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Deep Inelastic Scattering from Light Nuclei

Description: We review recent developments in the study of deep inelastic scattering from light nuclei, focusing in particular on deuterium, helium, and lithium nuclei. Understanding the nuclear effects in these systems is essential for the extraction of information on the neutron structure function.
Date: April 1, 2002
Creator: Melnitchouk, Wally & Thomas, A.W.
Partner: UNT Libraries Government Documents Department

Spin-dependent Forces of Quarks in Baryon

Description: Nonperturbative spin-dependent forces of quarks in a baryon are calculated directly from the QCD Lagrangian in the framework of the Field Correlator Method both for heavy and light quarks. Resulting forces contain terms of 5 different structures, only one being known before in asymptotic form. Perturbative terms obtained by the same method are standard and have different signs and structures with respect to the corresponding nonperturbative ones, implying possible cancellations for some baryonic states.
Date: March 1, 2002
Creator: Simonov, Yu A.
Partner: UNT Libraries Government Documents Department

Longitudinal phase space manipulation in energy recovering linac-driven free-electron lasers

Description: Energy recovering an electron beam after it has participated in a free-electron laser (FEL) interaction can be quite challenging because of the substantial FEL-induced energy spread and the energy anti-damping that occurs during deceleration. In the Jefferson Lab infrared FEL driver-accelerator, such an energy recovery scheme was implemented by properly matching the longitudinal phase space throughout the recirculation transport by employing the so-called energy compression scheme. In the present paper, after presenting a single-particle dynamics approach of the method used to energy-recover the electron beam, we report on experimental validation of the method obtained by measurements of the so-called ''compression efficiency'' and ''momentum compaction'' lattice transfer maps at different locations in the recirculation transport line. We also compare these measurements with numerical tracking simulations.
Date: November 1, 2002
Creator: Piot, P.; Douglas, D.R. & Krafft, G.A.
Partner: UNT Libraries Government Documents Department

Polarized Targets for the CLAS Detector at Jefferson Lab

Description: The CEBAF Large Acceptance Spectrometer is utilized for a wide ranging physics program at Jefferson Lab, including measurements of polarized structure functions and future tests of the Gerasimov-Drell-Hearn sum rule. To realize the entire extent of the program, polarized targets that can function inside the spectrometer without severely affecting its performance are necessary. In these proceedings, I describe a continuously polarized solid target of protons and deuterons that operated inside CLAS for a total of ten months from 1998 to 2001. The conceptual design of a frozen spin target that will more fully exploit the 4 pi acceptance of CLAS is also introduced.
Date: July 1, 2002
Creator: Keith, C.D.
Partner: UNT Libraries Government Documents Department

Light quark simulations with FLIC fermions

Description: Hadron masses are calculated in quenched lattice QCD in order to probe the scaling behavior of a novel fat-link clover fermion action in which only the irrelevant operators of the fermion action are constructed using APE-smeared links. Light quark masses corresponding to an m{sub pi}/m{sub p} ratio of 0.35 are considered to assess the exceptional configuration problem of clover-fermion actions. This Fat-Link Irrelevant Clover (FLIC) fermion action provides scaling which is superior to mean-field improvement and offers advantages over nonperturbative improvement, including reduced exceptional configurations.
Date: June 1, 2002
Creator: Zanotti, J.M.; Leinweber, D.B.; Melnitchouk, W.; Williams, A.G. & Zhang, J.B.
Partner: UNT Libraries Government Documents Department

Electroproduction of Photons and of Pawns on the Proton in Quadrimoment of Transfer Q2=1.0GeV2. Measure Cross Sections and Extraction of Polarizabilities Generalities; Electroproduction de Photons et de Pions sur le Proton au Quadrimoment de Transfert Q2=1.0GeV2. Mesure des Sections Efficaces et Extraction des Polarisabilites Generalisees

Description: In hadronic physics, the nucleon structure and the quarks confinement are still topical issues. The neutral pion electroproduction and virtual Compton scattering (VCS) reactions allow us to access new observables that describe this structure. This work is focused on the VCS experiment performed at Jefferson Lab in 1998.
Date: November 6, 2001
Creator: Laveissiere, Geraud
Partner: UNT Libraries Government Documents Department

