Early Stages Of Collision Research Articles

Colliding heavy nuclei take multiple identities on the path to fusion

The properties of superheavy elements probe extremes of physics and chemistry. They are synthesised at accelerator laboratories using nuclear fusion, where two atomic nuclei collide, stick together (capture), then with low probability evolve to a compact superheavy nucleus. The fundamental microscopic mechanisms controlling fusion are not fully understood, limiting predictive capability. Even capture, considered to be the simplest stage of fusion, is not matched by models. Here we show that collisions of 40Ca with 208Pb, experience an ‘explosion’ of mass and charge transfers between the nuclei before capture, with unexpectedly high probability and complexity. Ninety different partitions of the protons and neutrons between the projectile-like and target-like nuclei are observed. Since each is expected to have a different probability of fusion, the early stages of collisions may be crucial in superheavy element synthesis. Our interpretation challenges the current view of fusion, explains both the successes and failures of current capture models, and provides a framework for improved models.

Mantle Flow Deflected by Arc–Continent Collision and Continental Subduction in Eastern Indonesia

Abstract We investigate seismic anisotropy across southeastern Indonesia where the Indo-Australian plate subducts beneath and collides with the Sunda-Banda arc. Geochemical, geodetic, and tomographic studies reveal an along-strike transition from oceanic subduction to continental subduction and collision near central Flores that is due to a change of lithospheric composition in the subducting plate. To investigate the anisotropic fabric and dynamics of the upper mantle surrounding this young (∼5 Ma) arc–continent collision, we perform shear wave splitting analysis on local and teleseismic S waves recorded by an array of broadband seismometers that crosses the subduction–collision boundary. Seismic anisotropy inferred from our local S dataset shows that anisotropic sources above the slab extend to depths exceeding 100 km. Analysis of teleseismic SKS and SKKS waves reveal a shift in subslab fast axes from trench-parallel to trench-perpendicular near the ocean–continent boundary in the lower plate, which we relate to regional subslab mantle flow being deflected around the subducted continental lithosphere. Along-strike variations in anisotropic fast axes from teleseismic phases overlap with distinct structural and tectonic boundaries that divide distinct regions of the collision, implying the effects of the collision transcend any one dataset and highlighting the complexity of collisional boundaries. These results shed light on the interaction between tectonic structure and mantle dynamics in an emergent collision, and help constrain the nature of upper mantle deformation in the early stages of collision.

On transverse momentum broadening in real-time lattice simulations of the glasma and in the weak-field limit

In these proceedings, we report on our numerical lattice simulations of partons traversing the boost-invariant, non-perturbative glasma as created at the early stages of collisions at RHIC and LHC. Since these highly energetic partons are produced from hard scatterings during heavy-ion collisions, they are already affected by the first stage of the medium's time evolution, the glasma, which is the pre-equilibrium precursor state of the quark-gluon plasma. We find that partons quickly accumulate transverse momentum up to the saturation momentum during the glasma stage. Moreover, we observe an interesting anisotropy in transverse momentum broadening of partons with larger broadening in the rapidity than in the azimuthal direction. Its origin can be related to correlations among the longitudinal color-electric and color-magnetic flux tubes in the initial state of the glasma. We compare these observations to the semi-analytic results obtained by a weak-field approximation, where we also find such an anisotropy in a parton's transverse momentum broadening.

