| 41. |
- Hamberg, Mathias, et al.
(author)
-
Experimental studies of the dissociative recombination processes for the dimethyl ether ions CD3OCD2+ and (CD3)2OD
- 2010
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 514, s. A83-
-
Journal article (peer-reviewed)abstract
- Aims: Determination of branching fractions, cross sections and thermal rate coefficients for the dissociative recombination of CD3OCD2+ (0-0.3 eV) and (CD3)2OD+ (0-0.2 eV) at the low relative kinetic energies encountered in the interstellar medium. Methods: The measurements were carried out using merged electron and ion beams at the CRYRING storage ring, Stockholm, Sweden. Results: For (CD3)2OD+ we have experimentally determined the branching fraction for ejection of a single hydrogen atom in the DR process to be maximally 7% whereas 49% of the reactions involve the break up of the COC chain into two heavy fragments and 44% ruptures both C-O bonds. The DR of CD3OCD2+ is dominated by fragmentation of the COC chain into two heavy fragments. The measured thermal rate constants and cross sections are k(T) =1.7 ± 0.5 × 10−6(T/300)−0.77±0.01 cm3s−1, σ= 1.2 ± 0.4 × 10−15(Ecm[eV])−1.27 ± 0.01 cm2 and k(T) = 1.7 ± 0.6 × 10−6(T/300)−0.70±0.02 cm3s−1,σ= 1.7 ± 0.6 × 10−15(Ecm[eV])−1.20±0.02 cm2 for CD3OCD2+ and (CD3)2OD+, respectively.
|
|
| 42. |
- Aartsen, M. G., et al.
(author)
-
Computational techniques for the analysis of small signals in high-statistics neutrino oscillation experiments
- 2020
-
In: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 977
-
Journal article (peer-reviewed)abstract
- The current and upcoming generation of Very Large Volume Neutrino Telescopes - collecting unprecedented quantities of neutrino events - can be used to explore subtle effects in oscillation physics, such as (but not restricted to) the neutrino mass ordering. The sensitivity of an experiment to these effects can be estimated from Monte Carlo simulations. With the high number of events that will be collected, there is a trade-off between the computational expense of running such simulations and the inherent statistical uncertainty in the determined values. In such a scenario, it becomes impractical to produce and use adequately-sized sets of simulated events with traditional methods, such as Monte Carlo weighting. In this work we present a staged approach to the generation of expected distributions of observables in order to overcome these challenges. By combining multiple integration and smoothing techniques which address limited statistics from simulation it arrives at reliable analysis results using modest computational resources.
|
|
| 43. |
- Davidsson, Björn
(author)
-
Thermophysical Modelling and Mechanical Stability of Cometary Nuclei
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- Comets are the most primordial and least evolved bodies in the Solar System. As such, they are unique sources of information regarding the early history of the Solar System. However, little is known about cometary nuclei since they are very difficult to observe due to the obscuring coma. Indirect methods are therefore often used to extract knowledge about nucleus parameters such as size, shape, density, material strength, and rotational properties. For example, tidal and non-tidal splitting of cometary nuclei can provide important information about nuclear densities and material strengths, but only if the criteria for mechanical stability are well known. Masses and densities of cometary nuclei can also be obtained by studying orbital modifications due to non-gravitational forces, but only if the thermophysics of comets can be modelled accurately. A detailed investigation is made regarding the mechanical stability of small Solar System bodies. New expressions for the Roche distance are derived, as functions of the size, shape, density, material strength, rotational period, and spin axis orientation of a body. The critical rotational period for centrifugal breakup in free space is also considered, and the resulting formulae are applied to comets for which the size, shape and rotational period have been estimated observationally, in order to place constraints on their densities and material strengths. A new thermophysical model of cometary nuclei is developed, focusing on two rarely studied features - layer absorption of solar energy, and parallel modelling of the nucleus and innermost coma. Sophisticated modelling of radiative transfer processes and the kinetics of gas in thermodynamic non-equilibrium form the basis for this work. The new model is applied to Comet 19P/Borrelly, and its density is estimated by reproducing the non-gravitational changes of its orbit.
