Particle Production Mechanisms Research Articles

A laboratory study of the effect of velocity on Hallett–Mossop ice crystal multiplication

Laboratory experiments into the effect of ice multiplication in clouds by the Hallett–Mossop (H–M) mechanism have been extended to higher velocities than in earlier studies. The secondary ice particle production rate per milligram of rime accreted is found to be dependent on the velocity of the graupel pellet as it accretes supercooled droplets. In the range 1.5 to 12 m s −1, the maximum secondary ice particle ejection occurs at 6 m s −1 with a rate of one crystal per 70 mg of accreted rime. The relevance of these new results to the particle production mechanism and to behaviour in natural clouds is discussed.

Large scale rapidity correlations in heavy ion collisions

We discuss particle production mechanisms for heavy ion collisions. We present an argument demonstrating how the fluctuations of the number of produced particles in a series of classical emissions can account for Koba-Nielsen-Olesen scaling. We predict rapidity correlations in the particle production in the event-by-event analysis of heavy ion collisions on the rapidity scales of the order of $1/{\ensuremath{\alpha}}_{s}.$

On the competition between equilibrium and non-equilibrium particle production mechanisms in nucleus–nucleus collisions at [formula omitted

In nucleus–nucleus collisions at high energies a competition between different particle production mechanisms is possible. This competition can be studied assuming that experimental data for a given collision can be described using models based on different given hypotheses. An analysis of the particle production mechanisms based on different thermodynamic assumptions (global equilibrium, local equilibrium or non-equilibrium) is performed using (mainly) experimental data obtained from nucleus–nucleus collisions at 4.5A GeV/c . A competition between these mechanisms is observed in a given collision. Connections with the collision geometry are reported.

Systematic study of highpThadron spectra inpp,pA,andAAcollisions at ultrarelativistic energies

High-$p_T$ particle spectra in $p+p$ ($\bar p + p$), $p+A$ and $A+B$ collisions are calculated within a QCD parton model in which intrinsic transverse momentum, its broadening due to initial multiple parton scattering, and jet quenching due to parton energy loss inside a dense medium are included phenomenologically. The intrinsic $k_T$ and its broadening in $p+A$ and $A+B$ collisions due to initial multiple parton scattering are found to be very important at low energies ($\sqrt{s}<50$ GeV). Comparisons with $S+S$, $S+Au$ and $Pb+Pb$ data with different centrality cuts show that the differential cross sections of large transverse momentum pion production ($p_T>1$ GeV/$c$) in $A+B$ collisions scale very well with the number of binary nucleon-nucleon collisions (modulo effects of multiple initial scattering). This indicates that semi-hard parton scattering is the dominant particle production mechanism underlying the hadron spectra at moderate $p_T \stackrel{>}{\sim} 1$ GeV/$c$. However, there is no evidence of jet quenching or parton energy loss. Within the parton model, one can exclude an effective parton energy loss $dE_q/dx>0.01$ GeV/fm and a mean free path $\lambda_q< 7$ fm from the experimental data of $A+B$ collisions at the SPS energies. Predictions for high $p_T$ particle spectra in $p+A$ and $A+A$ collisions with and without jet quenching at the RHIC energy are also given. Uncertainties due to initial multiple scattering and nuclear shadowing of parton distributions are also discussed.

Inflation and preheating in nonoscillatory models

We study inflationary models in which the effective potential of the inflaton field does not have a minimum, but rather gradually decreases at large $\phi$. In such models the inflaton field does not oscillate after inflation, and its effective mass becomes vanishingly small, so the standard theory of reheating based on the decay of the oscillating inflaton field does not apply. For a long time the only mechanism of reheating in such non-oscillatory (NO) models was based on gravitational particle production in an expanding universe. This mechanism is very inefficient. We will show that it may lead to cosmological problems associated with large isocurvature fluctuations and overproduction of dangerous relics such as gravitinos and moduli fields. We also note that the setting of initial conditions for the stage of reheating in these models should be reconsidered. All of these problems can be resolved in the context of the recently proposed scenario of instant preheating if there exists an interaction ${g^2} \phi^2\chi^2$ of the inflaton field $\phi$ with another scalar field $\chi$. We show that the mechanism of instant preheating in NO models is much more efficient than the usual mechanism of gravitational particle production even if the coupling constant $g^2$ is extremely small, $10^{-14} \ll g^2 \ll 1$.

Higgs production in association with squark pairs in the Minimal Supersymmetric Standard Model at future hadron colliders

We study neutral and charged Higgs boson production in association with stop and sbottom squarks at the Large Hadron Collider, within the Supergravity inspired Minimal Supersymmetric Standard Model. The phenomenological relevance of such reactions is twofold. Firstly, they constitute a novel production mechanism of Higgs particles, either through a decay of a heavier (anti)squark into a lighter one or via a Higgs bremsstrahlung process. Secondly, their production rates are extremely sensitive to the values assumed by the five input parameters of the model, this possibly allowing one to put stringent constraints on the latter. After an exhaustive scan of the parameter space, we find that the majority of such processes could be detectable at high luminosity, provided \(\tan\beta\) is large, \(\tan\beta\gtrsim 30\) (except in the case of \(\tilde{t}_1\tilde{t}_1^*h\) and \(\tilde{t}_1\tilde{t}_2^*h\) final states, whose detection is also possible for smaller values), that the universal soft Supersymmetric breaking masses are in the ranges \(M_0 \lesssim 500\) GeV and \(M_{1/2} \lesssim 220\) GeV, and that the trilinear couplings are negative, \(A_0 < 0\). We also point out some sizable decay signatures and discuss their Standard Model (SM) backgrounds. Finally, we derive compact analytical formulae of the corresponding scattering matrix elements.

