## Sunday, 5 June 2016

### Warsaw Workshop on Non-Standard Dark Matter

For the last few days I've been at the Warsaw Workshop on Non-Standard Dark Matter. It's been very enjoyable! Plenty of interesting ideas, coffee, and social events. Yesterday I gave a short talk, trying to make the case for a dark matter direct detection search for the sidereal modulation signature. The general idea is that, if dark matter has self-interactions, the dark matter wind which strikes the Earth will interact with any Earth-captured dark matter, leading to a non-trivial spatial distribution which terrestrial detectors traverse throughout the day. I share the slides below this post. If nothing else you should click through to see some entertaining magnetohydrodynamic simulation animations!

By the way, as of this writing ATLAS+CMS have recorded about 2+2/fb of data (or 20 diphotons in alternative units):

We're quickly moving toward the position we were by Christmas last year (about 3+3/fb including the CMS $B=0$ data). If the 750 GeV diphoton resonance prevails in the new data we hope to know by the ICHEP on August 3-10. Some authors have taken to calling the would-be particle Ϝ, which is the archaic Greek letter "digamma" -- very fitting! We will see yet if this name becomes lore... I also quite like the following perhaps future update of the PDG from Strumia:

Slides

## Sunday, 24 April 2016

### Wrap-up

Readers might have noticed that this blog has slowed down lately. The reason is that I am in a transition period at the moment, wherein I hope to: see to completion three collaborative projects, attend two conferences in Europe (Warsaw Workshop on Non-Standard Dark Matter, and The Lindau Nobel Laureate Meeting), make it back in time for SUSY2016 in Melbourne, complete my PhD thesis (by August 1), move into a new position juggling research with industry work, and also go on a belated honeymoon... hence the blog will have to go on the backburner for now. In order to keep this format up I need to be in a stable routine, which was the case for the last year or so, but currently is not. Updates might still be made in the coming months, though certainly not in this same format. Until then...!
• April Fools came and went; see CERN's effort hereFermilab, arXiv here, and return of supersplit supersymmetry, this time (of course) linked to the 750 GeV diphoton excess.
• Dark Matter at the LHC Workshop ran from 30 March to 1 April (indico). LHCski: A first discussion of 13 TeV results, ran from 10-15 April (indico).
• In audio/video media:
• In Particular shorts: Graviton. [7:14]
• Art McDonald via Perimeter Institute: A Deeper Understanding of the Universe from 2 km Underground. [1:16:53]
• Jernej Kamenik via Laitn American Webinars: Update on the LHC diphoton excess. [1:11:58]
• Nima Arkani-Hamed via IAS Princeton: The Future of Particle Physics. [1:43:42]
• David Kaplan via Quanta: Is That 'Bump' a New Particle? [2:25]
• Don Lincoln via Fermilab: Theoretical physics: insider's tricks. [8:31]
• Perimeter Institute: Canadian Prime Minister Justin Trudeau Explains Quantum Computing. [1:08]
• Vsauce: How To Count Past Infinity. [23:45]

## Friday, 25 March 2016

### Friday wrap-up: Moriond...

The 50th anniversary Rencontres de Moriond (electroweak indicotwitterhashtag) was on over the past few weeks. Here's the updated logo [credit Strumia]:

Not sure how the 7 got in there... probably insignificant.
• The most anticipated results were updates on the 750 GeV diphoton saga. Slides from ATLAS and CMS are here and here.

There are excellent detailed write-ups at Résonaances and PhysicsMatt already, and I don't have much to add to these (for pop-sci articles see e.g. Guardian, symmetry, Scientific American). You should read them, if you haven't. In short, with the addition of new "B=0" data from CMS and an updated analysis from ATLAS, the excess is not going away. Below I reproduce one of the third-party combination plots published on PhysicsMatt which tells some of the story. On the left is the combination of previous data, and on the right after the Moriond update, assuming the Volksmodel $gg\to S\to \gamma\gamma$ and a narrow width:

One can see by eye that the reanalysis of the ATLAS 8 TeV data shows it is more consistent with a $gg\to S$ 13 TeV excess than previously believed, and there's an excess in the new CMS 13 TeV data in the right ballpark. Taken together this adds a little fuel to the fire.

