Individual Collision Events Research Articles

Experimental investigation of particle aggregation in a humid and turbulent environment

Aggregation of wetted particles in a turbulent flow is experimentally investigated. The particles settle through nearly isotropic turbulence inside a humid air chamber. The particles are pre-chilled so that wetting occurred even at sub-saturated humidity levels. Aggregates that fall out of the turbulent flow are physically sampled and their size distributions quantified to infer bulk sticking efficiencies at various humidity levels. Optical particle tracking techniques are used to detect individual time-resolved collision events, forming an independent measure of the sticking efficiency parameterized by a collision Stokes number. The two methods are contrasted and the limitations of each are discussed. The data are used to develop a new version of the early Costa sticking model, which is utilized by various volcanic ash transport codes. The key outcome of this work is a generalized Costa sticking model which incorporates the effect of the water film thickness coating the particles.

Coagulation rate coefficients for fractal-like agglomerates in the diffusive and ballistic limits

Predicting agglomeration dynamics in aerosol or colloidal systems requires accurate models for the agglomerate collision rate. In the present work, numerical simulations cover the ballistic and the diffusive collision regimes as limiting cases and are used to derive coagulation rate coefficients for a large number of agglomerate sizes and morphologies. The simulations account for both the translational and rotational motion of agglomerates and allow for the monitoring of individual collision events. The results suggest that the power law suffices to characterize the clusters’ morphologies and that representative collision radii of agglomerates are mainly controlled by two parameters: (i) the radius of gyration, Rg, and (ii) the morphology characterized by the fractal dimension Df. Collision radii for the diffusive regime are up to 10% above the corresponding ballistic results and cluster rotation also increases the effective collision radius. The latter effect can lead to differences of up to 16% in the diffusive regime. In contrast, interpenetration effects, i.e. a small cluster passing through a larger cluster without collision, are less pronounced in the diffusive limit. The results provide strong correlations between collision radii and key parameters for both regimes such that improved analytical expressions for the coagulation rate coefficients can be suggested. Compared to existing models for the ballistic and diffusive regimes, the proposed correlations offer reduced complexity across an extended range of the relevant parameter space.

Granular cooling of ellipsoidal particles in microgravity

A three-dimensional granular gas of ellipsoids is established by exposing the system to the microgravity environment of the International Space Station. We use two methods to measure the dynamics of the constituent particles and report the long-time development of the granular temperature until no further particle movement is detectable. The resulting cooling behavior can be well described by Haff’s cooling law with time scale τ. Different analysis methods show evidence of particle clustering towards the end of the experiment. By using the kinetic theory for ellipsoids we compare the translational energy dissipation of individual collision events with the overall cooling time scale τ. The difference from this comparison indicates how energy is distributed in different degrees of freedom including both translation and rotation during the cooling.

Short-time dynamics of a tracer in an ideal gas.

A small tagged particle immersed in a fluid exhibits Brownian motion and diffuses on a long timescale. Meanwhile, on a short timescale, the dynamics of the tagged particle cannot be simply described by the usual generalized Langevin equation with Gaussian noise, since the number of collisions between the tagged particle and fluid particles is rather small. On such a timescale, we should explicitly consider individual collision events between the tagged particle and the surrounding fluid particles. In this study we analyze the short-time dynamics of a tagged particle in an ideal gas, where we do not have static or hydrodynamic correlations between fluid particles. We perform event-driven hard-sphere simulations and show that the short-time dynamics of the tagged particle is correlated even under such an idealized situation. Namely, the velocity autocorrelation function becomes negative when the tagged particle is relatively light and the fluid density is relatively high. This result can be attributed to the dynamical correlation between collision events. To investigate the physical mechanism which causes the dynamical correlation, we analyze the correlation between successive collision events. We find that the tagged particle can collide with the same ideal-gas particle several times and such collisions cause a strong dynamical correlation for the velocity.

The relationship between collision metrics from micro-sensor technology and video-coded events in rugby union.