Higgs production in association with squark pairs in the Minimal Supersymmetric Standard Model at future hadron colliders

We study neutral and charged Higgs boson production in association with stop and sbottom squarks at the Large Hadron Collider, within the Supergravity inspired Minimal Supersymmetric Standard Model. The phenomenological relevance of such reactions is twofold. Firstly, they constitute a novel production mechanism of Higgs particles, either through a decay of a heavier (anti)squark into a lighter one or via a Higgs bremsstrahlung process. Secondly, their production rates are extremely sensitive to the values assumed by the five input parameters of the model, this possibly allowing one to put stringent constraints on the latter. After an exhaustive scan of the parameter space, we find that the majority of such processes could be detectable at high luminosity, provided tan(beta) is large, tan(beta) > 30 (except in the case of \tilde{t}_1\tilde{t}_1^* h and \tilde{t}_1\tilde{t}_2^* h final states, whose detection is also possible for smaller values), that the universal soft Supersymmetric breaking masses are in the ranges M_0 < 500 GeV and M_{1/2} < 220 GeV, and that the trilinear couplings are negative, A_0 < 0. We also point out some sizable decay signatures and discuss their Standard Model (SM) backgrounds. Finally, we derive compact analytical formulae of the corresponding scattering matrix elements.

Multiplex and Unfolding: Computer Simulation in Particle Physics

The ArgumentWhat kind of objects are computer programs used for simulation purposes in scientific settings? The current investigation treats a special case. It focuses on “event generators,” the program packages that particle physicists construct and use to simulate mechanisms of particle production. The paper is an attempt to bring the multiplex and unfolding character of such knowledge objects to the fore: Multiple meanings and functions are embodied in the object and can be drawn out selectively according to the requirements of a work setting. The object's conceptual complexity governs its application in some contexts, while the object is considered a mere “black box,” transparent and ready-to-hand, in others. These two poles span a full spectrum of object aspects, functions, and conceptions. Event generators are ideas turned into software, testing grounds for models, just a tool to study the performance of a detector, etc. The object's multiplex nature is submitted to negotiation among different actors.

Parametric resonance in an expanding universe

Parametric resonance has been discussed as a mechanism for copious particle production following inflation. Here we present a simple and intuitive calculational method for estimating the efficiency of parametric amplification as a function of parameters. This is important for determining whether resonant amplification plays an important role in the reheating process. We find that significant amplification occurs only for a limited range of couplings and interactions.

Inflaton decay and heavy particle production with negative coupling

We study the decay of the inflaton in a renormalizable two scalar theory. Since the dynamics of the system is dominated by states with large occupation numbers which admit a semiclassical description, the decay can be studied by solving the classical equations of motion on the lattice. Of particular interest is the case when the cross-coupling between the inflaton and the second scalar field is negative, which is naturally allowed in many realistic models. While the inflaton decays via parametric resonance in the positive coupling case we find that for negative coupling there is a new mechanism of particle production which we call negative coupling instability. Due to this new mechanism the variances of the fields grow significantly larger before the production is shut off by the backreaction of the created particles. We also find that heavy particles are produced much more efficiently with negative coupling, which is of prime importance for GUT baryogenesis. Using a simple toy model for baryogenesis and the results of our lattice simulations we show that for natural values of the cross-coupling enough 10^{14}GeV bosons are created to produce a baryon to entropy ratio consistent with observation. For positive coupling the value of the cross-coupling required to produce such massive particles is unnaturally large. In addition to our numerical results we obtain analytical estimates for the maximum variances of the fields in an expanding universe for all cases of interest in our model.

Modelling and simulation of activation product transport in helium cooling loops

This study presents models to describe the mobilisation, mass transport and deposition mechanisms of active and non-active species in the cooling loop of helium-cooled fusion power plants. A simple nuclear sputtering model has been developed in order to determine the active source term entering the coolant and the flux of sputtered particles ejected from the pipe surfaces. A time independent mass transfer model has been formulated to determine the transport and deposition of nuclides around the primary cooling circuit and the computer code HETRAN has been written to solve the mass transport of stable and active species at discrete locations around the cooling loop. The model describes the mass balance of each species accounting for the particle production and loss mechanisms in the coolant, the surface and the bulk solid. These models have been validated against experimental data and existing computer codes, where possible and found to be in good agreement with published work. Example calculations are performed, to determine the coolant, bulk solid and pipe surface atomic and activity concentrations for the primary circuit of the helium cooled SEAFP Plant Model 1.

