Large Impact Parameters Research Articles

Cooling efficiency in collisions between Pd $ \mathsf {_{13}}$ and He, Ne, Ar and Kr

The cooling of the metal cluster Pd13 in an atmosphere of rare gas has been studied by means of computer simulation. By simulation, the average energy transfer in collisions between one cluster and one gas atom has been obtained. Emphasis has been placed on conditions when the temperatures of the colliding species are almost equal. All modes of motion, inclusive the translation, must be considered in order to obtain vanishing energy transfer at equilibrium. A simulation scheme is presented by which the energy transfer is zero to the cluster when the gas and the cluster temperatures are equal. At equilibrium the energy transfer does however not vanish for all impact parameters. In the collisions with Pd13, the cluster is heated by collisions with a small impact parameter but equally cooled by collisions with a large impact parameter. Argon and krypton are found to cool Pd13 equally efficiently while neon and helium are less efficient cooling agents.

Observed effects of gravitational microlensing of stars by a spatial-temporal tunnel

We consider microlensing of stars by a spacetime tunnel (wormhole), which is manifest as a gravitational lens with negative mass and a circular caustic in the source plane. If a source with a small angular size does not cross the circular caustic, it is impossible to discriminate between gravitational lenses with positive and negative mass. Even for comparatively large stellar angular diameters, differences from a Schwarzschild lensing curve are on the order of the observational errors. When the angular size of a star is comparable to the radius of the Einstein cone, for a sufficiently large impact parameter, the shape of the observed light curves can be similar to that due to microlensing of a compact object surrounded by an extended gaseous envelope. Such an object can be easily distinguished from a negative-mass gravitational lens via analysis of chromatic and polarization effects of the lensing.

Search for gauge mediated SUSY breaking topologies in ee collisions at centre-of-mass energies up to 209 GeV

A total of 628$\invpb$ of data collected with the ALEPH detector at centre-of-mass energies from 189 to 209 GeV is analysed in the search for gauge mediated SUSY breaking (GMSB) topologies. These topologies include two acoplanar photons, non-pointing single photons, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and multi-leptons plus missing energy final states. No evidence is found for new phenomena, and lower limits on masses of supersymmetric particles are derived. A scan of a minimal GMSB parameter space is performed and lower limits are set for the next-to-lightest supersymmetric particle (NLSP) mass at 54$\gevcc$ and for the mass scale parameter $\Lambda$ at 10$\tevcc$, independently of the NLSP lifetime. Including the results from the neutral Higgs boson searches, a NLSP mass limit of 77$\gevcc$ is obtained and values of $\Lambda$ up to 16$\tevcc$ are excluded.

Quantum dynamics of the N(4S)+O2 reaction on the X 2A′ and a 4A′ surfaces: Reaction probabilities, cross sections, rate constants, and product distributions

We report real wave packet (WP) calculations of reaction probabilities, cross sections, rate constants, and product distributions of the reaction N(4S)+O2(X 3∑g−)→NO(X 2∏)+O(3P). We propagate initial WPs corresponding to several O2 levels, and employ reactant coordinates and a flux method for calculating initial-state-resolved observables, or product coordinates and an asymptotic analysis for calculating state-to-state quantities. Exact or J-shifting calculations are carried out at total angular momentum J=0 or J>0, respectively. We employ the recent X 2A′ S3 potential energy surface (PES) by Sayós et al. and the earlier a 4A′ PES by Duff et al. In comparing S3 results with the WP ones of a previous X 2A′ S2 PES, we find lower S3 energy thresholds and larger S3 probabilities, despite the higher S3 barrier. This finding is due to the different features of the doublet PESs in the reactant and product channels, at the transition state, and in the NO2 equilibrium region. We analyze the effects of the O2 initial level and show that tunneling through the S3 barrier enhance the room-temperature rate constant by ∼3.7 times with respect to the previous S2 WP rate. The agreement with the room-temperature experimental result is thus notably improved. The NO vibrational distribution is inverted and the rotational ones are strongly oscillating. We explain these nonstatistical results showing that the reaction partners approach each other with a large impact parameter. The WP vibrational distribution is however different from that observed, which is oscillating. WP calculations show that the new S3 PES describes accurately several features of the X 2A′ state, although a lowering of its barrier height by ∼0.56 kcal/mol should bring calculated and observed rate constants in full agreement.

