The 2008 DESY Theory Workshop focused on the nature of dark matter.
La recherche sur la matière noire arrive à un tournant
De nombreux indices montrent que l’Univers contient de la matière noire
composée de particules élémentaires inconnues, mais ces observations
laissent subsister bien des interrogations. Quelle est la nature microscopique de cette
matière noire non baryonique ? Jusqu’à quel point est-elle sombre,
froide et stable ? C’est autour de ces questions que s’est articulé l’atelier
de théorie 2008 de DESY « Dark matter at the Crossroads ». Alors que
la recherche sur la nature microscopique de la matière noire entre dans une phase
déterminante, l’atelier s’est attaché à rassembler les
résultats d’un grand nombre d’expériences avant de les confronter aux
There is overwhelming evidence that the universe contains dark matter made from
unknown elementary particles. Astronomers discovered more than 75 years ago that
spiral galaxies, such as the Milky Way, spin faster than allowed by the gravity of known
kinds of matter. Since then there have been many more observations that point to the
existence of this dark matter.
Gravitational lensing, for example, provides a unique probe of the distribution of
luminous-plus-dark matter in individual galaxies, in clusters of galaxies and in the
large-scale structure of the universe. The deflection of gravitational light depends
only on the gravitational field between the emitter and the observer, and it is
independent of the nature and state of the matter producing the gravitational field, so
it yields by far the most precise determinations of mass in extragalactic astronomy.
Gravitational lensing has established that, like spiral galaxies, elliptical galaxies
are dominated by dark matter.
Strong evidence for the fact that most of the dark matter has a non-baryonic nature
comes from the observed heights of the acoustic peaks in the angular power spectrum of
the cosmic microwave background measured by the Wilkinson Microwave Anisotropy Probe,
because the peaks are sensitive to the fraction of mass in the baryons. It turns out that
only about 4% of the mass of the universe is in baryons, whereas about 20% is in non-
baryonic dark matter – a finding that is also in line with inferences from
A host of candidates
This leaves some pressing questions. What is the microscopic nature of this non-
baryonic dark matter? Why is its mass fraction today about 20%? How dark is it? How cold
is it? How stable is it?
Progress in finding the answers to such questions provided the focus for the 2008 DESY
Theory Workshop, which was held on 29 September – 2 October.
Organized by Manuel Drees of Bonn, it sought to combine results from a range of experiments
and confront them with theoretical predictions. It is clear that the investigation of the
microscopic nature of dark matter has recently entered a decisive phase. Experiments are
being carried out around the globe to try to identify traces of the mysterious dark-matter
particles. Since the different theoretical candidates appear to have quite distinctive
signatures, there are good reasons to expect that from a combination of all of these
efforts a common picture will materialize within the next decade.
Theoretical particle physicists have proposed a whole host of candidates for the
constituents of non-baryonic dark matter, with fancy names such as axions, axinos,
gravitinos, neutralinos and lightest Kaluza–Klein partners. The best-motivated of
these occur in extensions of the Standard Model that have been proposed to solve other
problems besides the dark-matter puzzle. The axion, for example, arose in extensions
that aim to solve the strong CP problem. It later turned out to be a viable dark-
matter candidate if its mass is in the micro-electron-volt range. Gravitinos and
neutralinos, on the other hand, are the superpartners of the graviton and the neutral
bosons, respectively. They arise in supersymmetric extensions of the Standard Model,
which aim at a solution of the hierarchy problem and at a grand unification of the
strong and electroweak interactions. In fact, neutralinos are natural candidates for
dark matter because they have cross-sections of the order of electroweak interactions
and their masses are expected to be of the order of the weak scale (i.e. 100 GeV).
This leads to the fact that their relic density resulting from freeze-out in the early
universe is just right to account for the observed amount of dark matter.
Neutralinos belong to the class of weakly interacting massive particles (WIMPs). Such
particles seem to be more or less generic in extensions of the Standard Model at the
tera-electron-volt scale, but their stability (or a long enough lifetime) has to be
imposed. This is not necessary for super-weakly interacting massive particles
(superWIMPs), such as sterile neutrinos, gravitinos, hidden sector gauge bosons
(gauginos) and the axino. For example, unstable but long-lived gravitinos in the
5–300 GeV mass range are viable candidates for dark matter and provide a
consistent thermal history of the universe, including successful Big Bang
Detecting dark matter
Owing to their relatively large elastic cross-sections with atomic nuclei, WIMPs such
as neutralinos are good candidates for direct detection in the laboratory, yielding up to
one event per day, per 100 kg of target material. The expected WIMP signatures are
nuclear recoils, which should occur uniformly throughout the detector volume at a rate
that shows an annual flux modulation by a few per cent. Intriguingly, the DAMA experiment
in the Gran Sasso National Laboratory has seen evidence for such an annual modulation.
