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: http://zebu.uoregon.edu/~uochep/seminars/seminars-spr08.html
Дата изменения: Wed Sep 24 01:41:53 2008 Дата индексирования: Tue Oct 2 09:00:36 2012 Кодировка: Поисковые слова: total solar eclipse |
April 4, 2008 (Friday) Maxim Pospelov, Victoria University
April 7, 2008 (Monday) Andrea Pocar, Stanford
April 11, 2008 (Friday) Ricky Fok, University of Oregon
April 28, 2008 (Monday) Miguel Mostafa, University of Utah
May 12, 2008 (Monday) Weiming Yao, LBNL
June 2, 2008 (Monday) Jim Brau, University of Oregon
June 9, 2008 (Monday) Sourish Dutta
1:30 pm, 472 Willamette Hall
The Enriched Xenon Observatory (EXO) is a project aiming at detecting neutrinoless double beta decays of 136Xe using a novel approach to suppress backgrounds from trace radioactivity in the detector, especially gamma rays. A xenon-filled time projection chamber (TPC) supplemented with scintillation light readout detects energy and position of ionising particle interactions within its volume. When candidate BB events are recorded, the 136Ba ion daughters will also be identified, event by event, by means of optical spectroscopy. This coincidence technique would allow for a measurement of double beta decays virtually immune to external radioactive contaminations. The EXO collaboration is planning a ton-scale Xe detector using a phased approach. A smaller detector, EXO-200, employing 200 kg of enriched xenon (80% 136Xe) in liquid form within a TPC with scintillation readout and with no Ba identification, is in advanced stage of assembly. Its cryogenic and xenon handling systems are being re-assembled at the Waste Isolation Pilot Plant (WIPP) underground site in New Mexico. The central detector will be installed and the detector run by 2008. As a parallel effort to EXO-200, strategies for Ba tagging are being developed in the laboratory. I will present the EXO experiment in the context of neutrinoless double beta decay searches and describe the EXO-200 detector in detail, discussing its physics goals, experimental challenges, and schedule. I will also illustrate some of the most promising approaches for tagging single Ba ions produced in a ton-scale Xe detector, show milestone results achieved in laboratory setups, and discuss the EXO timeline for the near future.
4:00 pm, 472 Willamette Hall
Refreshments served at 3:45
1:30 pm, 472 Willamette Hall
Since the first detection of a cosmic ray event with energy above 10 20 eV in 1962, their nature and origin remain unknown. At these energies, it is expected that the cosmic ray flux undergo a strong suppression. Due to the extreme rarity of these ultra high energy cosmic rays, they must be observed indirectly through the observation of extensive air showers, and the lack of knowledge of hadronic interactions at these energies leads to inherent difficulties in characterising the properties of the primary particle. A new generation cosmic ray detector, the Pierre Auger Observatory, has been designed to study cosmic rays with energy above 10 18 eV and answer the crucial questions of ultra high energy cosmic ray physics. The Southern Observatory in Argentina has been collecting data since 2004 and, although still under construction, its exposure is already larger than that of any previous experiment. After three years of operation, we found strong indications that ultra high energy cosmic rays come from nearby, extragalactic sources, opening a window for charged particle astronomy. In this colloquium, I will describe the Pierre Auger Observatory in its astrophysical context, our most recent results, and the exciting prospects for the near future.
4:00 pm, 472 Willamette Hall
Refreshments served at 3:45
4:00 pm, 472 Willamette Hall
Refreshments served at 3:45
4:00 pm, 472 Willamette Hall
Oscillating scalar fields, with an oscillation frequency much greater than the expansion rate, have been proposed as models for dark energy. We examine these models, with particular emphasis on the evolution of the ratio of the oscillation frequency to the expansion rate. We show that this ratio always increases with time if the dark energy density declines less rapidly than the background energy density. This allows us to classify oscillating dark energy models in terms of the epoch at which the oscillation frequency exceeds the expansion rate, which is effectively the time at which rapid oscillations begin. There are three basic types of behavior: early oscillation models, in which oscillations begin during the matter-dominated era, late oscillation models, in which oscillations begin after scalar-field domination, and non-oscillating models. We examine a representative set of models (those with power-law potentials) and determine the parameter range giving acceptable agreement with the supernova observations. We show that a subset of all three classes of models can be consistent with the observational data.
4:00 pm, 472 Willamette Hall