Forward quark jets from protons shattering the color glass condensate.

We consider the single-inclusive minijet cross section in pA at forward rapidity within the color glass condensate model of high energy collisions. We show that the nucleus appears black to the incident quarks except for very large impact parameters. A markedly flatter p(t) distribution as compared to QCD in the dilute perturbative limit is predicted for transverse momenta about the saturation scale, which could be as large as Q(2)(s) approximately 10 GeV2 for a gold nucleus boosted to rapidity approximately 10 (as at the BNL-RHIC).

Rutherford Backscattering Angular Yields under Channeling Conditions for H+and H0Projectiles

The Rutherford backscattering angular scattering yields for 100 keV H + and H 0 projectiles from Si crystal was measured. The analysis of results using Monte Carlo simulations leads to the conclusion that H° beam suffers smaller dispersion than H + beam, when crossing crystal surface. It means that the H° scattering probability is smaller at large impact parameters. For greater penetration depth the channeling process turns out to be the same.

Classical and quantum cross-section for black hole production in particle collisions

We suggest a simple model to study the problem of the black hole production in particle collisions. The cross-section for the classical and quantum production is analysed within this model. In particular, the possibility to form a black hole in collision of low energy particles (or at large impact parameter) via the quantum tunneling mechanism is pointed out. It is found that, in this model, the geometric cross-section gives a good estimate for the production at low and high energies. We also reconsider the arguments in favor of exponential suppression for the production of trans-Planckian black hole and conclude that no such suppression in fact appears. Analyzing the probability for the black hole production we point out on the importance of the back-reaction and reevaluate the contribution of the black hole formed in gravitational collapse to the Euclidean path integral.

Evidence for highlyenergetic fragments followingelectron capture in O5+ + H2 collisions at low impactvelocities

Angular distributions of recoiling H2+ and H+ ionsfollowing collisions of slow O5+ and H2 were measured.The recoil ions originating from single or double electroncapture were detected at angles in the range20°-130°. At projectile velocities lower than0.1 au, two groups of peaks are clearly visible. A firststructure, centred at ~9.5 eV, corresponds to a doublecapture at relatively large impact parameters (b≈5 au).At higher fragment energies, another structure is clearlyobserved. The latter group of peaks shifts towards lowerenergies as the detection angle increases. Calculations usingclassical kinematics equations show that these peaks are due tosingle or double capture at very small impact parameters (b<1 au). Our calculations reproduce the position of the peaks, aswell as the relative cross sections as a function of thedetection angle.

A combined crossed-beam, ab initio, and Rice–Ramsperger–Kassel– Marcus investigation of the reaction of carbon atoms C(3Pj) with benzene, C6H6(X 1A1g) and d6-benzene, C6D6(X 1A1g)

The reactions of atomic carbon, C(3Pj), with benzene, C6H6(X 1A1g), and with d6-benzene, C6D6(X 1A1g) were investigated at twelve collision energies between 8.8 and 52.5 kJ mol−1 using the crossed molecular beams technique. Forward-convolution fitting of the data, high-level electronic structure calculations, and Rice–Ramsperger–Kassel–Marcus (RRKM) investigations on the singlet and triplet C7H6/C7D6 potential energy hyperface suggest that at low collision energies the chemical reaction dynamics are indirect and dominated by large impact parameters. As the collision energy increases, smaller impact parameters become more important, and the chemical dynamics is increasingly direct. At all collision energies, the reaction proceeds on the triplet surface via a barrierless addition of the carbon atom to form a bicyclic intermediate followed by ring opening of the initial collision complex to a seven-membered ring intermediate (cycloheptatrienylidene). The latter decomposes without exit barrier to the thermodynamically less stable 1,2-didehydrocycloheptatrienyl radical, C7H5(X 2B1)+H, and its deuterated C7D5(X 2B1)+D counterpart. The formation of a C7D6 adduct is observed as a second channel. The barrierless route for the destruction of benzene can help to model important pathways for the synthesis of higher polycyclic aromatic hydrocarbon derivatives in the interstellar medium, in outflows of dying carbon stars, in hydrocarbon-rich planetary atmospheres, as well as in oxygen-poor combustion flames.

