SCIPP Seminars

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 ARCHIVE FALL QUARTER 2008 Last updated:02/18/09 Tuesday October 7th, 10:30am Location: ISB 310 Nepomuk Otte Title: TBA In 2005 we observed a 3 sigma excess of pulsed gamma-ray emission from the Crab pulsar with the MAGIC air Cherenkov telescope. The hint of pulsed emission was a driving force to develop a new trigger system, which allowed us to lower the energy threshold of the telescope from 60 GeV to about 25 GeV. With the new trigger system we observed the Crab pulsar between October 2007 and February 2008, and detected the Crab pulsar with a significance of 6.4 standard deviations. This is the first time that a pulsar has been detected in gamma rays above 25 GeV. It is an important achievement because gamma rays of such energies provide direct information about the non-thermal processes in the magnetosphere of a pulsar. From previous non-detections above 100 GeV it is known that the spectral energy distribution of the Crab pulsar exhibits a steep turnover above 10 GeV. The point at which the spectrum turns over and the shape of the turnover provide information about the acceleration of particles in the magnetosphere. I also discuss the pulse profile, and compare it to EGRET data at lower energies. Tuesday October 14th, 10:30am Location: ISB 310 Matt Kistler (Ohio State University) "Supernovae, Neutrinos, and Exotic Core Collapses " The last few years have seen a number of exciting developments concerning core-collapse supernovae, including the discovery of over a dozen SN progenitor stars in nearby galaxies from archival searches. I will discuss ongoing work and prospects for the near future in determining which massive stars explode and what can be learned when they do. In particular, this will include information that can be obtained from the detection of neutrinos from SNe in the nearby universe and searches for stars that suddenly disappear after collapsing to a black hole. Tuesday October 28th, 10:30am Location: ISB 310 Dr. Andrew Ivanov (UC Davis) "The truth top quark is hiding " The existence of the top quark, by far the heaviest observed fundamental particle, is beyond any doubt now. The top quarks are copiously produced at Fermilab Tevatron, and the properties of the top quark are being measured with increasing precision. Due to its large mass the final products of top quark decays are very energetic, similar to those from decays of new exotic particles that we hope to discover.  Intriguingly, some of such exotic phenomena can have the same experimental event signatures, barely distinguishable from top. I will present recent results from CDF on searches for new physics processes, such as production of 4-th generation, supersymmetric and exotic mirror quarks, that could mimic top event signatures and be accessible at the Tevatron energies. Tuesday November 4th, 10:30am Location: ISB 310 TBD TBD ***PLEASE NOTE: SPECIAL DAY*** Monday, November 10th, 10:30am Location: ISB 310 Jason Steffen (FNAL) Constraints on axion-like particles and chameleons from the GammeV experiment I present the most recent results of both aspects of the GammeV experiment.  The first aspect is a search for axion-like particles using a variable baseline, "light-shining-through-a-wall" technique.  This search excludes the particle interpretation of the PVLAS signal with high confidence and sets new limits on axion-like particles with a laser experiment.  The second aspect of the GammeV experiment is a search for chameleon particles, scalar particles which may be responsible for the dark energy of the universe.  This is accomplished by looking for a characteristic afterglow signature from a "particle-in-a-jar" experiment whereby chameleon particles become trapped in a region with a high magnetic field and slowly decay into detectable photons.  This is the first use of this experimental technique to probe for these particles.  We place interesting limits on general chameleon models.  These limits are complimentary to other experiments, such as torsion pendulum experiments, which probe for forces that would result from new scalar particles.  I also discuss possible enhancements for experiments to probe for axion-like particles and chameleons. ***PLEASE NOTE: SPECIAL DAY*** Wednesday, November 19th, 10:30am Location: ISB 310 Aron Soha (UC Davis) "Searches for Exotic Physics at CDF" Are there hints of new physics in the large CDF Run II dataset? In this seminar, a collection of current discrepancies serves as a road map through the latest new physics search results from CDF. This includes signature-based and model-inspired searches, as well as more global search strategies. Tuesday November 25th, 10:30am Location: ISB 310 Wei-Ming Yao (LBNL) Standard Model Higgs Boson Searches at the Tevatron We review Standard Model Higgs Boson searches performed by CDF and D0 using up to 3.0 fb$^{-1}$ of the dataset accumulated from $p\bar p$ collisions at the center-of-mass energy of 1.96 TeV. The data are consistent with the background expectation so far. The limits and the future prospects of closing in on the SM Higgs production at the Tevatron will be presented. Tuesday December 2nd, 10:30am Location: ISB 310 Jason Nielsen (UCSC-SCIPP) "Anomalous multimuon events in CDF" The CDF collaboration recently reported an unexplained excess of dimuon events in which at least one muon is produced outside the beampipe.  This particular sample seems to be the source of several anomalous cross section results from CDF over the past few years.  I will provide a detailed pedagogical explanation of the result, particularly the motivation for and results of various cross checks in control samples.  I will also give a very short summary of some of the phenomenological models related to this experimental result. ***PLEASE NOTE SPECIAL DAY*** Monday December 8th, 10:30am Location: ISB 310 Victor Rykalin (NICADD Northern Illinois University) Development of Silicon Detectors for UNK, CMS, D0, ILC During this talk I"ll review some R&D work. A special attention will be given to development with Solid State Photomultiplier SSPM and to radiation tolerance of Silicon Strip Detectors SSD. A marriage of SSPM and extruded scintillator will be discussed. ***PLEASE NOTE SPECIAL DAY*** Wednesday December 10th, 10:30am Location: ISB 310 John Shlens (UCB-Salk) Exploring the network structure of the retina using maximum entropy methods All visual signals from the eye to the brain originate in the electrical activity of retinal ganglion cells (RGCs). Standard models implicitly assume that RGCs signal information independently of one another. However, several studies have demonstrated that significant concerted activity in pairs of RGCs may fundamentally alter visual signals. We recorded the electrical activity of several hundred RGCs in peripheral monkey retina under various forms of visual stimuli. Pairs of RGCs fired nearly simultaneously (i.e. synchronously) several-fold more often than expected by chance, indicating significant network interactions. Synchrony was localized and universal amongst cells of the same type indicating that it arises from local and highly stereotyped circuitry. To test whether concerted firing can be explained by known pairwise interactions, we used a maximum entropy approach borrowed from statistical mechanics to predict concerted activity. The model accurately reproduced the data. These results suggest that network interactions in the primate retina are well approximated by a nearest neighbor Ising model and concerted activity can be understood based on local interactions within a neural population. Questions? ***PLEASE NOTE: SPECIAL DAY, TIME, and LOCATION*** Thursday, December 11th, 12:00pm ISB 102 Tonia Venters (Department of Astronomy and Astrophysics, Kavli Institute for Cosmological Physics, The University of Chicago) "The Multi-wavelength View of the Gamma-ray Sky " The gamma-ray sky observed by the Energetic Gamma-ray Experiment Telescope(EGRET) encodes much information about the high-energy processes in the universe. Of the EGRET resolved sources, blazars comprise the class of gamma-ray emitters with the largest number of identified members. Unresolved blazars are expected to contribute significantly to the diffuse extragalactic gamma-ray emission. However, blazars are also broadband emitters (from radio to TeV energies), and as such the multi-wavelength study of blazars can provide insight into the high-energy processes of the universe.