arXiv subjects

J. M. Finn

Publications and source records attributed to J. M. Finn.

33 records · Page 2Linked to original sources

arXiv2012

New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei

We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

nucl-ex↗

arXiv2012

Virtual Compton Scattering and the Generalized Polarizabilities of the Proton at Q^2=0.92 and 1.76 GeV^2

Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson Lab using the exclusive photon electroproduction reaction (e p --> e p gamma). This paper gives a detailed account of the analysis which has led to the determination of the structure functions P_LL-P_TT/epsilon and P_LT, and the electric and magnetic generalized polarizabilities (GPs) alpha_E(Q^2) and beta_M(Q^2) at values of the four-momentum transfer squared Q^2= 0.92 and 1.76 GeV^2. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q^2-range, and point to their non-trivial behavior.

nucl-ex↗

arXiv2012

The Qweak Experiment: A Search for New Physics at the TeV Scale via a Measurement of the Proton's Weak Charge

We propose a new precision measurement of parity-violating electron scattering on the proton at very low Q^2 and forward angles to challenge predictions of the Standard Model and search for new physics. A unique opportunity exists to carry out the first precision measurement of the proton's weak charge, $Q_W =1 - 4\sin^2θ_W$. A 2200 hour measurement of the parity violating asymmetry in elastic ep scattering at Q^2=0.03 (GeV/c)^2 employing 180 $μ$A of 85% polarized beam on a 35 cm liquid Hydrogen target will determine the proton's weak charge with approximately 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of $Q_W$, based on the running of the weak mixing angle from the Z0 pole down to low energies, corresponding to a 10 sigma effect in this experiment.

physics.ins-det↗

arXiv2012

An Arbitrary Curvilinear Coordinate Method for Particle-In-Cell Modeling

A new approach to the kinetic simulation of plasmas in complex geometries, based on the Particle-in- Cell (PIC) simulation method, is explored. In the two dimensional (2d) electrostatic version of our method, called the Arbitrary Curvilinear Coordinate PIC (ACC-PIC) method, all essential PIC operations are carried out in 2d on a uniform grid on the unit square logical domain, and mapped to a nonuniform boundary-fitted grid on the physical domain. As the resulting logical grid equations of motion are not separable, we have developed an extension of the semi-implicit Modified Leapfrog (ML) integration technique to preserve the symplectic nature of the logical grid particle mover. A generalized, curvilinear coordinate formulation of Poisson's equations to solve for the electrostatic fields on the uniform logical grid is also developed. By our formulation, we compute the plasma charge density on the logical grid based on the particles' positions on the logical domain. That is, the plasma particles are weighted to the uniform logical grid and the self-consistent mean electrostatic fields obtained from the solution of the logical grid Poisson equation are interpolated to the particle positions on the logical grid. This process eliminates the complexity associated with the weighting and interpolation processes on the nonuniform physical grid and allows us to run the PIC method on arbitrary boundary-fitted meshes.

physics.plasm-ph↗

arXiv2011

Quasi-separatrix layers and three-dimensional reconnection diagnostics for line-tied tearing modes

In three-dimensional magnetic configurations for a plasma in which no closed field line or magnetic null exists, no magnetic reconnection can occur, by the strictest definition of reconnection. A finitely long pinch with line-tied boundary conditions, in which all the magnetic field lines start at one end of the system and proceed to the opposite end, is an example of such a system. Nevertheless, for a long system of this type, the physical behavior in resistive magnetohydrodynamics (MHD) essentially involves reconnection. This has been explained in terms comparing the geometric and tearing widths [1, 2]. The concept of a quasi-separatrix layer[3, 4] was developed for such systems. In this paper we study a model for a line-tied system in which the corresponding periodic system has an unstable tearing mode. We analyze this system in terms of two magnetic field line diagnostics, the squashing factor[3-5] and the electrostatic potential difference used in kinematic reconnection studies[6, 7]. We discuss the physical and geometric significance of these two diagnostics and compare them in the context of discerning tearing-like behavior in line-tied modes. [1] G. L. Delzanno and J. M. Finn. Physics of Plasmas, 15(3):032904, 2008. [2] Y.-M. Huang and E. G. Zweibel. Physics of Plasmas, 16(4):042102, 2009. [3] E. R. Priest and P. Démoulin. J. Geophys. Res., 100(A12):23443-23463, 1995. [4] P. Démoulin, J. C. Henoux, E. R. Priest, and C. H. Mandrini. Astron. Astrophys., 308:643-655, Apr. 1996. [5] V. S. Titov and G. Hornig. Advances in Space Research, 29(7):1087-1092, 2002. [6] Y. Lau and J. M. Finn. The Astrophysical Journal, 350:672-691, Feb. 1990. [7] Y. Lau and J. M. Finn. The Astrophysical Journal, 366:577-591, 1991.

