Large Impact Parameters Research Articles

Beam-size effect at colliders and correlations of particles in a beam

For several processes at colliding beams, macroscopically large impact parameters give an essential contribution to the standard cross section. These impact parameters may be much larger than the transverse sizes of the colliding bunches. In that case, the standard calculations have to be essentially modify. The corresponding formulae for such a beam-size effect were given twenty years ago without taking into account correlations of particle coordinates in the beams. In the present paper we derive formulae which necessary to take into account quantitatively the effect of particle correlations in the spectrum of bremsstrahlung as well as in pair production. Besides, we consider critically recent papers of Baier and Katkov [Phys. Rev. D {\bf 66}, 053009 (2002) and hep-ph/0305304] in which it was calculated a new additional ``subtraction term'' related to the coherent contribution into beam-size effect. We show that this result is groundless and point out the origin of the mistake.

ChandraandXMM‐NewtonObservations of the Double Cluster A1758

A1758 was classified by Abell as a single rich cluster of galaxies, but a ROSAT observation showed that this system consists of two distinct clusters (A1758N and A1758S) separated by approximately 8' (a projected separation of 2 Mpc in the rest frame of the clusters). Only a few galaxy redshifts have been published for these two clusters, but the redshift of the Fe lines in the Chandra and XMM-Newton spectra shows that the recessional velocities of A1758N and A1758S are within 2100 km s-1. Thus, these two clusters most likely form a gravitationally bound system, but our imaging and spectroscopic analyses of the X-ray data do not reveal any sign of interaction between the two clusters. The Chandra and XMM-Newton observations show that A1758N and A1758S are both undergoing major mergers. A1758N is in the late stages of a large impact parameter merger between two 7 keV clusters. The two remnant cores have a projected separation of 800 kpc. Based on the measured pressure jumps preceding the two cores, they are receding from one another at less than 1600 km s-1. The two cores are surrounded by hotter gas (kT = 9-12 keV) that was probably shock heated during the early stages of the merger. The gas entropy in the two remnant cores is comparable with the central entropy observed in dynamically relaxed clusters, indicating that the merger-induced shocks stalled as they tried to penetrate the high-pressure cores of the two merging systems. Each core also has a wake of low-entropy gas, indicating that this gas was ram pressure stripped without being strongly shocked. A1758S is undergoing a more symmetric (lower impact parameter) merger between two 5 keV clusters. The two remnant cores are nearly coincident as seen in projection on the sky. The two cores are surrounded by hotter gas (9-11 keV), which was probably shock heated during the merger. Based on the pressure jumps preceding the two cores, they must have a relative velocity of less than 1400 km s-1. Unlike A1758N, there is no evidence of wakes of low-entropy gas.

Ionization of helium by impact of negative B, O, and F ions

Single and double ionization processes for helium under the impact of B, O, and F anions were studied in the 1.1-1.9 a.u. velocity range. Coincidence cross sections were measured for direct ionization (DI), i.e., no projectile electron loss, and single (SL) and double projectile electron loss. The results show that DI is dominated by large impact parameters and SL is essentially the elastic scattering of the extra electron by the target. The cross section ratios taken between double and single helium ionization for single and double loss are projectile independent and grow almost linearly with the velocity.

Hyperthermal Reactions of O+(4S3/2) with CD4 and CH4: Theory and Experiment

Theoretical and experimental methods are applied to the study of the reaction dynamics in hyperthermal collisions of O+ (4S3/2) with methane. Measurements of the absolute reaction cross sections for the interaction of O+ with CD4 and CH4 were obtained at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) technique. Product recoil velocity distributions, using the GIB time-of-flight (TOF) methods, were determined for several product ions at selected collision energies. The main reaction channel, charge transfer, proceeds via large impact parameter collisions. A number of minor channels, involving more intimate collisions, were also detected. Ab initio electronic structure calculations have been performed with different levels of theory and basis sets, including high-level coupled-cluster calculations to determine the energies of reaction intermediates and transition states for reaction. Several reaction paths on both quartet and doublet electronic states of (O·CH4)+ are found, and these provide a reasonable qualitative interpretation of the experiments. Although most of the products can be produced via spin-allowed pathways, the appearance of CH3+ at low energies suggests that intersystem crossing plays some role.

