Events at Physics |
Events on Friday, April 10th, 2009
- Good Friday
- Special Astronomy Colloquium Talk
- Positron Annihilation Radiation from the Center of our Galaxy
- Time: 12:00 pm - 1:00 pm
- Place: 6515 Sterling Hall
- Speaker: Nidhal Guessoum, American University of Sharjah,
- Abstract: The staggering rate of positron production and annihilation in the
Galaxy (1043 e+/s) has been established for three decades now but has yet to find a convincing explanation, especially after the INTEGRAL satellite (launched in 2002) produced a radiation map showing the emission to be mostly coming from the galactic bulge (Knodlseder et al. 2004, Weidenspointner et al. 2008). While the physics of the annihilation of positrons has now mostly been understood (Guessoum, Jean, Gillard 2005), the origin and propagation of those positrons remain an unsettled question, although good progress has been made lately with interesting models and constraints presented.<br>
In this seminar I first review the current knowledge of that
positron-electron annihilation radiation, with what the spectroscopic analysis and the spatial mapping tell us about the media of annihilation (mainly) and to some extent about the sources of positrons. I explain the physical processes which the positrons undergo during their long lives and highlight the role of the interstellar gas, dust, and organic molecules. I then focus on the Galactic Center (with its supermassive black hole) and its potential contribution to explaining part, if not all, of the annihilation radiation observed at least in the Galactic bulge. Several specific proposals have recently been made for the production of positrons by the GC?s SMBH, and they will be critically reviewed on the basis of:a) the positrons? propagation away from the central compact source, given their initial kinetic energy; b) the constraint from direct annihilation emission in flight produced by relativistic positrons; c) estimates of the rates of positron production and other plausibility considerations. Finally, I briefly highlight the important issue of positron propagation from their production site(s), for which the specific physical process is uncertain (collisional or diffusional) and the conditions of the region (density, temperature, and magneticfield distributions) are complex.
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I conclude with some pointers for near-future research on the problem. - Theory/Phenomenology Seminar
- Light Higgsino-LSP Scenario in the BMSSM
- Time: 2:30 pm - 3:30 pm
- Place: 5280 Chamberlin Hall
- Speaker: Jeonghyeon Song, Konkuk University, Seoul, Korea
- Abstract: The modest addition of the dimension-5 term lambda(H_u.H_d)^2/M to the superpotential of the Minimal Supersymmetric Standard Model (MSSM) originated from physics beyond the MSSM (BMSSM) has a significant impact on the scenario of the Higgsino-dominated neutralino state being the lightest supersymmetric particle (LSP). It increases the mass difference between the LSP and the lighter chargino as well as that between the LSP and the second-lightest neutralino. This enhances the LHC discovery potential of the chargino and neutralino decays, producing more energetic charged leptons or pions than the decays without the BMSSM corrections. Furthermore, the coannihilation between the lighter chargino or second-lightest neutralino and the LSP is reduced substantially such that the LSP mass does not have to be very heavy. Consequently, an almost pure Higgsino LSP with its mass ~100 GeV in the BMSSM can account for all the relic density of cold dark matter in the Universe unless tan(beta) is too large.
- Host: F. Petriello
- Physics Department Colloquium
- What's so Cool about Ultra-Cold Neutrons
- Time: 4:00 pm
- Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
- Speaker: Brad Filippone, Caltech
- Abstract: Ultra-Cold Neutrons (UCN) are neutrons with small enough kinetic energy that they can be trapped in material bottles or by modest magnetic fields. With kinetic energies below 300 nano eV, UCN are ideal for studying the fundamental properties of the neutron. Precision studies of neutron decay can explore physics beyond the Standard Electroweak Model. In addition, highly sensitive searches for an Electric Dipole Moment of the neutron probe possible new sources of CP violation (Charge-conjugation and Parity) which could be responsible for the dominance of matter over antimatter observed in the Universe.
- Host: Ramsey-Musolf