Exact effective force between star-polymers in a $\Theta$-solvent

We re-examine here the computation of the effective force between two star-polymers of respective numbers of branches f₁ and f₂, immersed in a common -solvent. Such a force originates essentially from the repulsive three-body interactions. To achieve this, we take advantage of some established results using renormalization theory for three-dimensional star-polymers, or conformal invariance for two-dimensional ones. We first show that, in dimension d = 3, the force, , decreases with the center-to-center distance r as , with the exact universal amplitude . Second, in dimension d = 2, we find that the force decays more slowly as , with the exact universal amplitude . For high distances compared to the gyration radius, , of a single polymer chain at the -point, an exponential decay of the force is expected.Received: 3 February 2004, Published online: 24 May 2004PACS: 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 05.20.-y Classical statistical mechanics     

Some physics highlights from the EUROBALL spectrometer

Exotic, neutron-rich proton-induced spallation products of ²³²Th and ²³⁸U obtained from the PS Booster ISOLDE facility have been investigated by - , - coincidence and spectrum-multiscaling measurements. A new method for the reduction of isobaric contamination enabled to study the unknown region beyond ²⁰⁸Pb for the decay chain A = 217. A new isotope ²¹⁷Bi with a half-life of s was discovered and its -decay studied. For the first time, a half-life value of s for the -decay of ²¹⁷Po was measured.Received: 3 March 2003, Revised: 13 May 2003, Published online: 30 September 2003PACS: 23.20.Lv transitions and level energies - 23.60.+e decay - 27.80.+w 190 A 219 - 29.30.Kv X- and -ray spectroscopyV. Fedoseyev: Present address: ISOLDE, CERN-PPE, CH-1211, Geneva 23, Switzerland.M. Górska: Present address: GSI, Darmstadt, D-64220, Germany.M. Huhta: Present address: Nokia, Tampere, Finland.     

Calculation of the persistence length of a flexible polymer chain with short-range self-repulsion

For a self-repelling polymer chain consisting of n segments we calculate the persistence length , defined as the projection of the end-to-end vector on the direction of the j-th segment. This quantity shows some pronounced variation along the chain. Using the renormalization group and -expansion we establish the scaling form and calculate the scaling function to order . Asymptotically, the simple result emerges for dimension d = 3. Also away from the excluded-volume limit is found to behave very similar to the swelling factor of a chain of length . We carry through simulations which are found to be in good accord with our analytical results. For d = 2 both our and previous simulations as well as theoretical arguments suggest the existence of logarithmic anomalies.Received: 17 November 2003, Published online: 30 March 2004PACS: 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 05.10.Cc Renormalization group methods     

Dynamics of propylene glycol and its oligomers confined in clay

The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time , deduced from neutron scattering at a momentum transfer Q of about , is in perfect agreement with the dielectric -relaxation time, although neutron scattering does not only probe the main ( -) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature is proportional to Q ², indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that (or the dielectric -relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.25.Em Molecular liquids - 68.35.Ja Surface and interface dynamics and vibrations - 61.12.-q Neutron diffraction and scattering     

Scaling behavior of jamming fluctuations upon random sequential adsorption

It is shown that the fluctuations of the jamming coverage upon Random Sequential Adsorption (RSA) ( ), decay with the lattice size according to the power-law , with ,where D is the dimension of the substrate and is the fractal dimension of the set of sites belonging to the substrate where the RSA process actually takes place. This result is in excellent agreement with the figure recently reported by Vandewalle et al. [Eur. Phys. J. B 14, 407 (2000)], namely for the RSA of needles with D = 2 and , that gives . Furthermore, our prediction is in excellent agreement with different previous numerical results. The derived relationships are also confirmed by means of extensive numerical simulations applied to the RSA of dimers on both stochastic and deterministic fractal substrates.Received: 17 October 2003, Published online: 8 December 2003PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 02.50.-r Probability theory, stochastic processes, and statistics     

