arXiv subjects

X. Artru

Publications and source records attributed to X. Artru.

At least 19 recordsLinked to original sources

arXiv2023

String fragmentation of a quark pair with entangled spin states: application to $e^+e^-$ annihilation

We present a recursive quantum mechanical model for the fragmentation of a string stretched between a quark and an antiquark with entangled spin states. The quarks are assumed to be produced in the $e^+e^-$ annihilation process via the exchange of a virtual photon and the correlations between their spin states are described by a joint spin density matrix. The string fragmentation process is formulated at the amplitude level by using the splitting matrices of the recent string+${}^3P_0$ model of polarized quark fragmentation with pseudoscalar and vector meson emissions, and accounts for the systematic propagation of the spin correlations in the fragmentation chain. The model is formulated as a recursive recipe suitable for a Monte Carlo implementation. It reproduces the expected angular correlation, due to the Collins effect, between back-to-back pseudoscalar and/or vector mesons. For the latter, this correlation also involves the momenta of the decay products. We use the model for studying the sign of the Collins asymmetry for back-to-back vector and pseudoscalar mesons.

hep-ph↗

arXiv2023

Modeling spin effects in electron-positron annihilation to hadrons

The string+${}^3P_0$ model of spin-dependent hadronization is applied to the fragmentation of a string stretched between a quark and an antiquark with entangled spin states, assumed to be produced in the $e^+e^-$ annihilation process. The model accounts systematically for the spin correlations in the hadronization chain and is formulated as a recursive recipe suitable for the implementation in a Monte Carlo event generator. The recipe is applied to the production of two back-to-back pseudoscalar mesons produced in $e^+e^-$ annihilation, and it is shown to reproduce the form of the azimuthal distribution of the hadrons as expected in QCD.

hep-ph↗

arXiv2021

Production of vector mesons in the String+${}^3P_0$ model of polarized quark fragmentation

The production of vector mesons in the fragmentation process of polarized quarks is studied within the recursive String+${}^3P_0$ model, improving a previous version of the model in which the production of pseudoscalar mesons only was considered. Two types of couplings of the vector meson to quarks are introduced, their coupling constants being the additional free parameters of the model. The angular distribution of the decay products of the polarized vector meson is deduced from the spin density matrix of the meson and the spin information is propagated along the fragmentation chain taking into account the entanglement of spin states. The new model is implemented in a stand alone Monte Carlo program utilized to investigate in detail kinematic distributions and transverse spin asymmetries. The sensitivity of these observables to the new free parameters is discussed and the Monte Carlo results are compared with experimental data on transverse spin asymmetries.

hep-ph↗

arXiv2019

A simplified recursive ${}^3P_0$ model for the fragmentation of polarized quarks

We revisit our recursive model for the fragmentation of polarized quarks based on the string+${}^3P_0$ mechanism of $q\bar{q}$ pair creation. We make a different choice for one input function of the model that simplifies the implementation in a Monte Carlo program. No new parameters are introduced, and the relevant results are the same apart from the suppression of the spin-independent correlations between successive quarks. In addition, the present version is more suitable for an interface with external event generators. The theoretical aspects and the simulation results obtained with a stand alone program are discussed in detail and compared with those of the previous version of the model.

hep-ph↗

arXiv2018

Recursive model for the fragmentation of polarized quarks

We present a model for Monte Carlo simulation of the fragmentation of a polarized quark. The model is based on string dynamics and the ${}^3P_0$ mechanism of quark pair creation at string breaking. The fragmentation is treated as a recursive process, where the splitting function of the subprocess $q \to h + q'$ depends on the spin density matrix of the quark $q$. The ${}^3P_0$ mechanism is parametrized by a complex mass parameter $μ$, the imaginary part of which is responsible for single spin asymmetries. The model has been implemented in a Monte Carlo program to simulate jets made of pseudoscalar mesons. Results for single hadron and hadron pair transverse-spin asymmetries are found to be in agreement with experimental data from SIDIS and $e^+e^-$ annihilation. The model predictions on the jet-handedness are also discussed.

hep-ph↗

arXiv2017

Recursive Monte Carlo code for transversely polarized quark jet

We propose a Monte Carlo code for the simulation of the fragmentation process of polarized quarks into pseudoscalar mesons. Such process is generated recursively once the flavour, the energy and the spin density matrix of the initial quark are specified, performing a cascade of splittings of the type $q\rightarrow h+q'$, where $q, q'$ indicate quarks and $h$ the hadron with flavour content $q\bar{q}'$. Each splitting is generated using a splitting distribution which has been calculated in a string fragmentation framework including, for the first time, the ${}^3P_0$ mechanism. This mechanism involves a "complex mass" parameter responsible for transverse spin effects, such as the Collins effect. Results for single hadron and hadron pair analyzing power are found to be in agreement with experimental results from SIDIS and $e^+e^-$ annihilation.

hep-ph↗

arXiv2016

Transverse momentum correlations of quarks in recursive jet models

In the symmetric string fragmentation recipe adopted by PYTHIA for jet simulations, the transverse momenta of successive quarks are uncorrelated. This is a simplification but has no theoretical reason. Transverse momentum correlations are naturally expected, for instance, in a covariant multiperipheral model of quark hadronization. We propose a simple recipe of string fragmentation which lead to such correlations. The definition of the jet axis and its relation with the primordial transverse momentum of the quark is also discussed.

