{"id":3642,"date":"2016-06-17T00:52:53","date_gmt":"2016-06-17T00:52:53","guid":{"rendered":"http:\/\/www.particlebites.com\/?p=3642"},"modified":"2017-02-19T01:17:19","modified_gmt":"2017-02-19T01:17:19","slug":"respecting-your-elders","status":"publish","type":"post","link":"https:\/\/www.particlebites.com\/?p=3642","title":{"rendered":"Respecting your “Elders”"},"content":{"rendered":"

Theoretical physicists\u00a0have designed a new way\u00a0to explain the how dark matter interactions can\u00a0explain the observed amount of dark matter in the universe today.\u00a0This\u00a0elastically decoupling dark matter<\/strong> framework, is\u00a0a hybrid of\u00a0conventional and novel dark\u00a0matter\u00a0models.<\/p>\n

Presenting:<\/strong>\u00a0Elastically Decoupling Dark Matter
\nAuthors:<\/strong>\u00a0Eric Kuflik, Maxim Perelstein, Nicolas Rey-Le Lorier, Yu-Dai Tsai
\nReference:<\/strong>\u00a01512.04545<\/a>, Phys. Rev. Lett. 116, 221302 (2016)<\/a><\/div>\n

The particle identity of dark matter is one of the biggest open questions in physics. The simplest and most widely assumed\u00a0explanation is that dark matter is\u00a0a weakly-interacting massive particle (WIMP). Assuming that dark matter starts out in thermal equilibrium in the hot plasma of the early universe,\u00a0the present cosmic abundance of\u00a0WIMPs is set by the balance of two\u00a0effects:<\/p>\n

    \n
  1. When two WIMPs find each other, they can annihilate into ordinary matter. This depletes the number of WIMPs in the universe.<\/li>\n
  2. The universe is expanding, making it harder for WIMPs to find each other.<\/li>\n<\/ol>\n

    This process of “thermal freeze out” leads to an abundance of WIMPs controlled by the dark matter mass and interaction strength. The term ‘weakly-interacting massive particle’\u00a0comes from the observation that\u00a0dark matter of roughly the mass of the weak force particle\u00a0that interact through the weak\u00a0nuclear force\u00a0gives\u00a0the\u00a0experimentally measured dark matter density today.<\/p>\n

    \"Two<\/a>
    Two ways for a new particle, X, to produce the observed dark matter abundance: (left) WIMP annihilation into Standard Model (SM) particles versus (right) SIMP\u00a03-to-2 interactions that reduce the amount of dark matter.<\/figcaption><\/figure>\n

    More recently, physicists noticed that dark matter with very large\u00a0interactions with\u00a0itself<\/em> (not ordinary matter), can produce the\u00a0correct dark matter density in another way. These “strongly interacting\u00a0massive particle<\/a>” models\u00a0reduce\u00a0regulate the amount of dark matter through 3-to-2 interactions that reduce the total number of dark matter particles rather than annihilation into\u00a0ordinary matter.<\/p>\n

    The authors of\u00a01512.04545<\/a>\u00a0have proposed an intermediate road\u00a0that interpolates between these two types of dark matter. The “elastically decoupling dark matter” (ELDER) scenario. ELDERs have both of the above interactions: they can annihilate pairwise into ordinary matter, or sets of three ELDERs can turn into two ELDERS.<\/p>\n

    \"ELDER<\/a>
    Thermal history of ELDERs, adapted from 1512.04545.<\/figcaption><\/figure>\n

    The cosmic history of these ELDERs is as follows:<\/p>\n

      \n
    1. ELDERs are\u00a0produced in thermal bath\u00a0immediately after the big bang.<\/li>\n
    2. Pairs of ELDERS annihilate into ordinary matter. Like WIMPs, they\u00a0interact weakly with ordinary matter.<\/li>\n
    3. As the universe expands, the rate for annihilation into Standard Model particles\u00a0falls below the rate at which the universe expands<\/li>\n
    4. Assuming that the\u00a0ELDERs interact strongly amongst themselves, the 3-to-2 number-changing process still occurs.\u00a0Because this process distributes the\u00a0energy of 3 ELDERs in the initial state\u00a0to\u00a02 ELDERs in the final state, the two outgoing ELDERs have more kinetic energy: they’re hotter. This turns out to largely\u00a0counteract the effect of the expansion of the universe.<\/li>\n<\/ol>\n

      The\u00a0neat\u00a0effect here is\u00a0the abundance of ELDERs is actually set by the\u00a0interaction with ordinary matter, like WIMPs. However, because they have this 3-to-2 heating\u00a0period, they are able to produce\u00a0the observed present-day dark matter density\u00a0for very different choices of interactions. In this sense, the authors\u00a0show that this framework opens up a new “phase diagram” in the space of dark matter theories:<\/p>\n

      \"A<\/a>
      A “phase diagram” of dark matter models. The vertical axis represents the dark matter self-coupling strength while the horizontal axis represents the coupling to ordinary matter.<\/figcaption><\/figure>\n

      Background reading on dark matter<\/strong>:<\/p>\n