Irreversible adsorption from dilute polymer solutions

We study irreversible polymer adsorption from dilute solutions theoretically. Universal features of the resultant non-equilibrium layers are predicted. Two broad cases are considered, distinguished by the magnitude of the local monomer-surface sticking rate Q: chemisorption (very small Q) and physisorption (large Q). Early stages of layer formation entail single-chain adsorption. While single-chain physisorption times are typically micro- to milli-seconds, for chemisorbing chains of N units we find experimentally accessible times , ranging from seconds to hours. We establish 3 chemisorption universality classes, determined by a critical contact exponent: zipping, accelerated zipping and homogeneous collapse. For dilute solutions, the mechanism is accelerated zipping: zipping propagates outwards from the first attachment, accelerated by occasional formation of large loops which nucleate further zipping. This leads to a transient distribution of loop lengths s up to a maximum size after time t. By times of order the entire chain is adsorbed. The outcome of the single-chain adsorption episode is a monolayer of fully collapsed chains. Having only a few vacant sites to adsorb onto, late-arriving chains form a diffuse outer layer. In a simple picture we find for both chemisorption and physisorption a final loop distribution and density profile whose forms are the same as for equilibrium layers. In contrast to equilibrium layers, however, the statistical properties of a given chain depend on its adsorption time; the outer layer contains many classes of chain, each characterized by a different fraction of adsorbed monomers f. Consistent with strong physisorption experiments, we find the f values follow a distribution .Received: 13 January 2003, Published online: 8 July 2003PACS: 82.35.-x Polymers: properties; reactions; polymerization - 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 68.08.-p Liquid-solid interfaces     

Miscible displacement of non-Newtonian fluids in a vertical tube

The influence of rheology on the miscible displacement of a viscous fluid by a less viscous, Newtonian one in a vertical tube is studied experimentally as a function of the flow velocity. For Newtonian displaced fluids the transient residual film thickness is nearly of the tube radius at large viscosity ratios between the two fluids in agreement with experimental and numerical results from the literature. For shear-thinning fluids with a zero yield stress (mostly xanthan-water solutions), decreases down to of the radius for the most concentrated solutions. For fluids with a non-zero yield stess, further decreases down to 24-25% of the radius. The orders of magnitude of these values can be obtained through numerical simulations (commercial code) for the various types of fluids. Instabilities of the film at its boundary develop downstream and lead to a reduction of the final thickness of the film at longer times: this reduction is larger for lower viscosity ratios and larger velocities.Received: 15 February 2003, Published online: 8 July 2003PACS: 47.20.Gv Hydrodynamic stability: Viscous instability - 83.60.Wc Rheology: Flow instabilities     

Enhancement of the orientational order parameter of nematic liquid crystals in thin cells

We report measurements of birefringence of several nematic liquid crystals having transverse as well as longitudinal dipole moments in thin (1.4 to ) and thick (7 to cells. Rubbed polyimide-coated glass plates are used to get planar alignment of the nematic director in these cells. We find significant enhancement (6 to ) of ( , where S is the orientational order parameter) in thin cells in all compounds with aromatic cores even at temperatures far ( C) below the nematic-isotropic transition point. The enhancement is larger in compounds having several phenyl rings and lower if the number of phenyl rings is reduced. In a compound that does not have an aromatic core no significant enhancement is observed, implying that the strength of the surface potential depends on the aromaticity of the cores. Assuming a perfect orientational order at the surface, calculations based on the Landau-de Gennes theory show that the thickness averaged enhancement of S is sharply reduced as the temperature is lowered in the nematic phase. The measured order parameter S is further enhanced in thin cells because of the stiffening of the elastic constant which reduces the thermal fluctuations of the nematic director. The combined effect is however too small at low temperatures to account for the experimental data.Received: 22 February 2004, Published online: 24 May 2004PACS: 61.30.-v Liquid crystals - 61.30.Pq Microconfined liquid crystals: droplets, cylinders, randomly confined liquid crystals, polymer dispersed liquid crystals, and porous systems - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitionsSurajit Dhara: Present address: Department of Physics, Birla Institute of Technology and Science, Pilani 333 031, India.     

Can the glass transition in bulk polymers be modeled by percolation picture?

Recent observations (Eur. Phys. J. E 9, 135 (2002)) showed that the vitrification process, which sets in during the linear bulk methyl methacrylate (MMA) polymerization carried out below glass transition temperatures, can be modelled by static percolation picture. To generalize this observation for different kind of bulk linear or crosslinked polymers not enough data are present in the literature. To cover partly this deficit we studied the glass transition of MMA and styrene (Sty) crosslinking copolymerization in varying ratios of MMA and Sty. Both the fluorescence intensity I and the lifetime of pyrene (Py) used as a nanosecond in situ fluoroprobe were monitored during the gelation time. Both I and increase dramatically as a result of the reduced mobility of the probes trapped in the glassy regions, appearing near the glass transition point. The average size of the glassy regions just below, and the strength of the infinite network formed upon the connection of the glassy regions above the glass transition point obey power law relations. The data around were interpreted on the basis of the percolation theory and we observed that the corresponding exponents and give static percolation values independent of the polymer composition.Received: 9 July 2004, Published online: 1 October 2004PACS: 64.60.Ak Renormalization-group, fractal, and percolation studies of phase transitions - 64.70.Pf Glass transitions - 82.35.Jk Copolymers, phase transitions, structure     