This study aimed to determine the relationship between collision metrics from a commercially available micro-sensor technology unit (MST) and the count of collisions coded by expert video analysts in professional rugby union. Forty-four professional rugby union players wore MST units during match play. We analyzed 245 combined data files from 11 competitive matches, resulting in the inclusion of a total of 9202 individual collision events. Collision metrics (the count of collisions and the Collision Load™) were analyzed via the manufacturer's software. Each match was also video recorded and evaluated by two expert video analysts. Pearson's correlation coefficients were used to determine the relationship between the count of collisions coded by the expert video analysts, and both MST collision metrics. One-way ANOVA was used to determine whether differences in the Collision Load™ for individual collision events existed between different playing positions. Very large-nearly perfect correlations were observed between the count of collisions coded by the expert video analysts and both MST collision metrics (the count of collisions: r=0.91, 90% CI=0.89-0.93; the Collision Load™: r=0.89; 90% CI=0.87-0.91). Differences in the Collision Load™ for individual collision events were identified between different playing positions. Collision metrics registered by the MST software relate very strongly with the count of collisions coded by expert video analysts. The typical Collision LoadTM per individual collision event varies depending on player position. The application of automated collision detection for rugby union appears feasible.

Differentiation of metallic and dielectric nanoparticles in solution by single-nanoparticle collision events at the nanoelectrode

In this work, we demonstrate a highly effective method to generate and detect single-nanoparticle (NP) collision events on a nanoelectrode in aqueous solutions. The nanoelectrode of a nanopore–nanoelectrode nanopipette is first employed to accumulate NPs in solution by dielectrophoresis (DEP). Instead of using amperometric methods, the continuous individual NP collision events on the nanoelectrode are sensitively detected by monitoring the open-circuit potential changes of the nanoelectrode. Metallic gold NPs (GNPs) and insulating polystyrene (PS) NPs with various sizes are used as the model NPs. Due to the higher conductivity and polarizability of GNPs, the collision motion of a GNP is different from that of a PS NP. The difference is distinct in the shape of the transient potential change and its first time derivative detected by the nanoelectrode. Therefore, the collision events by metallic and insulating NPs on a nanoelectrode can be differentiated based on their polarizability. DEP induced NP separation and cluster formation can also be probed in detail in the concentrated mixture of PS NPs and GNPs.

On the Frequency Distribution of Neutral Particles from Low-Energy Strong Interactions

The rejection of the contamination, or background, from low-energy strong interactions at hadron collider experiments is a topic that has received significant attention in the field of particle physics. This article builds on a particle-level view of collision events, in line with recently proposed subtraction methods. While conventional techniques in the field usually concentrate on probability distributions, our study is, to our knowledge, the first attempt at estimating the frequency distribution of background particles across the kinematic space inside individual collision events. In fact, while the probability distribution can generally be estimated given a model of low-energy strong interactions, the corresponding frequency distribution inside a single event typically deviates from the average and cannot be predicted a priori. We present preliminary results in this direction and establish a connection between our technique and the particle weighting methods that have been the subject of recent investigation at the Large Hadron Collider.

Biomolecular detection at ssDNA-conjugated nanoparticles by nano-impact electrochemistry

We describe the use of ssDNA functionalized silver nanoparticle (AgNP) probes for quantitative investigation of biorecognition and real time detection of biomolecular targets using nano-impact electrochemistry. The method is based on measurements of the individual collision events between ssDNA aptamer-functionalized AgNPs and a carbon fiber miroelectrode (CFME). Specific binding events of target analyte induced collision frequency changes enabling ultrasensitive detection of the aptamer target in a single step. These changes are assigned to the surface coverage of the NP by the ssDNA aptamers and subsequent conformational changes of the aptamer probe which affect the electron transfer between the NP and the electrode surface. The method enables sensitive and selective detection of ochratoxin A (OTA), chosen here as a model target, with a limit of detection of 0.05nM and a relative standard deviation of 4.9%. The study provides a means of characterizing bioconjugation of AgNPs with aptamers and assessing biomolecular recognition events with high sensitivity and without the use of exogenous reagents or enzyme amplification steps. This methodology can be broadly applicable to other bioconjugated systems, biosensing and related bioanalytical applications.

