Wobbly Dark Matter Signals at Cherenkov Telescopes from Long-Lived Mediator Decays

Imaging Atmospheric Cherenkov telescope (IACT) searches for dark matter often perform observations in “wobble mode,” i.e., simultaneously collecting data from the signal region and from a corresponding background control region. This observation strategy is possibly compromised in scenarios where dark matter annihilates to long-lived mediators that can traverse astrophysical distances before decaying to produce the gamma rays observed by the IACTs. In this Letter, we show that this challenge comes with several interesting features and opportunities: in addition to signal contamination in the background control region, the gamma-ray spectrum changes with the observing direction angle and typically exhibits a hard excess at high energies. Such features represent a significant departure from the canonical picture and offer novel handles to identify a dark matter signal and to extract underlying dark matter parameters.

Figure 1

Schematic representation of the scenario studied in this Letter. The telescope performs observations in wobble mode, where the field of view contains two symmetric circular regions: the signal (ON) region, containing the signal source (here, a dwarf spheroidal galaxy), and a background control (OFF) region. We study the effects of dark matter annihilation in the ON region into long-lived mediators, $\varphi$, that can propagate to and decay in the OFF region.

Figure 2

The density of mediator particles as a function of $\theta$, normalized to 1 at the center of Draco ($\theta =0$), for instantaneous decay of the mediator in red, and decay lengths $l_d=0.1$, 1, 10 kpc in black, green, and blue, respectively, using $r_{\text{cutoff}}=0.05\mathrm{kpc}$. The green and gray bands represent the optimized ON and OFF regions ($w_{\text{opt}}=0.6°$, ${\theta }_{\text{opt}}=0.3°$) for CTA observations of Draco [8].

Figure 3

Contours of the ratio $\mathrm{\Phi }(E_{\gamma },\theta )/\mathrm{\Phi }(E_{\gamma },\theta =0)$, i.e., flux of photons of energy $E_{\gamma }$ from direction $\theta$ relative to the center of Draco, normalized to the flux from the dwarf center, for $m_{\chi }=1\mathrm{TeV}$, $m_{\varphi }=400\mathrm{GeV}$, $l_d=10\mathrm{kpc}$, and $r_{\text{cutoff}}=0.5\mathrm{kpc}$.

Figure 4

Signal from the ON region (green), OFF region (red), and the resulting background subtracted (ON-OFF) signal (black), with an arbitrary overall normalization, for $m_{\chi }=1\mathrm{TeV}$, $m_{\varphi }=400\mathrm{GeV}$, $l_d=10\mathrm{kpc}$, and $r_{\text{cutoff}}=0.5\mathrm{kpc}$. The dashed, solid, and dotted curves show results for $w_{\text{opt}}=0.5°,0.6°,0.7°$, respectively.

Figure 5

Same as Fig. 4, but with CTA instrumental effects taken into account (see text for details). Also shown (blue dotted curve) is the expected shape of the background (arbitrary normalization).