Tuesdays 10:30 - 11:30 | Fridays 11:30 - 12:30
Showing votes from 2016-08-26 12:30 to 2016-08-30 11:30 | Next meeting is Friday Aug 8th, 11:30 am.
We report constraints on spin-independent weakly interacting massive particle (WIMP)-nucleon scattering using a 3.35e4 kg-day exposure of the Large Underground Xenon (LUX) experiment. A dual-phase xenon time projection chamber with 250 kg of active mass is operated at the Sanford Underground Research Facility under Lead, South Dakota (USA). With roughly four-fold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeV/c^2, WIMP-nucleon spin-independent cross sections above 2.2e-46 cm^2 are excluded at the 90% confidence level.
Einstein's weak equivalence principle (EEP) can be tested through the arrival time delay between photons with different frequencies. Assuming that the arrival time delay is solely caused by the gravitational potential of the Milky Way, we show that a "nano-shot" giant pulse with an unresolved duration $\Delta t_{\rm{obs}}-\Delta t_{\rm{DM}}<0.4~\rm{ns}$ from the Crab pulsar poses a new upper limit on the deviation from EEP, i.e. $\Delta\gamma < 8\times 10^{-16}$. This result provides the hitherto most stringent constraint on the EEP, improving by at least 2 to 3 orders of magnitude from the previous results based on fast radio bursts.
Given the complexity of modern cosmological parameter inference where we are faced with non-Gaussian data and noise, correlated systematics and multi-probe correlated data sets, the Approximate Bayesian Computation (ABC) method is a promising alternative to traditional Markov Chain Monte Carlo approaches in the case where the Likelihood is intractable or unknown. The ABC method is called "Likelihood free" as it avoids explicit evaluation of the Likelihood by using a forward model simulation of the data which can include systematics. We introduce astroABC, an open source ABC Sequential Monte Carlo (SMC) sampler for parameter estimation. A key challenge in astrophysics is the efficient use of large multi-probe datasets to constrain high dimensional, possibly correlated parameter spaces. With this in mind astroABC allows for massive parallelization using MPI, a framework that handles spawning of jobs across multiple nodes. A key new feature of astroABC is the ability to create MPI groups with different communicators, one for the sampler and several others for the forward model simulation, which speeds up sampling time considerably. For smaller jobs the Python multiprocessing option is also available. Other key features include: a Sequential Monte Carlo sampler, a method for iteratively adapting tolerance levels, local covariance estimate using scikit-learn's KDTree, modules for specifying optimal covariance matrix for a component-wise or multivariate normal perturbation kernel, output and restart files are backed up every iteration, user defined metric and simulation methods, a module for specifying heterogeneous parameter priors including non-standard prior PDFs, a module for specifying a constant, linear, log or exponential tolerance level, well-documented examples and sample scripts. This code is hosted online at https://github.com/EliseJ/astroABC
In the near future, gravitational wave detection is set to become an important observational tool for astrophysics. It will provide us with an excellent means to distinguish different gravitational theories. In effective form, many gravitational theories can be cast into an f(R) theory. In this article, we study the dynamics and gravitational waveform of an equal-mass binary black hole system in f(R) theory. We reduce the equations of motion in f(R) theory to the Einstein-Klein-Gordon coupled equations. In this form, it is straightforward to modify our existing numerical relativistic codes to simulate binary black hole mergers in f(R) theory. We considered binary black holes surrounded by a shell of scalar field. We solve the initial data numerically using the Olliptic code. The evolution part is calculated using the extended AMSSNCKU code. Both codes were updated and tested to solve the problem of binary black holes in f(R) theory. Our results show that the binary black hole dynamics in f(R) theory is more complex than in general relativity. In particular, the trajectory and merger time are strongly affected. Via the gravitational wave, it is possible to constrain the quadratic part parameter of f(R) theory in the range |a_2|<10^{11} m^2 . In principle, a gravitational wave detector can distinguish between a merger of binary black hole in f(R) theory and the respective merger in general relativity. Moreover, it is possible to use gravitational wave detection to distinguish between f(R) theory and a non self-interacting scalar field model in general relativity.
As direct dark matter experiments continue to increase in size, they will become sensitive to neutrinos from astrophysical sources. For experiments that do not have directional sensitivity, coherent neutrino scattering (CNS) from several sources represents an important background to understand, as it can almost perfectly mimic an authentic WIMP signal. Here we explore in detail the effect of neutrino backgrounds on the discovery potential of WIMPs over the entire mass range of 500 MeV to 10 TeV. We show that, given the theoretical and measured uncertainties on the neutrino backgrounds, direct detection experiments lose sensitivity to light (~10 GeV) and heavy (~100 GeV) WIMPs with a spin-independent cross section below 10^{-45} cm^2 and 10^{-49} cm^2, respectively.
In recent literature one-loop tests of the higher-spin AdS$_{d+1}$/CFT$_d$ correspondences were carried out. Here we extend these results to a more general set of theories in $d>2$. First, we consider the Type B higher spin theories, which have been conjectured to be dual to CFTs consisting of the singlet sector of $N$ free fermion fields. In addition to the case of $N$ Dirac fermions, we carefully study the projections to Weyl, Majorana, symplectic, and Majorana-Weyl fermions in the dimensions where they exist. Second, we explore theories involving elements of both Type A and Type B theories, which we call Type AB. Their spectrum includes fields of every half-integer spin, and they are expected to be related to the $U(N)/O(N)$ singlet sector of the CFT of $N$ free complex/real scalar and fermionic fields. Finally, we explore the Type C theories, which have been conjectured to be dual to the CFTs of $p$-form gauge fields, where $p=\frac d 2 -1$. In most cases we find that the free energies at $O(N^0)$ either vanish or give contributions proportional to the free-energy of a single free field in the conjectured dual CFT. Interpreting these non-vanishing values as shifts of the bulk coupling constant $G_N\sim 1/(N-k)$, we find the values $k=-1, -1/2, 0, 1/2, 1, 2$. Exceptions to this rule are the Type B and AB theories in odd $d$; for them we find a mismatch between the bulk and boundary free energies that has a simple structure, but does not follow from a simple shift of the bulk coupling constant.
We provide a simple derivation of the equivalence between Einstein and Conformal Gravity (CG) with Neumann boundary conditions given by Maldacena. As Einstein spacetimes are Bach flat, a generic solution to CG would contain both Einstein and non-Einstein part. Using this decomposition of the spacetime curvature in the Weyl tensor, makes manifest the equivalence between the two theories, both at the level of the action and the variation of it. As a consequence, we show that the on-shell action for Critical Gravity in four dimensions is given uniquely in terms of the Bach tensor.