SCIPP Seminars




Last updated:12/15/10

Tuesday December 14, 10:00am - 12:00pm
Location: ISB 310
SCIPP Tech Meeting
Wednesday November 24, 12:00 - 1:00pm
Location: ISB 310
Yacine Ali-Haïmoud (Caltech)
Title: Fast and accurate primordial hydrogen recombination theory
Abstract: Cosmological hydrogen recombination has recently been the subject of renewed attention because of its importance for predicting the power spectrum of cosmic microwave background anisotropies. Correctly interpreting the upcoming data from the Planck satellite in terms of cosmological parameters indeed requires sub-percent accuracy in theoretical recombination histories. Two aspects are crucial to reach such an accuracy. At early times (z >~800), the dynamics of hydrogen recombination is controlled by the slow decays from the n=2 shell to the ground state, through two-photon decays from the 2s state and the highly self-absorbed Lyman alpha transition. Subtle radiative transfer effects must be accounted for in order to correctly calculate the rate of decays to the ground state of hydrogen. At late times, due to the decreasing abundance of free electrons and protons, an accurate recombination history must account for all the recombination pathways, and include excited states of hydrogen up to a very high principal quantum number n > 100. The cold radiation field at late times is not strong enough to maintain the angular momentum substates in statistical equilibrium, and they must therefore be followed separately. The traditional method of solution for the multi-level atom is very time consuming computationally and unpractical for inclusion in fast Markov chains for cosmological parameter estimation. In this talk I will present my recent work on a new method of solution, which allows to account for an arbitrarily large number of excited states, and is 5 to 6 orders of magnitude faster than the previously used method. I will also expose my recent work on radiative transfer effects and my upcoming  fast and highly accurate recombination code. 
Friday November 19, 11:00am
Location: SINSHEIMER 123
Dr. Evelyne Sernagor (Newcastle University)
Title: Multielectrode array applications for the investigation of retinal plasticity in health and disease
Abstract: Our laboratory investigates various aspects of neural plasticity. In this talk, I will review our two main reseach streams. The first one focuses on immature retinal waves and the underlying retinal connectivity. I will discuss our findings about the developmental changes in wave dynamics. In that context, I will introduce a novel experimental approach that enables recording retinal waves at unprecedented spatiotemporal resolution. This approach consists of a multielectrode array (MEA) with 4,096 active channels, the Active Pixel System MEA. The APS MEA allows us to record virtually from the entire neonatal mouse retina at near cellular resolution. APS MEA recordings reveal, for the first time, that the dynamics of the waves profoundly change when the main network drive of the waves switches from being cholinergic to becoming glutamatergic during the second postnatal week, during the critical period for the wiring of retinal projections in the brain.

    In the second part of the talk, I will introduce the Cone Rod Homeobox (Crx) knockout mouse, a model of retinal dystrophy. Using the Crx mouse, we investigate the retinal biocompatibily and electrical properties of electrodes made of carbon nanotubes (CNTs), a relatively new advanced material, as a potential choice for retinal prosthetic devices. Our findings suggest that CNT-based MEA devices may provide an excellent substrate to develop a new generation of retinal implant stimulating devices.
Wednesday November 10, 11:00am
Location: ISB 310
Samuel Lee (California Institute of Technology)
Title: Light Gravitinos at Colliders and Implications for Cosmology
Abstract: Light gravitinos, with mass in the eV to MeV range, are well-motivated in particle physics, but their status as dark-matter candidates is muddled by early-Universe uncertainties. Upcoming data from colliders may clarify this picture. Light-gravitino collider events should result in spectacular signals, including di-photons, delayed and non-pointing photons, kinked charged tracks, and heavy metastable charged particles. We find that the Tevatron with 20/fb and the 7 TeV LHC with 1/fb may both see evidence for hundreds of such events. Remarkably, this collider data is also well suited to distinguish between currently viable light-gravitino cosmological scenarios, with striking implications for structure formation, inflation, and other early-Universe cosmology. ion.
Monday November 1, 2:00pm
Location: ISB 310
Alexander Belikov (University of Chicago)
Title: Cosmological Dark Matter Annihilation
Abstract: In many scenarios, dark matter annihilates to produce photons and electrons/positrons at rates high enough to be potentially detectable by current experiments, such as the Fermi Gamma-Ray Space Telescope. I will review the calculation of the gamma-ray flux from the annihilation of extragalactic dark matter. The final state electrons and positrons can, through inverse Compton scattering, produce a flux of lower energy photons can significantly alter the shape of the spectrum, and, in a number of scenarios, enhance the possibility for detection. I will also discuss the effect of the annihilating dark matter on the process of cosmological reionization.
Thursday October 28, 2:00pm
Location: ISB 310
Kfir Blum (Weizmann Institute)
Title: Cosmic ray propagation time scales: lessons from radioactive nuclei and positron data


