Electron Mass Ratio Research Articles

AN OBSERVATIONAL DETERMINATION OF THE PROTON TO ELECTRON MASS RATIO IN THE EARLY UNIVERSE

In an effort to resolve the discrepancy between two measurements of the fundamental constant mu, the proton to electron mass ratio, at early times in the universe we reanalyze the same data used in the earlier studies. Our analysis of the molecular hydrogen absorption lines in archival VLT/UVES spectra of the damped Lyman alpha systems in the QSOs Q0347-383 and Q0405-443 yields a combined measurement of a (Delta mu)/mu value of (-7 +/- 8) x 10^{-6}, consistent with no change in the value of mu over a time span of 11.5 gigayears. Here we define (Delta mu) as (mu_z - mu_0) where mu_z is the value of mu at a redshift of z and mu_0 is the present day value. Our null result is consistent with the recent measurements of King et al. 2009, (Delta mu)/u = (2.6 +/- 3.0) x 10^{-6}, and inconsistent with the positive detection of a change in mu by Reinhold et al. 2006. Both of the previous studies and this study are based on the same data but with differing analysis methods. Improvements in the wavelength calibration over the UVES pipeline calibration is a key element in both of the null results. This leads to the conclusion that the fundamental constant mu is unchanged to an accuracy of 10^{-5} over the last 80% of the age of the universe, well into the matter dominated epoch. This limit provides constraints on models of dark energy that invoke rolling scalar fields and also limits the parameter space of Super Symmetric or string theory models of physics. New instruments, both planned and under construction, will provide opportunities to greatly improve the accuracy of these measurements.

Probing exciton-phonon interaction in AlN epilayers by photoluminescence

Deep ultraviolet (DUV) photoluminescence (PL) spectroscopy has been employed to investigate the exciton-phonon interaction in AlN. Longitudinal optical (LO) phonon replicas of free exciton recombination lines were observed in PL emission spectra, revealing the coupling of excitons with LO phonons. We have quantified such interaction by measuring Huang–Rhys factor based on polarization resolved DUV PL measurements. It was observed that the exciton-phonon coupling strength in AlN depends on the polarization configuration and is much larger in the direction with the electrical field (E⃗) of the emitted light perpendicular to the wurtzite c-axis (E⃗⊥c⃗) than in the direction of E⃗∥c⃗. Furthermore, a larger coupling constant was also measured in AlN than in GaN. The large effective hole to electron mass ratio in AlN, especially in the E⃗⊥c⃗ configuration, mainly accounts for the observed results.

Temperature equilibration in a fully ionized plasma: Electron-ion mass ratio effects

Brown, Preston, and Singleton (BPS) produced an analytic calculation for energy exchange processes for a weakly to moderately coupled plasma: the electron-ion temperature equilibration rate and the charged particle stopping power. These precise calculations are accurate to leading and next-to-leading order in the plasma coupling parameter and to all orders for two-body quantum scattering within the plasma. Classical molecular dynamics can provide another approach that can be rigorously implemented. It is therefore useful to compare the predictions from these two methods, particularly since the former is theoretically based and the latter numerically. An agreement would provide both confidence in our theoretical machinery and in the reliability of the computer simulations. The comparisons can be made cleanly in the purely classical regime, thereby avoiding the arbitrariness associated with constructing effective potentials to mock up quantum effects. We present here the classical limit of the general result for the temperature equilibration rate presented in BPS. In particular, we examine the validity of the m(electron)/m(ion)-->0 limit used in BPS to obtain a very simple analytic evaluation of the long-distance collective effects in the background plasma.

A comprehensive gyrokinetic description of global electrostatic microinstabilities in a tokamak

It is believed that low frequency microinstabilities such as ion temperature gradient (ITG) driven modes and trapped electron modes (TEMs) are largely responsible for the experimentally observed anomalous transport via the ion and electron channels in a tokamak. In the present work, a comprehensive global linear gyrokinetic model incorporating fully kinetic (trapped and passing) electrons and ions, actual ion to electron mass ratio, radial coupling, and profile variation is used to investigate the ITG driven modes and pure TEMs. These modes are found to exhibit multiscale structures in the presence of nonadiabatic passing electrons. The multiscale structure is related to the large nonadiabaticity of electrons in the vicinity of mode rational magnetic surfaces and leads to reduced mixing length estimates of transport compared to those obtained from adiabatic electron models.

