{"id":900,"date":"2014-06-05T03:04:56","date_gmt":"2014-06-05T03:04:56","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=900"},"modified":"2017-02-19T01:37:03","modified_gmt":"2017-02-19T01:37:03","slug":"dark-matter-shining-from-the-dwarfs","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=900","title":{"rendered":"Dark Matter Shining from the Dwarfs"},"content":{"rendered":"
\n\tTitle:<\/strong>\u00a0Dark Matter Constraints from Observations of 25 Milky Way Satellite Galaxies with the Fermi Large Area Telescope<\/ br>
\n\tAuthor:<\/strong>\u00a0FERMI-LAT Collaboration<\/ br>
\n\tPublished<\/strong>:\u00a0Phys.Rev.\u00a0D89<\/strong>\u00a0(2014) 042001\u00a0[arXiv:1310.0828<\/a>]\n<\/div>\n

Dark matter (DM) is `dark’ because it does not directly interact with light. \u00a0We suspect,\u00a0however, that dark matter does interact with other\u00a0Standard Model (SM) particles such as\u00a0quarks and leptons.\u00a0Since these SM particles\u00a0do <\/em>typically interact with\u00a0photons,\u00a0dark matter is indirectly luminous.\u00a0More specifically, when two dark matter particles\u00a0find each other and annihilate, their products\u00a0include a spectrum photons that can be detected by\u00a0telescopes. For typical `weakly-interacting massive particle’ DM candidates, these photons are in the GeV (\u03b3-ray) range.<\/p>\n

\"If<\/a>
If dark matter interacts with the Standard Model, e.g. quarks, then its annihilation products include a spectrum of photons. Here we schematically show DM annihilating into quarks which shower into other colored `partons’ (quarks and gluons) that, in turn, become color-neutral hadrons. These then decay into light hadrons; the lightest of which (the neutral pion \u03c0) decays into two photons.\u00a0Image adapted from D. Zeppenfeld (PiTP 05 lectures).<\/figcaption><\/figure>\n

This type of\u00a0indirect detection<\/strong>\u00a0is a\u00a0powerful handle to search for dark matter in the galaxy.\u00a0The most promising place to search for these annihilation products are places where we expect a high density of dark matter, such as the galactic center. In fact, there have been recent hints for precisely this signal (see, e.g. this astrobite<\/a>).\u00a0Unfortunately, the galactic center is a very complicated environment with lots of other sources of GeV-scale photons that can make a DM interpretation tricky without additional checks.<\/p>\n

Fortunately, there\u00a0are other\u00a0galactic objects that are dense with dark matter and have relatively little stellar (visible) matter:\u00a0dwarf spheroidals<\/strong>. These\u00a0satellite galaxies of the Milky Way are\u00a0ideal laboratories for dark\u00a0matter annihilation. While they have less dark matter density than the galactic center, they also have far fewer\u00a0background photons from ordinary matter.\u00a0Our tool of choice is the space-based\u00a0Fermi-Large Area Telescope<\/a>\u00a0which is sensitive to photons between\u00a00.03 —\u00a0300 GeV and surveys the entire sky every three hours.<\/p>\n

\"Fig<\/a>
Map of known dwarf spheroidals over a ‘heat map’ of Fermi\u00a0gamma-ray data. Image from\u00a01310.0828<\/a>.<\/figcaption><\/figure>\n

The photon flux from\u00a0dark matter annihilation is a product of three factors:<\/p>\n

\"Photon<\/a>
Photon flux from DM annihilation.<\/figcaption><\/figure>\n

The “particle physics factor” describes the\u00a0dark matter properties: its mass and annihilation rate. The $latex dN_\\gamma\/dE_\\gamma$ factor describes the spectrum of photons coming from\u00a0the DM annihilation products. The “astrophysics” factor is a line of sight integral along the dark matter density $latex \\rho$. Note that the $latex \\rho^2$\u00a0from this factor and the \u00a0$latex m_\\chi^{-2}$ in the particle physics factor\u00a0is simply the dark matter number density; the\u00a0photon flux depends on how\u00a0likely it is for DM particles to find each other. The astrophysics factor is sometimes called a J factor.\u00a0For\u00a0some of the dwarfs\u00a0astronomers can determine the J factor based on the kinematics of\u00a0the [few] stellar objects in the dwarf spheroidal.<\/p>\n