physics.plasm-ph↗

arXiv2011

The G0 Experiment: Apparatus for Parity-Violating Electron Scattering Measurements at Forward and Backward Angles

In the G0 experiment, performed at Jefferson Lab, the parity-violating elastic scattering of electrons from protons and quasi-elastic scattering from deuterons is measured in order to determine the neutral weak currents of the nucleon. Asymmetries as small as 1 part per million in the scattering of a polarized electron beam are determined using a dedicated apparatus. It consists of specialized beam-monitoring and control systems, a cryogenic hydrogen (or deuterium) target, and a superconducting, toroidal magnetic spectrometer equipped with plastic scintillation and aerogel Cerenkov detectors, as well as fast readout electronics for the measurement of individual events. The overall design and performance of this experimental system is discussed.

nucl-ex↗

arXiv2007

Regression with strongly correlated data

This paper discusses linear regression of strongly correlated data that arises, for example, in magnetohydrodynamic equilibrium reconstructions. We have proved that, generically, the covariance matrix of the estimated regression parameters for fixed sample size goes to zero as the correlations become unity. That is, in this limit the estimated parameters are known with perfect accuracy. Simple examples are shown to illustrate this effect and the nature of the exceptional cases in which the estimate covariance does not go to zero.

math.ST↗

arXiv2005

Noise Stabilized Random Attractor

A two dimensional flow model is introduced with deterministic behavior consisting of bursts which become successively larger, with longer interburst time intervals between them. The system is symmetric in one variable x and there are bursts on either side of x = 0, separated by the presence of an invariant manifold at x = 0. In the presence of arbitrarily small additive noise in the x direction, the successive bursts have bounded amplitudes and interburst intervals. This system with noise is proposed as a model for edge localized modes in tokamaks. Further, the bursts can switch from positive to negative x and vice-versa. The probability distribution of burst heights and interburst periods is studied, as is the dependence of the statistics on the noise variance. The modification of this behavior as the symmetry in x is broken is studied, showing qualitatively similar behavior if the symmetry breaking is small enough. Experimental observations of a nonlinear circuit governed by the same equations are presented, showing good agreement.

nlin.CD↗

arXiv2001

The quasielastic 2H(e,e'p)n reaction at high recoil momenta

The 2H(e,e'p)n cross section was measured in Hall A of the Thomas Jefferson National Accelerator Facility (JLab) in quasielastic kinematics (x=0.96) at a four-momentum transfer squared, Q^2=0.67 (GeV/c)^2. The experiment was performed in fixed electron kinematics for recoil momenta from zero to 550 MeV/c. Though the measured cross section deviates by 1-2 sigma from a state-of-the-art calculation at low recoil momenta, it agrees at high recoil momenta where final state interactions (FSI) are predicted to be large.

nucl-ex↗

arXiv2001

Magnetic Helix Formation Driven by Keplerian Disk Rotation in an External Plasma Pressure --- The Initial Expansion Stage

We study the evolution of a magnetic arcade that is anchored to an accretion disk and is sheared by the differential rotation of a Keplerian disk. By including an extremely low external plasma pressure at large distances, we obtain a sequence of axisymmetric magnetostatic equilibria and show that there is a fundamental difference between field lines that are affected by the plasma pressure and those are not (i.e., force-free). Force-free fields, while being twisted by the differential rotation of the disk, expand outward at an angle of $\sim 60^\circ$ away from the rotation axis, consistent with the previous studies. These force-free field lines, however, are enclosed by the outer field lines which originate from small disk radii and come back to the disk at large radii. These outer fields experience most of the twist, and they are also affected most by the external plasma pressure. At large cylindrical radial distances, magnetic pressure and plasma pressure are comparable so that any further radial expansion of magnetic fields is prevented or slowed down greatly by this pressure. This hindrance to cylindrical radial expansion causes most of the added twist to be distributed on the ascending portion of the field lines, close to the rotation axis. Since these field lines are twisted most, the increasing ratio of the toroidal $B_ϕ$ component to the poloidal component $B_{R,z}$ eventually results in the collimation of magnetic energy and flux around the rotation axis. We discuss the difficulty with adding a large number of twists within the limitations of the magnetostatic approximation.

