Baryon Number Flow in High-Energy Collisions

It is not obvious which partons in the proton carry its baryon number (BN). We present arguments that BN is associated with a specific topology of gluonic fields, rather than with the valence quarks. The BN distribution is easily confused with the difference between the quark and antiquark distributions. We argue, however, that they have quite different x-dependences. The distribution of BN asymmetry distribution is nearly constant at small x while q(x)-\bar q(x) \propto \sqrt{x}. This constancy of BN produces energy independence of the \bar pp annihilation cross section at high energies. Recent measurement of the baryon asymmetry at small x at HERA confirms this expectation. The BN asymmetry at mid-rapidities in heavy ion collisions is substantially enhanced by multiple interactions, as has been observed in recent experiments at the SPS. The same gluonic mechanism of BN stopping increases the production rate for cascade hyperons in a good accord with data. We expect nearly the same as at SPS amount of BN stopped in higher energy collisions at RHIC and LHC, which is, however, spread ove larger rapidity intervals.Comment: The estimated baryon stopping at RHIC is corrected in the Summar

A polarised QCD condensate: nu p elastic scattering as a probe of U_A(1) dynamics

U_A(1) dynamics have the potential to induce a polarised condensate inside a nucleon. The formation of this condensate is related to the realisation of U_A(1) symmetry breaking by tunneling processes such as instantons. If it is present, the polarised condensate induces a term in g_1 which has support only at x=0. Tunneling processes then induce a net transfer of ``spin'' from finite x to x=0. The polarised condensate may be measured by comparing the flavour-singlet axial charges which are extracted from polarised deep inelastic and nu p elastic scattering experiments.Comment: 13 pages LaTeX, Section 3 improved to include discussion of the 3 flavour quark instanton interaction; to appear in Mod. Phys. Lett.

Effects of sugars on lipid bilayers during dehydration - SAXS/WAXS measurements and quantitative model

We present an X-ray scattering study of the effects of dehydration on the bilayer and chain-chain repeat spacings of dipalmitoylphosphatidylcholine bilayers in the presence of sugars. The presence of sugars has no effect on the average spacing between the phospholipid chains in either the fluid or gel phase. Using this finding, we establish that for low sugar concentrations only a small amount of sugar exclusion occurs. Under these conditions, the effects of sugars on the membrane transition temperatures can be explained quantitatively by the reduction in hydration repulsion between bilayers due to the presence of the sugars. Specific bonding of sugars to lipid headgroups is not required to explain this effect

Kinetics of the lamellar gel-fluid transition in phosphatidylcholine membranes in the presence of sugars

Phase diagrams are presented for dipalmitoylphosphatidylcholine (DPPC) in the presence of sugars (sucrose) over a wide range of relative humidities (RHs). The phase information presented here, determined by small angle X-ray scattering (SAXS), is shown to be consistent with previous results achieved by differential scanning calorimetry (DSC). Both techniques show a significant effect of sucrose concentration on the phase behaviour of this phospholipid bilayer. An experimental investigation into the effect of sugars on the kinetic behaviour of the gel to fluid transition is also presented showing that increasing the sugar content appears to slightly increase the rate at which the transition occurs

Location of sugars in multilamellar membranes at low hydration

Severe dehydration is lethal for most biological species. However, there are a number of organisms which have evolved mechanisms to avoid damage during dehydration. One of these mechanisms is the accumulation of small solutes (e.g. sugars), which have been shown to preserve membranes by inhibiting deleterious phase changes at low hydration. Specifically, sugars reduce the gel to fluid phase transition temperatures of model lipid/water mixtures. However, there is a debate about the precise mechanism, the resolution of which hinges on the location of the sugars. In excess water, it has been observed using contrast variation SANS that the sugar concentration in the excess phase is higher than in the interlamellar region [Deme and Zemb, J. Appl. Crystallog. 33 (2000) 569]. This raises two questions regarding the location of the sugars at low hydrations: first, does the system phase separate to give a sugar/water phase in equilibrium with a lipid/water/sugar lamellar region (with different sugar concentrations); and second, is the sugar in the interlamellar region uniformly distributed, or does it concentrate preferentially either in close proximity to the lipids, or towards the center of the interbilayer region. In this paper we present the preliminary results of measurements using contrast variation SANS to determine the location of sugars in lipid/water mixtures