Cosmic Accelerators – Astronomy at the Highest Energies

Trajectory of a charged particle at a relativistic shock front.

High-energy astrophysics at MPIK is characterized by a very close cooperation between experimentalists coming from a particle physics background and the more theoretically oriented astrophysicists. They study the non-thermal phenomena in the Universe using the High-Energy Stereoscopic System H.E.S.S. to detect very-high-energy (VHE) gamma rays from the cosmos, and investigate the acceleration mechanisms in cosmic sources of high-energy particles.

Particles in the VHE range cannot be produced as thermal radiation, like the electromagnetic radiation in most other wavelength regimes; only in the Big Bang high enough temperatures were reached for a short time. Instead, collective non-thermal mechanisms are believed to be responsible for the acceleration: charged particles continue to gain energy by diffusively returning many times into the shock front of the giant shock waves generated in supernova explosions or in the plasma jets emerging from the immediate vicinity of the massive black holes at the centres of active galaxies. Considerable effort at the Institute is going into the modelling and theoretical description of processes within the different types of cosmic accelerators.

The VHE gamma rays detected on Earth are secondary products generated when the accelerated primary charged particles react with the ambient medium – either the interstellar gas or the ambient photon fields. In contrast to the charged particles, the gamma rays travel on a straight path from the source to the observer and allow imaging of sources and studying the processes in the acceleration region.

Since the first telescope became operational in 2002, H.E.S.S. has detected more than 60 VHE gamma-ray sources along the Milky Way such as supernova remnants or pulsar wind nebulae, that are mostly visible also at other wavelengths. Of special interest is the region of the Galactic Centre which harbours a supermassive black hole, whose position coincides with a strong gamma-ray source. Beyond our Galaxy, galaxies with active nuclei and starburst galaxies appear as faint objects in the VHE gamma light. Recently, H.E.S.S. successfully identified extremely luminous sources in the Large Magellanic Cloud. To understand the objects, a multi-wavelength analysis is required. In this context, special attention is paid to the infrared emission from dust which is a direct tracer of the interstellar matter with which cosmic rays interact to produce gamma rays.

Division Hofmann      Division Hinton

Theoretical Astrophysics (pdf)
H.E.S.S.: Cosmic Accelerators in the Light of Gamma Rays (pdf)

Cherenkov Telescopes

Observing gamma rays.

High-energy gamma rays from space – a trillion times more energetic than visible light – do not reach the Earth’s surface. Nevertheless, they can be sighted at the ground with the atmosphere serving as a detector. When entering the atmosphere, the gamma quanta collide with atomic nuclei producing cascades of electrically charged secondary particles, so-called particle showers. These emit faint bluish and extremely short flashes of light (Cherenkov light) which illuminate a circular area of about 250 m diameter on the ground and can be detected in dark, moonless nights with large reflector telescopes that are equipped with fast photosensors. To trace the exact direction the particle showers come from, they are observed stereoscopically by several telescopes simultaneously.

The High Energy Stereoscopic System H.E.S.S. consists of five telescopes, four of them being identical with 107 m2 mirror area each. They form the corners of a square of side 120 m. A camera – a matrix of 960 photosensors – in the focus of each mirror covers a large field of view of five degrees. Thus, the instrument is especially suited for sensitive sky surveys. H.E.S.S. was the first instrument to produce true images of astrophysical gamma-ray sources. In the centre of the array, a fifth, huge telescope with 614 m2 mirror area and a camera with 2048 pixels is operational since 2012. It strongly enhances the sensitivity of the system and extends the observable energy range to lower energies.

Preparations are underway for a next generation observatory with dramatically improved performance. The Cherenkov Telescope Array (CTA) will consist of two arrays, one in each hemisphere, with around 100 telescopes of three different sizes. CTA will bring much better resolution, higher sensitivity, a much wider energy range, and a collection area of many square kilometres at the highest energies. The MPIK instrumentation effort is on novel cameras for the different telescope types.

Division Hofmann      Division Hinton

H.E.S.S.: Cosmic Accelerators in the Light of Gamma Rays (pdf)
High Performance Cameras for the Cherenkov Telescopes of CTA (pdf)

Matter and Antimatter – Search for the Crucial Difference

Visualisation of a particle shower in the LHCb detector emerging from a proton-lead collision in the LHC.

There is no indication that anywhere in the visible Universe considerable amounts of antimatter exist. Since particles and antiparticles must have been created in equal amounts in the Big Bang, there must be a fundamental difference between them. Else, they would have completely annihilated, leaving a Universe filled with pure radiation. The LHCb experiment at the Large Hadron Collider (LHC) of CERN in Geneva searches for such differences in hadronic reactions. Besides many other particles, in proton-proton collisions so-called B mesons are created, heavy particles consisting of each a light quark and a heavy antiquark; and reversely for their antiparticles. Measurements of their decays that lead to equal amounts of matter and antimatter showed that there are processes in which antimatter disappears faster than matter. Physicists and electronic engineers at MPIK have developed, produced and tested radiation-proof electronics components for the silicon tracker of LHCb which records trajectories of charged particles. Presently, the scientists focus on the analysis of the huge amount of data from the first measurement period of LHC. The second measurement period at doubled energy is expected to provide further insight.

Ultracold negative ions will serve to cool antiprotons, which are a prerequisite for the production of cold antihydrogen. It will be used for direct high-precision studies of, e. g. the free fall of antimatter. In addition, measurements are ongoing of the magnetic moment of the antiproton and its charge-to-mass ratio in comparison to the proton.

Division Hofmann    Group Kellerbauer

The LHCb Experiment: B-Physics, Elementary Particle Physics at the Terascale (pdf)
Precision Measurements with Antimatter: Antihydrogen in the Laboratory (pdf)

The Early Universe – Elementary Particles at Highest Energies

The Universe’s first 3 minutes.

The Standard Model of elementary particle physics predicts some matter-antimatter asymmetry that, however, is many orders of magnitude smaller than the amount required to explain the observed matter abundance in the Universe. This symmetry violation must have occurred in the early Universe. A well motivated scenario for this, in which neutrinos play a crucial role, is the so-called leptogenesis which is explored by MPIK theorists and which is one of the motivations for GERDA. Here, the decay of heavy neutrinos plays a key role. Their existence explains the small but non-zero masses of the light neutrinos and their oscillations as well as the Dark Matter.

Immediately after the Big Bang, quarks and gluons were not yet combined to elementary particles. Instead, the extremely hot matter formed a so-called quark-gluon plasma. Also in very high-energy collisions of the nuclei of heavy elements such as lead, a quark-gluon plasma of the size of an atomic nucleus can be generated for an extremely short time span. In order to better understand this state, the particles formed in energetic proton-lead collisions are investigated with LHCb.

Division Hofmann    Division Lindner    Group Rodejohann

Theoretical Astroparticle Physics and Cosmology (pdf)
The LHCb Experiment: B-Physics, Elementary Particle Physics at the Terascale (pdf)



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