Saturday, 25 April 2015

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?
  • Scientific American has an article on self-interacting dark matter on the back of the Abell 3827 cluster "hint" from last week.
  • And now that we have warmed up with Hubble here are some photos from the week...

Saturday, 18 April 2015

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:

    Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

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

      Ceres' North Pole

Friday, 10 April 2015

Friday wrap-up: LHC first beam, mono-Z'...

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...

  • Protons circulated the LHC for the first time since long shutdown last Sunday! If you want to relive the day you can read backwards through the live blog. Beams circled both ways (first Beam 2 [anticlockwise] then Beam 1) at the injection energy of 450 GeV, after the collimators placed along the way were each opened up in turn. For ATLAS and CMS the collimators placed ~100m away created some spectacular beam splash events. Below are the first recorded splash events from CMS and from ATLAS.

    Images from the day can be found here, and you can check the current LHC status here (or a more detailed status here). Since then there have been a bunch of technical tests and splashes. There's also an article from CERN about the plan up until collisions. In the immediate future we wait for both beams to ramp-up to 6.5 TeV. The magnets are trained and ready as of Saturday, and first probe beam reached 6.5 TeV this morning...

  • An interesting observation which Strassler brought up, and I have noticed as well: there is much more talk about dark matter as a goal for LHC Run II, in lieu SUSY. The CERN press release wrote, "The Brout-Englert-Higgs mechanism, dark matter, antimatter and quark-gluon plasma are all on the menu for LHC season 2," and Nature wrote, "In 2012, more than two years after the machine first started up, LHC experiments found the long-awaited Higgs boson... The second run does not have such an obvious target. Instead, physicists will scour the data for signs of phenomena that do not fit with the standard model of particle physics in hopes of solving mysteries such as the origins of dark matter."
  • On that (dark matter at the LHC) front, the arXiv awoke on Wednesday to a new mono-X, i.e. these-three-preprints discussing the mono-Z'. The salient processes are the three depicted in the below diagram (where the leptons could be replaced by jets, or a thin jet if the Z' is light and boosted), plus dark Higgs-strahlung.

    It's a sensible thing to go and look for, and I'm surprised it's taken so long to be looked at in a general context... though obviously it was high time! Now inviting speculation on the next mono-X... mono-W'? mono-stoponium? neutrinoless mono-beta decay?
  • Lastly, as is the tradition, a space image... here is a gravitationally lensed galaxy nearly 12 billion light-years away, imaged by ALMA.

Friday, 3 April 2015

Friday wrap-up: LHC restart, dark photon, a drama in five acts...

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 the LHC restart, the short circuit was fixed on Monday by melting the offending metal fragment with an injection of 400 amps of current for a few milliseconds. Now the news is that "first beams could be circulating in the machine sometime between Saturday and Monday... Particle collisions at an energy of 13 TeV could start as early as June." 
  • The NA48/2 Collaboration has submitted their search for the dark photon in $\pi^0\to \gamma A'$ decays, ruling out the remaining parameter space for the dark photon as an explanation for the muon $(g-2)$ anomaly.

  • On the 'Evidence for dark matter in the inner Milky Way' front, we now have a 'Reply to Comment on "Evidence for dark matter in the inner Milky Way."' Will we see a 'Comment on "Reply to Comment on "Evidence for dark matter in the inner Milky Way"'? Stay tuned... 
  • In a blog post from Tim Head, it turns out that machine learning can do pretty well at telling the difference between interesting and uninteresting papers on the arXiv, using only the title and abstract as input.
  • Arkani-Hamed and Maldacena had a preprint out on Monday: "Cosmological Collider Physics." The paper is about how we might recognise the presence of new particles with inflaton interactions by their impact on primordial cosmological fluctuations. It is quite long (49+12 pages) and far outside my area, so I will leave it at that... I wonder if Tim Head's algorithm would classify the paper as interesting?
  • NPR ran a story on the CRAYFIS project aimed at detecting high-energy cosmic rays with a network of smartphones, bringing >7000 people to the service.
  • An arXiv preprint from Melbourne CoEPP and collaborators has pointed out that ATLAS and CMS have been led by theorists to perform searches for an unphysical dark matter EFT. To quote from the paper:

