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: Collisions, displaced Higgs 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...

• The LHC has seen collisions at injection energy, 450 GeV per beam. There's a little more you can read at symmetry magazine; still plenty of calibration to be done. Here's one of the events in the ATLAS detector, a far cry from the messy environment we'll see at 13 TeV...
  
  
  
  
  As well, CMS has a visualisation of one of their events on YouTube.

• Pheno 2015 happened this week in Pittsburgh. Definitely worth perusing the interesting talks on the indico page.

• I uploaded an arXiv preprint on Monday, "Constraining portals with displaced Higgs decay searches at the LHC." One of the primary purposes of the LHC is to study the properties of the newly discovered Higgs boson in great detail. Even though we have now measured its mass to within ~0.2%, it is still possible that it is decaying to exotic particles 20% of the time! So it is clearly sensible to search for exotic Higgs decays. I concentrated on one possibility: the decay to a pair of long-lived particles which each subsequently decay around 1 metre from the beam pipe...
  
  Such long-lived particles are well-motivated; typically all you need is an approximate symmetry (which is by definition technically natural à la 't Hooft) in your model which if exact would result in a stable particle. This appears to be coming into vogue at the moment as we see natural SUSY being pushed into compressed and long-lived areas of parameter space; for example, if you violate R-parity just slightly then the would-be neutralino dark matter candidate can become long-lived. I became interested in this sort of phenomenology from a much simpler standard model (SM) extension: by a real singlet scalar field $S$. In that case you can write down a potential term $\zeta \phi^\dagger\phi S^2$ which mixes the Higgs boson and a new mass eigenstate $s$ after symmetry breaking. As $\zeta\to 0$ the $S$ field decouples from the SM and becomes stable, so for small $\zeta$ it is long-lived. It is possible that the $S$ also directly couples to some dark sector uncharged under the SM forces, so-called Higgs portal models. Anyway, the Higgs we know and love can decay to two $s$ particles which, in the simplest case, decay to SM particles somewhere in the middle of the detector. Else the $s$ could have some complex cascade decay into hidden sector states which subsequently decay in the middle of the detector (often called hidden valley models).
  
  You can do a similar thing with a massive dark photon, the so-called vector portal. In fact, the possibilities are many and varied, which presents two complementary challenges: how do collaborations present their results in the most model-independent way possible? and how do phenomenologists reinterpret the results in the context of their own models? The point of my paper was to explore these questions...
  
  So I had a go at reinterpreting two searches already performed by ATLAS, in a very simple way: by running Monte Carlo simulations, calculating decay probabilities, and folding in the provided particle reconstruction efficiencies as a function of the decay distance of the long-lived particles. As phenomenologists we are reliant upon these provided efficiencies, as there is no tool available to reliably calculate them ourselves. As we move on the journey which constitutes the bulk of the paper, we learn some valuable life lessons about what efficiency table information the collaborations could provide to make the life of phenomenologists wanting to reinterpret their searches (and there are a few of us out there!) much easier. Those life lessons are summarised in the conclusion.
  
  As well, at the end of the day I was able to make my own contribution to the portal model exclusion space, with the pretty pictures below...
  
  
  
  
  
  
  

• There's an article at Nautilus on "The Admiral of the String Theory Wars" AKA Peter Woit. As might be intuited, the article describes the string theory wars around the time when Woit released his book, "Not Even Wrong." It touches on his arXiv trackback controversy and feud with Polchinski. (Also I learned that Motl once compared Woit to bin Laden...). Woit said a few words about the article on his blog. There is also some discussion there on the following...

• You should be able to find Amanda Peet's hour long Perimeter Institute public lecture on string theory on YouTube within a day or so.

• Anothere interesting post at Backreaction: Testing modified gravity with black hole shadows.

