Collision Pairs Research Articles

Search for resonanttt¯production in proton-proton collisions ats=8 TeV

A search is performed for the production of heavy resonances decaying into top-antitop quark pairs in proton-proton collisions at sqrt(s) = 8 TeV. Data used for the analyses were collected with the CMS detector and correspond to an integrated luminosity of 19.7 inverse femtobarns. The search is performed using events with three different final states, defined by the number of leptons (electrons and muons) from the t t-bar to W b W b decay. The analyses are optimized for reconstruction of top quarks with high Lorentz boosts, where jet substructure techniques are used to enhance the sensitivity. Results are presented for all channels and a combination is performed. No significant excess of events relative to the expected yield from standard model processes is observed. Upper limits on the production cross section of heavy resonances decaying to t t-bar are calculated. A narrow leptophobic topcolor Z' resonance with a mass below 2.4 TeV is excluded at 95% confidence level. Limits are also derived for a broad Z' resonance with a 10% width relative to the resonance mass, and a Kaluza-Klein excitation of the gluon in the Randall-Sundrum model. These are the most stringent limits to date on heavy resonances decaying into top-antitop quark pairs.

Pair creation in collision of γ-ray beams produced with high-intensity lasers.

Direct production of electron-positron pairs in two-photon collisions, the Breit-Wheeler process, is one of the basic processes in the universe. However, it has never been directly observed in the laboratory because of the absence of the intense γ-ray sources. Laser-induced synchrotron sources emission may open a way to observe this process. The feasibility of an experimental setup using a MeV photon source is studied in this paper. We compare several γ-ray sources and estimate the expected number of electron-positron pairs and competing processes by using numerical simulations including quantum electrodynamic effects.

Flow regime mapping of high inertial gas–liquid droplet microflows in flow-focusing geometries

Confined gas–liquid droplet microflows present a lot of new perspectives for microfluidic systems that require the presence of a gaseous phase. In addition to the benefits associated with the discretization of reactive and sensing processes, the highly inertial droplets generated in these systems can enable fast efficient mixing by pair collisions as well as high system throughput due to the short convective timescales involved in the droplet transport. Presented herein is mapping of the geometry-specific droplet generation from a binary gas–liquid flow for different flow-focusing configurations. The dynamic interactions of inertia, shear stress, viscous and surface tension forces create three unique regimes in the gas–liquid flow rate space, providing adaptable flow configuration to specific applications. Analytical investigation and numerical analyses involving governing forces are also introduced to predict the effective droplet diameter versus gas flow rate. We found that the experimental results were well matched to the analytical predictions within 10 % of uncertainty.

Search for a high-mass Higgs boson decaying to a W boson pair in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A search for a high-mass Higgs boson $H$ is performed in the $H\to{WW}\to\ell\nu\ell\nu$ and $H\to{WW}\to\ell\nu{qq}$ decay channels, using $pp$ collision data corresponding to an integrated luminosity of 20.3 fb$^{-1}$, collected at $\sqrt{s}=$ 8 TeV by the ATLAS detector at the Large Hadron Collider. No evidence of a high-mass Higgs boson is found. Limits on $\sigma_H\times \mathrm{BR}(H\to{WW})$ as a function of the Higgs boson mass $m_H$ are determined in three different scenarios: one in which the heavy Higgs boson has a narrow width compared to the experimental resolution, one a width increasing with the boson mass and modeled by the complex-pole scheme following the same behavior as in the Standard Model, and one for intermediate widths. The upper range of the search is $m_H=1500$ GeV for the narrow-width scenario and $m_H=1000$ GeV for the other two scenarios. The lower edge of the search range is $200-300$ GeV and depends on the analysis channel and search scenario. For each scenario, individual and combined limits from the two $WW$ decay channels are presented. At $m_H = 1500$ GeV, the highest mass point tested, $\sigma_H\cdot\mathrm{BR}(H\rightarrow WW)$ for a narrow-width Higgs boson is constrained to be less than 22 fb and 6.6 fb at the 95% CL for the gluon fusion and vector-boson fusion production modes, respectively.

Measurement of diffractive and exclusive processes with the ATLAS detector

The ATLAS Collaboration has carried out a study of di ractive dijet produc- tion in proton-proton collisions at a centre-of-mass energy of p s = 7 TeV at the LHC. The data distributions are compared with Monte Carlo models and the rapidity gap sur- vival probability has been estimated in the kinematic region with high di ractive contri- bution. Prospects for exclusive jet production studies with the forward proton tagging capability of the AFP sub-detector of ATLAS are also discussed. First results based on data taken jointly with the ATLAS and the LHCf detectors in a p+Pb run will also be shown. In addition, the measurement of the cross-section for the exclusive production of di-lepton pairs in pp collisions at p s = 7 TeV is discussed.