Charge transfer in slow collisions ofO8+andAr8+ions withH(1s)below2keV∕amu

We calculated the charge-transfer cross sections for ${\mathrm{O}}^{8+}+\mathrm{H}$ collisions for energies from $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$ to $2\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$, using the recently developed hyperspherical close-coupling method. In particular, the discrepancy for electron capture to the $n=6$ states of ${\mathrm{O}}^{7+}$ from the previous theoretical calculations is further analyzed. Our results indicate that at low energies (below $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$) electron capture to the $n=6$ manifold of ${\mathrm{O}}^{7+}$ becomes dominant. The present results are used to resolve the long-standing discrepancies from the different elaborate semiclassical calculations near $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$. We have also performed the semiclassical atomic orbital close-coupling calculations with straight-line trajectories. We found the semiclassical calculations agree with the quantal approach at energy above $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$, where the collision occurs at large impact parameters. Calculations for ${\mathrm{Ar}}^{8+}+\mathrm{H}$ collisions in the same energy range have also been carried out to analyze the effect of the ionic core on the subshell cross sections. By using diabatic molecular basis functions, we show that converged results can be obtained with small numbers of channels.

Beam-size effect and particle losses at B-factories KEKB and PEP-II

In the colliders, the macroscopically large impact parameters give a substantial contribution to the standard cross section of the e +e −→e +e −γ process. These impact parameters may be much larger than the transverse sizes of the colliding bunches. It means that the standard cross section of this process has to be substantially modified. In the present paper such a beam-size effect is calculated for bremsstrahlung at B-factories KEKB and PEP-II. We find out that this effect reduces beam losses due to bremsstrahlung by about 20%.

Reactions of O+ with CnH2n+2, n=2-4: a guided-ion beam study.

We have measured absolute reaction cross sections for the interaction of O(+) with ethane, propane, and n-butane at collision energies in the range from near thermal to approximately 20 eV, using the guided-ion beam (GIB) technique. We have also measured product recoil velocity distributions using the GIB time-of-flight (TOF) technique for several product ions at a series of collision energies. The total cross sections for each alkane are in excess of 100 A(2) at energies below approximately 2 eV, and in each case several ionic products arise. The large cross sections suggest reactions that are dominated by large impact parameter collisions, as is consistent with a scenario in which the many products derive from a near-resonant, dissociative charge-transfer process that leads to several fragmentation pathways. The recoil velocities, which indicate product ions with largely thermal velocity distributions, support this picture. Several product ions, most notably the C(2)H(3) (+) fragment for each of the alkanes, exhibit enhanced reaction efficiency as collision energy increases, which can be largely attributed to endothermic channels within the dissociative charge-transfer mechanism.

Plane Wave and Coulomb Asymptotics

A simple plane wave solution of the Schrödinger–Helmholtz equation is aquantum eigenfunction obeying both energy and linear momentum correspondenceprinciples. Inclusion of the outgoing wave with scattering amplitude fobeys unitarity and the optical theorem. By closely considering the standardasymptotic development of the plane wave, we show that there is a problemwith angular momentum when we consider forward scattering at the point ofclosest approach and at large impact parameter given semiclassically by(l + 1/2)/k where l is the azimuthal quantum number and may be large (JLeech et al, Phys. Rev. Lett. 88 257901 (2002)). The problem is resolved vianon-uniform, non-standard analysis involving the Heaviside step function,unifying classical, semiclassical and quantum mechanics, and the treatment isextended to the case of pure Coulomb scattering.

Ionization and Fragmentation Modes of Nucleobases after Collisions with Multiply Charged Ions

We studied multiply charged ion (MCI) induced ionization, excitation and fragmentation of the nucleobases uracil and thymine. Ions of different charge state at velocities between 0.2 and 0.4 atomic units were used as projectiles. By means of time-of-flight spectrometry of the collision products in a statistically representative mode as well as in triple coincidence mode, three different interaction types could be identified. Large impact parameters lead to “gentle” ionization of the molecules, whereas in close collisions mainly multifragmentation occurs. This gives rise to a bimodal mass-spectrum as found in fullerene fragmentation studies. An additional two-body breakup channel leading to intermediate fragment sizes was found for both nucleobases. From the coincidence data, the kinetic energy release of this channel could be obtained.

