[BDXlist] Inelastic DM Projections

[Gordan Krnjaic]

Hi Everyone,

To have a more productive discussion today, I want to show you some
projections for the inelastic scenario Eder discussed during the last
meeting.  For this scenario, the dark photon couples off-diagonally to chi1
and chi2 which are split in mass by some small amount -- the phenomenology
for this scenario is presented here https://arxiv.org/abs/1403.6826.

The key difference for our purposes is that the signal is not the recoil of
the target, but the detector-scale de-excitation of the excited state chi2
> chi1 e+e- where the e+/e- pair deposits ~ GeV scale energies inside the
detector for every scattering+de-excitation event, so the 3 event contours
are relevant here. Furthermore, the target serves merely to induce the
transition, so the scattering rates can be integrated all the way down to
E_recoil ~ 0 which greatly enhances the signal.

I should add that this scenario is just as generic as the elastic version
we're more familiar with. It has exactly the same field content and is more
general in some sense because there is no reason a-priori why the DM should
couple diagonally to the dark photon. Since the dark symmetry is broken and
the A' has a mass, both Dirac and Majorana mass terms are allowed for the
DM -- if both kinds are present, this interaction is obligatory.

Combined with the g-2 motivation plot (where we close most of the remaining
parameter space that's compatible with the dark-photon explanation (either
visibly or invisibly decaying) I think our case is still strong since the
effort has multiple well-defined targets to pursue -- incidentally, this is
no longer the case for the visibly decaying dark photon searches that
remain very popular (and funded!) .

There are some caveats which I hope to discuss during the meeting.

Cheers,
Gordan
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