Large Impact Parameters Research Articles

Geometry of ultraperipheral nuclear collisions

It is advocated that geometry of the interaction region of two heavy nuclei colliding at large impact parameters is important for the relative role of light-by-light scattering and QCD-initiated processes. Exclusive production of resonances is possible by dense electromagnetic fields in the interior space between the nuclei. The cross-section of the two-photon processes is evaluated and some examples are considered. It is speculated that the exclusive production of [Formula: see text]-mesons by “two-photon” processes forbidden by the Landau–Yang rule may be allowed within strong magnetic fields due to odd number of photons becoming involved.

A Mean-field Approach to Simulating the Merging of Collisionless Stellar Systems Using a Particle-based Method

Abstract We present a mean-field approach to simulating merging processes of two spherical collisionless stellar systems. This approach is realized with a self-consistent field (SCF) method in which the full spatial dependence of the density and potential of a system is expanded in a set of basis functions for solving Poisson’s equation. To apply this SCF method to a merging situation where two systems are moving in space, we assign the expansion center to the center of mass of each system, the position of which is followed by a massless particle placed at that position initially. Merging simulations over a wide range of impact parameters are performed using both an SCF code developed here and a tree code. The results of each simulation produced by the two codes show excellent agreement in the evolving morphology of the merging systems and in the density and velocity dispersion profiles of the merged systems. However, comparing the results generated by the tree code with those obtained with the softening-free SCF code, we found that in large impact parameter cases, a softening length of the Plummer type introduced in the tree code has an effect of advancing the orbital phase of the two systems in the merging process at late times. We demonstrate that the faster orbital phase originates from the larger convergence length to the pure Newtonian force. Other application problems suitable to the current SCF code are also discussed.

Roles of NΔ → NN and πN → Δ Reactions in Heavy-Ion Collisions at Intermediate Energies**Supported by the National Natural Science Foundation of China under Grant No. 11505150; the Yuncheng University Research Project under Grant No. YQ-2014014; the National Natural Science Foundation of China under Grant No. 11405278

Within the framework of the isospin-dependent transport model, the roles of the reactions NΔ → NN and πN → Δ are investigated through simulating heavy-ion collisions at 1000 MeV/nucleon. The absorption process NΔ → NN plays an important role for heavy impact systems and small impact parameters than for light impact systems and large impact parameters. The resorption process πN → Δ is of importance for heavy impact systems and large impact parameters than for light impact systems and small impact parameters. Thus the influences of the reaction NΔ → NN (πN → Δ) on pion production dynamics can be neglected in heavy-ion collisions for smaller (larger) impact parameters and light systems. It is the reaction πN → Δ that causes the anti-correlation of pions and nucleons in the rapidity dependence of the directed flow.

C ii] 158 μm Emission from z ∼ 4 H i Absorption-selected Galaxies

Abstract We report on a search for the [C ii] 158 μm emission line from galaxies associated with four high-metallicity damped Lyα absorbers (DLAs) at z ∼ 4 using the Atacama Large Millimeter/submillimeter Array (ALMA). We detect [C ii] 158 μm emission from galaxies at the DLA redshift in three fields, with one field showing two [C ii] 158 μm emitters. Combined with previous results, we now have detected [C ii] 158 μm emission from five of six galaxies associated with targeted high-metallicity DLAs at z ∼ 4. The galaxies have relatively large impact parameters, ≈16–45 kpc, [C ii] 158 μm line luminosities of (0.36–30) × 108 L ⊙, and rest-frame far-infrared properties similar to those of luminous Lyman-break galaxies, with star formation rates of ≈7–110 M ⊙ yr−1. Comparing the absorption and emission line profiles yields a remarkable agreement between the line centroids, indicating that the DLA traces gas at velocities similar to that of the [C ii] 158 μm emission. This disfavors a scenario where the DLA arises from gas in a companion galaxy. These observations highlight ALMA’s unique ability to uncover a high-redshift galaxy population that has largely eluded detection for decades.

