High Impact Parameter Research Articles

The LHCb vertex trigger

The second level trigger in LHCb (level 1) is based on the ability to reconstruct high impact parameter tracks in the vertex detector. The feasibility to reconstruct tracks using only the vertex detector in a time window of less than 1.7 ms is shown. Several algorithms under study to improve the performance of the vertex trigger are briefly described.

Quasiclassical trajectory calculations of collisional energy transfer in propane systems: Multiple direct-encounter hard-sphere model

Quasiclassical trajectory calculations of collisional energy transfer from highly vibrationally excited propane + rare gas systems are reported. This work extends our hard-sphere model (A. Linhananta and K. F. Lim, Phys. Chem. Chem. Phys., 2000, 2, 1385) to examine the variation of the internal energy during collisions with a rare bath gas. This was accomplished by recording the vibrational and rotational energy of propane after each atom–atom encounter during trajectory simulations of propane + rare gas systems. This provides detailed information of the energy flow during a collision. It was found that collisions with small number of encounters transfer energy efficiently, whereas those with many encounters do not. Detailed analyses reveal that the former collisions arise from trajectories with high initial impact parameter, whereas the latter have small initial impact parameter. The reason behind this is the dependence of collision energy transfer (CET) of large polyatomic molecules on their shape. This is connected to the well-known role of rotational energy transfer (RET) as a gateway for CET.

The structure of O3–CH4 and the implications for the O+CH4 precursor-initiated reaction

The rotational spectrum of the O3–CH4 complex has been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type rotation-inversion electric-dipole spectrum is observed, complicated by the nearly free internal rotation of the CH4 top and the inversion tunneling of the O3. The nuclear-spin statistics of the equivalent oxygen nuclei leads to only one tunneling component existing for each rotation–internal-rotation state, indicating that the transition state has a heavy-atom, C2v-symmetry geometry. The tunneling splitting is determined to be 30 to 40 MHz, dependent on the CH4 internal-rotor state. Only two of the three methane internal-rotor states have been assigned. These two states of A and F symmetry have asymmetric-rotor energy-level structures, weakly perturbed by the ozone-inversion tunneling. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal O atoms equidistant above and below this plane. The angle between the line joining the center of masses of the two subunits and the O3, C2 axis is 118.2(5)°, with the central O directed away from the CH4. The shortest O–C separation is 3.57 Å. The geometry of the complex suggests two outcomes for the reaction of an O atom produced by 267 nm photolysis of O3 in the complex (assuming that the initial O3 photodissociation dynamics are not perturbed by complexation), either nonreaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited, OH product.

Normal Vortex Interaction with a Circular Cylinder

Interaction of a columnar vortex with a long circular cylinder translated normal to the vortex axis is examined for the case where the cylinder diameter is much larger than the vortex core radius. The study focuses on understanding and quantifying the limitations of traditional vortex filament models arising from vortex-induced separation of the cylinder boundary layer and vortex core shape deformation. These limitations are examined over a wide range of values of the impact parameter, defined as the ratio of the ambient normal velocity past the cylinder to the maximum vortex azimuthal velocity. Filament model predictions of vortex displacement are compared to experimental data both before and after vortex-induced boundary-layer separation. Experimental data are presented showing the importance of the ambient normal velocity to the cylinder in delaying vortex-induced boundary-layer separation, so that for cases with high impact parameter the flow is governed by inviscid effects even as the vortex moves quite close to the cylinder surface. The inviscid shape deformation of the vortex core is modest in the cases examined, even for close vortex-cylinder interaction

The exoergic F+CH 4 reaction as an example of peripheral dynamics

Classical trajectory computations for thermal reactants on a six-atom potential show a forward scattering component which is correlated with the HF product being formed with high vibrational excitation. These trajectories are peripheral collisions where the F atom approaches CH 4 with a high impact parameter and reaction is through a nearly collinear F–H–C configuration with a stretched F–H bond. Other trajectories are well described by a hard-sphere model whose cutoff is below the range of peripheral collisions. Comparison is made with the F+H 2 and other reactions where nearly thermoneutral channels correlate with forward scattering.