Different effect of the hyperons $\Lambda$ and $\Xi$ on the nuclear core

We demonstrate the different effect of strange impurities ( and ) on the static properties of nuclei within the framework of the relativistic mean-field model. Systematic calculations show that the glue-like role of the -hyperon is universal for all -hypernuclei considered. However, the -hyperon has a glue-like role only for the protons distribution in nuclei, while for the neutrons distribution it plays a repulsive role. On the other hand, the -hyperon attracts the surrounding neutrons and reveals a repulsive force to the protons. Possible explanations of the above observation are discussed.Received: 10 March 2003, Revised: 6 November 2003, Published online: 5 May 2004PACS: 21.80. + a Hypernuclei - 24.10.Cn Many-body theory     

The $\eta_{c}(2980)$ formation in two-photon collisions at LEP energies

production in interactions has been detected via its decays into ,K ⁺ K ^- K ⁺ K ^- and in the data taken with the DELPHI detector at LEP1 and LEP2 energies. The two-photon radiative width averaged over all observed decay channels is = 13.9 2.0 (stat.) 1.4(syst.) 2.7 (BR) keV. No direct decay channel has been observed. An upper limit < 5.5 keV at 95% confidence level has been evaluated for this decay mode.Received: 3 July 2003, Published online: 7 November 2003     

Multiphoton radiation in leptonic <Emphasis Type="Italic">W</Emphasis>-boson decays

We explore CP violation in decay processes in the presence of the anomalous right-handed and couplings. The complex anomalous top coupling can be a source of new CP violation and may lead to a deviation of the observed weak phase in decays, which accounts for the present disagreement of the observed between and decays. Direct CP violation is also predicted. Received: 27 November 2002, Revised: 28 March 2003, Published online: 2 June 2003     

Modular construction of special mixed quantum states

For a homogeneous quantum network of N subsystems with n levels each we consider separable generalized Werner states. A generalized Werner state is defined as a mixture of the totally mixed state and an arbitrary pure state : with a mixture coefficient . For this density operator to be separable, will have an upper bound . Below this bound one should alternatively be able to reproduce by a mixture of entirely separable input-states. For this purpose we introduce a set of modules, each contributing elementary coherence properties with respect to a generalized coherence vector. Based on these there exists a general step-by-step mixing process for any . For being a cat-state it is possible to define an optimal process, which produces states right up to the separability boundary ( ).Received: 3 December 2002, Published online: 29 July 2003PACS: 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bells inequalities, GHZ states, etc.) - 03.67.-a Quantum information - 03.65.-w Quantum mechanics     

Boltzmann-Uehling-Uhlenbech calculation of total reaction cross-section and fragmentation cross-section

, and have been calculated via the BUU model with soft EOS and 0.8 times of . The density distribution without any adjustable parameters which comes from the RMF model has been introduced into the BUU calculation to replace the normally used one-parameter square-type distribution. The calculated results can reproduce the experimental data well for both halo- and stable-nuclei-induced reactions. Here or is calculated as the difference between of halo nucleus and core nucleus, by assuming . It indicates that this assumption works very well at high energy in the BUU calculation. More experimental measurements are necessary to test the validity of this assumption at intermediate energy.Received: 12 June 2003, Revised: 29 September 2003, Published online: 27 April 2004PACS: 24.10.-i Nuclear reaction models and methods - 25.60.Dz Interaction and reaction cross-sections - 25.60.Gc Breakup and momentum distributions - 27.20. + n      