Low-mass dimuon measurements in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC

Low-mass dimuon production, including light vector mesons ρ, ω, ϕ, provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions. In particular, strangeness production can be studied via ϕ meson measurements, while the detailed description of the full dimuon mass spectrum down to the kinematic threshold can be used to reveal in-medium modifications of hadron properties and the thermal emission arising from the medium. Measurements in pp and p-A systems, in absence of hot nuclear matter effects, must be used as a reference to test our knowledge of the processes expected to contribute to dilepton production. Dimuon production is studied with the ALICE apparatus at the LHC at forward rapidity (2.5 < y < 4) with the Muon Spectrometer. In this contribution, results on low-mass dimuon production are shown, for various center-of-mass energies per nucleon pair, in pp, p-Pb, and Pb-Pb collisions.

Analysis of charm pair production at the LHC

The Dokshitzer, Gribov, Lipatov, Alterelli, Parisi and Ciafaloni, Catani, Fiorani, Marchesini approaches to perturbative QCD evolution have been investigated by examining correlations of charmed hadron pairs in pp collisions at TeV. The theoretical models are compared to the data taken by the LHCb experiment. Differences in the parton kinematics between the two approaches are discussed. In general a model incorporating next-leading-order diagrams matched to parton showers describes the data best.

Theoretical model and simulations for a cw exciplex pumped alkali laser.

The Exciplex Pumped Alkali Laser (XPAL) system, which is similar to DPAL (Diode Pumped Alkali vapor Laser), has been demonstrated in mixtures of Cs vapor, Ar, with and without ethane. Unlike DPAL, it uses the broadband absorption blue satellite of the alkali D2 line, created by naturally occuring collision pairs. For example, Cs-Ar collision pairs have an absorption width which is as wide as the one of commercial semiconductor diode lasers. A continuous wave XPAL four-level theoretical model is presented in this paper. More factors are considered, such as the spectral dependence of pumped laser absorption for broadband pumping and the longitudinal population variation. Some intra-cavity details, such as longitudinal distributions of pumped laser and alkali laser, can also be solved well. The predictions of optical-to-optical efficiency as a function of temperature and pumped laser intensity are presented. The model predicts that there is an optimum value of temperature or pumped laser intensity. The analysis of the influence of cell length on optical-to-optical efficiency shows that a better performance can be achieved when using longer cell. The prediction of influence of Ar concentration and reflectivity of output coupler shows that higher optical-to-optical efficiency could be achieved if lower reflectivity of output coupler and higher Ar concentration are used. The optical-to-optical efficiency as high as 84% achieved by optimizing configuration with the pumped intensity of 5 × 10⁷ W/cm² presented shows that broadband pumped four-level XPAL system has a potential of high optical-to-optical efficiency.

Modeling of heavy-flavor pair correlations in Au-Au collisions at200AGeV at the BNL Relativistic Heavy Ion Collider

We study the nuclear modification of angular and momentum correlations between heavy quark pairs in ultrarelativistic heavy-ion collisions. The evolution of heavy quarks inside the thermalized medium is described via a modified Langevin approach that incorporates both elastic and inelastic interactions with the medium constituents. The spacetime evolution of the fireball is obtained from a (2+1)-dimensional viscous hydrodynamics simulation. The hadronization of heavy quarks is performed utilizing a hybrid model of fragmentation and coalescence. Our results show that the nuclear modification of the transverse momentum imbalance of D\bar{D} pairs reflects the total energy loss experienced by the heavy quarks and may help us probe specific regions of the medium. The angular correlation of heavy flavor pairs, especially in the low to intermediate transverse momentum regime, is sensitive to the detailed energy loss mechanism of heavy quarks inside the QGP.

Transition and Damping of Collective Modes in a Trapped Fermi Gas between BCS and Unitary Limits near the Phase Transition.

