{"id":3370,"date":"2015-10-08T06:59:22","date_gmt":"2015-10-08T06:59:22","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3370"},"modified":"2017-02-19T01:19:29","modified_gmt":"2017-02-19T01:19:29","slug":"dark-photons-from-the-center-of-the-earth","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3370","title":{"rendered":"Dark Photons from the Center of the Earth"},"content":{"rendered":"
Presenting:<\/strong>\u00a0Dark Photons from the Center of the Earth
\nAuthor: <\/strong>J. Feng, J. Smolinsky, P. Tanedo (disclosure: blog post is by an\u00a0author on the paper)<\/small>
\nReference<\/strong>: arXiv:1509.07525<\/a><\/div>\n

Dark matter may be collecting in the center of the Earth.\u00a0A recent paper\u00a0explores way to\u00a0detect its decay products\u00a0here on the surface.<\/p>\n

\"Dark<\/a>
Dark matter may collect in the Earth and annihilate in to dark photons, which propagate to the surface before decaying into pairs of particles that can be detected by a large-volume neutrino detector like IceCube. Image from arXiv:1509.07525.<\/figcaption><\/figure>\n

Our entire galaxy is gravitationally held together by a\u00a0halo of dark matter<\/a><\/strong>, whose particle properties remain\u00a0one of the\u00a0biggest open questions in high energy physics.\u00a0One class of theories\u00a0assumes that the dark matter\u00a0particles interact through a dark photon<\/strong>, a hypothetical particle which mediates a force analogous to how\u00a0the ordinary photon mediates electromagnetism.<\/p>\n

These theories also permit the\u00a0\u00a0ordinary and dark photons\u00a0to have a small quantum mechanical mixing. This effectively means that the dark photon can\u00a0interact\u00a0very<\/em> weakly with ordinary matter and\u00a0mediate interactions between ordinary matter and dark matter—this gives a handle for ways to detect dark matter.<\/p>\n

While most methods for detecting dark matter focus on\u00a0building detectors that are\u00a0sensitive to the “wind” of dark matter bombarding (and mostly passing through) the Earth as the solar system zooms through the galaxy, the authors of\u00a01509.07525<\/a>\u00a0follow up on an idea initially proposed in the\u00a0mid-80’s:\u00a0dark matter hitting the Earth might get stuck in the Earth’s gravitational potential and build up\u00a0in its core.<\/p>\n

These dark matter particles can then find each other and annihilate. If they annihilate into very weakly interacting particles, then these may be detected at the surface of the Earth. A typical example is dark matter annihilation into neutrinos.\u00a0In\u00a01509.07525<\/a>, the authors examine the case where the dark matter annihilates into\u00a0dark photons, which can\u00a0pass through the Earth as easily as a neutrino and decay into pairs of electrons or muons near the surface.<\/p>\n

These decays can be detected in\u00a0large neutrino detectors, such as the IceCube<\/a> neutrino observatory (previously featured in\u00a0ParticleBites<\/a>).\u00a0In the case where the dark matter is very heavy (e.g. TeV in mass) and the dark photons are very light (e.g. 200 MeV), these dark photons are very boosted and their decay products point back to the center of the Earth. This is a powerful discriminating feature\u00a0against background cosmic ray events. \u00a0The number of signal events expected\u00a0is\u00a0shown\u00a0in the following contour plot:<\/p>\n

\"Number<\/a>
Number of signal dark photon decays expected at the IceCube detector in the plane of dark photon mixing over dark photon mass. Image from arXiv 1509.07525. Blue region is in tension with direct detection bounds (from ariv:1507.04007), while the gray regions are in tension with beam dump and supernovae bounds, see e.g. arXiv:1311.029.<\/figcaption><\/figure>\n

While similar\u00a0analyses for dark\u00a0photon-mediated dark matter capture by celestial\u00a0bodies\u00a0and annihilation have been studied—see e.g. Pospelov et al<\/a>., Delaunay et al.<\/a>, Schuster et al.<\/a>, and Meade et al<\/a>.—the authors of 1509.07525\u00a0focus on the case of dark matter capture in the Earth (rather than, say, the sun) and subsequent annihilation to dark photons (rather than neutrinos).<\/p>\n

    \n
  1. The annihilation rate at the\u00a0center of the Earth is greatly increased do to Sommerfeld enhancement: because the\u00a0captured dark matter has very low velocity, it is\u00a0much more likely to annihilate with\u00a0other\u00a0captured dark matter particles due to mutual\u00a0attraction from\u00a0dark photon exchange.<\/li>\n
  2. This causes the Earth to\u00a0quickly saturate with dark matter, leading to larger annihilation rates\u00a0than one would naively expect in the case where the Earth were not\u00a0yet dark matter\u00a0saturated such that annihilation and capture occur at equal rates.<\/li>\n
  3. In addition\u00a0using directional information to identify signal events against cosmic ray backgrounds,\u00a0the authors identified kinematic quantities—the opening angle of the Standard Model decay products and the time delay\u00a0between them—as ways to further discriminate signal from background. Unfortunately\u00a0their analysis implies that these features lie just outside of the IceCube sensitivity to them.<\/li>\n<\/ol>\n

    Finally, the authors point out the possibility of large enhancements coming from the so-called dark disk<\/strong><\/a>, an enhancement in the low velocity phase space density\u00a0of dark matter. If that is the case, then the\u00a0estimated reach may increase by an order of magnitude.<\/p>\n

     <\/p>\n

    Suggested further reading:<\/strong><\/p>\n