As well, there are strong rumours that ATLAS are sitting on an analysis in which they relax some of their cuts (increasing acceptance of events), and that this alone bumps up the local (global) significance of the excess to ~4.7σ (>3σ) [see e.g. Résonaances and comment section]. If this is true then hep-ph might as well become hep-γγ...

For your interest see below some (obviously biased) surveys in the twittersphere. Clearly people are taking this seriously. If the rumoured ATLAS analysis is true I would give the 750 GeV excess a dice throw at sticking around.

• If you're out of ideas for how to explain the excess, then maybe you can find inspiration at snarXiv.
• About a month ago D0 announced observation of a tetraquark $X(5568)\to B_s^0\pi^\pm$ state. It received quite a bit of press. Here's the plot from the D0 preprint:

At Moriond LHCb announced that they see no evidence for such a tetraquark state (slides 22-24 here). A few days ago there was an LHC Seminar on the analysis. From what I can gather, there is some talk of bias introduced by a "cone cut" in the D0 analysis. In the Conf Note LHCb write:

In the D0 analysis, a requirement is imposed on the opening angle between the $B^0_s$ candidate and the companion pion in the plane of pseudorapidity and azimuthal angle [$\Delta R$]... No such requirement is imposed here, as $\Delta R$ is strongly correlated with $Q$ value and, when combined with kinematic requirements imposed by the LHCb detector acceptance, a cut on this variable can cause broad peaking structures.

There is speculation that this might have introduced some spurious shape or impacted the statistical interpretation somehow for D0. I find the following slide from the seminar rather telling.

Here $\rho_X$ is the fraction of $B_s^0$ coming from tetraquark decays. Could there be some major difference in the production of $X$ or of $B^0_s$ in a $p\bar{p}$ (as in D0) versus a $pp$ (as in LHCb) collider? Would love to hear from an expert. In the mean time we wait for results from ATLAS/CMS, and in particular CDF (the partner experiment to D0 at Tevatron) to tell us more. There's a pop-sci article at Scientific American here.
• LHC beam splashes tonight!
• In audio/video media:

## Sunday, 6 March 2016

### Friday wrap-up: LIGO, chasing the 750 GeV excess...

Apologies for the long hiatus -- other aspects of life have been getting in the way. Here is a summary of the last month or so...

• Of course the biggest news was the first observation of gravitational waves, a binary stellar-mass black hole system, and a binary black hole merger. Not bad for an 8 page paper! The signal is really quite striking; it's wonderful to see the agreement between the two detectors. I reproduce the observation plot below, just because one cannot admire it enough.

Interest in the finding was phenomenal; the Physical Review Letters server even crashed (they were getting 10k hits per minute). One can find plenty of explanations at various levels online: e.g. for the layperson see Quanta, or Brian Greene on the Late Show; for the more scientifically minded there exists a digestible summary of each paper by Christopher Berry; or for a colloquium-level talk see Barry Barish at CERN. Lastly you can enjoy the xkcd.

Here I just want to mention some interesting facts, taking as read the core ideas behind the phenomenon and the measurement. The event was actually observed before the first planned science run, during an engineering run. It was identified within 3 minutes and the decision was subsequently made to keep settings in place to take 16 more live days of data. This time period was chosen so that the data-driven background estimation could nail down the unlikeliness of the event to >5.1σ under the background-only hypothesis. Below is shown the event and background estimation; the detection is well in excess of 5.1σ, even including the event itself in the background estimation.

The data is in fact completely open and you could analyse it yourself! In addition to the GW150914 event there are also two others that rise somewhat above the background ("GW151012" and "GW151226"). You can see them by eye in the above plot. They are clearly not statistically significant enough to announce a discovery alone, but still they are tantalising... with room for improvement to design sensitivity (by a factor of ~2 which increases the spatial reach by 2^3) and the construction of a third detector in India to triangulate the signal, the future of gravitational wave astronomy is exciting.
• There's also that puzzling observation by Fermi of a gamma-ray burst 0.4s after the gravitational wave detection. There are good reasons for and against believing this was associated with the GW150914 event (see Quanta); the best way to tell is to just to wait and see if it happens again!
• On the 750 GeV diphoton excess you can read Jester's "750 ways to leave your lover" on various explanations.