ψ(2S) and J/ψ production in pp collisions at √s = 7, 8 and 13 TeV

Proton-proton collision is an essential part of high energy physics, as it is a baseline for nucleus-nucleus and hadron-nucleus collisions. Quark-gluon plasma, an exotic state of strongly interacting QCD matter, is routinely produced in heavy nuclei high-energy collisions. Charmonium, the bound states of $c\bar{c}$ produced at the early stages of collisions, are nice probes to examine the properties of matter through which they pass. The differential cross-sections of inclusive $\psi(2S)$ in pp collisions at energies of $\sqrt{s}$ = 7, 8 and 13 TeV have been studied. PYTHIA 8.1 event generator is used for the simulation of data. The differential cross-section of inclusive $J/\psi$ is also presented in order to examine the $\psi(2S)$ to $J/\psi$ ratio at similar energies. Both charmonium states are simulated in dimuon ($\mu^{+}\mu^{-}$) decay channel. The simulated data is also compared with the ALICE and LHCb experimental data based at LHC. We have plotted the differential cross-section as a function of transverse momentum ($p_T$) and rapidity $y$. PYTHIA simulation is found to be in good agreement with ALICE data for low $p_T$ interval, y interval close to midrapidity, while PYTHIA agrees with LHCb data at low $p_T$ interval. It is observed that the LHCb differential cross-section distributions of $J/\psi$ and $\psi(2S)$ as a function of y are lower compared to the ALICE data distributions and PYTHIA simulations. The $\psi(2S)$ to $J/\psi$ ratio is also well explained by PYTHIA and shows an increase in production of inclusive $J/\psi$ as compared to inclusive $\psi(2S)$.

Effects of multi-seamount subduction on accretionary wedge deformation: Insights from analogue modelling

Deformation patterns caused by the subduction of a single seamount or aseismic ridge have been well studied in analogue and numerical models. However, the effects of sequential multi-seamount subduction on accretionary wedge deformation have rarely been investigated in details. We performed a series of analogue modelling experiments of sequential subduction involving two seamounts of variable shape, spacing and rheological properties of strata to better understand the deformation mechanisms of an accretionary wedge with multi-seamount subduction. The results demonstrate that a seamount significantly hinders the seaward propagation of the accretionary wedge and facilitates lateral propagation. Two structural quiet zones form at the leading and trailing edges of the subducted seamount in the early stage of collision. As the seamount deeply penetrates into the wedge, the structural quiet zone in the leading edge is remoulded by a duplex structure, which may help the upward transport of deep subducted sediments back into the shallow area. Comparatively, the structural quiet zone in the trailing edge remains undeformed because it is situated in the stress shadow of the seamount. Deformation of the strata between two subducting seamounts may occur by thrusts laterally propagating into the seamount gap, which is facilitated by the décollement layer. The modelling results provide insights into the complex deformation mechanisms related to seamounts collision observed in the accretionary wedge offshore from the Costa Rica margin.

Orogenic Gold Mineralization of the Adycha Ore Region (East Yakutia, Russia): Geological Setting and Geochemical Features of Gold–Quartz Ores

The analysis of features of the geological structure of the Adycha gold placer region shows that flat structures of the early collision stage control the setting of the ore fields. The ores are similar to host rocks in the main components, lithofile trace elements, and REEs. The features of the trace-element pattern and indicative ratios agree with the suggested metamorphic origin of the main volume of fluids, their weakly reduced and almost neutral conditions, and sorption rock–fluid interaction. A group of elements, the distribution of which is significantly distinct from those of the host rocks, is determined. The geochemical type of the ore region is caused by Mo, Au, Bi, and Sb, the contents of which are higher in all studied ore objects. Chromium, As, Pb, and Te are present in all ore fields, but not in all objects. The Co–Ni–W–Zn–Cd–Ag, Co–Ni–W–In–Mn–Cu–Sr, and Sn–In–Mn–Cu–Zn–Cd–Sr–Ag associations supplement the geochemical type of the Delyuvial’ny, Lazo, and Sentachan ore fields, respectively. Our studies along with thermobarogeochemical data refine the metamorphic–magmatic model of the formation of the gold–quartz deposits of the Kular–Nera belt and outline approaches for the practical application of these features.

