Kinematic alpha effect in isotropic turbulence simulations

Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivity whose values are independent of the magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent coefficients are also consistent with expectations from the first order smoothing approximation. For small values of \Rm, alpha and turbulent diffusivity are proportional to \Rm. Over finite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic fluctuations. This suggests that small-scale dynamo-generated fields do not make a correlated contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter

Weighing neutrinos using high redshift galaxy luminosity functions

Laboratory experiments measuring neutrino oscillations, indicate small mass differences between different mass eigenstates of neutrinos. The absolute mass scale is however not determined, with at present the strongest upper limits coming from astronomical observations rather than terrestrial experiments. The presence of massive neutrinos suppresses the growth of perturbations below a characteristic mass scale, thereby leading to a decreased abundance of collapsed dark matter halos. Here we show that this effect can significantly alter the predicted luminosity function (LF) of high redshift galaxies. In particular we demonstrate that a stringent constraint on the neutrino mass can be obtained using the well measured galaxy LF and our semi-analytic structure formation models. Combining the constraints from the Wilkinson Microwave Anisotropy Probe 7 year (WMAP7) data with the LF data at z = 4, we get a limit on the sum of the masses of 3 degenerate neutrinos \Sigma m_\nu < 0.52 eV at the 95 % CL. The additional constraints using the prior on Hubble constant strengthens this limit to \Sigma m_\nu < 0.29 eV at the 95 % CL. This neutrino mass limit is a factor of order 4 improvement compared to the constraint based on the WMAP7 data alone, and as stringent as known limits based on other astronomical observations. As different astronomical measurements may suffer from different set of biases, the method presented here provides a complementary probe of \Sigma m_\nu . We suggest that repeating this exercise on well measured luminosity functions over different redshift ranges can provide independent and tighter constraints on \Sigma m_\nu .Comment: 14 pages, 7 figures, submitted to PR

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities (~1010 cm-3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength

Nonlinear absorption of InN/InGaN multiple-quantum-well structures at optical telecommunication wavelengths

We report on the nonlinear optical absorption of InN/ InxGa 1-x N (x=0.8,0.9) multiple-quantum-well structures characterized at 1.55 μm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2 ∼-9× 103 cm/GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption. © 2011 American Institute of Physics.Peer Reviewe

Observation of the Hadronic Transitions Chi_{b 1,2}(2P) -> omega Upsilon(1S)

The CLEO Collaboration has observed the first hadronic transition among bottomonium (b bbar) states other than the dipion transitions among vector states, Upsilon(nS) -> pi pi Upsilon(mS). In our study of Upsilon(3S) decays, we find a significant signal for Upsilon(3S) -> gamma omega Upsilon(1S) that is consistent with radiative decays Upsilon(3S) -> gamma chi_{b 1,2}(2P), followed by chi_{b 1,2} -> omega Upsilon(1S). The branching ratios we obtain are Br(chi_{b1} -> omega Upsilon(1S) = 1.63 (+0.35 -0.31) (+0.16 -0.15) % and Br(chi_{b2} -> omega Upsilon(1S) = 1.10 (+0.32 -0.28) (+0.11 - 0.10)%, in which the first error is statistical and the second is systematic.Comment: submitted to XXI Intern'l Symp on Lepton and Photon Interact'ns at High Energies, August 2003, Fermila

Measurement of Gamma_ee(J/psi), Gamma_tot(J/psi), and Gamma_ee[psi(2S)]/Gamma_ee(J/psi)

Using data acquired with the CLEO detector at the CESR e+e- collider at sqrt{s} = 3.773 GeV, we measure the cross section for the radiative return process e+e- --> gamma J/psi, J/psi --> mu+mu-, resulting in B(J/psi --> mu+mu-) x Gamma_ee(J/psi) = 0.3384 +- 0.0058 +- 0.0071 keV, Gamma_ee(J/psi) = 5.68 +- 0.11 +- 0.13 keV, and Gamma_tot(J/psi) = 95.5 +- 2.4 +- 2.4 keV, in which the errors are statistical and systematic, respectively. We also determine the ratio Gamma_ee[psi(2S)] / Gamma_ee(J/psi) = 0.45 +- 0.01 +- 0.02.Comment: 12 pages postscript,also available through http://www.lns.cornell.edu/public/CLNS/2005

A Study of Exclusive Charmless Semileptonic B Decays and Extraction of |V_{ub}| at CLEO

We have studied semileptonic B decay to the exclusive charmless states pi, rho/omega, eta and eta' using the full 15.5 fb^-1 CLEO Upsilon(4S) sample, with measurements performed in subregions of phase space to minimize dependence on a priori knowledge of the form factors involved. We find total branching fractions B(B^0 -> pi^-l^+nu) = (1.37 +- 0.15_stat +- 0.11_sys) x 10^-4 and B(B^0 -> rho^- l^+ nu) = (2.93 +- 0.37_stat +- 0.37_sys) x 10^-4. We find evidence for B^+ -> eta' l^+ nu, with B(B^+ -> eta' l^+ nu) = (2.66 +- 0.80_stat +- 0.56_sys) x 10^-4 and 1.20 x 10^-4 eta' l^+ nu) < 4.46 x 10^-4 (90% CL). We also limit B(B^+ -> eta l^+ nu) < 1.01 x 10^-4 (90% CL). By combining our B -> pi l nu information with unquenched lattice calculations, we find |V_ub| = (3.6 +- 0.4 +- 0.2 +0.6 -0.4) x 10^-3, where the errors are statistical, experimental systematic, and theoretical systematic, respectively.Comment: 35 pages, 15 figures; revise

New Measurements of Cabibbo-Suppressed Decays of D Mesons in CLEO-c

Using 281 pb^-1 of data collected with the CLEO-c detector, we report on first observations and new measurements of Cabibbo-suppressed decays of D mesons to 2, 3, 4, and 5 pions. Branching fractions of previously unobserved modes are measured to be: B(D^0\to pi^+pi^-pi^0pi^0)=(9.9\pm0.6\pm0.7\pm0.2\pm0.1)x10^-3, B(D^0\to\pi^+\pi^+\pi^-\pi^-\pi^0)=(4.1\pm0.5\pm0.2\pm0.1\pm0.0)x10^-3, B(D^+\to\pi^+\pi^0\pi^0)=(4.8\pm0.3\pm0.3\pm0.2)x10^-3, B(D^+\to\pi^+\pi^+\pi^-\pi^0)=(11.6\pm0.4\pm0.6\pm0.4)x10^-3, B(D^0\to\eta\pi^0)=(0.62\pm0.14\pm0.05\pm0.01\pm0.01)x10^-3, and B(D^0\to\omega\pi^+\pi^-)=(1.7\pm0.5\pm0.2\pm0.0\pm0.0)x10^-3. The uncertainties are from statistics, experimental systematics, normalization and CP correlations (for D^0 modes only). Improvements in other multi-pion decay modes are also presented. The D-->pi pi rates allow us to extract the ratio of isospin amplitudes A(Delta I=3/2)/A(\Delta I=1/2)=0.420\pm0.014(stat)\pm0.016(syst) and the strong phase shift of delta_I=(86.4+-2.8+-3.3) degrees, which is quite large and now more precisely determined.Comment: 9 pages postscript also available through http://www.lns.cornell.edu/public/CLNS/2005/, submitted to PR