{"id":1106,"date":"2014-09-22T08:38:16","date_gmt":"2014-09-22T08:38:16","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=1106"},"modified":"2015-03-10T02:01:37","modified_gmt":"2015-03-10T02:01:37","slug":"an-update-from-ams-02-the-detector-in-space","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=1106","title":{"rendered":"An update from AMS-02, the particle detector in space"},"content":{"rendered":"

Last Thursday,\u00a0Nobel Laureate Sam Ting<\/a> presented the latest results<\/a>\u00a0(CERN press release<\/a>) from the Alpha Magnetic Spectrometer<\/a>\u00a0(AMS-02) experiment, a particle detector attached to the International Space Station—think “ATLAS\/CMS in space.”\u00a0Instead of beams of protons,\u00a0the AMS detector examines cosmic rays in search of\u00a0signatures of new physics such as the products of\u00a0dark matter annihilation in our galaxy.<\/p>\n

\"from<\/a>
Image of AMS-02 on the space station, from NASA<\/a>.<\/figcaption><\/figure>\n

In fact, this is just the latest chapter in an ongoing mystery involving the energy spectrum of cosmic positrons. Recall that positrons are the antimatter versions\u00a0of electrons with identical properties except having opposite charge. They’re produced from known astrophysical processes when high-energy cosmic rays (mostly protons) crash into\u00a0interstellar gas—in this case they’re known as `secondaries’ because\u00a0they’re a product of the `primary’ cosmic rays.<\/p>\n

The dynamics of\u00a0charged particles in the galaxy are difficult to simulate due to the presence of intense and complicated magnetic fields. However,\u00a0the\u00a0diffusion models generically\u00a0predict that the\u00a0positron fraction<\/strong>—the number of positrons divided by the total number of positrons and electrons—decreases with energy. (This ratio of fluxes\u00a0is a nice quantity because some astrophysical uncertainties\u00a0cancel.)<\/p>\n

This prediction, however, is in stark contrast with the observed positron fraction from recent satellite experiments:<\/p>\n

\"AMS-02,<\/a>
Observed positron fraction from recent experiments compared to expected\u00a0astrophysical background (gray)\u00a0from\u00a0APS viewpoint article<\/a> based on\u00a0the 2013 AMS-02 results<\/a>\u00a0(data) and the analysis in 1002.1910<\/a>\u00a0(background).<\/figcaption><\/figure>\n

The rising fraction had been hinted\u00a0in balloon-based experiments for several decades, but the satellite experiments\u00a0have been able to demonstrate\u00a0this behavior\u00a0conclusively because they can\u00a0access higher energies.\u00a0In\u00a0their first\u00a0set of results<\/a> last year (shown above), AMS gave the most precise measurements of the positron fraction\u00a0as far as 350 GeV.\u00a0Yesterday’s announcement extended these results to 500 GeV and\u00a0added the following observations:<\/p>\n

First they\u00a0claim that they have measured the maximum of the positron fraction to be 275 GeV.\u00a0This is\u00a0close to the edge of the data they’re releasing, but the plot of\u00a0the positron fraction slope is slightly more convincing:<\/p>\n

\"From<\/a>
Lower: the latest positron fraction data from AMS-02 against a phenomenological model. Upper: slope of the lower curve.\u00a0From Phys. Rev. Lett. 113, 121101<\/a>. [Non-paywall summary<\/a>.]<\/figcaption><\/figure>The observation of a maximum in what was otherwise a fairly featureless rising curve is key for interpretations of the excess, as we discuss below.\u00a0A second observation\u00a0is a bit more curious:\u00a0while neither the electron nor the positron\u00a0spectra follow a simple power law, $latex \\Phi_{e^\\pm} \\sim E^{-\\delta}$, the total electron\u00a0or<\/em> positron flux does follow such a power law over a\u00a0range of energies.<\/p>\n
\"...\"<\/a>
Total electron\/positron flux weighted by the cubed energy and the fit to a\u00a0simple power law. From the\u00a0AMS press summary<\/a>.<\/figcaption><\/figure>\n

This is a little harder to interpret since\u00a0the flux form\u00a0electrons\u00a0also, in principle, includes different sources of background.\u00a0Note that this plot reaches higher energies than the positron fraction—part of the reason for this is that\u00a0it is more difficult to distinguish between electrons and\u00a0positrons at high energies. This is because the\u00a0identification depends on how the particle bends in the AMS magnetic field and higher energy particles bend less.\u00a0This, incidentally, is also why the FERMI data has much larger error bars\u00a0in the first plot above—FERMI doesn’t have its own magnetic field and must rely on that of the Earth\u00a0for charge discrimination.<\/p>\n

So what should one make of the latest results?<\/strong><\/p>\n

The most optimistic hope\u00a0is that this is a signal of dark matter, and at this point this\u00a0is more of a ‘wish’ than a deduction.\u00a0Independently of AMS, we know is that dark matter exists in a halo\u00a0that surrounds our galaxy.\u00a0The\u00a0simplest dark matter models also assume\u00a0that when two dark matter particles find each other in this halo, they can annihilate into\u00a0Standard Model particle–anti-particle pairs, such as electrons and positrons—the latter\u00a0potentially yielding the rising positron fraction signal seen by AMS.<\/p>\n

From a particle physics perspective,\u00a0this would be the most exciting possibility. The ‘smoking gun’ signature of such a scenario would be a steep drop in the positron fraction at the mass of the dark matter particle. This is because the annihilation occurs at low velocities so that the\u00a0energy of the\u00a0annihilation products is set by the dark matter\u00a0mass.\u00a0This is why the\u00a0observation of a maximum in the positron fraction is interesting:\u00a0the dark matter interpretation of this excess hinges\u00a0on how steeply the fraction drops off.<\/p>\n

There are, however, reasons to be skeptical.<\/p>\n