astro-ph↗

arXiv2000

Search for Quadrupole Strength in the Electro-excitation of the Delta(1232)

High-precision H(e,e'p)pi0 measurements at Q2=0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma* N->Delta transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole amplitude ratios, Rsm=(-6.5 +- 0.2{stat+sys} +- 2.5{mod}) % and Rem=(-2.1 +- 0.2{stat+sys} +- 2.0{mod}) %, are dominated by model error. Previous Rsm and Rem results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

nucl-ex↗

arXiv1999

Rossby Wave Instability of Thin Accretion Disks -- II. Detailed Linear Theory

In earlier work we identified a global, non-axisymmetric instability associated with the presence of an extreme in the radial profile of the key function ${\cal L}(r) \equiv (ΣΩ/κ^2) S^{2/Γ}$ in a thin, inviscid, nonmagnetized accretion disk. Here, $Σ(r)$ is the surface mass density of the disk, $Ω(r)$ the angular rotation rate, $S(r)$ the specific entropy, $Γ$ the adiabatic index, and $κ(r)$ the radial epicyclic frequency. The dispersion relation of the instability was shown to be similar to that of Rossby waves in planetary atmospheres. In this paper, we present the detailed linear theory of this Rossby wave instability and show that it exists for a wider range of conditions, specifically, for the case where there is a ``jump'' over some range of $r$ in $Σ(r)$ or in the pressure $P(r)$. We elucidate the physical mechanism of this instability and its dependence on various parameters, including the magnitude of the ``bump'' or ``jump,'' the azimuthal mode number, and the sound speed in the disk. We find large parameter range where the disk is stable to axisymmetric perturbations, but unstable to the non-axisymmetric Rossby waves. We find that growth rates of the Rossby wave instability can be high, $\sim 0.2 Ω_{\rm K}$ for relative small ``jumps'' or ``bumps''. We discuss possible conditions which can lead to this instability and the consequences of the instability.

astro-ph↗

arXiv1998

Quasielastic 12C(e,e'p) Reaction at High Momentum Transfer

We measured the 12C(e,e'p) cross section as a function of missing energy in parallel kinematics for (q,w) = (970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV). At w=475 MeV, at the maximum of the quasielastic peak, there is a large continuum (E_m > 50 MeV) cross section extending out to the deepest missing energy measured, amounting to almost 50% of the measured cross section. The ratio of data to DWIA calculation is 0.4 for both the p- and s-shells. At w=330 MeV, well below the maximum of the quasielastic peak, the continuum cross section is much smaller and the ratio of data to DWIA calculation is 0.85 for the p-shell and 1.0 for the s-shell. We infer that one or more mechanisms that increase with $ω$ transform some of the single-nucleon-knockout into multinucleon knockout, decreasing the valence knockout cross section and increasing the continuum cross section.

nucl-ex↗

arXiv1997

A Comparison of Polarization Observables in Electron Scattering from the Proton and Deuteron

Recoil proton polarization observables were measured for both the p($\vec {\rm e}$,e$^\prime\vec{\rm p}\,$) and d($\vec {\rm e}$,e$^\prime\vec{\rm p}\,)$n reactions at two values of Q$^2$ using a newly commissioned proton Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The hydrogen and deuterium spin-dependent observables $D_{\ell\ell}$ and $D_{{\ell}t}$, the induced polarization $P_n$ and the form factor ratio $G^p_E/G^p_M$ were measured under identical kinematics. The deuterium and hydrogen results are in good agreement with each other and with the plane-wave impulse approximation (PWIA).

nucl-ex↗

arXiv1997

Measurement of the Induced Proton Polarization P_n in the 12C(e,e'\vec{p}) Reaction

The first measurements of the induced proton polarization, P_n, for the 12C (e,e'\vec{p}) reaction are reported. The experiment was performed at quasifree kinematics for energy and momentum transfer (ω,q) \approx (294 MeV, 756 MeV/c) and sampled a recoil momentum range of 0-250 MeV/c. The induced polarization arises from final-state interactions and for these kinematics is dominated by the real part of the spin-orbit optical potential. The distorted-wave impulse approximation provides good agreement with data for the 1p_{3/2} shell. The data for the continuum suggest that both the 1s_{1/2} shell and underlying l > 1 configurations contribute.

nucl-ex↗