Influence of the beam-size effect on particle losses at B-factories PEP-II and KEKB

In the colliders, the macroscopically large impact parameters give a substantial contribution to the standard cross-section of the e +e −→e +e −γ process. These impact parameters may be much larger than the transverse sizes of the colliding bunches. It means that the standard cross-section of this process has to be substantially modified. In the present paper such a beam-size is calculated for bremsstrahlung at B-factories PEP-II and KEKB. We find out that this effect reduces beam losses due to bremsstrahlung by about 20%.

A crossed beam and ab initio study of the C 2(X 1Σ g+ /a 3∏ u) + C 2H 2(X 1Σ g+) reactions

The reaction dynamics of C 2 in the ground ( 1Σ g +) and first electronically excited ( 3∏ u) states with the simplest alkyne, acetylene C 2H 2(X 1Σ g +), were investigated in a crossed molecular beam setup at a nominal collision energy of 24.1 kJ mol −1. The experimental data expose the existence of a C 2/H exchange pathway to form C 4H + H. The experimental results were combined with electronic structure calculations on the singlet and triplet C 4H 2 surfaces. The reaction of C 2(X 1Σ g +) was found to proceed via indirect scattering dynamics through addition of C 2 to the carbon–carbon triple bond. On the triplet surface, C 2(a 3∏ u) adds also to the acetylenic triple bond. Dominated by large impact parameters, this process favors the formation of two short lived chain-like triplet C 4H 2 isomers. For both reactions, the identification of the butadiynyl radical as a relevant product presents an important step to refine the networks of reactions for the modeling of polycyclic aromatic hydrocarbons formation together with that of their hydrogen deficient precursors in the interstellar medium, in carbon-rich circumstellar envelopes and pre-planetary nebula, and in combustion flames.

Clustering of Galaxies atz ∼ 3 around the Probable Damped Lyα Absorber toward QSO APM 08279+5255

We present results on the clustering of Lyman break galaxies (LBGs) around a probable damped Ly? absorption line cloud (DLA) at zabs = 2.974 from deep UBVI images of the field containing the quasar APM 08279+5255 (zem = 3.91). The large area covered by our images, 0.31 deg2 or ~40 ? 40 Mpc comoving at redshift z = 3, and their depth, ?I,AB(sky) 27.6 mag arcsec-2, allow us to identify ~450 LBG candidates brighter than IAB = 24.80 at 2.75 < z < 3.25 both close (50 kpc) to the DLA line of sight and up to 20 Mpc (comoving) from the DLA, i.e., physically unrelated. LBG candidates were identified using photometric redshift techniques that include the I magnitude as a prior estimate in addition to the colors. The two are combined using Bayes's theorem. This helps to break the degeneracies that occur in a pure spectral template fitting scheme. The overall rms is ?z 0.15 at z ~ 3 based on our analysis of photometric redshifts in the HDF-N. From the redshift likelihood distributions, we selected LBG galaxies within a redshift slice of width Wz = 0.15( ?z) centered on the redshift of the DLA zabs. Within that redshift slice, we find an enhancement of galaxies near the DLA using both the surface density (?/?g 3) and an estimator of the three-dimensional spatial overdensity (n/g ~ 5 ? 3). The surface overdensity ?/?g is significant at the more than 95% significance level on scales 2.5 < r? < 5 Mpc comoving. The overdensity cannot be related to the QSO environment since the QSO is at zem = 3.91. These results imply that some DLA could reside in high-density regions. We search within 45'' from the line of sight for galaxies responsible for the DLA and find one candidate with zphot = 3.03 that is 26'' (145 kpc physical) away. From its magnitude I = 24.65 ? 0.2, its luminosity is MI,AB = -21.35. Due to its large impact parameter, however, this galaxy is not a likely candidate for the absorber.