• In video/audio media:
• Matt Strassler and Alan Boyle discussing the LHC at Virtually Speaking Science. [1 hour]
• The ALICE experiment and the quark-gluon plasma from CERN. [3 minutes]
• Chameleon Particles and Dark Energy with Clare Burrage on Sixty Symbols. [13 minutes]
• How scientists fairly obtain telescope time on numberphile. [8 minutes]
• The black hole information paradox with Amanda Peet. [2 minutes]

• Lastly, for contrast, here is a Dilbert cartoon and a sunset on Mars as photographed by the Curiosity rover...
  
  
  
  
  
  
  
  

Friday wrap-up: SHiP, portals...

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 SHiP fixed target experiment has uploaded its 200+ page physics case to the arXiv. Working at the intensity frontier, fixed target experiments collide very many (in this case) protons on a heavy target, with a detector placed some distance away after a significant amount of shielding. Because of the sheer number of protons on target, the setup is particularly sensitive to any ≲GeV scale extremely weakly interacting particles (large number × small number = detectable number!). The document is very comprehensive and speaks for itself; evidently the experiment has the capacity to explore some very interesting new physics scenarios, for example...
  
  If one modestly extends the standard model with a vector or a scalar field, it is always possible to write down gauge-invariant operators$$\epsilon F^{\mu\nu}F'^{\mu\nu},\\ \xi\phi^\dagger\phi S^2,$$where $F'^{\mu\nu}$ is a dark field strength operator and $S$ is a real singlet scalar. These are known as portal operators, and in the limit of very small $\epsilon$ or $\xi$ (which restores an enhanced Poincare symmetry and is therefore technically natural) the new states (referred to often as the dark photon and dark Higgs) are very long-lived and very weakly coupled to standard model states, so that they could still have gone undetected even if their masses are sub-GeV. If dark matter couples directly to these new states then they provide a "portal" from the standard model to the dark sector.
  
  So I was very interested to see the reach of the proposed experiment with respect to those portals; that reach is shown below as a function of mass for the case without dark matter, or with $m_{DM}>m_{A},m_S$ (the g* in the singlet case is proportional to the $\xi$ parameter above)...
  
  
  
  
  
  

  It is evident that the experiment would explore a significant amount of unexplored (not grey) parameter space (and the results are even stronger for a pseudoscalar). For the dark scalar case, the reach of the experiment comes from the unprecedented (in a fixed target experiment) number of B mesons produced, which can then subsequently decay to the light scalar state at a rate of one in a million or so. The states then live long enough to travel through the (~70m of) shielding before decaying in the detector.
  
  It is of note that unfortunately the widths and branching ratios of the scalar in the region $2m_\pi < m_S \lesssim 4$ GeV have large hadronic uncertainties, and the plot above must assume one theoretical prediction, so the story is not as clear-cut as it seems; luckily the experiment would be sensitive to many final states, and this goes some way to making the reach independent of this uncertainty. (These uncertainties do not exist for the dark photon thanks to measurements of our very own photon!). The most recent theoretical calculation for the dark scalar widths in this region is >20 years old. I wonder if lattice QCD could have something to say if it was applied to the problem?

• Quanta Magazine has a couple of interesting articles this week; on artificial intelligence, and entangled wormholes (first instalment of a series).

• Scientific American has an article on self-interacting dark matter on the back of the Abell 3827 cluster "hint" from last week.

• In video/audio media:
• Can LHC black holes destroy the universe? from Sabine Hossenfelder.
• CERN on LHCb and societal impacts. [2 and 4 minutes]
• Learning and unlearning to ride a reverse bike at SmarterEveryday. [8 minutes]
• The Cheryl Logic Problem at Numberphile. [11 minutes]
• First of Hubble memorable moments series. [5 minutes]
• The moment and a timelapse of the Calbuco eruption.

• NASA and ESA (and others!) are celebrating Hubble's 25th anniversary.
  
  
  

• And now that we have warmed up with Hubble here are some photos from the week...

• ... of the Calbuco eruption from Philip Oyarzo...
  
  
  
  

• ... Comet 67P/Churyumov–Gerasimenko from all the angles...
  
  
  

• ... and a glimpse of Ceres' bright spot from up close [gif].