A couplet from flavored dark matter
JHE
A couplet from flavored dark matter
Prateek Agrawal 2
Zackaria Chacko 0
Can Kilic 1
Christopher B. Verhaaren 0
0 Maryland Center for Fundamental Physics, Department of Physics, University of Maryland , College Park, MD, 20742-4111 U.S.A
1 Theory Group, Department of Physics and Texas Cosmology Center, The University of Texas at Austin , 2515 Speedway Stop C1608, Austin, TX, 78712-1197 U.S.A
2 Fermilab , P.O. Box 500, Batavia, IL, 60510 U.S.A
We show that a couplet, a pair of closely spaced photon lines, in the X-ray spectrum is a distinctive feature of lepton flavored dark matter models for which the mass spectrum is dictated by Minimal Flavor Violation. In such a scenario, mass splittings between different dark matter flavors are determined by Standard Model Yukawa couplings and can naturally be small, allowing all three flavors to be long-lived and contribute to the observed abundance. Then, in the presence of a tiny source of flavor violation, heavier dark matter flavors can decay via a dipole transition on cosmological timescales, giving rise to three photon lines. Two of these lines are closely spaced, and constitute the couplet. Provided the flavor violation is sufficiently small, the ratios of the line energies are determined in terms of the charged lepton masses, and constitute a prediction of this framework. For dark matter masses of order the weak scale, the couplet lies in the keV-MeV region, with a much weaker line in the eV-keV region. This scenario constitutes a potential explanation for the recent claim of the observation of a 3.5 keV line. The next generation of X-ray telescopes may have the necessary resolution to resolve the double line structure of such a couplet.
1 Introduction
3.1 Mass splittings
3.2 Relic abundance
3.3 Direct detection
3.4 Indirect detection
3.5 The couplet
3.6 The 3.5 keV line 4 Conclusions
Introduction
Increasingly precise cosmological measurements indicate that about 80% of the matter
density of the universe is composed of particles that are non-baryonic, and neutral under
both color and electromagnetic interactions. However, the precise nature of this dark
matter (DM) remains a mystery. One theoretically appealing possibility is that DM is
composed of Weakly Interacting Massive Particles (WIMPs), particles with mass of order
the weak scale that have interactions of weak scale strength with the standard model
(SM) fields. This scenario is compelling because, provided the WIMPs were in thermal
equilibrium with the SM at early times, just enough of them survive today as thermal
relics to account for the observed DM density.
While the WIMP framework requires that DM have interactions of weak scale strength
with the SM fields, efforts to produce it at high energy colliders have, so far, proven fruitless.
Likewise, efforts to directly detect DM in the laboratory through its scattering off nucleons,
in spite of the increased sensitivity of current experiments, have also been unsuccessful.
There are some tentative hints from indirect detection of DM annihilation to SM today,
but there is no conclusive signal. In the wake of these null results, DM scenarios that retain
the cosmological success of the WIMP framework while satisfying the current experimental
bounds have become increasingly compelling.
The matter fields of the SM (Q, U c, Dc, L and Ec) are known to come in three copies,
or flavors. Different flavors carry the same charges under the SM gauge groups, but have
different couplings to the Higgs, and so differ in their masses. One interesting possibility,
which has been receiving increased attention, is that DM, like the SM matter fields, also
comes in three flavors [1–11] or has flavor-specific couplings to the SM [12–15]. Specific
DM candidates of this type include sneutrino DM in supersymmetric extensions of the
SM [16], and Kaluza-Klein neutrino DM in theories with a universal extra dimension.
In [2], the simplest theories of flavored DM (FDM) were classified, and labelled as
lepton flavored, quark flavored or internal flavored, based on the form of the interactions of
the DM candidate with the SM fields. Models in which the DM has tree level interactions
with the SM leptons but not with the quarks — as in lepton FDM — can naturally
account for the observed abundance of DM while remaining consistent with all experimental
bounds [17–19]. The reason is that strong production at a hadron collider or scattering off
a nucleus rely on DM-quark interactions, which are loop suppressed in this scenario. In
addition, because the average number of photons generated in DM annihilation to hadrons
is much larger than in the case of DM annihilation to leptons, the limits from indirect DM
searches using gamma rays are also weaker. Some other indirect signals of DM annihilation,
such as the positron flux, are enhanced for lepton FDM, but regions of parameter space
for which the DM is a thermal relic remain viable.
In general the couplings of lepton FDM viol (...truncated)