Dynamical initialization and hydrodynamic modeling of relativistic heavy-ion collisions

We present a fully three-dimensional model providing initial conditions for energy and conserved charge density distributions in heavy ion collisions at RHIC Beam Energy Scan (BES) collision energies [C. Shen, B. Schenke, Dynamical initial state model for relativistic heavy-ion collisions, Phys. Rev. C97 (2) (2018) 024907. arXiv:1710.00881, doi:10.1103/PhysRevC.97.024907; C. Shen, B. Schenke, Initial state and hydrodynamic modeling of heavy-ion collisions at RHIC BES energies, PoS CPOD2017 (2018) 006. arXiv:1711.10544]. The model includes the dynamical deceleration of participating nucleons or valence quarks. It provides a realistic estimation of the initial baryon stopping during the early stage of collisions. We also present the implementation of the model with 3+1 dimensional hydrodynamics, which involves the addition of source terms that deposit energy and net-baryon densities produced by the initial state model at proper times greater than the initial time for the hydrodynamic simulation. The importance of this dynamical initialization stage on hadronic flow observables at the RHIC BES is quantified.

Formation of massive, dense cores by cloud–cloud collisions

Abstract We performed sub-parsec (∼ 0.014 pc) scale simulations of cloud–cloud collisions of two idealized turbulent molecular clouds (MCs) with different masses in the range of (0.76–2.67) × 10$^{4}\,M_{\odot }$ and with collision speeds of 5–30 km s−1. Those parameters are larger than in Takahira, Tasker, and Habe (2014, ApJ, 792, 63), in which study the colliding system showed a partial gaseous arc morphology that supports the NANTEN observations of objects indicated to be colliding MCs using numerical simulations. Gas clumps with density greater than 10−20 g cm−3 were identified as pre-stellar cores and tracked through the simulation to investigate the effects of the mass of colliding clouds and the collision speeds on the resulting core population. Our results demonstrate that the smaller cloud property is more important for the results of cloud–cloud collisions. The mass function of formed cores can be approximated by a power-law relation with an index γ = −1.6 in slower cloud–cloud collisions (v ∼ 5 km s−1), and is in good agreement with observation of MCs. A faster relative speed increases the number of cores formed in the early stage of collisions and shortens the gas accretion phase of cores in the shocked region, leading to the suppression of core growth. The bending point appears in the high-mass part of the core mass function and the bending point mass decreases with increase in collision speed for the same combination of colliding clouds. The higher-mass part of the core mass function than the bending point mass can be approximated by a power law with γ = −2–−3 that is similar to the power index of the massive part of the observed stellar initial mass function. We discuss implications of our results for the massive-star formation in our Galaxy.

Variability of orogenic magmatism during Mediterranean-style continental collisions: A numerical modelling approach

The relationship between magma generation and the tectonic evolution of orogens during subduction and subsequent collision requires self-consistent numerical modelling approaches predicting volumes and compositions of the produced magmatic rocks. Here, we use a 2D magmatic-thermomechanical numerical modelling procedure to analyse rapid subduction of a narrow ocean, followed by Mediterranean style collision, which is characterized by the gradual accretion of lower plate material and slab migration towards the orogenic foreland. Our results suggest that magmatism has a large-scale geodynamic effect by focusing deformation throughout the entire subduction and collision process. The rheological structure and compositional layering of the crust impose a key control on the distribution of magmatic rocks within the orogen. Compared to previous simplified homogeneous crustal models, a compositionally layered crust causes an increase in felsic material influx during continental collision and results in shallower magmatic sources that migrate with time towards the foreland. Changes in the deformation style may be locally driven by magma emplacement rather than by slab movement. Our modelling also demonstrates that the migration pattern of the deformation front and the magmatic arc relative to the location of the suture zone may be driven by lower crustal indentation in the overriding plate during early stages of collision. The modelling predicts a gradual change in magma source composition with time from typical calc-alkaline to ones associated with relamination and eduction during subduction, collision and slab detachment. This transition explains the compositional changes of magma their temporal and spatial migration, as well as the observed link with deformation in the Dinarides orogen of Central Europe selected as a case study.

PHENIX measurements of single electrons from charm and bottom decays at midrapidity in Au+Au collisions

Heavy quarks (charm and bottom) are a sensitive probe of the Quark-Gluon Plasma (QGP) created in high-energy nuclear collisions. The modification of their phase-space distribution in the QGP reflects the physical property of the QGP because they are generated in the early stage of collisions and subsequently propagate through the QGP. PHENIX has published the first measurement of the nuclear modification factors RAA of separated charm and bottom hadron decay electrons in minimum bias Au+Au collisions at sNN=200GeV using the 2011 data set. In these proceedings, we present RAA(c→e) and RAA(b→e) in most central Au+Au collisions at sNN=200GeV using 1/8 of our large 2014–2016 data set.