Ion-acoustic surface waves in a semi-bounded Lorentzian plasma

The dispersion properties of the ion-acoustic surface waves propagating on a boundary of semi-bounded Lorentzian (kappa) plasma is kinetically investigated. The real and the imaginary parts of the wave frequency are obtained as functions of the normalized wave number kxλe where kx is the x-component of the wave number and λe is the electron Debye length. The phase speed of the wave is found be decreased as the spectral index κ of the Lorentzian distribution function is decreased. In the long wavelength limit, it becomes a constant which depends on the value of κ. The wave displays the resonance at as expected in the case of a Maxwellian plasma. The imaginary part of the wave frequency appears to be negative which exhibits the linear wave dissipation in a collisionless plasma called Landau damping. The maximum damping rate is obtained as , where Mκ is a κ-dependent factor and m/M is the electron–ion mass ratio. Landau damping disappears as kxλe → ∞.

USES OF A SMALL FIELD VALUE WHICH FALLS FROM A METASTABLE MAXIMUM OVER COSMOLOGICAL TIMES

We consider a small, metastable maximum vacuum expectation value b0 of order of a few eV, for a pseudoscalar Goldstone-like field, which is related to the scalar inflaton field ϕ in an idealized model of a cosmological, spontaneously-broken chiral symmetry. The b field allows for relating semi-quantitatively three distinct quantities in a cosmological context. (a) A very small, residual vacuum energy density or effective cosmological constant of [Formula: see text], for λ ~ 3×10-14, the same as an empirical inflaton self-coupling. (b) A tiny neutrino mass, less than b0. (c) A possible small variation downward of the proton to electron mass ratio over cosmological time. The latter arises from the motion downward of the b field over cosmological time, toward a nonzero value. Such behavior is consistent with an equation of motion. We argue that hypothetical b quanta, potentially inducing new long-range forces, are absent, because of negative, effective squared mass in an equation of motion for b-field fluctuations. The assumed flatness of a potential maximum involves a small inverse-time parameter μ ≪ 1/t0, where t0 is the present age of the universe.

Submillimeter water and ammonia absorption by the peculiarz≈0.89 interstellar medium in the gravitational lens of the PKS 1830-211 system

Using the Atacama Pathfinder Experiment (APEX) telescope, we have detected the rotational ground-state transitions of ortho-ammonia and ortho-water toward the redshift ≈ 0.89 absorbing galaxy in the PKS 1830-211 gravitational lens system. We discuss our observations in the context of recent space-borne data obtained for these lines with the SWAS and Odin satellites toward Galactic sources. We find commonalities, but also significant differences between the interstellar media in a galaxy at intermediate redshift and in the Milky Way. Future high-quality observations of the ground-state ammonia transition in PKS 1830-211, together with inversion line data, will lead to strong constraints on the variation in the proton to electron mass ratio over the past 7.2 Gyr.

DO THE FUNDAMENTAL CONSTANTS CHANGE WITH TIME?

Comparisons between the redshifts of spectral lines from cosmologically-distant galaxies can be used to probe temporal changes in low-energy fundamental constants like the fine structure constant and the proton–electron mass ratio. Here, we review the results from, and the advantages and disadvantages of, the best techniques using this approach, before focussing on a new method, based on conjugate satellite OH lines, that appears to be less affected by systematic effects and hence holds much promise for the future.

Physics of collisionless reconnection in a stressed X-point collapse

Recently, magnetic reconnection during collisionless, stressed, X-point collapse was studied using kinetic, 2.5-dimensional, fully electromagnetic, relativistic particle-in-cell numerical code [D. Tsiklauri and T. Haruki, Phys. Plasmas 14, 112905 (2007)]. Here we finalize the investigation of this topic by addressing key outstanding physical questions: (i) Which term in the generalized Ohm’s law is responsible for the generation of the reconnection electric field? (ii) How does the time evolution of the reconnected flux vary with the ion-electron mass ratio? (iii) What is the exact energy budget of the reconnection process; i.e., in which proportion initial (mostly magnetic) energy is converted into other forms of energy? (iv) Are there any anisotropies in the velocity distribution of the accelerated particles? The following points have been established. (i) A reconnection electric field is generated by the electron pressure tensor off-diagonal terms, resembling to the case of tearing unstable Harris current sheet studied by the GEM reconnection challenge. (ii) For mi∕me⪢1, the time evolution of the reconnected flux is independent of ion-electron mass ratio. In addition, in the case of mi∕me=1, we show that reconnection proceeds slowly as the Hall term is zero; when mi∕me⪢1 (i.e., the Hall term is nonzero) reconnection is fast and we conjecture that this is due to magnetic field being frozen into electron fluid, which moves significantly faster than ion fluid. (iii) Within one Alfvén time, somewhat less than half (∼40%) of the initial total (roughly magnetic) energy is converted into the kinetic energy of electrons, and somewhat more than half (∼60%) into kinetic energy of ions (similar to solar flare observations). (iv) In the strongly stressed X-point case, in about one Alfvén time, a full isotropy in all three spatial directions of the velocity distribution is seen for superthermal electrons (also commensurate with solar flare observations).

