Searcharxiv⌕ Search

arXiv subjects

J. A. Crittenden

Publications and source records attributed to J. A. Crittenden.

11 recordsLinked to original sources
arXiv2021

Quantitative Assessment of Finite-element Models for Magnetostatic Field Calculations

We present quantitative means for assessing the numerical accuracy of static magnetic field calculations in finite-element models. Our calculations use the three-dimensional Opera simulation software suite of Dassault Syst`emes. Our need to assess the effects of fringe fields requires such a 3D algorithm. While we do discuss and compare our approach to a method of accuracy estimation used in the Opera post-processor, our methods are generally applicable to any model using relaxation techniques in finite-element systems. For purposes of illustration, we present modeling and analysis of two types of quadrupole electromagnets presently in operation in the south arc of the Cornell Electron Storage Ring (CESR). Calculations of field multipole expansion coefficients and numerical deviations from Maxwell's equations in source-free regions are discussed, with emphasis on the dependence of their accuracy on changes to the finite-element model. Successive refinement steps in the finite-element model for the non-extraction type of CESR south arc quadrupole achieve a reduction in the RMS value of the longitudinal component of the curl vector on the magnet axis by a factor of nearly 70 from $4.38 \times 10^{-2}$ T/m to $6.36 \times 10^{-4}$ T/m, which is 0.0021% of the field gradient. An accuracy of 2.9% is achieved for a dodecapole coefficient of $2.4 \times 10^{-4}$ of that of the quadrupole in the longitudinal integral of the vertical field gradient at a radius of 1 cm.

physics.acc-ph↗
arXiv2015

Measurement of Electron Trapping in the CESR Storage Ring

The buildup of low-energy electrons has been shown to affect the performance of a wide variety of particle accelerators. Of particular concern is the persistence of the cloud between beam bunch passages, which can impose limitations on the stability of operation at high beam current. We have obtained measurements of long-lived electron clouds trapped in the field of a quadrupole magnet in a positron storage ring, with lifetimes much longer than the revolution period. Based on modeling, we estimate that about 7% of the electrons in the cloud generated by a 20-bunch train of 5.3 GeV positrons with 16-ns spacing and $1.3x10^{11}$ population survive longer than 2.3 $μ$s in a quadrupole field of gradient 7.4 T/m. We have observed a non-monotonic dependence of the trapping effect on the bunch spacing. The effect of a witness bunch on the measured signal provides direct evidence for the existence of trapped electrons. The witness bunch is also observed to clear the cloud, demonstrating its effectiveness as a mitigation technique.

physics.acc-ph↗
arXiv2001

Experimental Results on the Diffractive Production of Light Vector Mesons at HERA

We discuss results on the diffractive production of the vector mesons rho, phi and omega reported by the H1 and ZEUS collaborations at HERA. Unique to such studies is the experimental accessibility to the polarization of the vector mesons and hence to the spin-density matrix elements arising in vacuum-exchange processes. We emphasize the relation between the observed dependence on momentum transfer and the polarization state of the vector meson. The diffractive nature of the production mechanism is investigated via extraction of the Pomeron trajectory at high Q^2. Flavor symmetry is observed in the phi/rho ratios in the same region of momentum transfer where the power-law scaling becomes similar. The multivariable helicity analyses impose stringent constraints on models for the vacuum-exchange production mechanism. Semi-exclusive photoproduction of transverse rho and phi mesons at momentum transfers far exceeding their mass scale exhibit a hard scaling behavior which appears to violate the QCD helicity selection rules in a two-gluon exchange model.

hep-ex↗
arXiv2001

Diffractive Production of Neutral Vector Mesons at THERA

We consider the contribution to our understanding of vacuum-exchange processes to be made by investigations at the proposed electron-proton collider THERA. Recent results have highlighted the value of such studies for testing quantum chromodynamical descriptions of both long-range and short-range strong interactions. Stringent quantitative constraints have been provided by exploiting the opportunity to correlate scaling behaviour with helicity selection in exclusive and semi-exclusive vector-meson production. After reviewing the progress achieved by the measurement programs presently being carried out by the H1 and ZEUS collaborations at HERA, we discuss the performance criteria imposed by such investigations on the THERA accelerator complex and on the detector design. We conclude that the study of vector-meson production will form an essential component of the THERA physics program beginning with the early turn-on stage of the machine and continuing throughout the achievement of its full high-luminosity potential.

hep-ex↗
arXiv2000

Measurements of Diffractive Vector-Meson Photoproduction at High Momentum Transfer from the ZEUS Experiment at HERA