We investigate the transition and damping of low-energy collective modes in a trapped unitary Fermi gas by solving the Boltzmann-Vlasov kinetic equation in a scaled form, which is combined with both the T-matrix fluctuation theory in normal phase and the mean-field theory in order phase. In order to connect the microscopic and kinetic descriptions of many-body Feshbach scattering, we adopt a phenomenological two-fluid physical approach, and derive the coupling constants in the order phase. By solving the Boltzmann-Vlasov steady-state equation in a variational form, we calculate two viscous relaxation rates with the collision probabilities of fermion’s scattering including fermions in the normal fluid and fermion pairs in the superfluid. Additionally, by considering the pairing and depairing of fermions, we get results of the frequency and damping of collective modes versus temperature and s-wave scattering length. Our theoretical results are in a remarkable agreement with the experimental data, particularly for the sharp transition between collisionless and hydrodynamic behaviour and strong damping between BCS and unitary limits near the phase transition. The sharp transition originates from the maximum of viscous relaxation rate caused by fermion-fermion pair collision at the phase transition point when the fermion depair, while the strong damping due to the fast varying of the frequency of collective modes from BCS limit to unitary limit.

An industry-scale modeling for the turbulence agglomeration of fine particles

Turbulence-induced agglomeration of fine particles/droplets in aerosol is important in various industrial and natural processes. Despite abundant theoretical and numerical studies on turbulent collision kernels, convincing numerical methods for industry-scale applications are still lacking. In this work, we develop a mixed Eulerian–Lagrangian model for the modeling of turbulence-induced agglomeration of fine particles/droplets. As advantages over Sommerfeld’s model, the present model searches for real collision pairs based on a new mesh-independent method and uses appropriate turbulent collision kernels to calculate the collision frequencies. Benchmark simulations are carried out and compared with the results provided by the well-characterized experiment by Duru et al.

A study of internal energy relaxation in shocks using molecular dynamics based models

Recent potential energy surfaces (PESs) for the N2 + N and N2 + N2 systems are used in molecular dynamics (MD) to simulate rates of vibrational and rotational relaxations for conditions that occur in hypersonic flows. For both chemical systems, it is found that the rotational relaxation number increases with the translational temperature and decreases as the rotational temperature approaches the translational temperature. The vibrational relaxation number is observed to decrease with translational temperature and approaches the rotational relaxation number in the high temperature region. The rotational and vibrational relaxation numbers are generally larger in the N2 + N2 system. MD-quasi-classical trajectory (QCT) with the PESs is also used to calculate the V-T transition cross sections, the collision cross section, and the dissociation cross section for each collision pair. Direct simulation Monte Carlo (DSMC) results for hypersonic flow over a blunt body with the total collision cross section from MD/QCT simulations, Larsen-Borgnakke with new relaxation numbers, and the N2 dissociation rate from MD/QCT show a profile with a decreased translational temperature and a rotational temperature close to vibrational temperature. The results demonstrate that many of the physical models employed in DSMC should be revised as fundamental potential energy surfaces suitable for high temperature conditions become available.

A novel simplified Bernoulli trials collision scheme in the direct simulation Monte Carlo with intelligence over particle distances

This work deals with the development of an intelligent inter-particle collision scheme using the Direct Simulation Monte Carlo (DSMC) method. Conventional DSMC collision schemes cannot perceive collision distances by their own routines and, consequently, must rely on other techniques to provide a choice of collision pairs with a smaller inter-particle space. Here, we propose a modification of the Simplified Bernoulli Trials (SBT) scheme, called the Intelligent Simplified Bernoulli Trials (ISBT) scheme, which can create pseudo-circular subcells that reduce approximately 25%–32% of the mean collision separation distance. The ISBT scheme tries to arrange the particle indexing and collision acceptance-rejection of the SBT scheme in a way that leads to the formation of virtual clusters. These inner-cell clusters then will cause the selection of the “near-neighbor” pair, which leads to smaller mean collision separation distances. Different low- and high-speed test cases, e.g., lid-driven cavity flow, steady hypersonic flow over a two-dimensional cylinder, and Mach 15.6 nitrogen flow over a 25°–55° axisymmetric biconic, are selected to assess the accuracy and efficiency of the ISBT collision scheme.