hep-ph↗

arXiv2010

Recursive fragmentation model with quark spin. Application to quark polarimetry

An elementary recursive model accounting for the quark spin in the fragmentation of a quark into mesons is presented. The quark spin degree of freedom is represented by a two-components spinor. Spin one meson can be included. The model produces Collins effect and jet handedness. The influence of the initial quark polarisation decays exponentially with the rank of the meson, at dierent rates for longitudinal and transverse polarisations

hep-ph↗

arXiv2010

Electron acceleration in a helical waveguide

Travelling wave in a helical wave guide is considered for electron acceleration. A first determination of the travelling wave modes using a partial wave expansion (PWE) and a point matching method is presented. It gives a rapid solution for moderate deformation of the guide relative to a straight cylinder. Strong deformations will give higher accelerating gradient but the PWE diverges. Two methods overcoming this difficulties are suggested.

physics.acc-ph↗

arXiv2009

"Shadowing" of the electromagnetic field of relativistic charged particles

In radiation processes such as a transition radiation, diffraction radiation, etc. based on relativistic electrons passing through or near an opaque screen, the electron self-field is partly shadowed after the screen over a distance of the order of the formation length $γ^2λ$. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. Absorbing and conductive half-plane screens were placed at various distances L before a standard DR source (inclined half-plane mirror). The radiation intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. It is worth noting that absorbing and conductive half-plane screens produce the same shadowing effect. The shadowing effect is responsible for a bound on the intensity of Smith-Purcell radiation.

physics.acc-ph↗

arXiv2009

The relevance of positivity in spin physics

Positivity reduces substantially the allowed domain for spin observables. We briefly recall some methods used to determine these domains and give some typical examples for exclusive and inclusive spin-dependent reactions.

hep-ph↗

arXiv2009

Particle Acceleration in a Helical Wave Guide

The electromagnetic wave field propagating in a helical wave guide is decomposed in an angular momentum basis. Eigenmodes are calculated using a truncation in $l$ and a discretisation of the boundary condition. Modes slightly slower than light are considered for relativistic particle acceleration.

physics.acc-ph↗

arXiv2009

Shadowing of the electromagnetic field of a relativistic electron

In coherent radiation sources (diffraction radiation, Smith-Purcell effect, etc.) based on relativistic electrons passing by a material radiator, the electron self-field is partly shadowed after each part of the radiator over a distance of the order of the formation length g2l. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. An absorbing half-plane screen was placed at various distances L before a standard DR source (inclined half-plane mirror). The DR intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. The shadowing effect can significantly reduce the total energy radiated in a long radiator.

physics.acc-ph↗

arXiv2008

Interference and Shadow Effects in the Production of Light by Charged Particles in Optical Fibers

A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound $-dE/dz\le C(Ze/b)^2$ for the particle energy loss is conjectured ($Ze$ is the particle charge, $b$ the impact parameter and $C$ a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith-Purcell radiation.

physics.ins-det↗

arXiv2008

Positivity domains for pairs of triples of spin observables

Positivity restrains the allowed domains for pairs or triples of spin observables in polarised reactions. Various domain shapes in ${1\over2}+{1\over2}\to{1\over2}+{1\over2}$ reactions are displayed. Some methods to determine these domains are mentioned and a new one based on the anticommutation between two observables is presented.

nucl-th↗

arXiv2008

Classical and quantum constraints in spin physics

Constraints on spin observables coming from discrete symmetries such as P, C, T and identical particles may be divided in two types: 1) classical ones, which insure the invariance of the cross sections under the symmetry operation; 2) non-classical ones, which can only be obtained at the level of amplitudes. Similarly, positivity constraints can be divided into classical and non-classical constraints. The former insure the positivity of the cross section for arbitrary individual polarisations of the external particles, the latter extend this requirement to the case of entangled external spins. The domain of classical positivity is shown to be dual to the domain of separability

hep-ph↗

arXiv2007

The High-Z hydrogen-like atom: a model for polarized structure functions

The Dirac equation offers a precise analytical description of relativistic two-particle bound states, when one of the constituent is very heavy and radiative corrections are neglected. Looking at the high-Z hydrogen-like atom in the infinite momentum frame and treating the electron as a "parton", various properties usually attributed to the quark distributions in the nucleon are tested, in particular: Bjørken scaling; charge, helicity, transversity and momentum sum rules; existence of the parton sea; Soffer inequality; correlation between spin and transverse momentum (Sivers and Boer-Mulders effects); transverse displacement of the center-of-charge and its connection with the magnetic moment. Deep inelastic experiments with photon or positron beams at MeV energies, analogous to DIS or Drell-Yan reactions, are considered.

nucl-th↗

arXiv2007

Particle Refraction, Reflection and Channeling by Laser Beams

It is shown that the charged particles are refracted and reflected on the boundary of field free and laser field regions in vacuum. Simple and transparent estimates are given which show the possibility of channeling of charged and neutral particles having polarizability by strong electromagnetic field of certain laser bunches just as by the field of orientated crystalline planes and axes. These processes can be applied for production of femtosecond sliced electron bunches, for measurement their length and particle distribution as well as for production of femtosecond X-ray and terahertz pulses using transition, channeling and other types of radiation.

physics.acc-ph↗