Heavy flavor measurements at the STAR experiment

In ultra-relativistic heavy-ion collisions at RHIC, a new state of nuclear matter with extreme properties is produced — the strongly interacting quark-gluon plasma. Heavy quarks, predominantly produced at early stages of collisions owing to their large masses, provide an exceptional probe for exploring the hot and dense medium created in such collisions. The Heavy Flavor Tracker and Muon Telescope Detector have been fully operational since 2014 and have significantly improved STAR’s capabilities in measuring both open and hidden heavy flavor hadrons in heavy-ion collisions. We present an overview of recent heavy flavor results obtained at the STAR experiment.

Entropy production and effective viscosity in heavy-ion collisions

Entropy production and an effective viscosity in central Au+Au collisions are estimated in a wide range of incident energies 3.3 GeV $\le \sqrt{s_{NN}}\le$ 39 GeV. The simulations are performed within a three-fluid model employing three different equations of state with and without deconfinement transition, which are equally good in reproduction of the momentum-integrated elliptic flow of charged particles in the considered energy range. It is found that more that 80\% entropy is prodused during a short early collision stage which lasts $\sim$1 fm/c at highest considered energies $\sqrt{s_{NN}}\ge$ 20 GeV. The estimated values of the viscosity-to-entropy ratio ($\eta/s$) are approximately the same in all considered scenarios. At final stages of the system expansion they range from $\sim$0.05 at highest considered energies to $\sim$0.5 lowest ones. It is found that the $\eta/s$ ratio decreases with the temperature ($T$) rise approximately as $\sim 1/T^4$ and exhibits a rather weak dependence on the net-baryon density.

Multiplicity distributions and fluctuations in proton-proton and heavy-ion collisions

The charged particle multiplicity is one of the simplest observable in collision of the hadrons, imposing constrains on the mechanisms of the particle production. Multiplicity distributions at lower energies have been studied earlier. In this work, it has been investigated how the multiplicity distributions and fluctuations have been explained in LHC energies. For this, a detailed study has been done from √s=7.7 GeV/A to √s= 2.76 TeV/A with event generators AMPT-Default and AMPT-String Melting. It will be also discussed how multiplicity distribution parameters are related to early stages of collision and how small-x helps to find information on initial states.

Ore-forming processes in the Drazhnoe gold–quartz deposit (Eastern Yakutia, Russia)

Themobarogeochemical investigations revealed that quartz from the Drazhnoe deposit was formed in mesothermal conditions at depths of 3–4 km from carbon dioxide–water fluids with wide salinity variations and an admixture of methane. Several types of fluids are distinguishable on the basis of the composition of extracts: hydrocarbonate–sodium, highly diluted, and late sulfate–hydrocarbonate–sodium with elevated salinity. Ore minerals precipitated in the thermostatic environments against the background of fluid heterogenization due to a probably significant pressure drop and mixing of different solutions. Metamorphic processes related to the early collision stage provided no substantial impact on the composition and potential of gold ore mineralization.

Long-range correlations in high multiplicity pp and pA collisions

This review summarizes recent discoveries in high-energy proton + proton and proton + nucleus collisions, with particular attention on the observation of long-range azimuthal correlations in high multiplicity collisions. These correlations, which resemble those seen in ultra-relativistic nucleus–nucleus collisions, provide a unique window into the physics of the very early collision stage in high energy nuclear interactions. Here we present a compilation of the most important experimental results and briefly discuss successes and challenges for a selection of theoretical approaches.

