Bayesian analysis of pentaquark signals from CLAS data

We examine the results of two measurements by the CLAS collaboration, one of which claimed evidence for a Theta(+) pentaquark, while the other found no such evidence. The unique feature of these two experiments was that they were performed with the same experimental setup. Using a Bayesian analysis, we find that the results of the two experiments are in fact compatible with each other, but that the first measurement did not contain sufficient information to determine unambiguously the existence of a Theta(+). Further, we suggest a means by which the existence of a new candidate particle can be tested in a rigorous manner.     

Spectral editing in (13)C MAS NMR under moderately fast spinning conditions

Novel procedures for the spectral assignment of peaks in high-resolution solid-state (13)C NMR are discussed and demonstrated. These methods are based on the observation that at moderate and already widely available rates of magic-angle spinning (10--14 kHz MAS), CH and CH(2) moieties behave to a large extent as if they were effectively isolated from the surrounding proton reservoir. Dipolar-based analogs of editing techniques that are commonly used in liquid-state NMR such as APT and INEPT can then be derived, while avoiding the need for periods of homonuclear (1)H--(1)H multipulse decoupling. The resulting experiments end up being very simple, essentially tuning-free, and capable of establishing unambiguous distinctions among CH, CH(2), and --C--/-CH(3) carbon sites. The principles underlying such sequences were explored using both numerical calculations and experimental measurements, and once validated their editing applications were illustrated on a number of compounds.     

Efficient Synthesis of 3,5‐Dicarbamoyl‐1,4‐dihydropyridines from Pyridinium Salts: Key Molecules in Understanding NAD(P)+/NAD(P)H Pathways

3,5‐Dicarbamoyl‐1,4‐dihydropyridines were prepared in high yields using a green protocol by reduction of the corresponding pyridinium salts in aqueous buffered sodium dithionite solutions. The pH value is a fundamental parameter for the reduction step and depends on the nature of substituent groups at positions 1, 3, and 5 of the pyridinium salts. These 3,5‐dicarbamoyl dihydropyridines show a lower tendency towards oxidation and a higher stability than N‐benzyl‐3‐carbamoyl‐1,4‐dihydropyridine at low pH values.     

Self‐Assembly of Decoupled Borazines on Metal Surfaces: The Role of the Peripheral Groups

Two borazine derivatives have been synthesised to investigate their self‐assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close‐packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl‐4‐phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1 , this is enough to weaken the molecule–substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2 , a stronger molecule–substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.     

Contact Problems for Interface Cracks under Harmonic Loading

The linear crack located at the bimaterial interface between dissimilar homogeneous linearly elastic materials under normally incident harmonic tension-compression wave is considered in the study. The problem is solved by the method of boundary integral equations using an iterative algorithm. The dynamic stress intensity factors are computed as functions of the loading frequency taking the contact interaction of the opposite crack's faces into account. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Weak and Ergodic Limit of the Spectral Shift Function

We investigate convergence properties of the spectral shift functions associated with a pair of Schrödinger operators with Dirichlet boundary conditions at the end points of a finite interval as the length of interval approaches infinity.     

Role of surface reconstructions in (111) silicon fracture

The (111) cleavage in crystalline silicon was investigated by hybrid quantum/classical atomistic simulations showing that its remarkable stability is largely due to asymmetric π-bonded reconstructions of the cleavage surfaces created by the advancing crack front. Further simulations show that the same reconstructions can induce an asymmetric dynamical response to added shear stress components. This explains why [211] upward steps are much more common than [211] downward steps on (111) cleavage surfaces, while "zigzag" cleavage with alternated (111) and (111) facets will still occur in crystal samples fractured under [110] uniaxial loading.     

Search for Theta++ pentaquarks in the exclusive reaction gammap-->K+K-p

The reaction gammap --> pK+K- was studied at Jefferson Lab with photon energies from 1.8 to 3.8 GeV using a tagged photon beam. The goal was to search for a Theta++ pentaquark, a narrow, doubly charged baryon state having strangeness S=+1 and isospin I=1, in the pK+ invariant mass spectrum. No statistically significant evidence of a Theta++ was found. Upper limits on the total and differential cross section for the reaction gammap --> K-Theta++ were obtained in the mass range from 1.5 to 2.0 GeV/c2, with an upper limit for a narrow resonance with a mass M(Theta++) = 1.54 GeV/c2 of about 0.15 nb, 95% C.L.. This result places a stringent upper limit on the Theta++ width Gamma(Theta++) <0.1 MeV/c2.