Two-dimensional fully kinetic simulations of driven magnetic reconnection with boundary conditions relevant to the Magnetic Reconnection Experiment

Two-dimensional fully kinetic simulations are performed using global boundary conditions relevant to model the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys Plasmas 4, 1936 (1997)]. The geometry is scaled in terms of the ion kinetic scales in the experiment, and a reconnection layer is created by reducing the toroidal current in the flux cores in a manner similar to the actual experiment. The ion-scale features in these kinetic simulations are in remarkable agreement with those observed in MRX, including the reconnection inflow rate and quadrupole field structure. In contrast, there are significant discrepancies in the simulated structure of the electron layer that remain unexplained. In particular, the measured thickness of the electron layers is 3–5 times thicker in MRX than in the kinetic simulations. The layer length is highly sensitive to downstream boundary conditions as well as the time over which the simulation is driven. However, for a fixed set of chosen boundary conditions, an extrapolation of the scaling with the ion to electron mass ratio implies that at realistic mass ratio both the length and width will be too small compared to the experiment. This discrepancy implies that the basic electron layer physics may differ significantly between MRX and the two-dimensional, collisionless simulations. The two leading possibilities to explain the discrepancy are weak Coulomb collisions and three-dimensional effects that are present in the experiment but not included in the simulation model.

Modeling Electrostatic Levitation of Dust Particles on Lunar Surface

This paper presents a simulation model on electrostatic levitation of lunar dust particles in the lunar terminator region. Full-particle particle-in-cell simulations are carried out using real ion to electron mass ratio to obtain plasma sheath, surface charging, and the transition point of surface electric field. Test particle simulations are carried out to simulate the levitation of dust particles from lunar surface. Results show that the dust levitation condition in the terminator region is sensitively influenced by the ambient plasma condition and surface charging, and the levitation altitude varies significantly even for small changes of the sun elevation angle.

Drift kink instability in the current sheet with a kappa-distribution

Superthermal particle distributions well-described by the family of κ-distributions have been observed in various astrophysical plasmas. In this paper, the drift kink instability in the current sheet with a κ-distribution is investigated in the framework of linear kinetic theory. The orbit integrals are treated numerically using the exact unperturbed particle orbits, and the resulting eigenvalue problem of the integro-differential equations is solved using the spectral method. The growth rate, eigenmode structure, and parametric dependencies of the kink mode are examined and compared with the case of the standard Harris current sheet. The results show that the drift kink instability in the κ-distribution current sheet resembles its counterpart in the standard Harris sheet, but has a smaller growth rate and real frequency for small value of κ. It is also demonstrated that a background population can enhance the growth rate of the kink mode, making the growth rate significant at the physical value of the ion-electron mass ratio.

Ordered structure formation in 2D mass asymmetric electron–hole plasmas

We study strong Coulomb correlations in dense two-dimensional electron–hole plasmas by means of direct path integral Monte Carlo simulations. In particular, the formation and dissociation of bound states, such as excitons, bi-excitons and many particle clusters, is analyzed and the density-temperature regions of their occurrence are identified. At high density, the Mott transition to the fully ionized state (electron–hole hexatic liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. For high enough values of M we observed the formation of Coulomb hole crystal-like structures.

Effect of the liquid-like ionic structure on the electron–ion energy relaxation timescales in dense plasmas

In a recent publication [J. Daligault, D. Mozyrsky, Phys. Rev. E 75 (2007) 026402], we derived a general expression for the electron–ion energy relaxation rate in plasmas which, as a result of the small electron–ion mass ratio, expresses the relaxation rate in terms of the low-frequency electronic density fluctuations. Here we propose a practical model for the electronic density fluctuations in dense plasmas and apply this model to the calculation of the electron–ion energy relaxation rate. We find that the rate is only scarcely affected by the underlying liquid-like ionic disorder typical of dense matter. Relaxation rates obtained are systematically slightly larger than those predicted by the Fermi Golden Rule formula, in contradiction with the coupled-modes' theory that predicts values an order of magnitude lower. We also find that the discontinuity of the rate at melting is tiny, in contrast with the sharp increase of the electrical conductivity.

On the Structure of Relativistic Collisionless Shocks in Electron-Ion Plasmas

Relativistic collisionless shocks in electron-ion plasma are thought to occur in the afterglow phase of Gamma-Ray Bursts (GRBs), and in other environments where relativistic flows interact with the interstellar medium. A particular regime of shocks in an unmagnetized plasma has generated much interest for GRB applications. In this paper we present ab-initio particle-in-cell simulations of unmagnetized relativistic electron-ion shocks. Using long-term 2.5-dimensional simulations with ion-electron mass ratios from 16 to 1000 we resolve the shock formation and reach a steady-state shock structure beyond the initial transient. We find that even at high ion-electron mass ratios initially unmagnetized shocks can be effectively mediated by the ion Weibel instability with a typical shock thickness of ~50 ion skin-depths. Upstream of the shock the interaction with merging ion current filaments heats the electron component, so that the postshock flow achieves near equipartition between the ions and electrons, with electron temperature reaching 50% of the ion temperature. This energy exchange helps to explain the large electron energy fraction inferred from GRB afterglow observations.