Improvements to a GPS radio occultation ray‐tracing model and their impacts on assimilation of bending angle

Several efforts are made to improve the accuracy and efficiency of a 2‐D GPS radio occultation forward ray‐tracing model. First, a large impact parameter offset existed in the forward model, caused by the inconsistency in the definition of the “occultation point” between the forward model and the GPS/MET data. The forward model is modified to be consistent with the GPS/MET data. Second, the errors in the simulation of the impact parameter and the bending angle due to the original starting point scheme of the ray integration are examined and a new starting point scheme is introduced to reduce the forward modeling errors. Third, to more accurately simulate the GPS/MET bending angle data, a derivation of the bending angle from a Doppler shift measurement under the application of Snell's law is incorporated into the ray‐tracing model. Fourth, a variable step size for the ray integration is proposed to the forward ray‐tracing model to reduce its computational cost. Assessment of the impacts of these modifications to the forward modeling of the bending angle and to the data assimilation of the bending angle is carried out with simulated bending angles and impact parameters derived from an accurate 3‐D ray‐tracing model. The 3‐D ray‐tracing model simulates a total of 133 occultation profiles with the actual satellite positions and velocities taken from the 133 GPS/MET occultations observed on 11 October 1995. The NCEP analysis at T126L28 resolution is used to represent the “true” refractivity field of the atmosphere. The bending angles and impact parameters in the 3‐D ray‐tracing model are derived following the same data processing procedure used in the GPS/MET experiment. All the input parameters (e.g., the tangent direction at the perigee point, etc.) required by the 2‐D ray‐tracing model are provided by the 3‐D ray‐tracing model. Numerical results with and without the above mentioned modifications to the 2‐D ray‐tracing model are examined and compared with the 3‐D ray‐tracing model results and the NCEP T126L28 analysis. It is shown that the modifications to the 2‐D ray‐tracing model significantly reduce the errors in the forward simulation of the bending angle and the impact parameter, and thus greatly improve the accuracy of the temperature, specific humidity, and surface pressure field analyses produced by GPS data assimilation, especially in middle and lower troposphere.

QCD saturation in the semi-classical approach

In this paper the semi-classical approach to the solution of non-linear evolution equation is developed. We found the solution in the entire kinematic region to the non-linear evolution equation that governs the dynamics in the high parton density QCD. The large impact parameter ( b t ) behaviour of the solution is discussed as well as the way how to include the non-perturbative QCD corrections in this region of b t . The geometrical scaling behaviour and other properties of the solution in the saturation (colour glass condensate) kinematic domain are analyzed. We obtain the asymptotic behaviour for the physical observables and found the unitarity bounds for them.

Dynamics of ionization processes studied with the COLTRIMS method—new insight into e–e correlation

Many particle dynamics in atomic and molecular physics has been investigated by using the COLTRIMS method. The method and its power is discussed. The COLTRIMS technique visualizes many-particle fragmentation processes in the eV and sub milli-eV regime and reveals like the bubble chamber the complete momentum pattern in atomic and molecular particles reactions. Complete differential cross sections in momentum space have been measured for the transfer ionization channel in fast four-body (p+He→H°+He 2++e) collisions. The correlated kinematical transfer ionization channel has been used to probe the highly correlated contributions to the asymptotic parts (high momenta and large nuclear impact parameters) of the He ground-state momentum wave function. In this reaction, one electron with selected initial momentum (2.5–7.5 a.u.) in the He ground state is kinematically captured by the proton (tunneling through the two-center barrier). The measured three-particle final-state momentum distributions show well-structured patterns, which reflect special features of the three-particle initial-state momentum wave function.