Sudakov effects in B→ πℓ νℓ form factors

In order to obtain fundamental information about the Standard Model of particle physics from experimental measurements of exclusive hadronic two-body B-decays we have to be able to quantify the non-perturbative QCD effects. Although approaches based on the factorization of mass singularities into hadronic distribution amplitudes and form factors provide a rigorous theoretical framework for the evaluation of these effects in the heavy quark limit, it is not possible to calculate the O( Λ QCD/ m b ) corrections in a model-independent way, because of the presence of non-factorizing long-distance contributions. It has been argued that Sudakov effects suppress these contributions and render the corresponding corrections perturbatively calculable in terms of the distribution amplitudes. In this paper we examine this claim for the simple and related example of semileptonic B→ π decays (which have similar long-distance contributions) and conclude that it is not justified. The uncertainties in our knowledge of the mesons' distribution amplitudes imply that the calculations of the form factors are not sufficiently precise to be useful phenomenologically. Moreover, it appears that a significant fraction of the contribution comes from the non-perturbative region of large impact parameters, and is therefore incalculable. We also raise a number of theoretical issues in the derivation of the underlying formalism. Our conclusion is therefore a disappointing one. For B-decays it is not possible to invoke Sudakov effects to calculate amplitudes for decays which have long-distance divergences (end-point singularities) in the standard hard-scattering approach.

Dynamics of H 2 formation on a graphite surface

We have studied the dynamics of energy-rich H 2 produced on a graphite surface in H(ad) + H(g) using a classical trajectory procedure. The majority of H(g) recombine with H(ad) on a subpicosecond timescale with a significant portion of the reaction exothermicity depositing in the H 2 vibration, leading to a vibrational population inversion. Vibrational excitation is strong when H(g) first approaches close proximity of the surface and rebounds toward H(ad) in a direct-mode mechanism. H(g) reaching close to the surface at large impact parameter tends to become trapped before rebounding to abstract H(ad), a complex-mode mechanism.

The Kinematic Connection between absorbing Gas toward QSOs and Galaxies at Intermediate Redshift

We present complementary data on five intermediate-redshift (0.44 ? z ? 0.66) Mg II-absorbing galaxies, combining high spatial resolution imaging from the Hubble Space Telescope, high-resolution quasi-stellar object (QSO) spectroscopy from the Keck High Resolution Echelle Spectrometer, and galaxy kinematics from intermediate-resolution spectroscopy using the Keck Low-Resolution Imaging Spectrometer. These data allow a direct comparison of the kinematics of gas at large galactocentric impact parameters with the galaxy kinematics obtained from the faint galaxy spectroscopy. All five galaxies appear to be relatively normal spirals, with measured rotation curves yielding circular velocities in the range 100 ? vc ? 260 km s-1. The QSO sight lines have projected impact parameters to the absorbing galaxies in the range 14.5 h-1 kpc ? d ? 75 h-1 kpc; the galaxies have inclination angles with respect to the line of sight ranging from 40? to 75?. We find that in four of the five cases examined, the velocities of all of the Mg II-absorption components lie entirely to one side of the galaxy systemic redshift. The fifth case, for which the galaxy is much less luminous than the other four, has narrow absorption centered at zero velocity with respect to systemic, despite having the largest disk inclination angle in the sample. These observations are consistent with rotation being dominant for the absorbing-gas kinematics; however, the total range of velocities observed is inconsistent with simple disk rotation in every case. Simple kinematic models that simultaneously explain both the systemic offset of the absorbing material relative to the galaxy redshifts and the total velocity width spanned by the absorption require either extremely thick rotating gas layers, rotation velocities that vary with z-height above the extrapolation of the galactic plane, or both. In any case, our small sample suggests that rotating halo gas is a common feature of intermediate-redshift spiral galaxies and that the kinematic signature of rotation dominates over radial infall or outflow even for gas well away from the galactic plane. We discuss possible explanations for this behavior and compare our observations with possible local analogs.