Comparing Galaxies and Lyα Absorbers at Low Redshift

A scenario is explored in which Lyα absorbers at low redshift arise from lines of sight through extended galaxy disks, including those of dwarf and low surface brightness galaxies. A population of galaxies is simulated based upon observed distributions of galaxy properties, and the gas disks are modeled using pressure and gravity confinement. Some parameter values are ruled out by comparing simulation results to the observed galaxy luminosity function, and constraints may be made on the absorbing cross sections of galaxies. Simulation results indicate that it is difficult to match absorbers with particular galaxies observationally since absorption typically occurs at high-impact parameters (>200 kpc) from luminous galaxies. Low-impact parameter absorption is dominated by low-luminosity dwarfs. A large fraction of absorption lines is found to originate from low surface brightness galaxies, so that the absorbing galaxy is likely to be misidentified. Low-redshift Lyα absorber counts can easily be explained by moderately extended galaxy disks when low surface brightness galaxies are included, and it is easily possible to find a scenario that is consistent with the observed galaxy luminosity function, with low-redshift Lyman limit absorber counts, and with standard nucleosynthesis predictions of the baryon density, ΩB.

WA92: a fixed target experiment to trigger on and identify beauty particle decays

We describe the detectors and trigger system used in the CERN WA92 experiment. The experiment was designed to study the production and decay of beauty particles from 350 GeV/ c π − interactions in copper and tungsten targets. Charged particle tracking is performed using the omega spectrometer. Silicon microstrip detectors are used to provide precise tracking information in the region of the production and the decay of heavy-flavoured particles and to trigger on the resulting high impact parameter tracks. The precision of vertex reconstruction corresponds to ±3.7% of the mean B-decay proper lifetime. Lepton and high transverse momentum hadron signals are also used in the trigger, which accepts 23% of B-decays and rejects 98.4% of non-beauty interactions.

The importance of high impact parameter interactions in the collision induced dissociation of protonated water clusters by argon using a Wien velocity filter

High impact parameter, i.e., glancing collisions, are of particular interest in ion beam experiments because ions experiencing such collisions remain available in the ion beam for further experiments. The collisional activation and dissociation processes for glancing collisions of protonated water clusters and argon have been studied with a new and simple, single-stage technique to detect fragment ions using a Wien velocity filter. The technique is specific with regard to the mass of a fragment ion and the mass of the parent from which it originates. A relation is derived and experimentally verified which governs the operation of the device. Absolute values of the attenuation cross section with argon of 11(6), 25(8), 36(11), 47(17), and 66(10) Å have been determined for H3O+, H5O2+, H7O3+, H9O4+, and H11O5+, respectively, at beam energies in the range of 300–1000 eV. Absolute values of the glancing collision induced dissociation cross section of 0.62(4), 4.3(1), and 9.1(6) Å2 have been determined for the detectable fragment ions of H5O2+, H7O3+, and H9O4+, respectively. Branching ratios upon activation by glancing collisions of 0.91:0.09:0 and 0.78:0.18:0.04:0 have been determined for successive loss of waters by H7O3+ and H9O4+, respectively. The one water loss channel predominates. A multiple collision analysis was performed which characterizes the fraction of parent ions which suffer a glancing collision without dissociating or being knocked out of the beam. Our results suggest that the ion beam which emerges from a collision cell can harbor a surprisingly large fraction of parent ions that have obtained a large amount of internal excitation, perhaps ∼0.7 eV per ion suffering a glancing collision.

