Wednesday, 24 September 2014

BICEP: what was wrong and what was right

As you already know, Planck finally came out of the closet.  The Monday paper shows that the galactic dust polarization fraction in the BICEP window is larger than predicted by pre-Planck models, as previously suggested by an independent theorist's analysis. As a result, the dust contribution to the B-mode power spectrum at moderate multipoles is about 4 times larger than estimated by BICEP. This implies that the dust alone can account for the signal strength reported by BICEP in March this year, without invoking a primordial component from the early universe. See the plot, borrowed from Kyle Helson's twitter, with overlaid BICEP data points and Planck's dust estimates.  For a detailed discussion of Planck's paper I recommend reading other blogs who know better than me, see e.g. here or here or here.  Instead, I will focus and the sociological and ontological aspects of the affair. There's no question that BICEP screwed up big time. But can we identify precisely which steps lead to the downfall, and which were a normal part of the scientific process?  The story is complicated and there are many contradicting opinions, so to clarify it I will provide you with simple right or wrong answers :)

  • BICEP Instrument: Right.
    Whatever happened one should not forget that,  at the instrumental level,  BICEP was a huge success. The sensitivity to B-mode polarization at  angular scales above a degree beats previous CMB experiments by an order of magnitude. Other experiments are following in their tracks, and we should soon obtain better limits on the tensor-to-scalar ratio.  (Though it seems BICEP already comes close to the ultimate sensitivity for single-frequency ground-based experiment, given the dust pollution demonstrated by Planck).  
  • ArXiv first: Right.
    Some complained that the BICEP result were announced before the paper was accepted in a journal. True, peer-review is the pillar of science, but it does not mean we have to adhere to obsolete 20th century standards. The BICEP paper has undergone a thorough peer-review process of the best possible kind that included the whole community. It is highly unlikely the error would have been caught by a random journal referee. 
  • Press conference: Right.  
    Many considered inappropriate that the release of the results was turned into a publicity stunt with a press conference, champagne, and  YouTube videos. My opinion is that,  as long as they believed the signal is robust, they had every right to throw a party, much like CERN did on the occasion of the Higgs discovery.  In the end  it didn't really matter. Given the importance of the discovery and how news spread over the blogosphere, the net effect on the public would be exactly the same if they just submitted to ArXiv.  
  • Data scraping: Right.
    There was a lot of indignation about the fact that, to estimate the dust polarization fraction in their field of view,  BICEP used preliminary Planck data digitized from a slide in a conference presentation. I don't understand what's the problem.  You should always use all publicly available relevant information; it's as simple as that. 
  • Inflation spin: Wrong.
    BICEP sold the discovery as the smoking-gun evidence for cosmic inflation. This narrative was picked by mainstream press, often mixing inflation with the big bang scenario. In reality, the primordial B-mode would be yet another evidence for inflation and a measurement of one crucial  parameter - the energy density during inflation. This would be of course a huge thing, but apparently not big enough for PR departments. The damage is obvious: now that the result does not stand,  the inflation picture and, by association,  the whole big bang scenario is  undermined in public perception. Now Guth and Linde cannot even dream of a Nobel prize, thanks to BICEP...  
  • Quality control: Wrong. 
    Sure, everyone makes mistakes. But, from what I heard, that unfortunate analysis of the dust polarization fraction based on the Planck polarization data was performed by a single collaboration member and never cross-checked. I understand  there's been some bad luck involved: the wrong estimate fell very close to the predictions of faulty pre-Planck dust models. But, for dog's sake, the whole Nobel-prize-worth discovery was hinging on that. There's nothing wrong with being wrong, but not double- and triple-checking crucial elements of the analysis is criminal. 
  • Denial: Wrong.
    The error in the estimate of the dust polarization fraction was understood soon after the initial announcement, and BICEP leaders  were aware of it. Instead of biting the bullet, they chose a we-stand-by-our-results story. This resembled a child sweeping a broken vase under the sofa in the hope that no one would notice...  

To conclude, BICEP goofed it up and deserves ridicule, in the same way a person slipping on a banana skin does. With some minimal precautions the mishap could have been avoided, or at least the damage could have been reduced. On the positive side, science worked once again, and  we all learned something. Astrophysicists learned some exciting stuff about polarized dust in our galaxy. The public learned that science can get it wrong at times but is always self-correcting. And Andrei Linde learned to not open the door to a stranger with a backpack.

Friday, 19 September 2014

Dark matter or pulsars? AMS hints it's neither.

Yesterday AMS-02 updated their measurement of cosmic-ray positron and electron fluxes. The newly published data extend to positron energies 500 GeV, compared to 350 GeV in the previous release. The central value of the positron fraction in the highest energy bin is one third of the error bar lower than the central value of the next-to-highestbin.  This allows the collaboration to conclude that the positron fraction has a maximum and starts to decrease at high energies :]  The sloppy presentation and unnecessary hype obscures the fact that AMS actually found something non-trivial.  Namely, it is interesting that the positron fraction, after a sharp rise between 10 and 200 GeV, seems to plateau at higher energies at the value around 15%.  This sort of behavior, although not expected by popular models of cosmic ray propagation, was actually predicted a few years ago, well before AMS was launched.  