As well, a comprehensive paper appeared on the arXiv reviewing some of the renormalisable and weakly-coupled explanations. In the authors' literature review they "found a wide range of mistakes or unjustified assumptions, which represent the main motivation that prompted this work." The suggestion is to utilise computational tools (e.g. SARAH) to automate the work which the phenomenologist should be doing anyway for a thorough analysis, and they provide 40 model files to match models already in the literature.

Let us review these "mistakes or unjustified assumptions"; we will refer to the resonant 750 GeV state as $S$ throughout, and consider models where the effective coupling of $S$ to the diphoton/digluon vertices are induced by a loop of gauge-charged fermion(s) or scalar(s) [of course there are explanations which do not fit into this framework]...

Next-to-leading-order (NLO) corrections to the $S$ decay widths matter. Compared to the LO result used in many papers, NLO corrections typically decrease $\Gamma(S\to\gamma\gamma)$ by O(10%), and N3LO corrections can increase $\Gamma(S\to gg)$ by a factor of almost 2. Overall this means $Br(S\to gg)/Br(S\to\gamma\gamma)$ is typically underestimated (for scalar $S$) by as much as a factor of 2 when using the LO estimate. This will change best fit regions and lead to stronger constraints from the dijet channel. Models which live on the edge of exclusion based on LO estimations may not survive.

It is often assumed that $S$ does not mix with the SM Higgs. But mixing is necessarily generated at some loop level. If a fermion is in the loop it is a three-loop effect (but with large Yukawas and strong gauge couplings at the vertices). If a scalar is in the loop it arises at one-loop. This contribution can be turned off by tuning a quartic term to zero [this is not stable under the renormalisation group evolution], but there is always a pure-gauge two-loop contribution. This effect should be acknowledged and checked for consistency.

Another common assumption is that the $S$ vev is zero. But since there is a $Sgg$ vertex the $S\to -S$ symmetry must be broken when expanded around the vacuum ($S=S_0 - \langle S_0 \rangle$). It is hard to imagine a non-finely-tuned potential with this property and a minimum at $\langle S_0 \rangle=0$. Another way to argue this is made in the paper: if the original state $S_0$ couples at a three-point vertex with a new fermion or scalar, then a tadpole term will induce a non-zero linear $S_0$ contribution which acts like a vev insertion.

Decay channels have been missed in some works which can significantly change conclusions.

In the proposed models it is necessary to have a rather large diphoton width. The authors identify three main methods for achieving this. There are worries with each of them which have not been addressed uniformly in the literature....

1. If fermion in the loop, then a large Yukawa coupling $yS\psi\bar{\psi}$. Typically these need to be O(1). Naturally, one should make sure perturbativity is under control when calculating the one-loop effective coupling. There exist papers which don't. As well, even if it remains pertubatively controlled at the 750 GeV scale, the renormalisation group evolution can evolve that Yukawa to the non-perturbative regime at some higher energy. This should be checked and at least acknowledged. [It is an interesting fact that this does not happen in the standard model for the top Yukawa; it is ~1 at the electroweak scale and shrinks with energy scale due to Higgs/top quark gauge contributions].

2. If scalar in the loop, then a large cubic term $\kappa S XX$. The authors point out that this generally leads to problems with stability of the electroweak vacuum. I will also add that if the cubic term is large (>TeV) compared to the desired sub-TeV particles, then it is likely that the vacuum potential must be somewhat tuned, and this will not be stable under radiative corrections.

