| 1. |
- Aamodt, K., et al.
(författare)
-
The ALICE experiment at the CERN LHC
- 2008
-
Ingår i: Journal of Instrumentation. - 1748-0221. ; 3:S08002
-
Forskningsöversikt (refereegranskat)abstract
- ALICE (A Large Ion Collider Experiment) is a general-purpose, heavy-ion detector at the CERN LHC which focuses on QCD, the strong-interaction sector of the Standard Model. It is designed to address the physics of strongly interacting matter and the quark-gluon plasma at extreme values of energy density and temperature in nucleus-nucleus collisions. Besides running with Pb ions, the physics programme includes collisions with lighter ions, lower energy running and dedicated proton-nucleus runs. ALICE will also take data with proton beams at the top LHC energy to collect reference data for the heavy-ion programme and to address several QCD topics for which ALICE is complementary to the other LHC detectors. The ALICE detector has been built by a collaboration including currently over 1000 physicists and engineers from 105 Institutes in 30 countries, Its overall dimensions are 16 x 16 x 26 m(3) with a total weight of approximately 10 000 t. The experiment consists of 18 different detector systems each with its own specific technology choice and design constraints, driven both by the physics requirements and the experimental conditions expected at LHC. The most stringent design constraint is to cope with the extreme particle multiplicity anticipated in central Pb-Pb collisions. The different subsystems were optimized to provide high-momentum resolution as well as excellent Particle Identification (PID) over a broad range in momentum, up to the highest multiplicities predicted for LHC. This will allow for comprehensive studies of hadrons, electrons, muons, and photons produced in the collision of heavy nuclei. Most detector systems are scheduled to be installed and ready for data taking by mid-2008 when the LHC is scheduled to start operation, with the exception of parts of the Photon Spectrometer (PHOS), Transition Radiation Detector (TRD) and Electro Magnetic Calorimeter (EMCal). These detectors will be completed for the high-luminosity ion run expected in 2010. This paper describes in detail the detector components as installed for the first data taking in the summer of 2008.
|
|
| 2. |
- Marzari, F., et al.
(författare)
-
Planetesimal Evolution in Circumbinary Gaseous Disks: A Hybrid Model
- 2008
-
Ingår i: The Astronomical Journal. - : American Astronomical Society. ; 681:2, s. 1599-1608
-
Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of planetesimals embedded in a circumbinary protoplanetary disk. A hybrid numerical approach is developed where the evolution of the gaseous component of the disk is computed with the hydrodynamical code FARGO while the planetesimal trajectories are computed with an N-body code. The local gas density and velocity derived from the hydrodynamical portion are used to calculate the drag force and the gravitational attraction of the disk on the planetesimals. We explore the effects of spiral density wave patterns and of the disk eccentricity, both excited by the binary tidal perturbations, on the dynamical evolution of planetesimal orbits. A new definition of osculating orbital elements is given to properly account for the gravitational attraction of the disk. The outcomes of the numerical simulations show that the pericenter alignment of the planetesimal orbits is a robust result. It occurs for different values of the binary eccentricity and surface density profiles of the disk. However, the pericenters are less collimated compared to early predictions based on codes adopting a stationary and axisymmetric approximation for the disk. In addition, the eccentricity values are higher and depend on the semimajor axis of the bodies. Both these effects favor higher relative velocities between colliding planetesimals, making accretion less likely than previously thought. Small 100 m size bodies (planetesimal precursors) have a very high inward drift rate that might lead to a high-density belt in the proximity of the inner border of the disk. Fast accretion into larger bodies might occur in this region.
|
|
| 3. |
- Thébault, Philippe, et al.
(författare)
-
Planet formation in the habitable zone of α Centauri B
- 2009
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 393:1, s. L21-L25
-
Tidskriftsartikel (refereegranskat)abstract
- Recent studies have shown that α Centauri B might be, from an observational point of view, an ideal candidate for the detection of an Earth-like planet in or near its habitable zone (0.5-0.9au). We study here if such habitable planets can form, by numerically investigating the planet-formation stage which is probably the most sensitive to binarity effects: the mutual accretion of km-sized planetesimals. Using a state-of-the-art algorithm for computing the impact velocities within a test planetesimal population, we find that planetesimal growth is only possible, although marginally, in the innermost part of the habitable zone (HZ) around 0.5au. Beyond this point, the combination of secular perturbations by the binary companion and gas drag drives the mutual velocities beyond the erosion limit. Impact velocities might later decrease during the gas removal phase, but this probably happens too late for preventing most km-sized objects to be removed by inward drift, thus preventing accretion from starting anew. A more promising hypothesis is that the binary formed in a crowded cluster, where it might have been wider in its initial stages, when planetary formation was ongoing. We explore this scenario and find that a starting separation roughly 15au wider, or an eccentricity 2.5 times lower than the present ones, is required to have an accretion-friendly environment in the whole HZ.
|
|
