Signatures of top flavored dark matter

We study the experimental signatures of top flavored dark matter (top FDM) in direct detection searches and at the LHC. We show that for a dark matter mass above 200 GeV, top FDM can be consistent with current bounds from direct detection experiments and relic abundance constraints. We also show that next generation direct detection experiments will be able to exclude the entire perturbative parameter region for top FDM. For regions of parameter space where the flavor partners of top FDM are not readily produced, the LHC signatures of top FDM are similar to those of other models previously studied in the literature. For the case when the flavor partners are produced at the LHC, we study their impact on a search based on transverse mass variables and find that they diminish the signal significance. However, when the DM flavor partners are split in mass by less than 120–130 GeV, the LHC phenomenology becomes very distinctive through the appearance of displaced vertices. We also propose a strategy by which all parameters of the underlying model can be experimentally determined when the flavor partners can be observed.

Figure 1

Feynman diagrams that contribute to the effective coupling to $\gamma /Z$ in the top FDM model. The vector boson lines in the loop diagrams can be attached to either the top quark or to the mediator $\varphi$ running in the loop.

Figure 2

The LUX bound on the coupling $\lambda$ for $m_{\varphi }=500\mathrm{GeV}$ calculated using the effective $Z$ coupling and charge term, with and without the addition of the dipole term.

Figure 3

The LUX bound on the coupling $\lambda$ for $m_{\varphi }=500\mathrm{GeV}$ calculated using the effective $Z$ coupling and charge term, with and without the addition of the effective coupling to $u\text{−}\overline{u}$.

Figure 4

The LUX exclusion region in the $\lambda \text{−}{m}_{\chi }$ plane ($\lambda \text{−}{m}_{\varphi }$ plane) for $m_{\varphi }=500\mathrm{GeV}$ ($m_{\chi }=200\mathrm{GeV}$). The correct relic abundance is obtained on the blue curve.

Figure 5

The LUX exclusion bounds in the $m_{\chi }\text{−}{m}_{\varphi }$ plane for $\lambda =0.5$ (green/darker shaded region) and $\lambda =0.6$ (blue/lighter shaded region). For each case, we also show the curve on which the correct relic abundance is obtained.

Figure 6

The current LUX exclusion bounds in the $m_{\chi }\text{−}{m}_{\varphi }$ plane (green/darker shaded region), where $\lambda$ is varied to give the correct relic abundance at each point. Contours of constant $\lambda$ are also shown for reference. The full parameter region in this plot (and beyond) can be excluded by the XENON1T experiment (blue/lighter shaded region).

Figure 7

Feynman diagrams for the pair production of the mediator $\varphi$ at the LHC.

Figure 8

$\varphi$-pair production cross section for $\lambda =0,0.5,1$ with $m_{{\chi }^{\prime }}=300\mathrm{GeV}$.

Figure 9

Feynman diagrams for $\varphi \text{−}{\chi }^{\prime }$ associated production and ${\chi }^{\prime }$-pair production at the LHC.

Figure 10

Top: $\varphi \text{−}{\chi }^{\prime }$ associated production cross section for $\lambda =1$ and two values of $m_{{\chi }^{\prime }}$. $\sigma$ scales like ${\lambda }^2$. Bottom: ${\chi }^{\prime }$-pair production cross section for $\lambda =1$ and two values of $m_{\varphi }$. $\sigma$ scales like ${\lambda }^4$.

Figure 11

The short and long decay modes for $\varphi$.

Figure 12

Branching ratio of the short decay mode of $\varphi$ as a function of the ratio $m_{\chi }/m_{{\chi }^{\prime }}$ for several values of $m_{{\chi }^{\prime }}$. When $m_{\varphi }-m_{\chi }$ drops below $m_t$, the short decay mode can only proceed through off-shell tops and therefore the branching ratio becomes negligible.

Figure 13

$M_{T2}$ histogram of background and the signal point $(m_{\varphi },m_{{\chi }^{\prime }},m_{\chi })=(500,400,200)\mathrm{GeV}$ with $\lambda =0.5$. The vertical line shows the best $M_{T2}$ cut. Background histograms are stacked while the signal spectrum is shown separately.

Figure 14

The decay length $c\tau$ of ${\chi }^{\prime }$ as a function of $m_{{\chi }^{\prime }}$ for fixed values of $m_{\varphi }=500\mathrm{GeV}$, $m_{\chi }=200\mathrm{GeV}$, and $\lambda =0.5$. The results at very low mass splitting should be regarded as an idealization only, since in this regime deviations from MFV can cause flavor-changing ${\chi }^{\prime }$ decays to become the dominant decay channel. The decay length changes by many orders of magnitude as $m_{{\chi }^{\prime }}-m_{\chi }$ goes through the $m_t$ and $m_W+m_b$ thresholds. The range in which the LHC experiments are expected to be most sensitive to displaced decays is indicated as a gray-shaded horizontal band.

Figure 15

The leading contribution to ${\chi }^{\prime }$ decay which is flavor preserving, without a top quark in the final state.

Figure 16

The ${\chi }^{\prime }$ enrichment factor, defined as the enhancement of the fraction of signal events containing at least one ${\chi }^{\prime }$ after demanding that both $b$-tagged jets in the event have $p_T<X$, with $X$ plotted on the $x$ axis. The red curve is obtained from a signal spectrum with $m_{{\chi }^{\prime }}-m_{\chi }<m_t$ while the green curve is obtained from a signal spectrum with $m_{{\chi }^{\prime }}-m_{\chi }>m_t$. As described in the main text, the ${\chi }^{\prime }$ content of the former signal point can be enhanced using such a cut with a low $X$ because ${\chi }^{\prime }$ decays when off-shell tops give rise to softer $b$-jets, whereas for the latter signal point when on-shell tops, the $b$-jets tend to be harder.