Particle Dispersion Simulation in Turbulent Flow Due to Particle-Particle and Particle-Wall Collisions

AbstractSimulation of the 3-D, fully developed turbulent channel flows laden with various mass loading ratios of particles is made using an Eulerian-Lagrangian approach in which the carrier-fluid flow field is solved with a low-Reynolds-number k-ε turbulence model while the deterministic Lagrangian method together with binary-collision hard-sphere model is applied for the solution of particle motion. Effects of inter-particle collisions and particle-wall collisions under different extents of wall roughness on particle dispersion are addressed in the study. A cost-effective searching algorithm of collision pair among particles is developed. It is found that the effects of inter-particle collisions on particle dispersion cannot be negligible when the ratio of the mean free time of particle to the mean particle relaxation time of particle is less or equal to O(10). In addition, the wall roughness extent plays an important role in the simulation of particle-wall collisions particularly for cases with small mass loading ratios.

Top quark pair production beyond NNLO

We construct an approximate expression for the total cross section for the production of a heavy quark-antiquark pair in hadronic collisions at next-to-next-to-next-to-leading order (N$^3$LO) in $\alpha_s$. We use a technique which exploits the analyticity of the Mellin space cross section, and the information on its singularity structure coming from large N (soft gluon, Sudakov) and small N (high energy, BFKL) all order resummations, previously introduced and used in the case of Higgs production. We validate our method by comparing to available exact results up to NNLO. We find that N$^3$LO corrections increase the predicted top pair cross section at the LHC by about 4% over the NNLO.

Two color pumping of the Rb D2 line laser (780 nm) through the photoassociation of Rb-Ar or Rb-Xe thermal pairs: Realization of a quantum efficiency above one

A two color process for optical pumping of the Rb D2 line laser (λ = 780 nm) in Rb vapor-rare gas mixtures is demonstrated to yield an overall quantum efficiency &amp;gt; 1. Combining photoexcitation of the Rb D2 line blue satellite in Rb-Xe or Rb-Ar mixtures with free ← free optical pumping of the A2Π12←X2Σ12+ transition at large internuclear separation (R &amp;gt; 5 Å) also increases the laser efficiency and pump absorption coefficient, relative to the single color, B ← X pumped system, by a factor of 1.7 and more than an order of magnitude, respectively. Simultaneous photopumping of Rb-Xe thermal collision pairs at 760 nm (D2 satellite) and 794.3 nm results in a 780 nm (D2 line) laser with a quantum efficiency of 1.008 which corresponds to the removal of ∼102 cm–1 (12.6 meV) of thermal energy per emitted photon.

Effect of the coagulation rate on the settling time of stationary vibrations of aerosol in an acoustic resonator

The effect of the coagulation rate of particles of a polydisperse gas suspension in an acoustic resonator on the settling time of stationary vibrations has been numerically investigated. The gassuspension dynamics is described by the system of equations of motion of multivelocity multitemperature continuum with allowance for the exchange in momentum and energy between the carrier medium and dispersed-phase fractions. The coagulation of particles of different fractions is described within the Lagrangian model, which takes pair collisions into account. The settling of vibrations in an acoustic resonator is described in dependence of change in the dispersivity of the vapor−droplet mixture in the wave field of a standing wave.

On the reactivity of ion pairs into different diatomic systems

SynopsisIon pair collisions are an important process in many astrophysical environments. We present ab initio calculations of highly excited states of C2+ and identify the ion pair channel C2+/C−. We use these results to interpret recent experiments on carbon cluster dissociation.

Margination Regimes and Drainage Transition in Confined Multicomponent Suspensions

A mechanistic theory is developed to describe segregation in confined multicomponent suspensions such as blood. It incorporates the two key phenomena arising in these systems at low Reynolds number: hydrodynamic pair collisions and wall-induced migration. In simple shear flow, several regimes of segregation arise, depending on the value of a "margination parameter" M. Most importantly, there is a critical value of M below which a sharp "drainage transition" occurs: one component is completely depleted from the bulk flow to the vicinity of the walls. Direct simulations also exhibit this transition as the size or flexibility ratio of the components changes.

Combustion simulations with accurate transport properties for reactive intermediates

The effect of employing accurate transport properties in simulations of freely propagating H2/O2/He and CH4/O2 flames is investigated. Transport properties for the collision pairs H2–H2, H–H2, and H–H are computed from published tables of collision integrals. Previous calculations in our group of transport properties for OH–He, H–H2O and H–O2 have been extended to higher temperatures. In addition, new calculations on H–He, O–He, H2–He, and H2O–He are reported. Flame simulations with two sets of transport data were compared. The first set contained transport properties obtained with the conventional method of employing parameterized Lennard-Jones (12-6) potentials, while in the second set transport properties for the above collision pairs were substituted. A modest increase in laminar flame speeds, comparable to differences found when employing different chemistry models, was found for both flames.