Long-lived B−L symmetric SSM particles at the LHC

We investigate the collider signatures of neutral and charged long-lived particles (LLPs), predicted by the supersymmetric $B\ensuremath{-}L$ extension of the Standard Model (BLSSM), at the Large Hadron Collider (LHC). The BLSSM is a natural extension of the minimal supersymmetric Standard Model (MSSM) that can account for nonvanishing neutrino masses. We show that the lightest right-handed sneutrino can be the lightest supersymmetric particle (LSP), while the next-to-the LSP (NLSP) is either the lightest left-handed sneutrino or the left-handed stau, which are natural candidates for the LLPs. We analyze the displaced vertex signature of the neutral LLP (the lightest left-handed sneutrino) and the charged tracks associated with the charged LLP (the left-handed stau). We show that the production cross sections of our neutral and charged LLPs are relatively large, namely of order $\mathcal{O}(1)\text{ }\text{ }\mathrm{fb}$. Thus, probing these particles at the LHC is quite plausible. In addition, we find that the displaced dilepton associated with the lightest left-handed sneutrino has a large impact parameter that discriminates it from other SM leptons. We also emphasize that the charged track associated with the left-handed stau has a large momentum with slow moving charged tracks, hence it is distinguished from the SM background, and therefore it can be accessible at the LHC.

Accurate closed-form trajectories of light around a Kerr black hole using asymptotic approximants

Highly accurate closed-form expressions that describe the full trajectory of photons propagating in the equatorial plane of a Kerr black hole are obtained using asymptotic approximants. This work extends a prior study of the overall bending angle for photons (Barlow et al 2017 Class. Quantum Grav. 34 135017). The expressions obtained provide accurate trajectory predictions for arbitrary spin and impact parameters, and provide significant time advantages compared with numerical evaluation of the elliptic integrals that describe photon trajectories. To construct approximants, asymptotic expansions for photon deflection are required in various limits. To this end, complete expansions are derived for the azimuthal angle as a function of radial distance from the black hole in the far-distance and closest-approach (pericenter) limits, and new coefficients are reported for the bending angle in the weak-field limit (large impact parameter).

The subleading eikonal in supergravity theories

In this paper we study the subleading contributions to eikonal scattering in (super)gravity theories with particular emphasis on the role of both elastic and inelastic scattering processes. For concreteness we focus on the scattering of various massless particles off a stack of Dp-branes in type II supergravity in the limit of large impact parameter b. We analyse the relevant field theory Feynman diagrams which naturally give rise to both elastic and inelastic processes. We show that in the case analysed the leading and subleading eikonal only depend on elastic processes, while inelastic processes are captured by a pre-factor multiplying the exponentiated leading and subleading eikonal phase. In addition to the traditional Feynman diagram computations mentioned above, we also present a novel method for computing the amplitudes contributing to the leading and subleading eikonal phases, which, in the large b limit, only involves knowledge of the onshell three and four-point vertices. The two methods are shown to give the same results. Furthermore we derive these results in yet another way, by computing various one-point amplitudes which allow us to extract the classical solution of the gravitational back reaction of the target Dp-branes. Finally we show how our expressions for the leading and subleading eikonal agree with the calculation of the metric and corresponding deflection angle for massless states moving along geodesics in the relevant curved geometry.

Molecular Emission from a Galaxy Associated with a z ∼ 2.2 Damped Lyα Absorber

Abstract Using the Atacama Large Millimeter/submillimeter Array, we have detected CO(3−2) line and far-infrared continuum emission from a galaxy associated with a high-metallicity ([M/H] = −0.27) damped Lyα absorber (DLA) at z DLA = 2.19289. The galaxy is located 3.″5 away from the quasar sightline, corresponding to a large impact parameter of 30 kpc at the DLA redshift. We use archival Very Large Telescope-SINFONI data to detect Hα emission from the associated galaxy, and find that the object is dusty, with a dust-corrected star formation rate of M⊙ yr−1. The galaxy’s molecular mass is large, Mmol = (1.4 ± 0.2) × 1011 × (α CO/4.3) × (0.57/r 31) M ⊙, supporting the hypothesis that high-metallicity DLAs arise predominantly near massive galaxies. The excellent agreement in redshift between the CO(3–2) line emission and low-ion metal absorption (∼40 km s−1) disfavors scenarios whereby the gas probed by the DLA shows bulk motion around the galaxy. We use Giant Metrewave Radio Telescope H i 21 cm absorption spectroscopy to find that the H i along the DLA sightline must be warm, with a stringent lower limit on the spin temperature of T s > 1895 × (f/0.93) K. The detection of C i absorption in the DLA, however, also indicates the presence of cold neutral gas. To reconcile these results requires that the cold components in the DLA contribute little to the H i column density, yet contain roughly 50% of the metals of the absorber, underlining the complex multi-phase nature of the gas surrounding high-z galaxies.