Possible High Ice Particle Production during Graupel–Graupel Collisions

Abstract In a cold room collisions of two ice spheres having different growth modes were studied to examine ice production. The production of ice particles through this process was high, with a maximum rate more than 800 per collision at −16°C. This ice particle production mechanism is different from the Hallett–Mossop ice multiplication process and may help to explain the high concentrations of ice crystals observed in maritime winter cumuli.

Deuteron and anti-deuteron production in CERN experiment NA44

The abundances of light nuclei probe the later stages of the evolution of a system formed in a relativistic heavy-ion collision. After the system has cooled and expanded, nucleons in close proximity and moving with small relative momenta coalesce to form nuclei. Light nuclei production enables the study of several topics, including the mechanism of composite particle production, freeze-out temperature, size of the interaction region, and entropy of the system. NA44 is the only relativistic heavy-ion experiment to have both deuteron and antideuteron results in both pA and AA collisions and the first CERN experiment to study the physics topics addressed by d and d production.

Study of very peripheral SPb and SS interactions at 200 GeV/c per nucleon

SPb and SS interactions at 200 A GeV/c, as measured by the NA36 experiment at CERN, have been classified according to the number of participant protons of the projectile, by means of a precise identification of the charge of the projectile fragment produced. Rapidity and transverse momentum spectra of π − associated with the production of Z = 10 to Z = 15 projectile fragments are presented and compared to three models with different mechanisms of final particle production.

MULTIPLICITY CHARACTERISTICS OF PARTICLES PRODUCED IN 12C-EMULSION COLLISIONS AT 4.5 A GeV/c

Experimental data on multiplicities and correlations of charged particles of different types produced in collisions of 4.5 A GeV/c carbon-12 with emulsion are reported and discussed. The data are compared with the results of other experiments on nucleus–nucleus and hadron–nucleus collisions. It is found that the particle production mechanism in nucleus–nucleus collisions is almost the same as in hadron–nucleus collisions. It is also observed that the shower particles' multiplicity distributions obey a KNO type scaling law, which supports the aforementioned result.

The transverse profile of a color flux tube

The p T spectra of soft mesons produced in high-energy collisions contain information on the transverse motion of quarks and antiquarks which form these mesons. Using these data, we study the transverse profile of the color flux tube in the context of the Schwinger's mechanism for particle production by representing the transverse profile in terms of an effective transverse scalar potential. We find that the classical turning point for the potential is of the order of 0.6 fm. However, the potential flattens out considerably beyond that point. Consequently, the wave function decays surprisingly slowly in the transverse direction as r − 3 2 and extends much beyond the radius of the flux tube.

Fluxes of material in the Arabian Sea and Bay of Bengal — Sediment trap studies

In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean. In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively. Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal. The mechanisms of particle production and transport, however, differ in both the regions. In the Arabian Sea, average annual fluxes are over 50gm-2y-1 in the western Arabian Sea and less than 27gm-2 y-1 in the central part. Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods. High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening. In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm-2y-1) and are least in the southern part of the Bay (37gm-2y-1). Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system. Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers. Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive. Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents.

Universal properties of π and η spectra in nuclear collisions

The universal properties of the invariant transverse mass spectra of produced poins and etas in proton-nucleus and nucleus-nucleus collisions are discussed within the Quark Gluon String Model in a large range of bombarding energies starting from E/A ⋍ 1 GeV . A unique mechanism of particle production and building-up the transverse mass of produced mesons is suggested. A possible extension of this universal behaviour of transverse mass spectra to other heavy and strange hadrons is discussed shortly.

Poincaré covariant particle dynamics. I. Intranuclear cascade model.

To analyze the high energy heavy ion reactions performed at Lawrence Berkeley BEVALAC, Brookhaven AGS, and CERN SPS we present a dynamical model that is entirely Poincar\'e covariant. The description of strong interactions by Lorentz-scalar quasipotentials makes possible a relativistic extension of the cascade concept without losing its inherent simplicity. No field degrees of freedom appear explicitly, neither in elastic nucleon scattering nor in particle creation processes. Although various formalisms describe directly interacting relativistic particles with identical solutions for the two-particle case, they all suffer from different problems in many-particle systems. The basis of our approach is a Hamiltonian formulation for N pointlike nucleons, moving unconstrained in an 8N-dimensional phase space. One additional Lorentz scalar is introduced for an appropriate parametrization of all trajectories, defining also the connection to the proper times of the individual interacting nucleons. The creation of particles, for which Hamiltonian dynamics does not provide a generic mechanism, is incorporated phenomenologically as a ``perturbative'' process. We describe heavy ion collisions as a sequence of two-particle reactions without any additional fit to experimental data. Pion yield and mass spectra of the heavy fragments agree reasonably well with BEVALAC results. We do not compare to experimental high energy data because the employed mechanism of particle production is certainly improper in that regime. However, the numerical results give an impression on how the reaction evolves.

Time evolution of the quark-gluon plasma

We review progress in our understanding the production and time evolution of the quark gluon plasma starting with boost invariant initial conditions in a filed theory model based on the Schwinger mechanism of particle production via tunneling.