Quantum Effects on Collisional Orientations in Dense Plasmas

Quantum effects on the orientation phenomena for the 1s → 2p±1 collisional excitations in electron-hydrogenic ion scatterings are investigated in dense plasmas. An effective pseudopotential model taking into account both the quantum-mechanical effect and the plasma screening effect is applied to describe the electron-ion interaction potential in dense plasmas. The semiclassical straight-line trajectory method is applied to the motion of the projectile electron in order to investigate the variation of the orientation parameter as a function of the scaled impact parameter, thermal de Broglie wavelength, Debye length, and projectile energy. The result shows that the preference for the 1s → 2p-1 transition is significantly reduced with increasing projectile energy. For large impact parameters, the quantum mechanical effect reduces the preference for the 1s → 2p-1 transition. However, the preference for the 1s → 2p-1 transition is also increased with increasing quantum mechanical effect for small impact parameters

Coulomb versus nuclear break-up of 11Be halo nucleus in a nonperturbative framework

The 11Be break-up is calculated at 41 MeV per nucleon incident energy on different targets using a nonperturbative time-dependent quantum calculation. The evolution of the neutron halo wave function shows an emission of neutron at large angles for grazing impact parameters and at forward angles for large impact parameters. The neutron angular distribution is deduced for the different targets and compared to experimental data. We emphasize the diversity of diffraction mechanisms, in particular we discuss the interplay of the nuclear effects such as the towing mode and the Coulomb break-up. A good agreement is found with experimental data.

A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. III. 1,2-butadiene, H2CCCH(CH3) (X 1A′)—a non-Rice–Ramsperger–Kassel–Marcus system?

Crossed molecular beam experiments were conducted to investigate the reaction of ground state carbon atoms, C(3Pj), with 1,2-butadiene, H2CCCH(CH3) (X 1A′), at three collision energies of 20.4 kJ mol−1, 37.9 kJ mol−1, and 48.6 kJ mol−1. Ab initio calculations together with our experimental data reveal that the reaction is initiated by a barrier-less addition of the carbon atom to the π system of the 1,2-butadiene molecule. Dominated by large impact parameters, C(3Pj) attacks preferentially the C2–C3 double bond to form i1 (mechanism 1); to a minor extent, small impact parameters lead to an addition of atomic carbon to the C1–C2 bond yielding i2 (mechanism 2). Both cyclic intermediates i1 and i2 ring open to triplet methylbutatriene complexes i3′ (H2CC*CCH(CH3)) and i3″, (H2CCC*CH(CH3)); C* denotes the attacked carbon atom. i3′ is suggested to decay nonstatistically prior to a complete energy randomization via atomic hydrogen loss forming 1- and 4-methylbutatrienyl CH3CCCCH2 (X 2A″) and HCCCCH(CH3) (X 2A″), respectively. The energy randomization in i3″ is likely to be complete. This isomer decomposes via H atom loss to 3-vinylpropargyl, H2CCCC2H3(X 2A″), as well as 1- and 4-methylbutatrienyl radicals. In high-density environments such as the inner regions of circumstellar envelopes of carbon stars and combustion flames, these linear C5H5 isomers might undergo collision induced isomerization to cyclic structures like the cyclopentadienyl radical. This isomer is strongly believed to be a key intermediate involved in the production of polycyclic aromatic hydrocarbon molecules and soot formation. These characteristics make the reactions of atomic carbon with C4H6 isomers compelling candidates to form C5H5 isomers in the outflow of AGB stars and oxygen-deficient hydrocarbon flames.

Unimolecular decomposition of chemically activated triplet C4HD3 complexes: A combined crossed-beam and ab initio study

The crossed molecular beam technique was utilized to investigate the reaction of ground state carbon atoms, C(3Pj), with d3-methylacetylene, CD3CCH(X1A1), at an average collision energy of 21.1 kJ mol−1. Product angular distributions and time-of-flight spectra were recorded. Only the deuterium loss was observed; no atomic hydrogen emission was detected experimentally. Forward-convolution fitting of the data combined with electronic structure calculations show that the reaction is indirect and initiated by a barrierless interaction of the carbon atom to the π-system of the methylacetylene molecule. Reactions with large impact parameters yield a triplet trans-methylpropene-1-diylidene complex whereas—to a minor amount—the formation of a triplet methylcyclopropenylidene intermediate is governed by smaller impact parameters. Both collision complexes rearrange via hydrogen migration and ring opening, respectively, to two distinct triplet methylpropargylene intermediates. A deuterium atom loss via a tight transition state located about 30 kJ mol−1 above the n-C4H3 product is a likely reaction pathway. The formation of the thermodynamically less stable cyclic isomer remains to be investigated closer. The D atom loss pathway represents an entrance barrierless and exothermic route to synthesize an extremely reactive C4H3 hydrocarbon radical in combustion processes and extraterrestrial environments.