Combined Experimental and Computational Investigation of the Elementary Reaction of Ground State Atomic Carbon (C; 3P j) with Pyridine (C5H5N; X1A1) via Ring Expansion and Ring Degradation Pathways.

We explored the elementary reaction of atomic carbon (C; 3P j) with pyridine (C5H5N; X1A1) at a collision energy of 34 ± 4 kJ mol-1 utilizing the crossed molecular beams technique. Forward-convolution fitting of the data was combined with high-level electronic structure calculations and statistical (RRKM) calculations on the triplet C6H5N potential energy surface (PES). These investigations reveal that the reaction dynamics are indirect and dominated by large range reactive impact parameters leading via barrier-less addition to the nitrogen atom and to two chemically nonequivalent "aromatic" carbon-carbon bonds forming three distinct collision complexes. At least two reaction pathways through atomic hydrogen loss were identified on the triplet surface. These channels involve multiple isomerization steps of the initial collision complexes via ring-opening and ring expansion forming an acyclic 1-ethynyl-3-isocyanoallyl radical (P1; 2A″) and a hitherto unreported seven-membered 1-aza-2-dehydrocyclohepta-2,4,6-trien-4-yl radical isomer (P3; 2A), respectively. For RRKM calculations at zero collision energy, representing conditions in cold molecular clouds, the ring expansion product P3 is formed nearly exclusively for the atomic hydrogen loss channel, but based on these computations, the molecular fragmentation channel forming acetylene (C2H2) plus 3-cyano-2-propen-1-ylidene (P6; 3A″) accounts for nearly all of the degradation products of the reaction of atomic carbon with pyridine, proposing a destruction pathway of interstellar pyridine, which may account for the absence in the detection of pyridine in the interstellar medium. These results are also discussed in light of the isoelectronic carbon-benzene (C6H6; X1A1) system with important implications to the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons (NPAHs) in the interstellar medium compared to mass growth processes of PAH counterparts through ring expansion.

Simulating the Galaxy Cluster “El Gordo”: Gas Motion, Kinetic Sunyaev–Zel’dovich Signal, and X-Ray Line Features

Abstract The massive galaxy cluster “El Gordo” (ACT-CL J0102–4915) is a rare merging system with a high collision speed suggested by multi-wavelength observations and theoretical modeling. Zhang et al. propose two types of mergers, a nearly head-on merger and an off-axis merger with a large impact parameter, to reproduce most of the observational features of the cluster using numerical simulations. The different merger configurations of the two models result in different gas motion in the simulated clusters. In this paper, we predict the kinetic Sunyaev–Zel’dovich (kSZ) effect, the relativistic correction of the thermal Sunyaev–Zel’dovich (tSZ) effect, and the X-ray spectrum of this cluster, based on the two proposed models. We find that (1) the amplitudes of the kSZ effect resulting from the two models are both on the order of ΔT/T ∼ 10−5; but their morphologies are different, which trace the different line-of-sight velocity distributions of the systems; (2) the relativistic correction of the tSZ effect around 240 GHz can be possibly used to constrain the temperature of the hot electrons heated by the shocks; and (3) the shift between the X-ray spectral lines emitted from different regions of the cluster can be significantly different in the two models. The shift and the line broadening can be up to ∼25 eV and 50 eV, respectively. We expect that future observations of the kSZ effect and the X-ray spectral lines (e.g., by ALMA, XARM) will provide a strong constraint on the gas motion and the merger configuration of ACT-CL J0102–4915.

Scattering of point particles by black holes: Gravitational radiation

Gravitational waves can teach us not only about sources and the environment where they were generated, but also about the gravitational interaction itself. Here we study the features of gravitational radiation produced during the scattering of a point-like mass by a black hole. Our results are exact (to numerical error) at any order in a velocity expansion, and are compared against various approximations. At large impact parameter and relatively small velocities our results agree to within percent level with various post-Newtonian and weak-field results. Further, we find good agreement with scaling predictions in the weak-field/high-energy regime. Lastly, we achieve striking agreement with zero-frequency estimates.

