Research: Dark Matter
The nature of Dark Matter (DM) and also of Dark Energy in the Universe
are among the most exciting questions of astroparticle physics. Dark
Matter and Dark Energy are actually the main constituents of the
Universe as shown in Figure 1. The evidence for DM is based on
the gravitational effects of this new form of matter, for example on
the motion of visible stars, dust, or gas in galaxies (including the
Milky Way), the motion of galaxies in clusters, the evolution of
matter structure in the Universe, the temperature fluctuations in the
cosmic microwave background, or Big Bang nucleosynthesis. A very
active on-going experimental research program has a good potential for
significant discoveries in the coming decade.
Figure 1: The composition of the known
Universe: shown are the contributions from Dark
Energy, Dark Matter and ordinary matter made of
Standard Model particles.
Dark Matter is most likely composed of some type of particle which
does not interact electromagnetically or strongly. However, it is a
well motivated assumption that Dark Matter has in addition to gravity
also some other (presumable weak) interaction with normal matter,
i.e. is a Weakly Interacting Massive Particle (WIMP). Such particles
are predicted by many extensions of the Standard Model, and in this
case the abundance of Dark Matter particles is predicted to be in
roughly the range required by observations due to thermal production
in the early Universe.
The WIMP hypothesis can be tested with three complementary methods:
(a) search for the interaction of DM particles in underground
detectors (``direct detection''), (b) search for the products of
annihilations or decays of DM particles e.g., inside the sun, or at
the centre of our galaxy (``indirect detection''), and (c) direct
production of DM particles at high energy particle collider
experiments. In a given model the strength of the signals in these
three observational chanels can be related in principle, as
illustrated in Figure 2. In all three of these avenues
substantial experimental progress is expected within the next few
years. The group performs phenomenological research relating possible
signals, exploring the complementarity of the various observational
channels, and constructing models able to explain all available data
on Dark Matter.
Figure 2: Interaction of WIMP dark matter with
Standard Model particles. In a given model for the cross sections
relevant for thermal production in the early Universe, for indirect
detection, for direct detection, and for production in collider
experiments are related to each other.
Many new projects for direct DM detection experiments are under way or
in preparation (e.g., CDMS-II, XENON100, COUPP, PICASSO, ZEPLIN). The
next generation of experiment aims at ton-scale detectors (e.g.,
SuperCDMS, EURECA, XENON, LUX, ArDM, WARP, DEAP). These experiments
will improve the sensitivity for WIMP cross sections by 4-5 orders of
magnitude with respect to current bounds, and cover most of the
interesting parameter space of common DM models. In particular,
Supersymmetric DM will be crucially tested. Within the group analyses
of available data from DM direct detection experiments are performed,
as well as prospective sensitivity studies for future experiments are
carried out. This is done in close contact with the XENON group at in
the same division.
In indirect DM detection various techniques are explored to
look for annihilation or decay products of DM, including cosmic ray
experiments on satellites (e.g., EGRET, PAMELA, FERMI-LAT) or on balloon
flights (e.g., ATIC, HEAT, PPB-BETS), atmospheric Cerenkov telescopes (HESS,
MAGIC), or neutrino telescopes (SuperKamiokande, IceCube, ANTARES, KM3NET).
Various signatures can be explored, such as high energetic anti-matter
particles (positrons, anti-protons, anti-deuterons), gamma rays, radio
emission, or high energy neutrinos. Many of these experiments are
expected to deliver results within the next few years.
The Large Hadron Collider (LHC) at CERN is at the focus of the
high-energy physics community. This experiment is expected to shed
light on physics at the energy scale of several 100~GeV, responsible
for electro-weak symmetry breaking. Several arguments suggest that
physics beyond the Standard Model should show up at that scale. Many
models for this new physics (such as, e.g., Supersymmetry) offer a
candidate particle for DM. Therefore, one may hope that results from
LHC will provide another piece of information on the DM
problem. Typically, DM will show up as missing energy in decay chains
of heavy particles. This signature per se is rather indirect and it
will be necessary to relate possible findings of this kind at LHC to
signals from direct or indirect DM detection experiments, aiming at
consistent interpretations in terms of DM by taking into account all
available information from non-accelerator experiments.
An important approach in the field of DM will be to combine data from
direct searches, indirect detection, and collider experiments. This
will be essential in order to gain confidence in a DM interpretation
of any positive signal. All three experimental avenues towards DM
identification will provide rather indirect and vague information.
Therefore, exploring complementary data and performing consistency
checks will be necessary. In a specific model it might be possible to
obtain definite predictions for the various signatures of DM, and such
models might be identified (or falsified) through combined analyses.
Apart from WIMPs, there are also other very promising candidates for
Dark Matter, such as axions, keV sterile neutrinos, gravitinos and
others. In these cases experimental signatures can be very different,
and the phenomenology of such candidates is explored within the group.
Theoretical work in the field includes construction of models which
predict the various dark matter candidates. The corresponding research
covers classical WIMP candidates with new physics at the electroweak
scale, as well as non-WIMP candidates. The ultimate goal will be to
obtain a consistent theory beyond the Standard Model, which provides a
DM particle in agreement with all available data.