Friday, 13 February 2015

Friday wrap-up: Planck, long-lived particles, dark matter, neutrinos...

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.

  • As for the rest 2015 Planck release, the base cosmological parameters have hardly changed (see e.g. Table 1 of this paper). ΛCDM of course continues to explain everything extremely well. The $\Sigma m_\nu$ upper limit is down to 0.17 eV (Planck TT, TE, EE+lowP+BAO) at best, almost at a level to rule out an inverted hierarchy ($\Sigma m_\nu\gtrsim 0.1$ eV). $N_{eff}$ is still at 3.04 ± 0.18 (Planck TT, TE, EE+lowP+BAO), consistent with the standard value 3.046. There is no longer a hint of an excess there. As for inflation, Jester weighed in, and Planck present their results in this paper:


    Looks like the canonical Starobinsky $R^2$ inflation (1980) is taking the lead, but time will tell... experiments will need to probe at the level r~0.001 to test it.
  • BaBar released a new paper, "Search for Long-Lived Particles in e+e− Collisions". No significant signal is observed. It seems worth mentioning that they see a high local significance of events in the dimuon mode at $m_{\mu\mu}\approx 212$ MeV, which interestingly is very close to the HyperCP anomaly, but they say it is consistent with background from photon conversions. Anyway, from the point of view of a phenomenologist I especially like the effort BaBar has put into presenting their results in a model independent way. For example:

    1. They provide upper limits on $\sigma(e^+e^-\to LX)\mathcal{B}(L\to f)\epsilon(f)$, where $L$ is the long-lived particle and $f$ is the final state, which are completely agnostic of the production mechanism.
    2. They provide limits on $\mathcal{B}(B\to LX_s)\mathcal{B}(L\to f)$ where $X_s$ is "strange stuff", which reduces significantly some theoretical uncertainties for exotic $B$ decays and increases the possible signal yield.
    3. They look at six different two-body final states.
    4. On top of this, they will provide the full efficiency as a function of $m$, $c\tau$ and $p_T$ in supplementary material, so that their results can be reinterpreted.

    Very well done from BaBar. Their measurement has implications for a very simple extension of the Standard Model with a real singlet scalar mixing with the Higgs (a Higgs portal). I have quickly reinterpreted their Fig. 3 bounds (1 cm < $c\tau_L$ < 100 cm) on the $\mu^+\mu^-$, $\pi^+\pi^-$ and $K^+K^-$ final states, assuming the limit lines are $\mathcal{B}(B\to LX_s)\mathcal{B}(L\to f)\lesssim 10^{-6.5}$...



    The above plot shows exclusions (within solid lines) for the model as a function of light scalar mass and mixing (see a previous paper of ours). The incremental shadings represent different lifetime regions and the shaded region between 0.28 GeV and 4 GeV masses indicates a very uncertain region for branching predictions; between we choose the most recent calculation (still >20 years old!) below 1.4 GeV and a perturbative calculation above. The approximate exclusion from BaBar is shown as the purple dashed line. Indeed they are exploring previously unexplored parameter space of interest for an inflationary model of Bezrukov/Gorbunov (between black dashed lines). They are limited by difficult backgrounds between 0.37 GeV and 0.86 GeV masses. The purple dotted line indicates the region they would have excluded if they could limit $\mu^+\mu^-$ and $\pi^+\pi^-$ final states to the same level in that region. 
    I am interested to get my hands on the supplementary material when it is released.
  • new paper published in Nature Physics (not on the arXiv [edit: now it is]) this week is doing the rounds; it infers that there must be dark matter in the inner region (within the solar circle) of our own galaxy. It's behind a paywall, but there is a press release here. The main plot is below.