3. Instead of relying on a large Yukawa or cubic, increase the charge coupling the loop fermion/scalar to gluons/photons or have more particles in the loop. Papers exist with Q≥5 and N=9000. However, such changes induce a large correction to the gauge coupling renormalisation group running above threshold, and can lead to high-energy electroweak behaviour which is ruled out, or worse to a Landau Pole at energies below 1 TeV.
• My conclusion from all this: even if one does not object to the phenomena of ambulance chasing [I personally do not object to the principle], one should object to the lack of quality that seems to go along with it. It is a problem that assumptions are made and effects are overlooked which change conclusions considerably. It is a problem that lower quality papers are (at least for a good while) cited on par with better considered ones. It is a problem that we are mostly seeing the same idea embedded into different models with no qualitatively new observations. In addition, it is frustrating that (non-)participation in trending topics has implications for whether you can continue to make your way in the field, especially for early career researchers.
• On a lighter note, see the arXiv preprint "A Theory of Ambulance Chasing" by Mihailo Backović (a 750 GeV ambulance chaser himself!) for a bit of fun, where he attempts to model the total number of papers on a trending topic as a function of time. For the diphoton excess: "It follows that if the interest scales as an inverse power law in time, the cumulative number of papers on a topic is well described by a di-gamma function, with a distinct logarithmic behavior at large times." "Di-gamma" is just brilliant. A (testable) prediction of this model is that "the total number of papers will not exceed 310 by June 1, 2016". If you feel like you have a better model, then throw your hat into the ring!
• The HEP Postdoc Project has appeared. To quote the website: "The HEP Postdoc Project intends to be a tool for Postdocs, or even PhD students, in the area of High Energy Physics... When an applicant accepts an offer, she/he is lacking, however, important information about the senior researchers in the corresponding institution... The goal of the HEP Postdoc Project is to fill this gap. Please, send us your opinions on senior high energy physicists you have interacted with in the past..."
• CERN is doing an "In Theory" series of articles on the CERN Theory department. The first-two installments are "Welcome to the Theory corridor" and "why bother with theoretical physics?"
• Conferences/workshops:
• CoEPP Annual Workshop 2016 (indico)
• LHC Performance Workshop (indico)
• UCLA Dark Matter 2016 (agenda)
• CERN Winter School on Supergravity, Strings, and Gauge Theory 2016 (indico)
• Stories of Australian Science: Looking for dark matter in a gold mine.
• Sabine Hossenfelder via aeon: The superfluid Universe.
• SciAm: Physicist Sabine Hossenfelder Fears Theorists, Lacking Data, May Succumb to "Wishful Thinking".
• Smashpipe: Who's winning the string wars and why should you care? [Part 1 and Part 2]
• Quanta: From Einstein’s Theory to Gravity’s Chirp.
• Lawrence Krauss via New Yorker: Do the New, Big-Money Science Prizes Work?
• symmetry magazine: The ABCs of particle physics.
• In audio/video media:
• Recordings of talks from "Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics" [In my opinion, more conferences/workshops should record and make public their talks like this].
• CMS Experiment: An introduction to the CMS Experiment at CERN. [7:25]
• Katherine Freese at Perimeter Institute: The Dark Side of the Universe. [1:03:16]
• Gianfranco Bertone: The Quest for Dark Matter. [1:00:23]
• Camilo Garcia-Cely: Phenomenology of Left-Right Symmetric Dark Matter. [1:06:30]
• Gero von Gersdoff: Light by light scattering and the 750 GeV diphoton excess. [58:00]
• The Good Stuff: What the Heck is Dark Matter? [12:02]
• Stephen Sekula SMU Godbey Lecture: "The Tail of the Lion: 100 Years of General Relativity, the Scientific Theory of Space and Time" [1:10:05]
• La physique autrement: Physics and caffeine. [9:12]

## Friday, 29 January 2016

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

## Friday, 15 January 2016

### Friday wrap-up: gravitational* waves?...

A short list this week. And the next post will be in two weeks...

• The 6th International Workshop on High Energy Physics in the LHC Era was on from 6-12th Jan [indico].
• Scientific American: "Stephen Hawking's New Black-Hole Paper, Translated: An Interview with Co-Author Andrew Strominger"
• Sabine Hossenfelder via Quanta: "String Theory Meets Loop Quantum Gravity"
• Nautilus: "Beauty Is Physics’ Secret Weapon," an interview with Frank Wilczek.
• In audio/video media:
• Hitler doesn't get a postdoc in High Energy Theory. [3:49]
• NASA: Fermi Sharpens its High-Energy View. [5:30]
• Sixty Symbols: Hairy Black Holes and Super Selfies. [7:11]
• Physics Girl: 5 amazing stars we’ve discovered in space. [7:00]
• SpaceX: "The Falcon has landed" Recap of Falcon 9 launch and landing. [3:37]
• The collaborative diphoton project mentioned in the previous two posts is gaining some momentum. Watch this space...

## Saturday, 9 January 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.
• 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]