Crossed Molecular Beam Study of H+CH4 and H+CD4 Reactions: Vibrationally Excited CH3/CD3 Product Channels

We study the H+CH4/CD4→H2/HD+CH3/CD3 reactions using the time sliced velocity map ion imaging technique. Ion images of the CH3/CD3 products were measured by the (2+1) resonance enhanced multi-photon ionization (REMPI) detection method. Besides the CH3/CD3 products in the ground state, ion images of the vibrationally excited CH3/CD3 products were also observed at two collision energies of 0.72 and 1.06 eV. It is shown that the angular distribution of the products CH3/CD3 in vibrationally excited states gradually vary from backward scattering to sideways scattering as the collision energy increases. Compared to the CH3/CD3 products in the ground state, the CH3/CD3 products in vibrationally excited states tend to be more sideways scattered, indicating that larger impact parameters play a more important role in the vibrationally excited product channels.

Central and peripheral interactions of hadrons

Surprisingly enough, the ratio of elastic to inelastic cross sections of proton interactions increases with energy in the interval corresponding to ISRrightarrow LHC (i.e. from 10 to 10^4 GeV). That leads to special features of their spatial interaction region at these and higher energies. Within the framework of some phenomenological models, we show how the particular ranges of the transferred momenta measured in elastic scattering experiments expose the spatial features of the inelastic interaction region according to the unitarity condition. The difference between their predictions at higher energies is discussed. The notion of central and peripheral collisions of hadrons is treated in terms of the impact-parameter description. It is shown that the shape of the differential cross section in the diffraction cone is mostly determined by collisions with intermediate impact parameters. Elastic scattering at very small transferred momenta is sensitive to peripheral processes with large impact parameters. The role of central collisions in the formation of the diffraction cone is less significant.

On the star-forming ability of Molecular Clouds

The star-forming ability of a molecular cloud depends on the fraction of gas it can cycle into the dense-phase. Consequently, one of the crucial questions in reconciling star-formation in clouds is to understand the factors that control this process. While it is widely accepted that the variation in ambient conditions can alter significantly the ability of a cloud to spawn stars, the observed variation in the star-formation rate in nearby clouds that experience similar ambient conditions, presents an interesting question. In this work we attempted to reconcile this variation within the paradigm of colliding flows. To this end we develop self-gravitating, hydrodynamic realisations of identical flows, but allowed to collide off-centre. Typical observational diagnostics such as the gas-velocity dispersion, the fraction of dense-gas, the column density distribution ({\small N-PDF}), the distribution of gas mass as a function of $K$-band extinction and the strength of compressional/solenoidal modes in the post-collision cloud were deduced for different choices of the impact parameter of collision. We find that a strongly sheared cloud is terribly inefficient in cycling gas into the dense phase and that such a cloud can possibly reconcile the sluggish nature of star-formation reported for some clouds. Within the paradigm of cloud-formation via colliding flows this is possible in case of flows colliding with a relatively large impact parameter. We conclude that compressional modes - though probably essential - are insufficient to ensure a relatively higher star-formation efficiency in a cloud.

Theoretical study on collision dynamics of H+ + H2O at low energies

ABSTRACTScattering dynamics of a water molecule in collision with proton is studied based on a time-dependent density functional theory and coupled with the molecular dynamics method, in which the electrons are described by quantum mechanics and the nuclei are described by classical mechanics. Four different incident directions at 46 eV are chosen in order to investigate the orientations effect, and the energy-dependent effect in low energy region is explored under impact energies 27, 36 and 46 eV. Reaction channels, scattering angles and energy loss of protons are calculated. The differences between those characteristics are unobvious in large impact parameters, which are irrespective of the incident orientations due to weak projectile-target interaction. In small impact parameters, the results strongly depend on the collision energy and orientation.

Electromagnetic processes in ultra-peripheral Pb+Pb collisions with ATLAS

The large equivalent-photon fluxes accompanying Pb ion beams at the LHC initiate photon–photon interactions, which occur when the colliding nuclei have large impact parameter (ultra-peripheral collisions). The ATLAS collaboration has carried out a range of measurements related with two-photon interactions at high energies using recently recorded Pb+Pb collision data taken at sNN=5.02 TeV.