Ne++C60 collisions: The dynamics of charge and energy transfer, fragmentation, and endohedral complex formation

Interaction of Ne+ with C60 has been studied for collision energies ranging up to 100 eV. The dominant process is charge transfer, with C60+ accounting for 85% to 100% of the total product signal in this energy range. At 25 eV collision energy, C60-2n+ dissociative charge transfer products appear, accounting for ∼13% of the total product signal at high energies. At ∼25 eV, NeC60+, NeC58+, and NeC56+ all appear together, followed by a series of NeC60-2n+ products where n increases with increasing collision energy. These products are believed to be endohedral complexes, Ne@C60-2n+, where neon is physically trapped inside the fullerene cage. The production efficiency of the endocomplexes reaches a maximum of ∼1.2% at 60 eV, and decreases to 0.5% at 100 eV. Modeling the results leads to the following picture of Ne++C60 collision dynamics: Charge transfer occurs in nearly all collisions, and is the only significant process for high impact parameters. At collision energies below ∼25 eV, translational-to-internal energy transfer is inefficient, even for low impact parameter collisions. At higher energies, the inelasticity jumps to near 100%, leading to fragmentation of the nascent products. Both endohedral penetration and efficient energy transfer appear to require bond rupture in the C60.

Polarizational bremsstrahlung in collisions of slow atomic particles

The polarizational bremsstrahlung problem is considered in collisions of slow atomic particles. It is demonstrated that the polarizational bremsstrahlung dominates in the region of high impact parameters and small photon frequencies over the well-known molecular orbital X-ray emission. The contribution of the polarizational bremsstrahlung to the total radiation spectrum is significant over a wide frequency range.

A trigger for beauty

The possibility of B-meson experiments, in a fixed-target high-energy proton machine (Tevatron) is discussed. Compared to a B-meson factory experiment, it can produce 10 5 B B 's per hour, using 10 8 protons per second, but it suffers from high background and needs high selectivity to cope with the million times higher interaction rate. To overcome these difficulties a technique called the “optical trigger for beauty” is proposed, based on the detection of Cherenkov photons produced in a 2 mm thick LiF crystal, through a fast photodetector. Its virtue is that it is opaque to minimum-bias events originating in a small target, but sensitive to the high impact parameter B-meson decay charged particles from a secondary vertex. Calculations and first simulations results give a good efficiency for B-meson detection. A multistep trigger, combining the “optical trigger” and a tracking detector, allows significant selection and a consequent enrichment of the data sample. Taking into account its fast response (∼ 1 ns), the above considerations can be extended to other hadronic machines, especially those with high-rate environments such as the LHC or SSC.

Null geodesics in black hole metrics with non-zero cosmological constant

We study the radial motion along null geodesics in the Reissner-Nordstrom-de Sitter and Kerr-de Sitter space-times. We analyze the properties of the effective potential and we discuss circular orbits. We find that: 1) the radius of circular photon orbits in the Reissner-Nordstrom-de Sitter space-times does not depend on the cosmological constant. We show also how this is related to properties of the optical reference geometry. 2) For a specific range of the cosmological constant, photons with high impact parameter may travel radially between the cosmological horizon and the black hole horizon in the equatorial plane of the Kerr-de Sitter space-times.

Integral and differential alignment of atomic inner-shell vacancies in the semiclassical approximation

The semiclassical approximation (SCA) is used to obtain general expressions for the statistical tensors describing the polarisation state of vacancies in inner shells of atoms produced by fast proton or ion impact. The asymptotic behaviour of alignment is examined in the range of high impact parameters. The restrictions imposed on the statistical tensors by the symmetry of the experiment and by the additional symmetry introduced by the various SCA versions are discussed. The integral and differential alignment of vacancies in the L and M shells of atoms is calculated systematically allowing for the Coulomb deflection of ions by the target nucleus. The deflection is allowed for by a tangential approximation of hyperbolic trajectory. The atom is described by the Hartree-Salter self-consistent-field model. The results of the calculation are compared with the experimental and calculated data published elsewhere. The effect of the screening of nuclear charge of an ion by its electrons on the differential alignment is investigated.

