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PEBS: Introduction
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Disclaimer: This page is intended as a general introduction to PEBS physics. Detailed information can be found on the Aachen PEBS physics pages.

This page is under construction
Dark Matter: a key challenge for the XXI century physicist.

What do we know about it?

-It influences the way light propagates through the universe
Hubble dark matter
On this picture taken by the Hubble Space Telescope, the darker ring is most likely due to the presence of a heavy dark matter ring. See here for more details .

-It influences the rotation curves of the galaxies
Chandra dark matter
It has been known for long (F. Zwicky emitted the idea in 1935) that the rotation curves of spiral galaxies are inconsistent with the mass distribution of standard matter within the galaxy. A halo of dark matter is the most commonly accepted explanation for flat rotation curve like the one of NGC 3198, from van Albada et al, Astrophys. J. 1 259 (1985).
See e.g. here for a short introduction .

-It forms haloes around galaxies
Chandra dark matter
A composite image made with data from the Chandra X-ray telescope and Hubble. When two clusters of galaxies collide, dark matter halos (blue) continue their way unscathed while standard matter (pink) interacts and is left behind. A full explanation can be found here here and a movie showing how the image is build here.

But the main question about dark matter is its nature! What is something we don't know made of? This question opens the way to an almost infinite number of suggestions. The most obvious thing to do would be to track the existence of a new particle. A particle that isn't described by the standard model of particle physics. There are some hints that this new particle should be a Weakly Interacting Massive Particle (WIMP), among which the neutralino, the supersymmetric partner of the neutrino is the most widely discussed. But thorists can provide ask for any kind of new particle Kaluza-Klein particles, axions, sterile neutrinos, etc...
Supersymmetric Particles
Whatever the nature of the DM particles, they are likely to decay or to annihilate into standard model particles which can be detected in cosmic rays. In 2008 a key mesurement performed by the PAMELA satelite [1]showed an exess of positrons in the cosmics rays spectrum. This feature has been heavily discussed. While the most probable explanation would be the presence of an astrophysical positron source, such as a pulsar, it could also be due to dark matter particles decaying or annihilating into positrons.
Pamela Results

PAMELA data show an excess of positrons in cosmic rays for E>10 GeV that is incompatible with the GALPROP propagation model.

In parallel to those indirect dark matter searches, a large effort is done on direct searches for DM, among them a few like DAMA show some (faint) hints for the existence of DM.
In order to test this poitro excess further, it has to be measured more accurately. PEBS is a detector whose main goal is a precise measurement of the positron fraction in cosmic rays up to an energy of 1.8 TeV. It is scheduled to fly from the south pole in (austral) summer 2013-2014 at an altitude of 40 km.
NASA balloon at the south pole NASA balloon trajectory
The two main challenges in such a measurement are the huge proton background (Np /Ne- at 100 GeV) and the separation between positrons and electrons. The background protons will be rejected combining the information of the Transition Radiation Detector and the Electric CALorimeter. Positrons will be separated from electrons using the Scintillating Fiber Tracker in a strong magnetic field provided by a superconducting magnet.
At EPFL and ETHZ, we're building the ECAL which comprises scintillating bars with embedded wavelength shiting fibres, sandwhiched between tungsten absorber plates. The light output will be read at the end of the fiber by Silicon Photomultipliers (SiPM). The goal of the ECAL is a precice measurement of the positron or electron energy up to 1.8 TeV and a proton background rejection of 103