The terminal rise velocity of 10-100 microm diameter bubbles in water.

Single bubbles of very pure N2, He, air and CO2 were formed in a quiescent environment in ultra-clean water, with diameters ranging from 10 to 100 mum. Their terminal rise velocities were measured by high-speed video microscopy. For N2, He and air, excellent agreement with the Hadamard-Rybczynski (H-R) equation was observed, indicating that slip was occurring at the liquid-vapor interface. For CO2 bubbles with diameters less than 60 microm, the terminal rise velocities exceeded those predicted by the H-R equation. This effect was ascribed to the enhanced solubility of CO2 compared with the other gases examined. The presence of a diffusion boundary layer may be responsible for the increased terminal velocity of very small CO2 bubbles.     

Soft gluon corrections to large-pT direct photon and dilepton production

QCD O(α_s²) corrections to large-p_T real and virtual photon production from scattering in the quark-gluon channel are studied in the soft gluon limit. In this limit we carefully preserve the analytic structure of amplitudes in the small-momenta integration regions. The result is a rather large correction to the Born term which significantly improves the agreement with data on p + p → γ + X. The application of the approach to other processes is discussed and the dominant terms of the O(α_s²) correction to e⁺e^? → three jets are reproduced.     

Nd: Laser induced absorption in semiconductors and aqueous PrCl3 and NdCl3

The insertion of a multi-photon absorber within a laser cavity has practical importance in providing stabilised laser amplitude and variable pulse duration. We report the relative non-linear absorption coefficients of semiconductors Si, CdSe, CdTe, and GaAs at 1.064m using a Q-switched Nd:YAG laser. The measurements on Si were carried out from 77 to 450° K, and the results confirm a stepwise process giving rise to induced absorption. The effects of the spatial and temporal distributions of the laser emission on the absolute non-linear coefficient values are briefly discussed. Preliminary observation of stepwise multi-photon absorption in aqueous PrCl₃ and NdCl₃ measured with a pulsed Nd:glass laser is also reported. The possibility of using a combination of inducible and saturable absorbers, placed within the laser cavity, as a means of providing laser intensities confined to a narrow range centred about a pre-set intensity, is raised.Alfred P. Solan Foundation Fellow.     

On stationary modes in inviscid rotating fluids

This paper studies the spectrum of the generator of the group of motions of an inviscid fluid in a slowly rotating container. It is shown that for all containers the spectrum is $\text{{ix ¦ x ? [? 2, 2]}}$. Analogous results are obtained for axisymmetric motions of a large rotating ring of fluid (the “Poincaré problem”), and a family of examples exhibiting various mixtures of continuous and point spectrum is given for this case.     

Measuring the transverse polarization of quarks in the proton

The transverse spin polarization distribution of quarks in a proton contributes at leading twist to the angular distribution of lepton pair production in doubly polarized proton-proton collisions. Although this distribution is straightforward to define in the QCD-improved parton model and represents a fundamental quark degree of freedom, it has never been measured. Moreover, deeply inelastic electroproduction experiments are only sensitive to this distribution at the higher twist level, and then mix this contribution with others, including the one coming from multiparton initial states. Reviewing this, we conclude that the Drell-Yan process is the best way to measure the quark transverse spin polarization distribution.Unité propre du CNRS