We discuss recent preliminary results on the diffractive photoproduction of {\rhoz}, $ϕ$, and {\jpsi} mesons at high momentum transfer reported by the ZEUS collaboration at HERA. A special-purpose calorimeter served to tag the quasi-real photons ($\qsq<0.01 \gevsq$) in the process $γ+ p \to {\rm VM}+ Y$, where $Y$ represents a dissociated state of the proton. The resulting range in photon-proton center-of-mass energy extends from 80 to 120 \gev. The differential cross sections $\frac{\Dσ}{\D t}$ were obtained in the region ${\abst}>1.2 \gevsq$, where $t$ denotes the squared momentum transfer to the proton. The measurements provide good sensitivity to the observed power-law dependence $\frac{\Dσ}{\D t} \propto {(-t)}^{-n}$. Over the region in momentum transfer covered by the data, the power is found to be approximately $n\simeq 3$ for {\rhoz} and $ϕ$ photoproduction, and approximately $n\simeq 2$ for {\jpsi} photoproduction.

hep-ex↗
arXiv1999

Topical Results on Vector-Meson Production from the HERA Collider Experiments

The HERA collider experiments H1 and ZEUS have established extensive measurement programs for diffractive vector-meson production processes during the first six years of their operation. The results provide stringent phenomenological tests of quantum chromodynamical descriptions of hard diffraction. We discuss recent topical results on Upsilon photoproduction, on decay-angle analyses of rho^0, phi and J/psi electroproduction and on rho^0 and phi photoproduction at high momentum transfer.

hep-ex↗
arXiv1999

Results on Diffractive Processes from the HERA Collider Experiments

We review topical results on diffractive processes from the experiments H1 and ZEUS at the HERA electron-proton collider. Emphasis is placed on the phenomenological and experimental consequences of the discoveries at HERA for the proposed electron/polarized-ion collider EPIC.

hep-ex↗
arXiv1998

Scale Issues in High-Energy Diffractive Vector-Meson Production

Recent measurements of diffractive vector-meson production with the general-purpose detectors H1 and ZEUS in electron-proton interactions with 300 GeV center-of-mass energy at the HERA accelerator complex have stimulated great interest in the question of whether perturbative QCD (pQCD) can provide an accurate description of such diffractive processes. The high flux of quasi-real photons from the 27.5 GeV electron beam has allowed high-statistics studies of vector-meson photoproduction to be compared to the deeply inelastic processes at high photon virtuality Q^2 >> Λ^2_QCD. Special-purpose electron detectors at small scattering angle have been used to identify a transition region, 0.2 < Q^2 < 2 GeV^2, where the pQCD prediction for the dominance of point-like configurations of qqbar-pairs is verified. Data samples characterized by high momentum transfer to the 820 GeV initial-state proton, 1 < |t| < 13 GeV^2, have now become available, and the results lend support to the proposition that pQCD calculations should accurately describe the inelastic vector-meson production processes in high-|t| photoproduction. Measurements of the energy dependence and the t-dependence of the elastic cross sections for rho, omega, phi, and J/psi production have provided tests of the proposal that the mass (or size) of the vector meson provides a third scaling variable for studying the transition to the domain of pQCD applicability. A preliminary result on elastic upsilon production has also been announced by the ZEUS collaboration. These issues of scale will be discussed in the context of topical phenomenological models, comparing the HERA results and those from fixed-target experiments at lower energy to the wealth of calculations available in the contemporary literature.

hep-ex↗
arXiv1997

Recent Results from Investigations of Diffractive Vector Meson Production with the ZEUS Detector at HERA

We present results from recent investigations of diffractive vector meson production using the ZEUS detector at HERA. These consist of measurements of rho production in the region of photon virtuality 0.25 < Q^2 < 0.85 GeV^2, of rho, phi, and J/psi production for values of the momentum transfer to the proton 0.3 < |t| < 4.0 GeV^2, and of elastic J/psi production both in photoproduction and for Q^2 > 2 GeV^2.

hep-ex↗
arXiv1997

Exclusive Production of Neutral Vector Mesons at the Electron-Proton Collider HERA

The first five years of operation of the multi-purpose experiments ZEUS and H1 at the electron-proton storage ring facility HERA have opened a new era in the study of vector-meson production in high-energy photon-proton interactions. The high center-of-mass energy available at this unique accelerator complex allows investigations in hitherto unexplored kinematic regions, providing answers to long-standing questions concerning the energy-dependence of the rho, omega, phi, and J/psi production cross sections. The excellent angular acceptance of these detectors, combined with that of specialized tagging detectors at small production angles, has permitted measurements of elastic and inelastic production processes for both quasi-real photons and those of virtuality exceeding the squared mass of the vector meson. This report provides a quantitative picture of the present status of these studies, comparing them to the extensive measurements in this field at lower energies and summarizing topical developments in theoretical work motivated by the new data.

hep-ex↗