    As an example of a problem encountered with an $SU(2)_L$ violating EFT, consider the following operator:$$\frac{1}{\Lambda^2}(\overline{\chi}\gamma^\mu \chi)(\overline{u}\gamma_\mu u+\xi\overline{d}\gamma_\mu d)$$This Lagrangian violates $SU(2)_L$, unless $\xi=1$. The case of unequal $u$ and $d$ couplings was considered in Ref. [13], where a very strong constructive(destructive) “interference effect” was found for $\xi=-1(+1)$, the degree of which depends on the energy scale. The analysis of Ref. [13] was subsequently repeated by the LHC experimental collaborations ATLAS [14, 15] and CMS [16, 17]. We shall demonstrate that the large cross section enhancement for $\xi=+1$ is in fact due the production of longitudinally polarized W’s as a result of breaking gauge invariance.
  • APS has spotlighted some lattice QCD results from investigators including CoEPP Adelaide node members which indicate that the Λ(1405) resonance has a molecular quark pair+triplet structure (arXiv version).
  • Scientific American has a story on dark matter and the dinosaurs, inspired by the Rampino paper from February. They are refreshingly skeptical, and they mention the work of Randall-Reece and Abbas-Abbas, which was my main problem with prior articles -- overall a nice piece of science journalism!
  • In video/audio media
  • I have only seen one April Fools' arXiv article making the rounds this week: A Farewell to Falsifiability. From the abstract, "... some far-thinking physicists have proposed instead that we should give up on the notion of Falsifiability itself. We endorse this suggestion but think it does not go nearly far enough. We believe that we should also dispense with other outdated ideas, such as Fidelity, Frugality, Factuality and other "F" words. And we quote a lot of famous people to support this view." There is some discussion on Peter Woit's blog as to whether this article from Ashoke Sen (Milner Prize winner) is also to be taken as an April Fools' submission, or just some pop-sci...
  • This week I became aware of a 1979 document on the CERN server from Ellis et al. called "Can one tell QCD from a hole in the ground? : a drama in five acts." It is indeed a drama in five acts, about QCD, with illustrations and figures!

  • Lastly, some loosely science-related images from the week...

    Here is a photo from César Cantú of lightning in the ash cloud of Mexico's Colima volcano (hat-tip Bad Astronomy).

    IMG_0360 2400x1600.jpg

    Typhoon Maysak from Samantha Cristoforetti on the International Space Station.

Wednesday, 1 April 2015

arXiv-watch: Jan-Mar 2015

The top five cited articles (according to INSPIRE) of the last three months overall and of the last six months in hep-ph.

Hot topics overall are clearly the Planck results, and Fermi LAT.

Joint Analysis of BICEP2/Keck Array and Planck Data
BICEP2 and Planck Collaborations (P. A. R. Ade (Cardiff U.) et al.). Feb 2, 2015. 17 pp.
Published in Phys.Rev.Lett. 114 (2015) 10, 101301
DOI: 10.1103/PhysRevLett.114.101301
e-Print: arXiv:1502.00612 [astro-ph.CO] | PDF

Fermi Large Area Telescope Third Source Catalog
Fermi-LAT Collaboration. Jan 8, 2015. 99 pp.
e-Print: arXiv:1501.02003 [astro-ph.HE] | PDF


In hep-ph, hot topics are the galactic centre excess, B→K*μμ, and non-perturbative inflation.

WIMPs at the Galactic Center
Prateek Agrawal (Fermilab), Brian Batell (CERN), Patrick J. FoxRoni Harnik (Fermilab). Nov 10, 2014. 34 pp.
FERMILAB-PUB-14-411-T, CERN-PH-TH-2014-219
e-Print: arXiv:1411.2592 [hep-ph] | PDF