Dark matter at the University of Bologna

Research into  dark matter is carried out by the Astronomy and Physics Departments of the University of Bologna, in collaboration with the Astronomical Observatory of Bologna, national research organisations such as ASI (the Italian Space Agency ), CNR (National Research Council) and INFN (National Institute of Nuclear Physics) and international research organisations.

One of the most important proofs of the presence of dark matter in the spiral galaxies comes from the curves that describe the rotational speed of galactic matter as a function of the distance from the centre of the galaxy itself (see movement of stars in a galaxy). The curves are obtained from observations of spectral lines emitted by ionized gas (Alpha line), by neutral hydrogen gas (line at 21 cm wavelength) and by molecular gas (CO), measuring the displacement of the frequency of the line (due to the Doppler effect). While the ionized gas and the molecular gas are found in the brighter, central parts of the galaxies, neutral hydrogen extends well beyond the luminous disk. This allows us to trace the rotation curves at large distances from the centre of the galaxies.

• Rotation curves of spiral galaxies:
researchers at the Astronomical Observatory of Bologna, in collaboration with the Kapteyn Astronomical Institute of the Groningen University (Netherlands), have performed research programmes on dark matter in the galaxies for the past thirty years. These programmes are mainly based on observations with the radiotelescopes of Westerbork (Netherlands) and VLA (USA) of the 21cm line of neutral hydrogen in spiral galaxies of various morphological types and various brightnesses. Recent studies have shown that the rotation curve of the spiral galaxy NGC 5055 agrees with the presence of an extensive halo of dark matter.

The search for dark matter of an elementary particle type can be either direct or indirect. Direct experimental methods are based on the possible interaction of dark matter particles inside a detector, or which crosses the detector. Indirect studies try to detect, for example, neutrinos generated in the decay of hadrons produced in particle-antiparticle annihilations (of dark matter) which may occur at the centre of the Earth, the Sun or the Galaxy.
In addition, experiments with very high energy accelerators are performed to look for new elementary particles which might make up a part of particle dark matter.

The Physics Department carries out dark matter research through laboratory experiments without accelerators (MACRO, SLIM,  ANTARES and AMS), and experiments with accelerators (LEP, LHC, Tevatron).

	Fig. 1: Photo of the MACRO detector. (Credit: MACRO experiment)

• MACRO (1989 - 2000):
this was a large-size detector situated in the underground Gran Sasso laboratories. One of the aims of the experiment was to detect rare events in the penetrating cosmic radiation MACRO was looking directly for magnetic monopoles and nuclearites and indirectly for neutralinos.
Nuclearites would be objects like atomic nuclei, but containing stype quarks as well as u and d types found in ordinary matter. These quarks should be free to move around within a nuclearite, while being "confined" there. The nuclei of ordinary matter, instead, are formed of u and d type quarks which can move only inside a proton or a neutron. Neutralinos would be neutral
supersymmetric elementary particles; these could be one of the main components of dark matter. Neutralinos could collide with the atomic nuclei of a celestial body like the earth or the sun, and could remain trapped in their centre by the force of gravity. A neutralino and an antineutralino could annihilate, giving rise to pions and thus to muon neutrinos. MACRO looked for a flux of muon-neutrinos coming from the centre of the earth and the sun, setting important limits.

Fig. 2: The Cosmic Physics Laboratory of Chacaltaya, La Paz, Bolivia, where the SLIM experiment. (Credit: SLIM experiment)

SLIM (2001 - ....): this experiment consists of 400 m2of nuclear track detectors (CR39 and lexan), situated in the High AltitudeLaboratory of Chacaltaya, La Paz, Bolivia, 5230 m above sea level. The main objective of the experiment is the direct search for magnetic monopoles and nuclearites of intermediate mass. The first results are expected at the end of 2005.

AMS (2005 - ....): AMS (Alpha Magnetic Spectromer) is a cosmic ray detector. From 2006 it will be situated in the International Space Station (ISS) orbiting the Earth. Amongst other things, the apparatus will include silicon detectors for tracing charged particles, a system to measure time of flight, and a superconductor magnet which can deflect particles entering in opposite directions depending on the sign of their charge. The experiment's main aim is a direct search for antimatter and an indirect search for dark matter.

Fig. 4: The International Space Station (ISS) where the AMS detector will be installed. (Credit: AMS experiment)

The supersymmetric particles which could make up part of the Universe's dark matter can be investigated with the following accelerator experiments:

• DELPHI, L3 and OPAL (1989 - 2000):
These detectors, operating at the CERN LEP collider were looking for neutralinos in reactions of the type     e⁺ +  e¯ --> neutralino + ....   . No events of this or other types were found; therefore they set a lower limit on the mass and on the interaction cross section for the production of neutralinos and other particles.

• CDF (1989 - ....):
CDF is a detector at the Fermilab Tevatron collider in Chicago. The Tevatron accelerates protons and antiprotons up to 1 TeV and then makes them collide at the centre of the CDF detector. Scientists were researching supersymmetric particles and other particles produced in proton-antiproton collisions; these have not yet been found but the research continues with the Tevatron at higher energies and intensities and with an improved CDF detector.

• ZEUS (1998 - ....):
Here too, at the DESY laboratory in Hamburg, scientists are searching for many types of new particles produced in positron-proton collisions at high energies. The performance of the collider and of the ZEUS detector have improved.

• ATLAS, CMS and ALICE (2007 - ....):
The future experiments ATLAS, CMS and ALICE at the huge new LHC collider at CERN will search for supersymmetric particles and many other possible particles.

A theory group, in collaboration with a group from the CNR of Bologna involved in the PLANCK experiment, is studying the implications of dark matter and dark energy with regard to the results of experiments on the microwave cosmic background radiation and its anisotropies .The experiments have provided detailed images of the Universe when it had a temperature of about 3000 degrees. The ever-improving precision and resolution with which these anisotropies are detected gives access to primordial information on the dark components of the Universe.