Kang-Redner Anomaly in Cluster-Cluster Aggregation

The large time, small mass, asymptotic behavior of the average mass distribution \pb is studied in a $d$-dimensional system of diffusing aggregating particles for $1\leq d \leq 2$. By means of both a renormalization group computation as well as a direct re-summation of leading terms in the small reaction-rate expansion of the average mass distribution, it is shown that \pb \sim \frac{1}{t^d} (\frac{m^{1/d}}{\sqrt{t}})^{e_{KR}} for $m \ll t^{d/2}$, where $e_{KR}=\epsilon +O(\epsilon ^2)$ and $\epsilon =2-d$. In two dimensions, it is shown that \pb \sim \frac{\ln(m) \ln(t)}{t^2} for $m \ll t/ \ln(t)$. Numerical simulations in two dimensions supporting the analytical results are also presented.Comment: 11 pages, 6 figures, Revtex

Statistique mensuelle de la viande. 1968 N° 4 APRIL-AVRIL = Monthly statistiques of meat. 1968 No. 4 April

In high energy experiments such as active beam dump searches for rare decays and missing energy events, the beam purity is a crucial parameter. In this paper we present a technique to reject heavy charged particle contamination in the 100 GeV electron beam of the H4 beam line at CERN SPS. The method is based on the detection with BGO scintillators of the synchrotron radiation emitted by the electrons passing through a bending dipole magnet. A 100 GeV pi- beam is used to test the method in the NA64 experiment resulting in a suppression factor of 10−5 while the efficiency for electron detection is 95%. The spectra and the rejection factors are in very good agreement with the Monte Carlo simulation. The reported suppression factors are significantly better than previously achieved.ISSN:0168-9002ISSN:1872-957

Hunting down the X17 boson at the CERN SPS

Indexación ScopusRecently, the ATOMKI experiment has reported new evidence for the excess of e+e- events with a mass ∼ 17 MeV in the nuclear transitions of 4He, that they previously observed in measurements with 8Be. These observations could be explained by the existence of a new vector X17 boson. So far, the search for the decay X17 → e+e- with the NA64 experiment at the CERN SPS gave negative results. Here, we present a new technique that could be implemented in NA64 aiming to improve the sensitivity and to cover the remaining X17 parameter space. If a signal-like event is detected, an unambiguous observation is achieved by reconstructing the invariant mass of the X17 decay with the proposed method. To reach this goal an optimization of the X17 production target, as well as an efficient and accurate reconstruction of two close decay tracks, is required. A dedicated analysis of the available experimental data making use of the trackers information is presented. This method provides independent confirmation of the NA64 published results [1], validating the tracking procedure. The detailed Monte Carlo study of the proposed setup and the background estimate show that the goal of the proposed search is feasible. © 2020, The Author(s).https://link-springer-com.recursosbiblioteca.unab.cl/article/10.1140%2Fepjc%2Fs10052-020-08725-

Strong matrix effect in low-energy He+ ion scattering from carbon

In low-energy ion scattering the contribution of neutralization processes to the scattered ion yield is very important in quantification. Neutralization of low-energy (1-3.5 keV) He+ ions by carbon is found to be much stronger for graphitic than for carbidic carbon. The ion fraction for graphitic carbon for 2.5 keV 3He+ scattering over 136° is about 60 times lower than that for carbidic carbon. For the 4He+ isotope the effect is even larger. Such a strong matrix effect for one element has not been measured before in low-energy (1-3.5 keV) inert-gas ion scattering. The neutralization behaviour is discussed in terms of a special quasi-resonant neutralization process for graphite

Mechanism of neutralization in low-energy He+ ion scattering from carbidic and graphitic carbon species on rhenium

In general, the absence of matrix effects in low-energy ion scattering makes quantification possible. Ion fractions, important in quantification, are obtained by measuring scattered ion yields as a function of primary energy. Differences in ion fraction and final energy (strong matrix effect) are observed in 1-3.5 keV He+ scattering from graphitic and carbidic carbon on a polycrystalline rhenium ribbon. These results are explained by a special quasi-resonance neutralization for graphite because of the large width of the sp band of graphite, extending towards the He 1s energy level

Substrate properties of C′-methyl UTP derivatives in T7 RNA polymerase reactions. Evidence for N-type NTP conformation

AbstractThe number of synthetic UTP analogues containing methyl groups in different positions of the ribose moiety were tested as substrates for T7 RNA polymerase (T7 RNAP). Two of these compounds (containing substituents in the 5′ position) were shown to be weak substrates of T7 RNAP. 3′Me-UTP was neither substrate nor inhibitor of T7 RNAP while 2′Me-UTP was shown to terminate RNA chain synthesis. Conformational analysis of the analogues and parent nucleotide using the force-field method indicates that the allowed conformation of UTP during its incorporation into the growing RNA chain by T7 RNAP is limited to the χ angle range of 192–256° of N-type conformation

Oxidation of carbidic carbon on a rhodium surface

Different mechanisms of at. carbon and oxygen recombination on a rhodium surface are studied with Auger electron spectroscopy (AES) and XPS. The kinetics of adsorbed carbidic carbon oxidn. (carbon coverage qC ~ 0.1-0.3 ML(monolayer)) by gas-phase oxygen that proceeds by a Langmuir-Hinshelwood reaction mechanism, provides the value of the activation energy for recombination (Erecact ~ 170 +- 20 kJ/mol). Erecact depends slightly on the carbon coverage. An Eley-Rideal type of reaction was obsd. for adsorbed oxygen and at. gas-phase carbon recombination which occurs in a dynamic regime. The low value found for the activation energy (near zero) is consistent with the mechanism that this exothermic reaction is too fast for energy dissipation into the substrate; the energy is mainly transferred into translational, vibrational energy of CO. [on SciFinder (R)