Before I get to the point, let's have a brief summary. In 2008 the PAMELA experiment observed a steep rise of the cosmic ray positron fraction between 10 and 100 GeV. Positrons are routinely produced by scattering of high energy cosmic rays (secondary production), but the rise was not predicted by models of cosmic ray propagations. This prompted speculations of another (primary) source of positrons: from pulsars, supernovae or other astrophysical objects, to  dark matter annihilation. The dark matter explanation is unlikely for many reasons. On the theoretical side, the large annihilation cross section required is difficult to achieve, and it is difficult to produce a large flux of positrons without producing an excess of antiprotons at the same time. In particular, the MSSM neutralino entertained in the last AMS paper certainly cannot fit the cosmic-ray data for these reasons. When theoretical obstacles are overcome by skillful model building, constraints from gamma ray and radio observations disfavor the relevant parameter space. Even if these constraints are dismissed due to large astrophysical uncertainties, the models poorly fit the shape the electron and positron spectrum observed by PAMELA, AMS, and FERMI (see the addendum of this paper for a recent discussion). Pulsars, on the other hand, are a plausible but handwaving explanation: we know they are all around and we know they produce electron-positron pairs in the magnetosphere, but we cannot calculate the spectrum from first principles.

But maybe primary positron sources are not needed at all? The old paper by Katz et al. proposes a different approach. Rather than starting with a particular propagation model, it assumes the high-energy positrons observed by PAMELA are secondary, and attempts to deduce from the data the parameters controlling the propagation of cosmic rays. The logic is based on two premises. Firstly, while production of cosmic rays in our galaxy contains many unknowns, the production of different particles is strongly correlated, with the relative ratios depending on nuclear cross sections that are measurable in laboratories. Secondly, different particles propagate in the magnetic field of the galaxy in the same way, depending only on their rigidity (momentum divided by charge). Thus, from an observed flux of one particle, one can predict the production rate of other particles. This approach is quite successful in predicting the cosmic antiproton flux based on the observed boron flux. For positrons, the story is more complicated because of large energy losses (cooling) due to synchrotron and inverse-Compton processes. However, in this case one can make the  exercise of computing the positron flux assuming no losses at all. The result correspond to roughly 20% positron fraction above 100 GeV. Since in the real world cooling can only suppress the positron flux, the value computed assuming no cooling represents an upper bound on the positron fraction.

Now, at lower energies, the observed positron flux is a factor of a few below the upper bound. This is already intriguing, as hypothetical primary positrons could in principle have an arbitrary flux,  orders of magnitude larger or smaller than this upper bound. The rise observed by PAMELA can be interpreted that the suppression due to cooling decreases as positron energy increases. This is not implausible: the suppression depends on the interplay of the cooling time and mean propagation time of positrons, both of which are unknown functions of energy. Once the cooling time exceeds the propagation time the suppression factor is completely gone. In such a case the positron fraction should saturate the upper limit. This is what seems to be happening at the energies 200-500 GeV probed by AMS, as can be seen in the plot. Already the previous AMS data were consistent with this picture, and the latest update only strengthens it.

So, it may be that the mystery of cosmic ray positrons has a simple down-to-galactic-disc explanation. If further observations show the positron flux climbing  above the upper limit or dropping suddenly, then the secondary production hypothesis would be invalidated. But, for the moment, the AMS data seems to be consistent with no primary sources, just assuming that the cooling time of positrons is shorter than predicted by the state-of-the-art propagation models. So, instead of dark matter, AMS might have discovered models of cosmic-ray propagation need a fix. That's less spectacular, but still worthwhile.

Thanks to Kfir for the plot and explanations. 

Sunday, 7 September 2014

Weekend Plot: ultimate demise of diphoton Higgs excess

This weekend's plot is the latest ATLAS measurement of the Higgs signal strength μ in the diphoton channel:

Together with the CMS paper posted earlier this summer, this is probably the final word on Higgs-to-2-photons decays in the LHC run-I. These measurements have had an eventful history. The diphoton final state was one of the Higgs discovery channels back in 2012. Initially, both ATLAS and CMS were seeing a large excess of the Higgs signal strength compared to the standard model prediction. That was very exciting, as it was hinting at new charged particles  with masses near 100 GeV. But in nature, sooner or later, everything has to converge to the standard model.  ATLAS and CMS chose different strategies to get there. In CMS, the central value μ(t) displays an oscillatory behavior, alternating between excess and deficit. Each iteration brings it closer to the standard model limit μ = 1, with the latest reported value of μ= 1.14 ± 0.26.  In ATLAS, on the other hand, μ(t) decreases monotonically, from μ = 1.8 ± 0.5 in summer 2012 down to μ = 1.17 ± 0.27 today (that precise value corresponds to the Higgs mass of 125.4 GeV, but from the plot one can see that the signal strength is similar anywhere in the 125-126 GeV range). At the end of the day, both strategies have led to almost identical answers :)