One may use the morphology—or spatial distribution of dark matter—to help subtract background photons and fit\u00a0data.\u00a0For this\u00a0ParticleBite we won’t discuss this step further except to emphasize that these fits are where all the astrophysics “muscle”\u00a0enters. Each dwarf individually sets bounds on the dark matter profile, but one can\u00a0combine (or “stack”) these results\u00a0into a combined bound for\u00a0each\u00a0DM annihilation final state. The bounds differ\u00a0depending on these annihilation\u00a0products because\u00a0each type of particle produces a different spectrum of photons that must be re-fit relative to the background. The dark\u00a0matter mass\u00a0controls the\u00a0energy with which the ‘primary’ annihilation products are produced so that heavier dark matter masses yield more energetic photons.<\/p>\n

\"blahblah\"<\/a>
Combined dwarf spheroidal bounds on the annihilation cross section (roughly the\u00a0rate of DM annihilation) as a function of the dark matter mass for a\u00a0choice of\u00a0DM annihilation products. Image from\u00a01310.0828<\/a>.<\/figcaption><\/figure>\n

In the above plots,\u00a0the green and yellow bands represent the approximate expected 1\u03c3 and 2\u03c3\u00a0sensitivity while the\u00a0solid black line is the observed bound.\u00a0There is a\u00a0slight excess\u00a0at lower\u00a0masses, though the most optimistic excess in the b-quark channel has a significance of TS ~ 8.7, where TS is a ‘test statistic’ measure\u00a0introduced in the paper. The relevant comparison is that TS ~ 25 is the standard Fermi uses for a discovery, so this excess should be understood\u00a0to be fairly modest. (Note that the paper also notes that the\u00a0statistical analysis underestimates statistical significance so that if one were to convert this into\u00a0p<\/em>-values or\u00a0\u03c3, one would overestimate the significance.)<\/p>\n

Note further that the “stacked” analysis is most sensitive to those dwarfs with the largest J factors. Of these, half\u00a0showed an excess while the other half\u00a0were consistent with no excess.<\/p>\n

The most\u00a0important feature of the above plots is the horizontal dashed line. This line represents the dark matter annihilation cross section (“annihilation\u00a0rate”) that one\u00a0predicts based on the requirement\u00a0that\u00a0the observed dark matter density is set by this annihilation process. (There are ways around this, but it remains the simplest and most natural possibility.) The relevant bounds on the dark matter models, then, comes from looking at the point where the solid line and the dashed horizontal line meet.\u00a0Dark matter masses to the left (i.e. less than) this value are\u00a0disfavored in the\u00a0simplest\u00a0models.<\/p>\n

For example, for dark matter that annihilates to b-quarks,\u00a0one finds that the dwarf spheroidals\u00a0set a\u00a0lower limit on the dark matter mass of around 10 Gev. We note that this bound based on 4 years of Fermi data is\u00a0weaker<\/em> than the\u00a0previously published 2 year results due, in part, to\u00a0a revised analysis.<\/p>\n

The future?<\/strong>\u00a0A gamma ray excess\u00a0in the galactic center\u00a0(see, e.g.\u00a0this astrobite<\/a>) may\u00a0possibly be interpreted\u00a0as a signal of dark matter with mass of around 40 GeV annihilating into b<\/em> quarks. At the moment the dwarf spheroidal bounds are to weak to probe this region. Will it ever? Since Fermi samples the entire sky,\u00a0any newly identified dwarf spheroidal (e.g. from the Sloan Digital Sky Survey)\u00a0automatically makes the full 4 year dataset for that dwarf available. Since the\u00a0bounds scale like $latex \\sqrt{N}$ (in the DM mass range below 200 GeV), one may roughly estimate the future sensitivity to the 40 GeV mass range as requiring 16 times more data. If we consider the next 4 years (doubling the\u00a0observation time), this would require roughly\u00a04\u00a0times more dwarfs to be identified. (See, e.g. this talk<\/a> for a discussion.)<\/p>\n

Further\u00a0reading:\u00a0<\/strong>some useful references for indirect detection of dark matter<\/p>\n