Little Higgs model effects in γγ→γγ\gamma \gamma \to \gamma \gamma

Though the predictions of the Standard Model (SM) are in excellent agreement with experiments there are still several theoretical problems associated with the Higgs sector of the SM, where it is widely believed that some ``{\it new physics}'' will take over at the TeV scale. One beyond the SM theory which resolves these problems is the Little Higgs (LH) model. In this work we have investigated the effects of the LH model on \gggg scattering \cite{Choudhury:2006xa}.Comment: Talk given at LCWS06, Bangalore, 4 pages (style files included

Testing Reionization with Gamma Ray Burst Absorption Spectra

We propose to study cosmic reionization using absorption line spectra of high-redshift Gamma Ray Burst (GRB) afterglows. We show that the statistics of the dark portions (gaps) in GRB absorption spectra represent exquisite tools to discriminate among different reionization models. We then compute the probability to find the largest gap in a given width range [Wmax, Wmax + dW] at a flux threshold Fth for burst afterglows at redshifts 6.3 < z < 6.7. We show that different reionization scenarios populate the (Wmax, Fth) plane in a very different way, allowing to distinguish among different reionization histories. We provide here useful plots that allow a very simple and direct comparison between observations and model results. Finally, we apply our methods to GRB 050904 detected at z = 6.29. We show that the observation of this burst strongly favors reionization models which predict a highly ionized intergalactic medium at z~6, with an estimated mean neutral hydrogen fraction xHI = 6.4 \pm 0.3 \times 10^-5 along the line of sight towards GRB 050904.Comment: 5 pages, 3 figures, revised to match the accepted version; major change: gap statistics is now studied in terms of the flux threshold Fth, instead of the observed J-band flux FJ; MNRAS in pres

High Power, Continuous-wave Supercontinuum Generation in Highly Nonlinear Fibers Pumped with High Order, Cascaded Raman Fiber Amplifiers

A novel method for efficient generation of high power, equalized continuous-wave supercontinuum source in an all conventional silica fiber architecture is demonstrated. Highly nonlinear fiber (HNLF) is pumped in its anomalous dispersion region using a novel, high power, L-band laser. The L-band laser encompasses a 6th order cascaded Raman amplifier which is pumped with a high power Ytterbium doped fiber laser and amplifies a low-power, tunable L-band seed source. The supercontinuum generated 35W of power with ~40% efficiency. The Supercontinuum spectrum was measured to have a high degree of flatness of better than 5 dB over 400 nm of bandwidth (1.3 - 1.7 micron, limited by spectrum analyzer range) and a power spectral density in this region of >50 mW/nm. The extent of the SC spectrum is estimated to be upto 2 micronComment: 6 pages, 5 figure

Probing large distance higher dimensional gravity from lensing data

The modifications induced in the standard weak-lensing formula if Newtonian gravity differs from inverse square law at large distances are studied. The possibility of putting bounds on the mass of gravitons from lensing data is explored. A bound on graviton mass, esitmated to be about 100 Mpc$^{-1}$ is obtained from analysis of some recent data on gravitational lensing.Comment: 6 pages, 1 figure, added reference

Unitarity constraints on the stabilized Randall-Sundrum scenario

Recently proposed stabilization mechanism of the Randall-Sundrum metric gives rise to a scalar radion, which couples universally to matter with a weak interaction ($\simeq 1$ TeV) scale. Demanding that gauge boson scattering as described by the effective low enerrgy theory be unitary upto a given scale leads to significant constraints on the mass of such a radion.Comment: 10 page Latex 2e file including 4 postscript figures. Accepted in Journal of Physics

Total cross sections for neutron-nucleus scattering

Systematics of neutron scattering cross sections on various materials for neutron energies up to several hundred MeV are important for ADSS applications. Ramsauer model is well known and widely applied to understand systematics of neutron nucleus total cross sections. In this work, we examined the role of nuclear effective radius parameter (r$_0$) on Ramsauer model fits of neutron total cross sections. We performed Ramsauer model global analysis of the experimental neutron total cross sections reported by W. P. Abfalterer, F. B. Bateman, {\it et. al.,}, from 20MeV to 550MeV for nuclei ranging from Be to U . The global fit functions which can fit total cross section data over periodic table are provided along with the required global set of parameters. The global fits predict within $\pm 8$ deviation to data, showing the scope for improvement. It has been observed that a finer adjustment of r$_0$ parameter alone can give very good Ramsauer model description of neutron total scattering data within $\pm 4$ deviation. The required r$_0$ values for Ramsauer model fits are shown as a function of nuclear mass number and an empirical formula is suggested for r$_0$ values as a function of mass number. In optical model approach for neutron scattering, we have modified the real part of Koning-Deleroche potentails to fit the neutron total cross sections using SCAT2 code. The modified potentails have a different energy dependence beyond 200MeV of neutron energy and fit the total cross sections from Al to Pb.Comment: 9 pages, 20figures, Poster number ND-1457, ND2010 Conference in Kore

The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal

Observations of redshifted 21cm radiation from HI at high redshifts is an important future probe of reionization. We consider the Multi-frequency Angular Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint function of the angular multipole l and frequency separation \Delta\nu. The signal at two different frequencies is expected to get decorrelated as \Delta\nu is increased, and quantifying this decorrelation is particularly important in deciding the frequency resolution for future HI observations. This is also expected to play a very crucial role in extracting the signal from foregrounds as the signal is expected to decorrelate much faster than the foregrounds (which are largely continuum sources) with increasing \Delta\nu. In this paper we develop formulae relating the MAPS to different components of the three dimensional HI power spectrum taking into account HI peculiar velocities. We show that the flat-sky approximation provides a very good representation over the angular scales of interest, and a final expression which is very simple to calculate and interpret. We present results considering two models for the HI distribution, namely, (i) DM: where the HI traces the dark matter and (ii) PR: where the effects of patchy reionization are incorporated through two parameters. We find that while the DM signal is largely featureless, the PR signal peaks at the angular scales of the individual bubbles, and the signal is considerably enhanced for large bubble size. For most cases of interest at l \sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less, whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence also carries an imprint of the bubble size and the bias, and is expected to be an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted versio