Coulomb dissociation of halo nuclei

Coulomb dissociation of one neutron halo nuclei 11Be and 15,19C has been studied at energies around 70 A MeV by kinematically complete measurements of breakup particles at RIKEN. The experiment of 11Be breakup on a Pb target aims at studying the characteristic features of low-lying E1 strenght. In particular the effects of higher order excitation and nuclear contribution have been closely studied by using the impact parameter analysis. We have found that these contributions are very small, and the selection of the large impact parameter is useful to extract the spectroscopic information of the halo around state. We have then successfully applied this analysis method to 15C and 19C, where we have extracted the spectroscopic factor for the s-wave halo configuration. We also show preliminary results on the breakup of 11Be on a carbon target in order to study the nuclear breakup mechanism and low-lying discrete states of the halo nucleus.

Monte Carlo simulation of charged particle diffusion through an ionized gas: contributions of simultaneous multiple collisions

When a charged particle moves through fully or partially ionized plasmas, energy loss and momentum transfer occur through various kinds of elastic and inelastic collisions. In ordinary gases, composed primarily of neutral particles, the interactions are mainly close binary collisions, being the impact parameter well shorter than the mean free path. In a gas composed of charged particles, neglecting simultaneous multiple collisions can be a very poor approximation because the electromagnetic forces decrease much more slowly with distance than do the forces between neutral atoms. Here we present a Monte Carlo code, where the effects of large impact parameter multiple scattering are considered.

Performance of the NOMAD-STAR detector

The NOMAD-STAR detector is a silicon vertex detector installed in the NOMAD spectrometer at the CERN SPS neutrino beam. It consists of four layers of a passive boron carbide target with a total mass of 45 kg and five layers of 600 single-sided silicon microstrip detectors covering a total area of 1.14 m 2 . About 11,500ν μ charged current interactions were reconstructed in the fiducial volume of NOMAD-STAR from the neutrino run in 1998. The potential use of silicon detectors for ν μ(ν e)↔ν τ oscillations depends on the observation of the τ candidates by the experimental signature of a large impact parameter, in the case of the one prong decay of the τ, or a double vertex, in the case of the three prong decay. The main aim of NOMAD-STAR is to measure the impact parameter and vertex distributions of charged current interactions, which constitute the main backgrounds for the oscillation signals, to understand the significance of a potential signal in a future experiment. The present paper describes the experience gained in the operation of this silicon vertex detector, and the performance achieved with it.

Impact parameter dependence of electron capture in slow O5+ + Hecollisions

Angular distributions of recoiling He2+ andHe+ions following collisions of slow O5+ions and He atoms were measured, for projectile energies in the range100–2500 eV. The recoil ions were detected at angles from 20° to130°.Two groups of peaks are clearly visible, corresponding tosingle and double electron capture. Highly energetic He2+ andHe+ions were observed at forward angles. It has already been shown in previous works on the C5+ and B5+ + He collisions thatrecoiling He+targets mainly originate from a single electron capture on the n = 2or 3 orbitals of the projectile, whereas He2+ions are due to a double electron capture on 2ℓnℓ′(n ≥ 2)configurations. The population of such configurations is confirmed by our model calculationsbased on classical kinematics equations. From this model, charge exchange probabilitiesP (b),where bis the impact parameter, were deduced for the explored projectile energies. Toexplain the large kinetic energies for recoil targets, we had to invoke electroncapture processes under small impact parameter conditions (b < 1 au).At 2500 eV, while the capture events occurring at large impact parameters are notdetected in the present measurements, electron capture in collisions at smallimpact parameters is evidenced. At the lowest projectile energy (100 eV) wehave access to the whole range of impact parameters. The contribution ofsmall impact parameters is shown to be dominant at this energy.

Collective and quantum effects on elastic electron–ion collisions in strongly coupled semiclassical plasmas

Collective plasma screening and quantum-mechanical effects on elastic electron–ion collisions in strongly coupled semiclassical plasmas are investigated using the second-order eikonal method. The effective screened potential model taking into account both the plasma screening and quantum effects is applied to describe the elastic electron–ion interactions in strongly coupled semiclassical plasmas. The impact-parameter trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal cross section as a function of the impact parameter, thermal de Broglie wavelength, Debye length, and collision energy. It is found that the collective plasma screening and quantum effects reduce the elastic electron–ion scattering cross section for small impact parameters. However, the quantum effects are found to be less important than the plasma screening effects for large impact parameters.