Measuring the number concentration of arbitrarily-shaped gold nanoparticles with surface plasmon resonance microscopy

Molar concentration of gold nanoparticles is one of the most critical parameters of gold colloids in order to develop their applications in sensing, diagnostics and nanomedicine. Previous methods often stand just for gold nanoparticles with regular shape and narrow size distribution. In the present work, we proposed an absolute quantification method that determined the molar concentration of gold nanoparticles with arbitrary shapes and polydisperse sizes. This approach involved the real time monitoring and counting of individual nanoparticles collision events, from which the quantification of molar concentration was achieved using a theoretical model consisting of Fick’s laws of diffusion and Stokes-Einstein equation. The determination of spherical gold nanoparticles concentration resulted in excellent agreement with traditional spectrometry method. It was further demonstrated that the present approach can be expanded to determine the molar concentration of gold nanoparticles with arbitrary shapes and poly-diversed distributions.

Preparations for the public release of high-level CMS data

The CMS Collaboration, in accordance with its commitment to open access and data preservation, is preparing for the public release of up to half of the reconstructed collision data collected in 2010. Efforts at present are focused on the usability of the data in education. The data will be accompanied by example applications tailored for different levels of access, including ready-to-use web-based applications for histogramming or visualising individual collision events and a virtual machine image of the CMS software environment that is compatible with these data. The virtual machine image will contain instructions for using the data with the online applications as well as examples of simple analyses. The novelty of this initiative is two-fold: in terms of open science, it lies in releasing the data in a format that is good for analysis; from an outreach perspective, it is to provide the possibility for people outside CMS to build educational applications using our public data. CMS will rely on services for data preservation and open access being prototyped at CERN with input from CMS and the other LHC experiments.

Toward particle-level filtering of individual collision events at the Large Hadron Collider and beyond

Low-energy strong interactions are a major source of background at hadron colliders, and methods of subtracting the associated energy flow are well established in the field. Traditional approaches treat the contamination as diffuse, and estimate background energy levels either by averaging over large data sets or by restricting to given kinematic regions inside individual collision events. On the other hand, more recent techniques take into account the discrete nature of background, most notably by exploiting the presence of substructure inside hard jets, i.e. inside collections of particles originating from scattered hard quarks and gluons. However, none of the existing methods subtract background at the level of individual particles inside events. We illustrate the use of an algorithm that will allow particle-by-particle background discrimination at the Large Hadron Collider, and we envisage this as the basis for a novel event filtering procedure upstream of the official reconstruction chains. Our hope is that this new technique will improve physics analysis when used in combination with state-of-the-art algorithms in high-luminosity hadron collider environments.

Multiple cosmic collisions and the microwave background power spectrum

Collisions between cosmic bubbles of different vacua are a generic feature of false vacuum eternal inflation scenarios. While previous studies have focused on the consequences of a single collision event in an observer's past, we begin here an investigation of the more general scenario allowing for many "mild" collisions intersecting our past light cone (and one another). We discuss the general features of multiple collision scenarios and consider their impact on the cosmic microwave background (CMB) temperature power spectrum, treating the collisions perturbatively. In a large class of models, one can approximate a multiple collision scenario as a superposition of individual collision events governed by nearly isotropic and scale-invariant distributions, most appearing to take up less than half of the sky. In this case, the shape of the expected CMB temperature spectrum maintains statistical isotropy and typically features a dramatic increase in power in the low multipoles relative to that of the best-fit $\Lambda$CDM model, which is in tension particularly with the observed quadrupole. We argue that this predicted spectrum is largely model-independent and can be used to outline features of the underlying statistical distributions of colliding bubbles consistent with CMB temperature measurements.

DNA Analysis by Application of Pt Nanoparticle Electrochemical Amplification with Single Label Response

This study demonstrates a highly sensitive sensing scheme for the detection of low concentrations of DNA, in principle down to the single biomolecule level. The previously developed technique of electrochemical current amplification for detection of single nanoparticle (NP) collisions at an ultramicroelectrode (UME) has been employed to determine DNA. The Pt NP/Au UME/hydrazine oxidation reaction was employed, and individual NP collision events were monitored. The Pt NP was modified with a 20-base oligonucleotide with a C6 spacer thiol (detection probe), and the Au UME was modified with a 16-base oligonucleotide with a C6 spacer thiol (capture probe). The presence of a target oligonucleotide (31 base) that hybridized with both capture and detection probes brought a Pt NP on the electrode surface, where the resulting electrochemical oxidation of hydrazine resulted in a current response.

