Relativistic Structure of the Deuteron: 1.Electro-disintegration and y-scaling

Realistic solutions of the spinor-spinor Bethe-Salpeter equation for the deuteron with realistic interaction kernel including the exchange of pi, sigma, omega, rho, eta and delta mesons, are used to systematically investigate relativistic effects in inclusive quasi-elastic electron-deuteron scattering within the relativistic impulse approximation. Relativistic y-scaling is considered by generalising the non relativistic scaling function to the relativistic case, and it is shown that y-scaling does occur in the usual relativistic scaling variable resulting from the energy conservation in the instant form of dynamics. The present approach of y-scaling is fully covariant, with the deuteron being described by eight components, viz. the 3S_1^{++}, 3S_1^{--}, 3D_1^{++}, 3D_1^{--}, 3P_1^{+-}, 3P_1^{-+}, 1P_1^{+-}, 1P_1^{-+} waves. It is demonstrated that if the negative relative energy states 1P_1, 3P_1 are disregarded, the concept of covariant momentum distributions N(p_0,p), with p_0=M_D/2-\sqrt{p^2+m^2}, can be introduced, and that calculations of lectro-disintegration cross section in terms of these distributions agree within few percents with the exact calculations which include the 1P_1, 3P_1 states, provided the nucleon three momentum |p|\<= 1 GeV/c; in this momentum range, the asymptotic relativistic scaling function is shown to coincide with the longitudinal covariant momentum distribution.Comment: 32 LaTeX pages, 18 eps-figures. Final version to appear in Phys. Rev.

Tensor Polarization of the phi meson Photoproduced at High t

As part of a measurement of the cross section of $\phi$ meson photoproduction to high momentum transfer, we measured the polar angular decay distribution of the outgoing $K^+$ in the channel $\phi \to K^+K^-$ in the $\phi$ center-of-mass frame (the helicity frame). We find that s-channel helicity conservation (SCHC) holds in the kinematical range where $t$-channel exchange dominates (up to $-t \sim 2.5$ GeV$^2$ for $E_{\gamma}$=3.6 GeV). Above this momentum, $u$-channel production of a $\phi$ meson dominates and induces a violation of SCHC. The deduced value of the $\phi NN$ coupling constant lies in the upper range of previously reported values.Comment: 6 pages; 5 figure

The pathogen Neisseria meningitidis requires oxygen, but supplements growth by denitrification. Nitrite, nitric oxide and oxygen control respiratory flux at genetic and metabolic levels

The human pathogen Neisseria meningitidis is the major causative agent of bacterial meningitis. The organism is usually treated as a strict aerobe and is cultured under fully aerobic conditions in the laboratory. We demonstrate here that although N. meningitidis fails to grow under strictly anaerobic conditions, under oxygen limitation the bacterium expresses a denitrification pathway (reduction of nitrite to nitrous oxide via nitric oxide) and that this pathway supplements growth. The expression of the gene aniA, which encodes nitrite reductase, is regulated by oxygen depletion and nitrite availability via transcriptional regulator FNR and two-component sensor-regulator NarQ/NarP respectively. Completion of the two-step denitrification pathway requires nitric oxide (NO) reduction, which proceeds after NO has accumulated during batch growth under oxygen-limited conditions. During periods of NO accumulation both nitrite and NO reduction are observed aerobically, indicating N. meningitidis can act as an aerobic denitrifier. However, under steady-state conditions in which NO is maintained at a low concentration, oxygen respiration is favoured over denitrification. NO inhibits oxidase activity in N. meningitidis with an apparent Ki NO = 380 nM measured in intact cells. The high respiratory flux to nitrite after microaerobic growth and the finding that accumulation of the denitrification intermediate NO inhibits oxygen respiration support the view that denitrification is a pathway of major importance in N. meningitidis