« fall 2014 - spring 2015 - summer 2015 »

This week | Next week | This semester | All future | Print view

This week | Next week | This semester | All future | Print view

Wednesday, January 21st 2015

1:25 pm:

Wednesday, January 28th 2015

1:25 pm:

Wednesday, February 4th 2015

1:25 pm:

Wednesday, February 11th 2015

1:25 pm:

Wednesday, February 25th 2015

1:25 pm:

A Type-I X-ray burst is the thermonuclear shell flash that happens on the surface of a neutron star. Studies on these bursts are of importance for understanding neutron stars in binary systems, nuclear reaction networks and dense matter at low temperature. I will discuss a subset of X-ray bursts that is powerful to lift up the photosphere with the simulations based on a new 1D turbulence model. The model is different in a sense that the turbulence is generated by a stochastic process and the stability of the system. The results will be compared with KEPLER's.

Wednesday, March 4th 2015

1:25 pm:

Low-mass ( 8-10 solar masses) supernovae (ECSNe) which result from the collapse of O-Ne-Mg cores triggered by electron capture could provide a source of elements heavier than iron. The yields of nucleosynthesis for a 8.8 solar mass ECSN was studied based on the hydrodynamic simulation with state-of-art treatment for neutrino transport. In terms of the yield relative to iron, this nucleosynthesis is dominated by Sc and Co, and is very different from that for a regular supernova (heavier than 10 solar masses). Further, some proton-rich nuclei could be synthesized in significant amounts even in ECSN ejecta with equal number of protons and neutrons. Neutrino interactions on light nuclei are observed to smooth the yields of heavy nuclei. These results are analyzed using the theory of quasi-equilibrium nucleosynthesis. Due to their relatively clear nucleosynthetic signatures, the contributions of ECSNe could be inferred from the abundances in some extremely metal-poor stars with high [Sc/Fe] and [Co/Fe].

Wednesday, March 11th 2015

1:25 pm:

Observations strongly indicate that chemical enrichment in nearby dwarf spheroidal galaxies (dSphs) remains inhomogeneous until the end of their star formation despite their small size. Motivated by this unsolved problem, I built a chemical evolution model for Fornax, the brightest dSph in Milky Way, based on its star formation history. I simulated stochastic and inhomogeneous mixing of newly-synthesized elements by supernovae and compared the results with the observed metallicity distributions and scatter in abundances of individual elements of e.g., Mg, Si, Ca, Ti, and Fe. This approach not only can test supernova nucleosynthesis models, but also provides insights into mixing of supernova ejecta with the interstellar medium. I found that this mixing depends on large-scale gas flows, and the differences between environments of core-collapse and Type Ia supernovae.

Wednesday, March 18th 2015

1:25 pm:

Wednesday, March 25th 2015

1:25 pm:

Wednesday, April 1st 2015

1:25 pm:

Understanding the origin of the short-lived radio isotopes (SLR) that were present in the early solar system (ESS) is crucial in understanding the events leading to the formation of the Solar System. Many of the SLR require a nucleosynthetic event within about a million years before the formation of the Solar System. We show that low mass compact progenitors of can self-consistently account of some of the very short-lived isotopes such as ^{41} Ca, ^{53} Mn, and ^{60} Fe. Furthermore, neutrino-induced spallation can also account for ^{10} Be in the ESS, which until now were thought to be produced only by cosmic ray irradiation.

Wednesday, April 8th 2015

1:25 pm:

Wednesday, April 15th 2015

1:25 pm:

Basis Light Front Quantizition (BLFQ) is a nonperturbative treatment of quantum field theory, which adopts light-front quantization and the Hamiltonian formalism. BLFQ offers a first principles solution to many outstanding puzzles in nuclear and particle physics. BLFQ can also be used to study time-dependent scattering processes (tBLFQ). As its first realistic application, we apply principles of tBLFQ to lepton pair production in ultraperipheral heavy ion collisions. Due to the strong electromagnetic fields generated in heavy ion collision, perturbative approaches may lose its prediction power especially if two heavy ions collide with small impact parameter. As a non perturbative approach, tBLFQ takes into account the production and multiple-scattering of the leptons by the strong electromagnetic fields generated in the collisions inherently. Preliminary results with highly truncated basis will be presented.

Wednesday, April 22nd 2015

1:25 pm:

Wednesday, April 29th 2015

1:25 pm:

This talk will describe the calculations of iso-vector and iso-scalar nucleon charges g_A, g_S and g_T. These quantities are needed to probe new physics in precision experiments of neutron decay and electric dipole moment nEDM. Discussion of lattice QCD results will address systematic uncertainties associated with the lattice volume, lattice spacing, quark mass and renormalization of the novel CP violating operators. I will provide results for the quark electric dipole moment contribution to nEDM and the status of the calculations of the quark chromoelectric dipole moment being calculated by the LANL-RBC-UKQCD collaboration.