We take a fresh look at high energy radioactive nuclei data reported in the 90's and at the positron data recently reported by PAMELA. Our aim is to study the model independent implications of these data for the propagation time scales of cosmic rays in the Galaxy. Considering radioactive nuclei, using decaying charge to decayed charge ratios -- the only directly relevant data available at relativistic energies -- we show that a rigidity independent residence time is consistent with observations. The data for all nuclei can be described by f_{s,i}=(t_i/100 Myr)^{0.7}, where f_{s,i} is the suppression of the flux due to decay and t_i is the observer frame lifetime for nucleus specie i. Considering positron measurements, we argue that the positron flux is consistent with a secondary origin. Comparing the positron data with radioactive nuclei at the same energy range, we derive an upper bound on the mean electromagnetic energy density traversed by the positrons, \bar U_T<1.25 eV/cm^3 at a rigidity of R=40 GV. Charge ratio measurements within easy reach of the AMS-02 experiment, most notably a determination of the Cl/Ar ratio extending up to R\sim100 GV, will constrain the energy dependence of the positron cooling time. Such constraints can be used to distinguish between different propagation scenarios, as well as to test the secondary origin hypothesis for the positrons in detail.

Monday October 25, 2:00pm
Location: ISB 310
Chiara Caprini (CEA, France)
Title: Gravitational waves from first order phase transitions
Abstract: First order phase transitions in the early universe give rise to a stochastic background of gravitational waves. Three main processes lead to the production of the gravitational wave signal: the collision of the broken phase bubbles, the turbulence in the primordial plasma stirred by the bubble collision, and the magnetic fields amplified by the turbulence. In this seminar I will show how the main features of the gravitational wave spectrum can be predicted by simple, general arguments based on the sources properties, such as their time evolution and space structure. I will discuss detection prospects, in particular for the electroweak and the QCD phase transitions, which can generate a GW signal which is potentially detectable respectively by the space interferometer LISA and by pulsar timing arrays.
Tuesday October 19, 2:00pm
Location: ISB 310
Indresh Dwivedi (Tata Institute of Fundamental Research)
Title: Internal Structure and Appearance of a Naked Singularity during a Gravitational Collapse
Abstract: Singular regions of space time (either naked or black hole) have extreme density, pressure, and curvature and are important from both theoretical and observational point of view. Energy theorems predict the existence of such regions under generic conditions but say nothing about their nature. Structure of such regions of space time if covered (black hole) is invisible to outside world. In cases otherwise the internal structure is visible and rich. In an interesting case a naked singularity (formed during a dust collapse) would appear to a distant observer as an expanding luminous ball. Study of observable signature of such regions (naked singularity) to a distant observer from escaping causal particles could have important physical and observational implications.
Tuesday October 5, 10:30am
Location: ISB 310
Title: Detector Performance Improvements using Wafer Thinning and Singulating Techniques
Abstract: I will present ways in which the performance of silicon detectors and arrays is being improved. Some factors known to degrade detector and array performance will be outlined.  Factors outlined will be those that can be affected by thinning and singulation. It will be shown that the processes can be effectively scaled from an R&D lab to a high volume production environment.
Thursday September 9, 11:00am
Location: ISB 310
Tobi Szuts (Program in Biophysics, Harvard University)
Title: Wireless recording of neural activity in the visual cortex
Abstract:   Conventional neural recording systems restrict behavioral experiments to a flat indoor environment compatible with the cable that tethers the subject to the recording instruments. To overcome these constraints, we developed a wireless multi-channel system for recording neural signals. The device takes up to 64 voltage signals from samples each at 20 kHz, time-division multiplexes them onto a single output line, and transmits that output by radio frequency to a receiver and recording computer up to >60 m away. The system introduces less than 4 μV RMS of electrode-referred noise, comparable to wired recording systems and considerably less than biological noise. The system has greater channel count or transmission distance than existing telemetry systems. The wireless system has been used to record from the visual cortex during unconstrained conditions. Outdoor recordings show V1 activity is modulated by nestbuilding activity. During unguided behavior indoors, neurons responded rapidly and consistently to changes in light level, suppressive effects were prominent in response to an illuminant transition, and firing rate was strongly modulated by locomotion. Neural firing in the visual cortex is relatively sparse and moderate correlations are observed over large distances, suggesting that synchrony is driven by global processes.
Tuesday September 7, 10:30am
Location: ISB 310
John Jaros (SLAC)
Title: "A Search for Massive Photons at Jefferson Laboratory "

Abstract: Does Nature have additional, massive U(1) gauge bosons? String theories, among others, predict such particles. General considerations lead us to believe they would kinetically mix with our photon, which would induce small couplings to electric charge. Recent astrophysical evidence for high energy electrons and positrons in the cosmic rays might be evidence for dark matter annihilations into heavy photons which subsequently decayed into high energy lepton pairs. This talk will discuss a proposal to search for massive photons in a new experiment at Jefferson Laboratory. Heavy photons would be produced in electron bremstrahlung on heavy targets, and decay to electron-positron pairs. They can be distinguished from the abundant QED trident background by virtue of their invariant mass and finite decay lengths. The talk will review motives for the search, the proposed apparatus, simulated performance, and how well we hope to do.