Comment on “the effect of reagent rotation on steric requirements of elementary exchange reactions” by H. Kornweitz, A. Persky and R.D. Levine

The conclusion that reagent rotation in O+HCl enhances reactivity at high impact parameter, but suppresses reactivity at low impact parameter has been examined. It is found instead that rotation enhances reactivity as effectively for low as for high impact parameter.

Precompound analyses ofNi58−64(n, px)reactions

Data for the inclusive (n, $\mathrm{p}x$) reaction for 60 MeV neutrons on $^{58,60,62,64}\mathrm{Ni}$ are presented. Angular distributions for $Q$-value bins show a decrease of the forward peaking of the proton emission with increasing neutron excess. A marked dependence of the angle integrated spectra on ($N\ensuremath{-}Z$) is observed. The data are compared to hybrid and geometry dependent hybrid model calculations. Although these results predict the general shape of the spectra, they do not account for ($N\ensuremath{-}Z$) dependence. A neutron skin, as given by a hydrodynamic model, is added to the geometry dependent hybrid model, which predicts a higher precompound decay for high impact parameters. These results agree much better with the $N\ensuremath{-}Z$ dependence of the data.NUCLEAR REACTIONS $^{58,60,62,64}\mathrm{Ni}$(n,p), ${\ensuremath{\epsilon}}_{\mathrm{n}}=60$ MeV, measured $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}$, $\frac{d\ensuremath{\sigma}}{d\ensuremath{\epsilon}}$; analyze via hybrid and geometry dependent hybrid models; explore importance of neutron skin.

Towards a new absorption model for hadron collisions

The cross sections for meson-exchange reactions induced by pions are calculated with a method which assumes that, for high angular momentum or high impact parameter b, the Born approximation is valid and that for small impact parameter b the reaction amplitude vanishes. For treating intermediate values b we introduce a step-function η( b) which goes from zero to one at b∼ R within an interval d. Both R and d are slowly varying functions of s, which are assumed to be the same for all pion-induced reactions. The expressions for the Born approximation are completely determined by the coupling constants of the exchange particles with the reaction partners. For reactions in which the coupling constants are given, the only unknown input are the values of R and d. The latter ones are determined by fitting the pion charge-exchange reaction at several energies. The main features of many other pion-exchange reactions are then well reproduced by this method.

Analysis of theHe+on He Collision

This paper is the second of two consecutive papers dealing with resonant electron capture in ${\mathrm{He}}^{+}$ on He collisions. The first of these, by Lockwood, Helbig, and Everhart, describes differential measurements of electron capture probability ${P}_{0}$ made over a wide range of incident energy and scattering angle. The present paper treats these data within the framework of an improved theory for charge transfer due to Bates and McCarroll. The theory requires a knowledge of the energy levels of the ${\mathrm{He}}_{2}^{+}$ molecular ion at all internuclear distances. Using values of these energy levels given by Lichten, the present paper computes the location of the maxima and minima of ${P}_{0}$ at all impact parameters and velocities. The repulsive potential energy of the ${\mathrm{He}}_{2}^{+}$ system is also calculated here so that these impact parameters can be related to the scattering angle. These results allow the values of ${P}_{0}$ to be predicted over a wide range of impact parameters (0.05 to 1.4 \AA{}), scattering angles (\textonehalf{} to 4\ifmmode^\circ\else\textdegree\fi{}), and energies (0.4 to 1000 keV). There is fair quantitative agreement with the data curves. However, a detailed comparison of theory and data shows discrepancies. At high velocities and low impact parameters there is rather good agreement and there is again fair agreement at low velocities and large impact parameters. In between, however, at intermediate impact parameters the data curves are such that the interaction appears to be different depending on whether the velocity is above or below a critical value of about 6\ifmmode\times\else\texttimes\fi{}${10}^{7}$ cm/sec. This behavior of the data is not predicted by the present theory.