matter as many-quark states, from Sabine Hossenfelder and Naomi Lubick via New Scientist.
        • The current issue of Scientific American is a special issue on Einstein. [You should be able to access the articles through a University subscription].

      Friday, 21 August 2015

      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 ReleasesCERN 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.
      • 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.

      Friday, 14 August 2015

      Friday wrap-up: various links...

      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..

      Slow news week, or maybe I was just too busy...?
      • The LHCb pentaquark discovery has been published in PRL. There's a Viewpoint article from Kenneth Hicks here.
      • The BOOST2015 7th International Workshop on Boosted Object Phenomenology was on this week (indico/hashtag).
      • The 43rd SLAC Summer Institute is going on at the moment: The Universe of Neutrinos (indico).
      • Blog post at Backreaction: Why do some people assume that the Planck length/time are the minimum possible length and time?
      • John Preskill muses on Kitaev, Moore, and Read's shared ICTP Dirac Medal and anyons, the two-dimensional cousins to our fermions and bosons.
      • A couple of articles from Shannon Hall at Nautilus: Is It Time to Embrace Unverified Theories? and; 6 Graphs That Showed Landmark Discoveries—but Were Later Debunked.
      • In video/audio media:
        • Symmetry, a dance-opera film, premiered this week; trailers at the link.
        • Big Bang Aftershock, on the BICEP2 discovery and fallout. [40 minutes]
        • Uranium: Twisting the Dragon's Tail. Hosted by Derek from Veritasium, and researched by my officemate Rebecca Leane! [Link for Australians; 50 minutes]
        • How Does Symmetry Shape Nature’s Laws? from Quanta. [2 minutes]
        • What is Dark Matter and Dark Energy? for laymen, from Nova Project. [6 minutes]
        • What Has New Horizons Taught Us About Pluto? at It's Okay to be Smart. [6 minutes]
        • Prime knots at Numberphile. [7 minutes]
        • Tour Ceres, from Nasa JPL. [2 minutes]
      • On 13 Aug, Rosetta witnessed Comet 67P/Churyumov–Gerasimenko traversing its perihelion.


      • Those in the Northern Hemisphere were lucky to have an almost-new moon for the Perseids this year. (The shot below is from Ruslan Merzlyakov).


        Check out the slideshow at space.com.