Thermal photons as a quark-gluon plasma thermometer reexamined

Photons are a penetrating probe of the hot medium formed in heavy-ion collisions, but they are emitted from all collision stages. At photon energies below 2-3 GeV, the measured photon spectra are approximately exponential and can be characterized by their inverse logarithmic slope, often called "effective temperature" $T_\mathrm{eff}$. Modelling the evolution of the radiating medium hydrodynamically, we analyze the factors controlling the value of $T_\mathrm{eff}$ and how it is related to the evolving true temperature $T$ of the fireball. We find that at RHIC and LHC energies most photons are emitted from fireball regions with $T{\,\sim\,}T_\mathrm{c}$ near the quark-hadron phase transition, but that their effective temperature is significantly enhanced by strong radial flow. Although a very hot, high pressure early collision stage is required for generating this radial flow, we demonstrate that the experimentally measured large effective photon temperatures $T_\mathrm{eff}{\,>\,}T_\mathrm{c}$, taken alone, do not prove that any electromagnetic radiation was actually emitted from regions with true temperatures well above $T_\mathrm{c}$. We explore tools that can help to provide additional evidence for the relative weight of photon emission from the early quark-gluon and late hadronic phases. We find that the recently measured centrality dependence of the total thermal photon yield requires a larger contribution from late emission than presently encoded in our hydrodynamic model.

Collision Dynamics of Protonated N-Acetylmethionine with Singlet Molecular Oxygen (a1Δg): The Influence of the Amide Bond and Ruling Out the Complex-Mediated Mechanism at Low Energies

It has been proposed (J. Phys. Chem. B 2011, 115, 2671) that the ammonium group is involved in the gas-phase reaction of protonated methionine (MetH(+)) with singlet oxygen (1)O2, yielding hydrogen peroxide and a dehydro compound of MetH(+) where the -NH3(+) transforms into cyclic -NH2-. For the work reported, the gas-phase reaction of protonated N-acetylmethionine (Ac-MetH(+)) with (1)O2 was examined, including the measurements of reaction products and cross sections over a center-of-mass collision energy (Ecol) range from 0.05 to 1.0 eV using a guided-ion-beam apparatus. The aim is to probe how the acetylation of the ammonium group affects the oxidation chemistry of the ensuing Ac-MetH(+). Properties of intermediates, transition states, and products along the reaction coordinate were explored using density functional theory calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. Direct dynamics trajectory simulations were carried out at Ecol of 0.05 and 0.1 eV using the B3LYP/4-31G(d) level of theory. In contrast to the highly efficient reaction of MetH(+) + (1)O2, the reaction of Ac-MetH(+) + (1)O2 is extremely inefficient, despite there being exoergic pathways. Two product channels were observed, corresponding to transfer of two H atoms from Ac-MetH(+) to (1)O2 (H2T), and methyl elimination (ME) from a sulfone intermediate complex. Both channels are inhibited by collision energies, becoming negligible at Ecol > 0.2 eV. Analysis of RRKM and trajectory results suggests that a complex-mediated mechanism might be involved at very low Ecol, but direct, nonreactive collisions prevail over the entire Ecol range and physical quenching of (1)O2 occurs during the early stage of collisions.

Λc analysis with the ALICE detector at the LHC

ALICE is the LHC experiment dedicated to the study of the Quark Gluon Plasma (QGP) in high-energy nuclear collisions. Due to their large mass and their production at the early collision stage, heavy quarks (charm and beauty) are well-suited probes to study the properties of the QGP. The open charm baryon Λc is reconstructed in ALICE in the decay channels Λc+ → π+ + K− + p and Λc+ → K0S + p, exploiting both topological selections and particle identification. We report about the analysis strategy to observe the signal in pp collisions for both decay channels and then show the invariant mass plots where the Λc peak is clearly visible.