                      

    The authors have compiled an exhaustive list of rotation curve measurements (red), as well as gathered a set of models for the baryonic contribution from which they form their "baryonic bracket" (grey). The lower panel shows that all baryonic-only models are already ruled out at 5-sigma by the time we arrive at our galactic radii. Hence there must be a dark matter component in the inner Milky Way. They do not appeal to any dark matter density profile, so in that sense it is a model-independent result (but of course it is not independent of the baryonic models!). Nevertheless you can see that a typical Navarro-Frenk-White profile models the residuals extremely well. This is nice to know.

    It is interesting that you can already infer dark matter just with measurements of the rotation curve within the solar circle, and I suppose the result may also be useful to constrain dark matter distributions of interest to direct and indirect detection. I do not think it is "the first observational proof of the presence of dark matter in the innermost part of the Milky Way" as the press release claims (the paper does not claim this). It is obvious that you need some dark matter component in the Milky Way to explain the rotation curve measurements at large galactic radii, and nobody thinks that all that dark matter is accumulated beyond 8 kpc! Those measurements are already observational evidence for dark matter in the inner Milky Way. Regardless, Hooper et al. already saw dark matter at the galactic centre...
  • IceCube have released their results on the flavour ratio of astrophysical neutrinos above 35 TeV. They release a very nice plot which fits the observed flavour ratio at Earth:


    Averaging neutrino oscillations over astronomical distances would give a value in the blue triangle for any flavour ratio at the source. The blue circle $\approx(1:1:1)_{Earth}$ marks the expected value for pion decays as the dominant source. Any measurement inconsistent with the (very thin) blue triangle would be a signal of new neutrino physics, such as neutrino decay, sterile neutrinos, or CPT violation (see for example here and here). Tommasso discusses it a little more here. [Edit: there is a short article at the IceCube web site also.]
  • If you'd like to read some more on the "Firewall Phenomenology with Astrophysical Neutrinos" paper that appeared on hep-ph last week, Bee at Backreaction has written a nice summary. The paper shows that IceCube's PeV neutrinos could be explained by a suitable emission spectra from black hole firewalls.
  • CMS is alive! Magnet comes on next week.
  • Physicist Val Fitch, whose discovery (along with James Cronin) of CP violation in a 1964 experiment won him the 1980 Nobel Prize, has died at the age of 91. Read about his life and contributions here.
  • Steven Weinberg has written a history book, released a few weeks ago. According to the About, "To Explain the World is a sweeping, ambitious account of how difficult it was to discover the goals and methods of modern science, and the impact of this discovery on human knowledge and development." For some reason it even garnered praise from Ian McEwan.
  • IPMU has recently published a 26 page transcript [pdf] of a conversation with Edward Witten after he was awarded a Kyoto Prize last year for "Outstanding Contributions to the Development of Mathematical Sciences through the Exploration of Superstring Theory". Peter Woit highlights some excerpts and talks about it on his blog.
  • This blog post has a gif showing the history of planet detection in 1 minute, and the exoplanet gold rush starting at the year ~2000, with some discussion. Here's where we were by the end of last year:

  • On fake academic journals and "Rogeting".
  • SpaceX launched the Deep Space Climate Observatory (DSCOVR) on Wednesday. You can watch the launch and read a little about it at space.com. DSCOVR will sit at the L1 Lagrange point between the Earth and the Sun, observe the climate, and serve as an early alert "buoy" for geomagnetic storms. The satellite is the resurrection of a previous project championed by Al Gore; he wrote, "DSCOVR has embarked on its mission to further our understanding of Earth and enable citizens and scientists alike to better understand the reality of the climate crisis and envision its solutions. DSCOVR will also give us a wonderful opportunity to see the beauty and fragility of our planet and, in doing so, remind us of the duty to protect our only home."
  • Finishing up with some photos from space for the week:
    • Rosetta took an incredible shot of its comet. Tomorrow it will make its closest approach yet at 6 km above the surface, the Sun at its back.

      comet
    • Lastly, here is an image of galaxies SDSSCGB 8842.3 and SDSSCGB 8842.4 as pictured by Hubble and released on Monday 

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