Resolving the Image of Gamma‐Ray Burst Afterglows with Gravitational Microlensing

Microlensing of a gamma-ray burst afterglow by an intervening star can be used to infer the radial structure of the afterglow image. Near the peak of the microlensing event, the outer edge of the image is more highly magnified than its central region, whereas the situation is reversed at later times because of the rapid radial expansion of the image on the sky. Thus, the microlensed afterglow light curve can be inverted to recover the self-similar radial intensity profile of the afterglow image. We calculate the expected errors in the recovered intensity profile as a function of the number of resolution elements, under the assumption that the afterglow and microlensing event parameters are known. For a point-mass lens and uniform source, we derive a simple scaling relation between these parameters and the resultant errors. We find that the afterglow need not be monitored for its entire duration; rather, observations from the peak magnification time tpeak of the microlensing event until ~7tpeak are sufficient to resolve the majority of the afterglow image. Thus, microlensing events can be alerted by relatively infrequent observations of afterglows and then monitored intensively, without significant loss of information about the afterglow intensity profile. The relative intensity profile of ~1% of all afterglows can be measured with 10 resolution elements to an accuracy of (1%) in the optical and (10%) in the infrared, using 4 m class telescopes. Weak microlensing events with large impact parameters are more common; we estimate that for ~10% of afterglows the image profile may be inverted to a fractional accuracy 20% through frequent optical observations. We also calculate the effects of external shear due to the host galaxy or a binary companion, as well as contamination by background light from the host galaxy.

Reaction of acetaldehyde cations with water: The effects of CH3CHO+ vibrational mode and impact parameter on reactivity and product branching

Scattering of mode-selectively excited acetaldehyde cations from D2O was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by CH3CHO+ vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly mode-specific dependence on CH3CHO+ vibrational state.

Gas Distribution and Starburst Activity in the Widely Separated Interacting Galaxy Pair NGC 6670

We present high-resolution H I 21 cm line, 20 cm radio continuum, and CO (1–0) line observations of the luminous infrared galaxy pair NGC 6670. NGC 6670 consists of two edge-on disk galaxies (NGC 6670E and NGC 6670W) with a projected nuclear separation of ~16 kpc. While there are no optically identified tidal features and the molecular disks are not distorted much, we have detected a 90 kpc long H I tail, which suggests that the galaxies are interacting and have already experienced at least one close encounter. The galaxies appear to be gravitationally bound and in a prograde-prograde orbit, which is the most efficient for producing tidal tails. Our observations suggest that the H I at larger galactic radii has been ejected to form the tails and the remnant H I disks have been perturbed by the interaction. In particular, we find that the H I disk of NGC 6670E appears to have been nearly destroyed. We conclude that the previous encounter between the galaxies had a large impact parameter and that the system is still in an early stage of interaction. Even though NGC 6670 is in an early stage of interaction, we find that there is evidence for nuclear starbursts already present. The CO (1–0) observations show large molecular gas reservoirs in the central regions, and the 20 cm radio continuum observations reveal enhanced star formation activity in the nuclear regions of both galaxies. The spatial extent of the 20 cm emission and the far-infrared–radio correlation further rule out active galactic nuclei as the source of the IR luminosity from NGC 6670. We estimate the ratio LIR/M, which is often used as an indicator of star formation efficiency, in the nuclear regions of NGC 6670E and NGC 6670W to be 18 and 11 L⊙/M⊙, respectively. The nuclear star formation efficiency of these two galaxies has been elevated to the level observed in other nearby starburst galaxies (LIR/M > 10 L⊙/M⊙). Other indicators of starburst activity, such as CO brightness temperature and infrared surface brightness, are also observed.

Multistep Coulomb and nuclear breakup of one-nucleon halo nuclei

Coulomb dissociation of one neutron halo nuclei ‘IBe and 15,1QC has been studied at energies around 70A MeV by kinematically complete measurements of breakup particles at RIKEN. The experiment of l1Be breakup on a Pb target aims at studying the characteristic features of low-lying El strength. In particular the effects of higher order excitation and nuclear contribution have been closely studies 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 ground state. We have then successfully applied this analysis method to l”C and “C, where we have extracted the spectroscopic factor for the s-wave halo configuration. We also show preliminary results on the breakup of “Be on a carbon target in order to study the nuclear breakup mechanism and low-lying discrete states of the halo nucleus.