The $ B\to\pi K$ puzzle and its relation to rare B and K decays

The standard-model interpretation of the ratios of charged and neutral rates, R _c and R _n, respectively, points towards a puzzling picture. Since these observables are affected significantly by colour-allowed electroweak (EW) penguins, this puzzle could be a manifestation of new physics in the EW penguin sector. Performing the analysis in the R _n- R _c plane, which is very suitable for monitoring various effects, we demonstrate that we may, in fact, move straightforwardly to the experimental region in this plane through an enhancement of the relevant EW penguin parameter q. We derive analytical bounds for q in terms of a quantity L, which measures the violation of the Lipkin sum rule, and point out that strong phases around are favoured by the data, in contrast to QCD factorisation. The modes imply a correlation between q and the angle that, in the limit of negligible rescattering effects and colour-suppressed EW penguins, depends only on the value of L. Concentrating on a minimal flavour-violating new-physics scenario with enhanced Z ⁰ penguins, we find that the current experimental values on require roughly . As the data give , L has either to move to smaller values once the data improve or new sources of flavour and CP violation are needed. In turn, the enhanced values of L seen in the data could be accompanied by enhanced branching ratios for the rare decays , , and . Most interesting turns out to be the correlation between the modes and , with the latter depending approximately on a single scaling variable .Received: 8 September 2003, Published online: 12 November 2003     

Strange particle production at RHIC in the dual parton model

We compute the mid-rapidity densities of pions, kaons, baryons and antibaryons in Au-Au collisions at GeV in the dual parton model supplemented with final state interaction (comovers interaction). The ratios ( ) increase between peripheral ( ) and central ( collisions by a factor 2.4 (2.0) for the , 4.8 (4.1) for the and 16.5 (13.5) for the . The ratio increases by a factor 1.3 in the same centrality range. A comparison with the available data is presented.Received: 28 April 2003, Published online: 11 July 2003     

Monte Carlo simulation of particle aggregation and gelation: II. Pair correlation function and structure factor

Diffusion-limited cluster aggregation and gelation are studied using lattice and off-lattice Monte Carlo simulations. The pair correlation function g(r) and the structure factor S(q) of the particle gels were investigated as a function of the volume fraction ( ) and time. At volume fractions below , the gel structure is fractal on small length scales with . g(r) shows a weak minimum at the correlation length ( ), before reaching the average concentration at large length scales. The cut-off function of g(r) varies during the aggregation process, but at a given , where is the gel time, it is a universal function of . At high volume fractions, the structure is dominated by excluded-volume interactions, while at low volume fractions, it is determined by the connectivity.Received: 27 April 2004, Published online: 26 October 2004PACS: 64.60.Ak Renormalization-group, fractal, and percolation studies of phase transitions - 02.70.Uu Applications of Monte Carlo methods     

Energy dependence of the Λ/Σproduction cross-section ratio in p-p interactions

The production of the - and -hyperons has been measured via the reaction at the internal COSY-11 facility in the excess energy range between 14 and 60 MeV. The transition of the cross-section ratio from about 28 at MeV to the high-energy level of about 2.5 is covered by the data showing a strong decrease of the ratio between 10 and 20 MeV excess energy. Effects from the final-state interactions in the p- channel seem to be much smaller than in the p- channel. Estimates of the effective range parameters are given for the and the systems.PACS: 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 13.75.Ev Hyperon-nucleon interactions - 13.85.Lg Total cross-sections - 25.40.Ep Inelastic proton scattering     

\alpha-decay spectroscopy of light odd-odd Bi isotopes - II: 186Bi and the new nuclide 184Bi

Alpha-decay of the new nuclide ¹⁸⁴Bi has been studied in the complete-fusion reaction ⁹³Nb(⁹⁴Mo, 3n)¹⁸⁴Bi at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of recoil- , recoil- - analysis and excitation functions measurements. Two -decaying isomeric states in ¹⁸⁴Bi with half-life values of 13(2) ms and 6.6(1.5) ms were identified. The -branching ratio of ¹⁸⁰Tl was deduced for the first time as . Improved data on the fine-structure -decay of ¹⁸⁶Bi were obtained in the ⁹³Nb(⁹⁵Mo, 2n)¹⁸⁶Bi reaction. A similarity of the decay energies and half-life values of ^184,186Bi is pointed out and a possible explanation for this effect is suggested.Received: 14 March 2003, Revised: 29 April 2003, Published online: 2 September 2003PACS: 23.60.+e decay - 27.70.+q - 27.80.+w K. Heyde: Present address: EP-ISOLDE, CERN, CH-1211, Geneva 23, Switzerland.     

Search for anomalous weak dipole moments of the $\tau$ lepton

The anomalous weak dipole moments of the lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155 pb^-1. Tau leptons produced in the reaction at energies close to the mass are studied using their semileptonic decays to , , or . The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment, , , and , are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95% CL): , , , and .Received: 25 September 2002, Published online: 29 August 2003