|
|
| 44. |
- Hultman, John
(author)
-
Dissipative formation of galaxies in hierarchical cosmology
- 1998
-
Doctoral thesis (other academic/artistic)abstract
- A description and detailed tests of a numerical code for simulations of galaxy formation is presented. The dynamics is followed with fully adaptive and Lagrangian Smooth Particle Hydrodynamics (SPH) combined with a gravitational tree method. The severe numerical problem of sub-resolution clustering is solved by merging of gas particles.The code is used for Hydrodynamical simulations of the formation of galaxies in the mass range 109 - 1013M☉. The effect of an ambient UV radiation field, and the effect of metal enrichment of halo gas due to star formation and stellar evolution, are investigated. The UV radiation field is found to strongly affect galaxies with circular velocities less than ≈ 50 km/s. The effect is, however, not large enough to reconcile hierarchical clustering models with observations.I present numerical simulations of galaxy formation in a CDM scenario, including star formation, using the SPH and tree method on scales ranging from 20 Mpc down to less than 1 kpc. The gas component is treated as a two-phase medium, governed by thermal instability and supernova feedback. For a certain degree of supernova evaporation, several general properties of elliptical galaxies are reproduced, like shapes, r1/4-profiles, half-light radii, slow rotation and anisotropic velocity dispersions. A Faber-Jackson -like relation is observed, being of the form M ~ Σ2.6.No disk shaped objects form. The elliptical objects form through a hierarchical, star-forming "inside-out" collapse. The SFR has a rapidly raising and later exponentially decaying behaviour in individual objects. The stellar contribution to the closure density is around one percent. The co-moving average SFR shows an increasing behaviour with redshift. Low mass objects are strongly suppressed. Generally, the more massive objects are in place at z ~ 0.5 - 2. Although many problems and questions still remain, the overall impression is that surprisingly many properties of ellipticals are reproduced.
|
|
| 45. |
- Sandin, Christer
(author)
-
A Study of Grain Drift in C Stars : Theoretical Modeling of Dust-Driven Winds in Carbon-Rich Pulsating Giant Stars
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- A major fraction of stars will pass through a short period of dramatic events in their final evolutionary stage. Low- to intermediate-mass stars, studied here, are stripped of their outer parts in a slow massive wind. This mass loss reshapes both the star and the surrounding medium. The formation of the wind is a consequence of the non-linear interaction of a number of physical processes. Stellar pulsations and efficient dust formation are examples of such key processes. Time-dependent theoretical models, in combination with observations, are useful tools for understanding these winds.The main object of this thesis has been the physical improvement of a theoretical wind model. Here the coupling between the dust and gas in the wind is studied in further detail, allowing drift. The methods that have been developed earlier to describe the micro-physical interaction are overviewed and summarized. Previously dust has often been assumed to move at the same velocity as gas. New time-dependent wind models are presented where grain drift has been treated self-consistently. Specifically, the coupling between dust and gas in the wind has been modeled more realistically, with descriptions of both the modified momentum and energy balances, and drift dependent dust formation. The results of these new ``drift models'' have been compared with the results of non-drift models. A general result of the study is that the effects of drift are significant and difficult to predict if a simple analytical theory is used. It has been found that dust in drift models tends to accumulate in certain dense regions, an accumulation that was not possible without drift. Moreover the new models show an increased variability in the wind structure. The use of drift in dust formation tends to markedly increase the produced dust. Some sets of model parameters lead to a wind without including drift, but a corresponding wind does not form when drift is included -- and vice versa. The effects of drift are important and can probably not be ignored in realistic models.
|
|
| 46. |
- Örndahl, Eva
(author)
-
Quasar host galaxies at intermediate and high redshifts
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts.An imaging campaign of a large sample of quasars at intermediate redshift (0.4 < z < 0.8) was carried out at optical wavelengths using the Nordic Optical Telescope, and is studied in this thesis together with two smaller samples. The joint material forms more than half of the total number of observed sources in this redshift interval and increases the number of resolved radio-quiet hosts at z>0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation.With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditions. Bright, compact host galaxies showing signs of merging and interaction were detected in this way for three quasars at z~2.2, which were observed with the ESO 3.6 m telescope. The method is not restricted to host galaxy analysis but can be utilized in other applications as well, provided that the underlying extended source can be described by an analytical model.
|
|
| 47. |
- Östman, Linda, et al.