A guiding centre direct implicit scheme for magnetized plasma simulations

A new implicit fully electromagnetic particle in cell method is presented in which the full ion dynamic and all the electron guiding centre drifts of a magnetized collisionless plasma are retained. This code retains the inertial effects on the electrons. It must be used for time steps larger or so than an electron cyclotron period, it is only valid for an electron plasma beta β e < 1 . Therefore, it is efficient only with strongly magnetized electrons. On the opposite, the constraint on the ions β i < m i / m e (where m i / m e is the ion to electron mass ratio) is very weak. This method allows for the simulation of an ion-beam instability (triggering electrostatic ion cyclotron waves and filamentation of the plasma), the propagation of MHD waves, and ion-temperature anisotropies (mirror and/or cyclotron waves), at a relatively low cost. This method could become a useful tool for the modelization of space plasmas, quite complementary with other methods such as Hall-MHD, hybrid codes, or other implicit particle codes.

PRECISION MEASUREMENT BASED ON ULTRACOLD ATOMS AND COLD MOLECULES

Ultracold atoms and molecules provide ideal stages for precision tests of fundamental physics. With microkelvin neutral strontium atoms confined in an optical lattice, we have achieved a fractional resolution of 4 × 10-15 on the 1S0–3P0 doubly forbidden 87 Sr clock transition at 698 nm. Measurements of the clock line shifts as a function of experimental parameters indicate systematic errors below the 10-15 level. The ultrahigh spectral resolution permits resolving the nuclear spin states of the clock transition at small magnetic fields, leading to measurements of the 3P0 magnetic moment and metastable lifetime. In addition, photoassociation spectroscopy performed on the narrow 1S0–3P1 transition of 88 Sr shows promise for efficient optical tuning of the ground state scattering length and production of ultracold ground state molecules. Lattice-confined Sr 2 molecules are suitable for constraining the time variation of the proton–electron mass ratio. In a separate experiment, cold, stable, ground state polar molecules are produced from Stark decelerators. These cold samples have enabled an order-of-magnitude improvement in the measurement precision of ground state, Λ doublet microwave transitions in the OH molecule. Comparing the laboratory results to those from OH megamasers in interstellar space will allow a sensitivity of 10-6 for measuring the potential time variation of the fundamental fine structure constant Δα/α over 1010 years. These results have also led to improved understandings of the molecular structure. The study of the low magnetic field behavior of OH in its 2Π3/2 ro-vibronic ground state precisely determines a differential Landé g factor between opposite parity components of the Λ doublet.

Trions in cylindrical nanowires with a dielectric mismatch

We investigated the lowest energy levels of trions (charged excitons) in freestanding nanowires with strong lateral carrier confinement. Within the adiabatic approximation, the three-particle problem reduces to an effective two-dimensional Schr\"odinger equation for the relative motion which is solved numerically. Dielectric mismatch effects are taken into account, which results in a distorted Coulomb interaction between the charged particles. We obtain the ``bright'' singlet and triplet trion binding energies and we found that the negatively charged exciton is always less stable than the positively charged exciton in a wire with a hole to electron mass ratio $\ensuremath{\sigma}&gt;1$. The pair correlation functions and the conditional probabilities are calculated, which visualizes the correlation between the particles in the wire.

High-accuracy calculations in the H+2 molecular ion: towards a measurement of mp/me

We report on our recent advances in the calculation of the energy levels of the H+2 molecular ion, including relativistic and radiative corrections. These theoretical efforts are linked to the prospect of obtaining a new determination of the proton to electron mass ratio mp/me through precise vibrational spectroscopy of H+2. We describe the setup of our experiment, aiming at a measurement of the L = 2, υ = 0 → L = 2, υ = 1 two-photon transition at 9.166 μm using a phase-locked quantum cascade laser as excitation source.PACS Nos.: 31.15.Pf, 31.30.Jv, 32.10.Hq

Correlation effects in partially ionized mass asymmetric electron-hole plasmas

The effects of strong Coulomb correlations in dense three-dimensional electron-hole plasmas are studied by means of unbiased direct path integral Monte Carlo simulations. The formation and dissociation of bound states, such as excitons and biexcitons, is analyzed and the density-temperature region of their appearance is identified. At high density, the Mott transition to the fully ionized metallic state (electron-hole liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. Above a critical value of about M=80 formation of a hole Coulomb crystal was recently verified [Bonitz, Phys. Rev. Lett. 95, 235006 (2005)] which is supported by additional results. Results are related to the excitonic phase diagram of intermediate valent Tm[Se,Te], where large values of M have been observed experimentally.