...but the evidence for an actual drop remains underwhelming...
In April 2013, the Alpha Magnetic Spectrometer (AMS-02), a gadget carried by the International Space Station that looks for dark matter and other things and whose data are being evaluated by Nobel prize winner Sam Ting (MIT) and his folks, reported intriguing observations that were supposed to grow to a smoking fun proving that dark matter exists and is composed of heavy elementary particles:
AMS-02 seems to overcautiously censor solid evidence for dark matterI had various reasons for these speculative optimistic prophesies – including Sam Ting's body language. It just seemed that he knew more than he was saying and was only presenting a very small, underwhelming part of the observations.
AMS: the steep drop is very likely there
Recall that among these high energy particles, there are both electrons and positrons. The positrons are more exotic and may be produced by pulsars – which is a boring explanation. However, they may also originate from the annihilation of dark matter "WIMP" particles. If that's so, the dark matter particle physics predicts that the positron fraction increases as you increase the energy of the electrons and positrons. But at some moment, when the energy reaches a few hundreds of \(\GeV\) or so, the positron fraction should stop growing and steeply drop afterwards.
Was that observed in 2013? Has it been observed by now? Finally, today, AMS-02 published a new paper in prestigious PRL:
High Statistics Measurement of the Positron Fraction in Primary Cosmic Rays of \(0.5\)–\(500\GeV\) with the Alpha Magnetic Spectrometer on the International Space Station (PRL)The PRL abstract says that for the first time, they see that for the first time, they observe the positron fraction's increase with energy to stop somewhere, approximately at \(200\GeV\) although e.g. interactions.org puts the place at \(275\pm 32\GeV\). Moreover, the derivative of the number of positrons with respect to the energy exceeds the same derivative for electrons around dozens of \(\GeV\) which makes it more likely that these lower but high-energy positrons indeed directly originate from a high-energy source and not from deceleration.
CERN story, CERN press release + PDF supplement, copy at interactions.org, APS, NBC, Symmetry Magazine
While the claim about the end of the increase of the positron fraction agrees with the graphs like those above (other graphs show the positron fraction stabilized at \(0.15\) between \(190\) and \(430\GeV\) or so), I find the "end of the increase" or a "potentially emerging decrease" tantalizing but still unspectacularly weak and inconclusive. Indeed, the "straight decrease" itself still seems to be unsupported. Even if we were shown these graphs in April 2013, and we were shown a bit less than that, I would have thought that Sam Ting's hype was probably a bit excessive.
Just to be sure, the behavior in the graphs is compatible with a (below) \(1\TeV\) dark matter particle like a neutralino (supersymmetry's most convincing dark matter candidate), and indeed, I tend to think that this is what actually exists and will emerge at the LHC, too. Incidentally, some sources tell us that the LHC is back to business after the upgrade. It's a bit exaggeration of the actual ongoing "business" but let's hope that in April 2015, the \(13\TeV\) and perhaps \(14\TeV\) collisions will start smoothly and abruptly.
SUSY and related scenarios predicts the positron fraction as a function of energy that looks like the red graph above (two values of the neutralino mass are depicted). Up to a certain energy, it looks just like what AMS-02 has already shown us – which is good news for WIMP and/or SUSY – but we still haven't seen the dramatic drop yet. Of course, it's conceivable that Ting et al. are still hiding something they already have – and maybe have had already in April 2013. Maybe the hiding game is needed for the continued funding of their experiment. But this is just another speculation.
The neverending story that takes place at the ISS is described in this musical video clip.