Wednesday, May 6th 2015

1:25 pm:

Friday, September 18th 2015

11:15 am:

We point out that high-energy neutrinos produced by relativistic jets can be annihilated with thermal neutrinos emitted by the associated accretion disks in gamma-ray bursts and core-collapse supernovae. For a broad range of conditions, the emerging all-flavor spectrum for neutrinos of E >∼ 0.1 PeV is modified by a factor E^(−n) with n ≈ 0.4–0.5. Flavor evolution of accretion-disk neutrinos does not affect this result but can change the emerging flavor composition of high-energy neutrinos. The above signatures provide potential tests of detailed models by IceCube.

Friday, September 25th 2015

11:15 am:

Friday, October 2nd 2015

11:15 am:

Friday, October 9th 2015

11:15 am:

Friday, October 16th 2015

11:15 am:

Friday, October 23rd 2015

11:15 am:

Nuclei colliding at very high energy create a strong, quasi-classical gluon field during the initial phase of their interaction. We present an analytic calculation of the initial space-time evolution of this field in the limit of very high energies using a formal recursive solution of the Yang-Mills equations. We provide analytic expressions for the initial chromo-electric and chromo-magnetic fields and for their energy-momentum tensor. In particular, we discuss event-averaged results for energy density and energy flow as well as for longitudinal and transverse pressure of this system. Our results are generally applicable if the time is less than the inverse of the saturation scale Q_s. The transverse energy flow of the gluon field exhibits hydrodynamic-like contributions that follow transverse gradients of the energy density. In addition, a rapidity-odd energy flow also emerges from the non-abelian analog of Gauss' Law and generates non-vanishing angular momentum of the field. We will discuss the space-time picture that emerges from our analysis and its implications for observables in heavy ion collisions.

Friday, October 30th 2015

11:15 am:

Friday, November 6th 2015

11:15 am:

Friday, November 13th 2015

11:15 am:

It is generally assumed that neutrino wave packets (WPs) do not overlap when they emerge from the source. We examine this assumption by modeling neutrinos as Gaussian WPs. A 3D solution with proper spherical shape has been derived to describe the propagation of a massless Gaussian WP. By using this 3D solution, we define the volume "occupied" by each WP to be the region in which the probability of finding the neutrino is 90%. A numerical factor is further defined as an indicator of the extent to which the WPs overlap in space. In particular, we consider the overlap among those neutrinos with mean energies differing by no more than the intrinsic energy uncertainty in a WP. The overlap factor depends on how sharp the transverse momentum distribution of the initial WP is and the differential production rate of the neutrino source. The estimate of the factor suggests that such WP overlap is significant for solar and supernova neutrinos and is potentially significant for neutrinos from radioactive sources and fission reactors. We raise the question of whether the ferminonic nature of neutrinos affect experimental observables if such overlap is significant.

Friday, November 20th 2015

11:15 am:

Using fluid/gravity correspondence, we study all-order resummed hydrodynamics in a weakly curved spacetime. The underlying microscopic theory is a finite temperature \mathcal{N}=4 super-Yang-Mills theory at strong coupling. To linear order in the amplitude of hydrodynamic variables and metric perturbations, the fluid's stress-energy tensor is computed with derivatives of both the fluid velocity and background metric resummed to all orders. In addition to two viscosity functions, we find four curvature induced structures coupled to the fluid via new transport coefficient functions, which were referred to as gravitational susceptibilities of the fluid (GSF). We analytically compute these coefficients in the hydrodynamic limit, and then numerically up to large values of momenta. We extensively discuss the meaning of all order hydrodynamics by expressing it in terms of the memory function formalism, which is also suitable for practical simulations. We also consider Gauss-Bonnet correction in the dual gravity, which is equivalent to some 1/N corrections in the dual CFT. To leading order in the Gauss-Bonnet coupling, we find that the memory function is still vanishing.

Friday, November 27th 2015

11:15 am:

Friday, December 4th 2015

11:15 am:

I will briefly review the Color Glass Condensate framework, which was proposed to describe the high energy limit of hadronic wave functions. Its classical realization, the McLerran-Venugopalan[MV] model will be used to study the early time dynamics of relativistic heavy ion collisions. Within the MV model, we solve the classical Yang-Mills equations analytically via a near-field(small-\tau) power series expansion. Energy-momentum tensor of the gluon field is calculated to all orders under a leading Q^2 approximation. Our calculations favor an early pressure isotropization at the time of the scale Q_s^{-1}.

Friday, December 11th 2015

11:15 am:

A Type-I X-ray burst is the thermonuclear runaway that occurs on the surface of a neutron star in a binary system. Studies on these bursts are of great importance for understanding neutron stars, nuclear reactions and the equation of state of dense matter at low temperature. I will discuss a subset of X-ray bursts, photospheric radius expansion bursts, that is powerful to lift up the photosphere of the star with the simulations based on a new 1D turbulence model, ODT. The model is different in that the turbulent motion is implemented according to a stochastic process and an eddy event is represented by a measure-preserving map. I will compare the light curve, abundances, and turbulent motion development with a KEPLER model in which the traditional mixing length theory is applied. The light curves of both models will be compared with observational data.

Friday, December 18th 2015

11:15 am:

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.