However, there is some tension with other direct-detection experiments. Theoretical
studies have revealed that interpretation in terms of a low-mass (5–50 GeV)
WIMP is marginally compatible with the current limits from other experiments. In contrast
to DAMA, which looks just for scintillation light, most of the latter exploit at least
two observables out of the set (phonons, charge, light) to reconstruct the nuclear
Many different techniques based on cryogenic detectors (e.g. the Cryogenic Dark Matter
Search), noble liquids (e.g. the XENON Dark Matter Project) or even bubble chambers, are
currently employed to search for WIMPs via direct detection. Detectors with directional
sensitivity (e.g. the Directional Recoil Identification From Tracks experiment) may not
only have a better signal-to-background discrimination but may also be capable of
measuring the local dark-matter, phase-space distribution. In summary, these direct
experiments are currently probing some of the theoretically interesting regions for WIMP
candidates. The next generation of experiments may enter the era of WIMP (astro)
The axion is another dark-matter candidate for which there are ongoing direct-
detection experiments. Both the Axion Dark Matter Experiment (ADMX) in the US and the
Cosmic Axion Research with Rydberg Atoms in a Resonant Cavity (CARRACK) experiment in
Japan exploit a cooled cavity inside a strong magnetic field to search for the
stimulation of a cavity resonance from a dark-matter axion–photon conversion in the
microwave frequency region, corresponding to the expected axion mass. While they differ
in their detector technology – ADMX uses microwave telescope technology whereas
CARRACK employs Rydberg atom technology – both experiments are designed to cover
the 1–10 μeV mass range. Indeed, if dark matter consists just of axions
then it should soon be found in these experiments. The CERN Axion Solar Telescope,
meanwhile, is looking for axions produced in the Sun.
There are also of course possibilities for indirect detection. Dark matter may not be
absolutely dark. In fact, in regions where the dark-matter density is high (e.g. in the
Earth, in the Sun, near the galactic centre, in external galaxies), neutralinos or other
WIMPs may annihilate to visible particle–antiparticle pairs and lead to signatures
in gamma-ray, neutrino, positron and antiproton spectra. Moreover, superWIMPs (e.g.
gravitinos), may also leave their traces in cosmic-ray spectra if they are not absolutely
Interestingly, the Payload for Antimatter Matter Exploration and Light-Nuclei
Astrophysics (PAMELA) satellite experiment recently observed an unexpected rise in the
fraction of positrons at energies of 10–100 GeV, thereby confirming earlier
observations by the High Energy Antimatter Telescope balloon experiment. In addition, the
Advanced Thin Ionization Chamber balloon experiment has reported a further anomaly in the
electron-plus-positron flux, which can be interpreted as the continuation of the PAMELA
excess to about 800 GeV. The quantification of these excesses is still quite
uncertain, not least because of relatively large systematic uncertainties. It is well
established that they cannot be explained by the standard mechanism, namely the secondary
production of positrons arising from collisions between cosmic-ray protons and the
interstellar medium within our galaxy. However, a very conventional astrophysical source
for them could be nearby pulsars.
On a more speculative level, these observations have inspired theorists to search for
pure particle-physics models that accommodate all results. Generically, interpretations
in terms of WIMP annihilation seem to be disfavoured, because they require a huge
clumpiness of the Milky Way dark-matter halo, which is at variance with recent numerical
simulations of the latter. This constraint is relaxed in superWIMP scenarios, where the
positrons may be produced in the decay of dark-matter particles (e.g. gravitinos).
It is clear that one of the keys to understanding the origin of the excess in the
positron fraction is the accurate, separate measurement of positron and electron fluxes,
which can be done with further PAMELA data and with the Alpha Magnetic Spectrometer
satellite experiment. Furthermore, distinguishing different interpretations of the
observed excesses requires a multimessenger approach (i.e. to search for signatures in
the radio range, synchrotron radiation, neutrinos, antiprotons and gamma rays).
Fortunately the Fermi Gamma-Ray Space Telescope is in orbit and taking data (CERN Courier November
2008 p13). Together with other cosmic-ray experiments it will probe interesting
regions of parameter space in WIMP and superWIMP scenarios of dark matter.
Dark matter at colliders
Clearly, at colliders the existence of a dark-matter candidate can be inferred only
indirectly from the apparent missing energy, associated with the dark-matter particles,
in the final state of the collision. However, such a measurement can be made with
precision and under controlled conditions. To extract the properties, such as the mass,
of dark-matter particles, these final-state measurements have to be compared with
predictions from theoretical models. In a supersymmetric extension of the Standard Model,
for example, with the neutralino as the lightest superpartner, experiments at the LHC
would search for signatures from the cascade decay of gluinos and squarks into gluons,
quarks, leptons and neutralinos. This would show up as large missing transverse-energy in
events with some jets and leptons. The endpoints in kinematic distributions could then be
used for the determination of the dark-matter candidate’s mass, which could be compared
with the mass determined eventually by measurements of recoil energy in direct-detection
This complementarity between direct, indirect and collider searches for dark matter
is essential. Although collider experiments might identify a dark-matter candidate and
precisely measure its properties, they will not be able to distinguish a cosmologically
stable particle from one that is long-lived but unstable. In turn, direct detection
cannot tell definitely what kind of WIMP has been observed. Moreover, in many superWIMP
dark-matter scenarios a direct detection is impossible, while detection at the LHC may
be feasible. For example, if the lightest superpartner is a gravitino (or hidden
gaugino) and the next-to-lightest is a charged lepton, experiments at the LHC may search
for the striking signature of a displaced vertex plus an ionizing track.
In many cases, however, precision measurements from a future electron–positron
collider seem to be necessary to exploit fully the
collider–cosmology–astrophysics synergy. In addition “low-energy photon-
collider” experiments – such as the Axion-Like Particle Search at DESY, the GammeV
experiment at Fermilab and the Optical Search for QED magnetic birefringence, axions and
photo regeneration at CERN, where the interactions of intense laser beams with strong
electromagnetic fields are probed – may give viable insight into the existence of
very lightweight, axion-like, dark-matter candidates.
In summary, there is evidence for non-baryonic dark matter that is not made of any
known elementary particle. We are today in the exploratory stage to figure out its
microscopic nature. Many ideas are currently being explored in theories and in
experiments, and more will come. Nature has given us a few clues that we need to exploit.
The data coming soon from accelerators, and from direct and indirect detection
experiments, will be the final arbiter.
For more information about the workshop, see http://th-workshop2008.desy.de.