Event indexing systems for efficient selection and analysis of HERA data

The design and implementation of two software systems introduced to improve the efficiency of offline analysis of event data taken with the ZEUS Detector at the HERA electron–proton collider at DESY are presented. Two different approaches were made, one using a set of event directories and the other using a tag database based on a commercial object-oriented database management system. These are described and compared. Both systems provide quick direct access to individual collision events in a sequential data store of several terabytes, and they both considerably improve the event analysis efficiency. In particular the tag database provides a very flexible selection mechanism and can dramatically reduce the computing time needed to extract small subsamples from the total event sample of the experiment.

Improved methods of measurement and analysis of conversion electron and β-particle spectra

A general statistical test of the stability of measurement conditions was demonstrated on the β-spectra of 241Pu cumulated during four years. The α- and γ-ray spectroscopy indicated stability of the 241Pu source. Monte Carlo modelling of individual collision events clarified the role of electron scattering and energy losses within a radioactive source down to energies of several hundreds of eV. The impact ionization by β-particles of carbon and oxygen atoms in a surface contamination layer on the 241Pu and 63Ni sources was observed.

Results from the experiment E895 at the BNL AGS

We present some of the latest results from the E895 experiment conducted at the BNL AGS accelerator. Au+Au collisions were recorded by the EOS Time Projection Chamber (TPC) at beam energies of 2, 4, 6, and 8 A GeV. The TPC detector permitted the reconstruction of individual collision events with almost 4π acceptance and good particle identification. This capability allowed E895 to study global observables and two particle correlations with respect to symmetries of the event. Flow excitation functions are examined and discussed in the context of the Nuclear Equation of State.

Dissociation Scheme of Highly Charged Triatomic Molecules

Ionization of triatomic molecules in collision with 120 keV Ar8+ and the subsequent molecular fragmentation has been studied by means of a position sensitive time-of-flight (TOF) technique. The triple coincidence signals for these molecules are identified for the dissociation channels of (CO2)n+ where n = 3–6. The dissociation of (NO2)n+ where n = 3–5 has also been studied. The velocity vector of each fragment ion is determined for individual collision events and the angle between the velocity vectors is calculated. For the highly ionized molecules (n = 6), the angle is consistent with the angle of the neutral molecule, both in the case of linear and bent molecules. For the lower charge states considerable deviation from the neutral molecule bond angle has been found. These results suggest that production of higher charge states is favorable for the fulfillment of a pure Coulombic dissociation scheme.

Solar Neutrino Experiments: The Next Generation

In the next few years, three massive new solar neutrino detectors will generate large amounts of precise data that should have a major impact on our understanding of how the Sun shines and how neutrinos behave. They are Super Kamiokande, in the mountains west of Tokyo; the Sudbury Neutrino Observatory (SNO) in a northern Ontario mine and Borexino, in the Apennines east of Rome. Each of these detectors was conceived and is being built by a sizable international collaboration. Each is housed in an underground laboratory shielded from cosmic ray products other than neutrinos and very energetic muons by a mile or so of earth. Super Kamiokande, the most massive of the three, is a 50-kiloton water-Cerenkov detector. (See figure 1 and the cover of this issue.) In all of these new detectors, sophisticated electronics will record and analyze the individual neutrino collision events. Each detector will register more neutrino interactions in two months than all of the previous solar neutrino experiments have detected in a quarter of a century.

Analytical treatment of high-energy nucleus-nucleus collisions including mean field effects: Boltzmann equation approach.

We study nucleus-nucleus collisions at high energy on the basis of Boltzmann's kinetic theory under the assumption that the incident energy per nucleon is high enough for nucleon-nucleon collisions to dominate the strength of the mean field so that the latter may be treated perturbatively. Then, by use of multiple collision expansion as well as linearization techniques and the eikonal approximation, we obtain the stationary distribution function from the Boltzmann equation with the nuclear mean field as external force. The distribution consists of an infinite sum over multiple collision components. Their momentum dependent parts obey a transport equation linear in the nuclear mean force field, and can be evaluated analytically. They differ from the solution of a Fokker-Planck-type equation without external forces, obtained previously, only by a shift in the momentum caused by the nuclear mean field. The present investigation confirms a recently developed phenomenological approach which has been successful in explaining the sideward kinetic energy flow of the particles emitted in individual nucleus-nucleus collision events.