Top-Flavoured Dark Matter in Dark Minimal Flavour Violation

We study a simplified model of top-flavoured dark matter in the framework of Dark Minimal Flavour Violation. In this setup the coupling of the dark matter flavour triplet to right-handed up-type quarks constitutes the only new source of flavour and CP violation. The parameter space of the model is restricted by LHC searches with missing energy final states, by neutral $D$ meson mixing data, by the observed dark matter relic abundance, and by the absence of signal in direct detection experiments. We consider all of these constraints in turn, studying their implications for the allowed parameter space. Imposing the mass limits and coupling benchmarks from collider searches, we then conduct a combined analysis of all the other constraints, revealing their non-trivial interplay. Especially interesting is the combination of direct detection and relic abundance constraints, having a severe impact on the structure of the dark matter coupling matrix. We point out that future bounds from upcoming direct detection experiments, such as XENON1T, XENONnT, LUX-ZEPLIN, and DARWIN, will exclude a large part of the parameter space and push the DM mass to higher values.

NP interaction production channel.

Figure :
Cross section for tt final state, mixing angles set to zero. [ATLAS collaboration '14] Dλ The phenomenologically interesting region is mχ ≤ 1 TeV.

Constraints from SUSY Searches at LHC
Too large couplings D λ,ii would exclude nearly all of parameter space.

Flavour Constraints from Neutral Meson Mixing
[UTfit collaboration '14] No mesons with top quark are possible, the only constraints come from D mesons.
⇒ not too strong The NP contribution has to be smaller than experimental bounds.

DM Constraints from Observed Relic Abundance
Depending on mass splitting several freeze out scenarios are possible.
If DM mass is below top mass several channels drop out.

⇒ different impact on parameters
Co-annihilation has to be just as large as to produce the correct relic density.

Combined Analysis of Constraints
A combination of relic abundance and direct detection constraints confine Θ 13 to a narrow interval around the "perfect" cancellation point.
The lower and upper bounds on the DM mass become more serious, since the parameters do not only have to fulfill relic abundance constraints.
The combined analysis clearly prefers top flavoured DM.   [ATLAS collaboration '14] Study the process pp → φφ → qqχχ.

Constraints from SUSY Searches at LHC
Depending on decay product of φ we detect either a top signature or a jet (+ E T ).

Inspiration from SUSY searches at LHC
⇒ Upper bounds on CS of both tt and dijet signals. Stronger exclusion bounds on model.
Too large couplings D λ,ii would exclude nearly all of parameter space.
Most serious constraints come from dijet final state.
⇒ Safe parameter space:

Influence of Mixing Angles on LHC production
Mixing angles shift influences between couplings D λ,ii .
⇒ For big splitting in the couplings, mixing angles can cause big shifts in cross sections.
For our choice of m φ bounds from tt final state cause no constraints.
⇒ Mixing angles can cause no problem with this choice of safe parameter space.

Flavour Constraints from Neutral Meson Mixing
[UTfit collaboration '14] No mesons with top quark are possible, the only constraints come from D mesons.
⇒ not too strong The NP contribution has to be smaller than experimental bounds.
The higher the splitting ∆ ij = D λ,ii − D λ,jj , the more we will see the constraints on the mixing angle θ ij .  Coannihilation CS has to be just large enough to produce the correct relic density (we allow for a 10% tolerance interval): σv eff ,exp = 2.2 × 10 −26 cm 3 /s.

DM Constraints from Observed Relic Abundance
Depending on the mass splitting of the different DM flavours several freeze out scenarios are possible.
For a DM mass below the top quark mass this decay channel drops out.
⇒ CS formula and hence impact on parameters can be quite different Extreme example: only χ t present at freeze out with DM mass below top mass threshold:

Quasi Degenerate Freeze Out (QDF) Szenario
All DM flavours are present at the freeze out.
We require the mass splitting to be less than 1% (significantly smaller than Tf ) for this to happen.
This guarantees top flavoured DM (see direct detection section for motivation).
Constraint cuts out valid area for D λ,ii depending on m φ and mχ.
Lower bound on mχ due to upper limits for D λ,ii , depending on m φ .

Figure :
Valid points in quasi degenerate freeze out scenario.

Single Flavour Freeze Out (SFF) Szenario
Only mχ present at freeze out.
We require the mass splitting to be more than 10% (significantly bigger than Tf ) for this to happen.
This guarantees top flavoured DM (see direct detection section for motivation).
Constraint cuts out valid area of parameters depending on m φ and mχ, with significant effect on mixing angles.
In addition to lower bound, we also find an upper bound on mχ due to upper and lower (from mass splitting condition) limits for D λ,ii , depending on m φ .

DM Bounds from Direct Detection Experiments
Many contributions to total WIMPnucleon cross section:

DM Bounds from Direct Detection Experiments
[LUX collaboration '15] All contributions have to combine to a WIMP-nucleon cross section below the LUX bounds.
All contributions are positive, only the Z-penguin with the neutron is negative ⇒ saves the day. Largest contribution comes from tree level process. Largest negative term is hence interference term of tree level and neutron Z-penguin.
⇒ serious constraints on θ13 For higher couplings the cancellation gets more complicated.
For too large couplings the cancellation is no longer possible at all → excluded.
Top-flavoured DM is the natural choice: ⇒ Tree level contribution small ⇒ Neutron Z-penguin contribution large.