Dark Matter from the Sky

Another highlight of the ongoing ENTApP Dark Matter Workshop was the session devoted to indirect dark matter detection. So far, dark matter has been observed only via the gravitational attraction it exerts. However, according to the most popular scenario, dark matter was once in a thermal equilibrium with the ordinary matter. If this is true, there must be some interactions between the two, which we could in principle detect. There are three complementary approaches one can try:

1. Colliders. Massive, weakly interacting particles produced in collisions could leave missing energy signatures.
2. Direct detection. A flux of dark matter particles going through the Earth could trigger a signal in an underground detector.
3. Indirect detection. Radiation produced in dark matter annihilations could produce characteristic astrophysical signatures.

The last point was discussed last thursday in a session led by Gianfranco Bertone. We were offered a beautiful survey of different strategies to observe a signal from dark matter in the sky. Although, on average, the dark matter annihillation rate is small today (obviously, dark matter is no longer in a thermal equilibrium), there could be overdense regions where it is large enough to produce a tell-tale signal. The observers thus turn their attention to the places where dark matter could clump: the galaxy center, black holes or the center of the Sun.

How can we see dark matter? First of all, with our own eyes, or a slightly enhanced version thereof. By detecting photons. There are several ways dark matter annihilation can produce photons. First, dark matter could directly annihilate into a photon pair. This would produce a beautiful sharp line in the gamma-ray spectrum, at the energy corresponding to the mass of the dark matter particle. A smoking gun, indeed. The problem is that dark matter is dark and does not couple directly to photons. Any such process would proceed via quantum loops and could be too much suppressed. Therefore we should keep our eyes open to other signatures. Another process to focus on is the secondary radiation: decays of the annihilation products (pions, for example) into photons. This produces a featureless spectrum that is harder, but not impossible to distinguish from the astrophysical background. Finally, there is the final state radiation when charged particles produced in the annihilation radiate photons. All this is subject to active experimental studies. Our galaxy center was recently probed by the satellite gamma-ray observatory EGRET and the ground based gamma-ray telescope HESS. More accurate measurements in the 10MeV to 100 GeV band will be performed by GLAST, whose launch is scheduled for this autumn.

The photon window is not the only one to look through. Much hope is put in neutrino experiments. Dark matter is expected to accrete on the centre of the Sun and annihilate into highly energetic neutrinos. Those can escape the Sun and be detected by neutrino telescopes like AMANDA or ICE-CUBE who can distinguish them from MeV neutrinos copiously produced in nuclear reactions. Positrons and antiprotons from the galactic halo could also be messengers of the dark side.

The problem with astrophysics is that it is a dirty business. In fact, several experiments have already detected signals above the estimated background. HEAT observed an excess of positrons at 10 Gev, see the figure. EGRET and HESS report an excess of gamma-rays from the galactic center. INTEGRAL sees a 511 keV line signifying a suspicious positron activity in the galactic center. These signals can be interpreted as a hint of dark matter (needless to say, different experiments point to a dark matter particle of different mass). But just as well they can be interpreted as a hint of complicated astrophysical dynamics that has not been understood well enough. Because of that, indirect detection will hardly ever be considered as an evidence for dark matter, unless a very clean signal is observed. But combined with collider studies and theoretical models, it can provide important hints about the nature of dark matter and its distribution in our galaxy.

There is a chapter on indirect detection in a Phys.Rept review by Gianfranco&co.