Respecting your “Elders”

Theoretical physicists have designed a new way to explain the how dark matter interactions can explain the observed amount of dark matter in the universe today. This elastically decoupling dark matter framework, is a hybrid of conventional and novel dark matter models.

Presenting: Elastically Decoupling Dark Matter
Authors: Eric Kuflik, Maxim Perelstein, Nicolas Rey-Le Lorier, Yu-Dai Tsai
Reference: 1512.04545, Phys. Rev. Lett. 116, 221302 (2016)

The particle identity of dark matter is one of the biggest open questions in physics. The simplest and most widely assumed explanation is that dark matter is a weakly-interacting massive particle (WIMP). Assuming that dark matter starts out in thermal equilibrium in the hot plasma of the early universe, the present cosmic abundance of WIMPs is set by the balance of two effects:

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

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’ comes from the observation that dark matter of roughly the mass of the weak force particle that interact through the weak nuclear force gives the experimentally measured dark matter density today.

Two ways for a new particle, X, to produce the observed dark matter abundance: (left) conventional WIMP annihilation into Standard Model (SM) particles versus (right) strongly-Interacting 3-to-2 interactions that reduce the amount of dark matter.
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 3-to-2 interactions that reduce the amount of dark matter.

More recently, physicists noticed that dark matter with very large interactions with itself (not ordinary matter), can produce the correct dark matter density in another way. These “strongly interacting massive particle” models reduce regulate the amount of dark matter through 3-to-2 interactions that reduce the total number of dark matter particles rather than annihilation into ordinary matter.

The authors of 1512.04545 have proposed an intermediate road that 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.

ELDER scenario
Thermal history of ELDERs, adapted from 1512.04545.

The cosmic history of these ELDERs is as follows:

  1. ELDERs are produced in thermal bath immediately after the big bang.
  2. Pairs of ELDERS annihilate into ordinary matter. Like WIMPs, they interact weakly with ordinary matter.
  3. As the universe expands, the rate for annihilation into Standard Model particles falls below the rate at which the universe expands
  4. Assuming that the ELDERs interact strongly amongst themselves, the 3-to-2 number-changing process still occurs. Because this process distributes the energy of 3 ELDERs in the initial state to 2 ELDERs in the final state, the two outgoing ELDERs have more kinetic energy: they’re hotter. This turns out to largely counteract the effect of the expansion of the universe.

The neat effect here is the abundance of ELDERs is actually set by the interaction with ordinary matter, like WIMPs. However, because they have this 3-to-2 heating period, they are able to produce the observed present-day dark matter density for very different choices of interactions. In this sense, the authors show that this framework opens up a new “phase diagram” in the space of dark matter theories:

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.
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.

Background reading on dark matter:

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Flip Tanedo

Assistant Professor at University of California, Riverside
Flip is an assistant professor in theoretical physics at the University of California, Riverside. He previousy completed a Bachelors of Science at Stanford, Masters degrees at Cambridge and the IPPP in Durham, and a Ph.D at Cornell. He has been supported by a Goldwater scholarship, a Marshall scholarship, an NSF Gradaute Research Fellowship, a Paul & Daisy Soros fellowship, and a UCI Chancellor's ADVANCE fellowship. He was a participant in the original Communicating Science 2013 workshop which led to the creation of ParticleBites. His research focuses on models and signatures of physics beyond the Standard Model, including dark matter, supersymmetry, and extra dimensions. Much of his creative thinking is done while swimming or driving along Southern California's freeways.

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2 Replies to “Respecting your “Elders””

  1. Wouldn’t the cross-section of the 3-to-2 interaction decrease due to expansion faster than the cross-section of SM freeze-out would? Naively one would think the former requires 3 wimps, so 3 cdm particles would have a harder time to find each other in an expaded universe than 2.

    1. Hi Dan, yes: there’s a balancing act. When the universe is hot, it’s not hard for three wimps to find each other since there’s a thermal background of them—that is, they pop in and out of the cosmic soup. As the universe cools below the mass of the wimps, then it indeed becomes very rare for three wimps to find each other.

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