(author)
-
Limiting the dimming of distant type Ia supernovae
- 2005
-
In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :2, s. 5-
-
Journal article (peer-reviewed)abstract
- Distant supernovae have been observed to be fainter than what is expected in a matter dominated universe. The most likely explanation is that the universe is dominated by an energy component with negative pressure, namely dark energy. However, there are several astrophysical processes that could, in principle, affect the measurements and in order to be able to take advantage of the growing supernova statistics, the control of systematic effects is crucial. We discuss two of these; extinction due to intergalactic grey dust and dimming due to photon-axion oscillations and show how their effect on supernova observations can be constrained using observed quasar colours and spectra. For a wide range of intergalactic dust models, we are able to rule out any dimming larger than 0.2 magnitudes for a type Ia supernova at z = 1. The corresponding limit for intergalactic Milky Way type dust is 0.03 magnitudes. For the more speculative model of photons mixing with axions, we find that the effect is independent of photon energy for certain combinations of parameter values and a dimming as large as 0.6 magnitudes cannot be ruled out. These effects can have profound implications for the possibility of constraining dark energy properties using supernova observations.
|
|
| 48. |
- Abazov, V. M., et al.
(author)
-
Measurement of the t-channel single top quark production cross section
- 2010
-
In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 682:4-5, s. 363-369
-
Journal article (peer-reviewed)abstract
- The DO Collaboration reports direct evidence for electroweak production of single top quarks through the t-channel exchange of a virtual W boson. This is the first analysis to isolate an individual single top quark production channel. We select events containing an isolated electron or muon, missing transverse energy, and two, three or four jets from 2.3 fb(-1) of p (p) over bar collisions at the Fermilab Tevatron Collider. One or two of the jets are identified as containing a b hadron. We combine three multivariate techniques optimized for the t-channel process to measure the t- and s-channel cross sections simultaneously. We measure cross sections of 3.14(-0.80)(+0.94) pb for the t-channel and 1.05 +/- 0.81 pb for the s-channel. The measured t-channel result is found to have a significance of 4.8 standard deviations and is consistent with the standard model prediction.
|
|
| 49. |
- Abazov, V. M., et al.
(author)
-
Search for single top quarks in the tau plus jets channel using 4.8 fb(-1) of p(p)over-bar collision data
- 2010
-
In: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 690:1, s. 5-14
-
Journal article (peer-reviewed)abstract
- We present the first direct search for single top quark production using reconstructed tau leptons in the final state. The search is based on 4.8 fb(-1) of integrated luminosity collected in p (p) over bar collisions at root s= 1.96 TeV with the D0 detector at the Fermilab Tevatron Collider. We select events with a final state including an isolated tau lepton. missing transverse energy, two or three jets, one or two of them being identified as b quark jet. We use a multivariate technique to discriminate signal from background. The number of events observed in data in this final state is consistent with the signal plus background expectation. We set in the tau + jets channel an upper limit on the single top quark cross section of 7.3 pb at the 95% C.L. This measurement allows a gain of 4% in expected sensitivity for the observation of single top production when combining it with electron + jets and muon + jets channels already published by the DO Collaboration with 2.3 fb(-1) of data. We measure a combined cross section of 3.84(-0.83)(+0.89) pb, which is the most precise measurement to date.
|
|
| 50. |
- Abazov, V. M., et al.
(author)
-
Search for the Associated Production of a b Quark and a Neutral Supersymmetric Higgs Boson that Decays into τ Pairs
- 2010
-
In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 104:15
-
Journal article (peer-reviewed)abstract
- We report results from a search for production of a neutral Higgs boson in association with a b quark. We search for Higgs decays to tau pairs with one tau subsequently decaying to a muon and the other to hadrons. The data correspond to 2: 7 fb(-1) of p (p) over bar collisions recorded by the D0 detector at.. root s = 1.96 TeV. The data are found to be consistent with background predictions. The result allows us to exclude a significant region of parameter space of the minimal supersymmetric model.
|
|
