Exothermic double-disk dark matter

Anniversary: On July 16th, not only my namesakes had their name day ;-) but we also celebrated the 68th anniversary of the Trinity Test, the beginning of the nuclear age. See Wikipedia, YouTube, The Bulletin. The concentration and speed of the deaths was scary during the following month or so but the event finally saved millions of lives during the (shortened) World War II as well as the 68 years that followed.

This blog entry is about a very similar topic as the previous one; the paper released today also tries to incorporate the hints from the dark matter direct search experiments, both positive and negative ones. However, in their

Exothermic Double-Disk Dark Matter,

Matthew McCullough of MIT and Lisa Randall of Harvard adopt a very different philosophy about the inner structure of dark matter. It isn't composed of individual structureless particles that arise from some symmetries. Instead, in agreement with two 2013 papers that Lisa co-wrote, this dark matter may be composed of rather symmetry-uninspired particles but what makes them special is that they are subject to internal interactions, they live almost just like the visible baryonic matter we know.




The type of dark matter they propose is DDDM, the double-disk dark matter. A more accurate name of ExoDDDM which combines the virtues of DDDM with the exothermic dark matter is the directly detectable, decomposable, dominantly deexcitable, doubly degenerate, delta-depositing double-disk da-da-da [2010] dark matter or DDDDDDDDDDDDDDDM for short. I guess that this acronym by itself will triple the number of readers who will actually open the paper. ;-)




DDDM refers to "double disks" because unlike ordinary dark matter that forms halos, DDDM tends to form disks similar to the galactic disks of the visible matter we know (stars in the Milky Way) which is only possible because of the mutual interactions between the dark matter particles – they may lose energy while keeping the angular momentum, and therefore rearranging themselves into disks.

When there are interactions inside dark matter, it's also possible to imagine that the dark matter particles come in excited states. If they're long-lived, we may imagine that the extra excitation energy is released when this composite dark matter collides with the nucleons, e.g. in the underground experiments.



Kraftwerk (The Power Plant Band) musically explain that the excitation energy of ExoDDDM may contribute to the observed radioactivity.

You see that there are some parameters here that may be adjusted or fudged. But the nice part of the story is that such a model of ExoDDDM, combining the ideas from both previous paragraphs, may explain the observations from both sides of the dark matter wars. CDMS-silicon events are OK while xenon remains largely blind to ExoDDDM – recall the apparent xenophobia of the dark matter. You will have to read the paper to see why the visibility of ExoDDDM is lower when xenon is used.

An animate, complex dark matter sector is surely a cute idea, something that may be considered natural for many reasons. On the other hand, simple isolated dark matter particles may be viewed as less analogous to the visible matter but more minimal and more economic. I think that we don't have enough data – or sufficiently deep ideas – to reliably pick between these two paradigms.

On the other hand, I would probably bet against the validity of Lisa's and Matthew's particular model. It just seems to add more new fields and parameters than the number of extra features or experimental facts that it explains. The ExoDDDM dark sector requires one to add a new unbroken, long-range \(U(1)_D\) gauge group along with a broken \(U(1)'\) and four fields with the following charges:\[

\begin{array}{c|rrrr}
& C & Y_1 & Y_2 & H'\\
\hline
U(1)' & 0 & 1 & -1 & 2\\
U(1)_D & 1 & 1 & 1 & 0
\end{array}

\] along with the natural kinetic and interaction terms for these fields. The terms are natural but the parameters to adjust are numerous:\[

\eq{
\LL &= \LL_{\rm SM} + \LL_{\rm Kin} +\epsilon' F'_{\mu\nu}F^{\mu\nu}+\delta m^2 Z'_{\mu}Z^\mu - V(H_D,H^*_D)-\\
&- m_C \bar C C - m_Y (\bar Y_1 Y_1+\bar Y_2 Y_2) - (\lambda H' \bar Y_1 Y_2+{\rm h.c.})
}

\] Try to count these parameters. So the animate dark matter paradigm seems to lead to non-robust models and it's somewhat hard to fall in love with a specific one, like this one.

So although the dark matter sector may be given a similar a priori opportunity to form complex structures just like the visible sector, Nature doesn't have a duty to serve this entitlement. It is perfectly possible that interaction-free dark matter is preferred. Different parts of the collection of fields in Nature may have very different fates. We know it's true. For example, in the \(E_8\times E_8\) heterotic string vacua, only one of the two \(E_8\) factors is broken to GUT and then to the Standard Model group. It is perfectly possible that we live in a heterotic world where the other \(E_8\) remains unbroken and its gaugino condensate sparks SUSY breaking instead. Different parts of the physical world may play – and usually do play – different roles so I surely don't think it is our "duty" to think that the dark sector must be fully analogous to the visible one.