Upper crustal deformation in continent‐continent collision: A case study from the Bernard nappe complex (Valais, Switzerland)

AbstractThe Penninic nappes in the Swiss Alps formed during continental collision between the Adriatic and European plates in Cenozoic times. Although intensely studied, the finite geometry of the basement‐bearing Penninic nappes in western Switzerland has remained a matter of debate for decades (e.g., “Siviez‐Mischabel dilemma”) and the paleogeographic origin of various nappes has been disputed. Here, we present new structural data for the central part of the Penninic Bernard nappe complex, which contains pre‐Permian basement and Permo‐Mesozoic metasedimentary units. Our lithological and structural observations indicate that the discrepancy between the different structural models proposed for the Bernard nappe complex can be explained by a lateral discontinuity. In the west, the presence of a Permian graben caused complex isoclinal folding, whereas in the east, the absence of such a graben resulted mainly in imbricate thrusting. The overall geometry of the Bernard nappe complex is the result of three main deformation phases: (1) detachment of Mesozoic cover sediments along Triassic evaporites (Evolène phase) during the early stages of collision, (2) Eocene top‐to‐the‐N(NW) nappe stacking (Anniviers phase), and (3) subsequent backfolding and backshearing (Mischabel phase). The southward localized backshearing is key to understand the structural position and paleogeographic origin of units, such as the Frilihorn and Cimes Blanches “nappes” and the Antrona ophiolites. Based on these observations, we present a new tectonic model for the entire Penninic region of western Switzerland and discuss this model in terms of continental collision zone processes.

Generation of Arc and Within-plate Chemical Signatures in Collision Zone Magmatism: Quaternary Lavas from Kurdistan Province, Iran

New whole-rock and Sr–Nd isotopic analyses of Quaternary lavas from Kurdistan Province, western Iran, shed light on the nature of orogenic plateau magmatism during continental collision and the possible role of accessory minerals during mantle partial melting in this tectonic setting. The sampled lavas are from the Turkish–Iranian plateau within the Arabia–Eurasia collision zone. Compositions are typically basanite, hawaiite and alkali basalt, with minor rhyolite. Most of the basic samples from the Qorveh–Bijar region have elevated abundances of large ion lithophile elements (LILE) and light rare earth elements (LREE) (e.g. 76 ppm < La < 165 ppm), with steep REE patterns: 45 < La/Yb < 101. Two flows from Takab in the north of the study area have higher K2O/Na2O and Rb than the Qorveh–Bijar samples, but lower LREE and Ba. Sr–Nd isotope values for all the basic samples plot close to Bulk Silicate Earth, with 143Nd/144Nd between 0·51263 and 0·51276, and 87Sr/86Sr between 0·70467 and 0·70600; this is similar to Quaternary alkali basalts from the Iran–Turkey borderlands to the NW, but distinct from a more depleted source melting elsewhere in the same collision zone; for example, at Mount Ararat and the Kars plateau (Turkey). Crustal contamination does not appear to be an important factor affecting magma composition. The range of chemical signatures suggests variable melting of at least two distinct sources. One inferred source produced melts with La/Nb ranging from ∼3·5 to ∼1·2, which is unusual for volcanic rocks that are otherwise coeval. We interpret this variation as the result of depletion of a K-richterite- and rutile-bearing source during melting in the garnet stability field. The importance of this result is that the size of the negative Nb anomaly and fractionation of LILE may depend on K-richterite and rutile in the source mineralogy, rather than simply the bulk abundances of incompatible trace elements in the source region. We infer the presence of phlogopite in a second mantle source, the melting of which produced the more potassic lavas from Takab. Lithosphere delamination or slab break-off mechanisms for triggering melting are problematic in the study area, as the lithosphere is reportedly ∼150–200 km thick. Late Cenozoic extension has not been recognized, and so extension is unlikely as a cause of melting. The NW–SE alignment of volcanic centres, parallel to the regional structural grain, does imply a structural control, at least on the final eruption site. Eurasian mantle lithosphere was probably metasomatized by fluids derived from Mesozoic–early Cenozoic subduction and the early stages of collision. More recent convergence has had the potential to cause further fluxing and melting of the re-fertilized lithosphere, with rapid magma ascent assisted by transcurrent motion along the orogen. It is also possible that the negative dT/dP section of the amphibole peridotite solidus was crossed as a result of lithospheric thickening in the collision zone (i.e. compression melting rather than decompression melting). Exhaustion of accessory phases may also help explain larger-scale transitions from arc to within-plate chemistry in orogens, such as the Paleogene Arabia–Eurasia system, and the Mesozoic–Cenozoic magmatism of eastern China.