Effects of magnetic field on plasma evolution in relativistic heavy-ion collisions

Very strong magnetic fields can arise in non-central heavy-ion collisions at ultrarelativistic energies, which may not decay quickly in a conducting plasma. We carry out relativistic magnetohydrodynamics (RMHD) simulations to study the effects of this magnetic field on the evolution of the plasma and on resulting flow fluctuations in the ideal RMHD limit. Our results show that magnetic field leads to enhancement in elliptic flow for small impact parameters while it suppresses it for large impact parameters (which may provide a signal for initial stage magnetic field). Interestingly, we find that magnetic field in localized regions can temporarily increase in time as evolving plasma energy density fluctuations lead to reorganization of magnetic flux. This can have important effects on chiral magnetic effect. Magnetic field has non-trivial effects on the power spectrum of flow fluctuations. For very strong magnetic field case one sees a pattern of even-odd difference in the power spectrum of flow coefficients arising from reflection symmetry about the magnetic field direction if initial state fluctuations are not dominant. We discuss the situation of nontrivial magnetic field configurations arising from collision of deformed nuclei and show that it can lead to anomalous elliptic flow. Special (crossed body-body) configurations of deformed nuclei collision can lead to presence of quadrupolar magnetic field which can have very important effects on the rapidity dependence of transverse expansion (similar to {\it beam focusing} from quadrupole fields in accelerators).

Probabilistic Approach to the Scattering of Gravitational Centers

Some controversial results that arise in the problem of the scattering of gravitational objects are analyzed in this paper. First, the approach leading to possible use of the approximation of pairwise (binary) collisions in the theory of scattering in plasmas and in gravitational systems is examined. It is stressed that this possibility is not a consequence of neglecting long-range (distant) collisions. Next, results obtained for the number of scattering events of a test particle in an infinite gravitational medium with a fixed velocity change are discussed. The conclusion that the function describing the number of particle collisions is divergent is shown to be incorrect. This is because the finite time duration of collisions with large impact parameters is neglected. In concluding, we show that the previously proposed variant of a modified Holtsmark distribution is erroneous.

Thermodynamically-consistent semi-classical ℓ-changing rates

We compare the results of the semi-classical (SC) and quantum-mechanical (QM) formalisms for angular-momentum changing transitions in Rydberg atom collisions given in a series of papers by Vrinceanu et al, most recently Vrinceanu et al (2012 Astrophys. J. 747 56), with those of the SC formalism using a modified Monte Carlo realization. We find that this revised SC formalism agrees well with the QM results. This provides further evidence that the rates derived from the QM treatment are appropriate to be used when modeling recombination through Rydberg cascades, an important process in understanding the state of material in the early universe. The rates for derived from the QM formalism diverge when integrated to sufficiently large impact parameter, b. Further to the empirical limits to the b integration suggested by Pengelly and Seaton (1964 Mon. Not. R. Astron. Soc. 127 165), we suggest that the fundamental issue causing this divergence in the theory is that it does not fully cater for the finite time taken for such distant collisions to complete.

Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-line Spectroscopy. II. Methods and Models∗

Abstract We present basic data and modeling for a survey of the cool, photoionized circumgalactic medium (CGM) of low-redshift galaxies using far-UV QSO absorption-line probes. This survey consists of “targeted” and “serendipitous” CGM subsamples, originally described in Stocke et al. (Paper I). The targeted subsample probes low-luminosity, late-type galaxies at z < 0.02 with small impact parameters ( 〈 ρ 〉 = 71 kpc), and the serendipitous subsample probes higher luminosity galaxies at z ≲ 0.2 with larger impact parameters ( 〈 ρ 〉 = 222 kpc). Hubble Space Telescope and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H i and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H i absorption complexes, we confirm the CGM cool gas mass of Paper I, but this value is significantly smaller than that found by the COS-Halos survey. We trace much of this difference to the specific values of the low-z metagalactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain log ( M / M ⊙ ) = 10.5 ± 0.3 , or ∼30% of the total baryon reservoir of an L ≥ L * , star-forming galaxy.