Excitations of the nucleon with dynamical fermions

Description: We measure the spectrum of low-lying nucleon resonances using Bayesian fitting methods. We compare the masses obtained in the quenched approximation to those obtained with two flavours of dynamical fermions at a matched lattice spacing. At the pion masses employed in our simulations, we find that the mass of the first positive-parity nucleon excitation is always greater than that of the parity partner of the nucleon.
Date: June 1, 2002
Creator: Maynard, C.M. & Richards, D.G.
Partner: UNT Libraries Government Documents Department

DC Scanning Field Emission Microscope Integrated with Existing Scanning Electron Microscope

Description: Electron field emission (FE) from broad-area metal surfaces is known to occur at much lower electric field than predicted by Fowler-Nordheim law. Although micron or submicron particles are often observed at such enhanced field emission (EFE) sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission, a DC scanning field emission microscope (SFEM) has been built as an extension to an existing commercial scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDX/EDS) for emitter characterization. In the SFEM chamber of ultra high vacuum ({approx}10-9 Torr), a sample is moved laterally in a raster pattern (2.5 mm step resolution) under a high voltage anode micro-tip for field emission detection and localization. The sample is then transferred under vacuum by a hermetic retractable linear transporter to the SEM chamber for individual emitter site characterization. Artificial marks on the sample surface serve as references to convert x, y coordinates of emitters in the SFEM chamber to corresponding positions in the SEM chamber with a common accuracy of {+-}100-200 mm in x and y. Samples designed to self-align in sample holders are used in each chamber, allowing them to retain position registration after non-in situ processing to track interesting features. No components are installed inside the SEM except the sample holder, which doesn't affect the routine operation of the SEM. The apparatus is a system of low cost and maintenance and significant operational flexibility. Field emission sources from planar niobium, the material used in high-field RF superconducting cavities for particle accelerator, have been studied after different surface preparations, and significantly reduced field emitter density has been achieved by refining the preparation process based on scan results. Scans ...
Date: March 1, 2002
Creator: Wang, Tong; Reece, Charles E. & Sundelin, Ronald M.
Partner: UNT Libraries Government Documents Department

Hydrogen Contamination of Niobium Surfaces

Description: The presence of hydrogen is blamed for dramatic reductions in cavity Q's. Hydrogen concentration is difficult to measure, so there is a great deal of Fear, Uncertainty, and Doubt (FUD) associated with the problem. This paper presents measurements of hydrogen concentration depth profiles, commenting on the pitfalls of the methods used and exploring how material handling can change the amount of hydrogen in pieces of niobium. Hydrogen analysis was performed by a forward scattering experiment with Helium used as the primary beam. This technique is variously known as FRES (Forward Recoil Elastic Scattering), FRS, HFS (Hydrogen Forward Scattering), and HRA (Hydrogen Recoil Analysis). Some measurements were also made using SIMS (Secondary Ion Mass Spectrometry). Both HFS and SIMS are capable of measuring a depth profile of Hydrogen. The primary difficulty in interpreting the results from these techniques is the presence of a surface peak which is due (at least in part) to contamination with either water or hydrocarbons. With HFS, the depth resolution is about 30 nm, and the maximum depth profiled is about 300 nm. (This 10-1 ratio is unusually low for ion beam techniques, and is a consequence of the compromises that must be made in the geometry of the experiment, surface roughness, and energy straggling in the absorber foil that must be used to filter out the forward scattered helium.) All the observed HFS spectra include a surface peak which includes both surface contamination and any real hydrogen uptake by the niobium surface. Some contamination occurs during the analysis. The vacuum in the analysis chamber is typically a few times 10{sup -6} torr, and some of the contamination is in the form of hydrocarbons from the pumping system. Hydrocarbons normally form a very thin (less than a monolayer) film which is in equilibrium between arrival rate and ...
Date: October 1993
Creator: Nguyen-Tuong, Viet & Doolittle, Lawrence
Partner: UNT Libraries Government Documents Department

The CLAS Forward Electromagnetic Calorimeter

Description: The CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab utilizes six iron-free superconducting coils to provide an approximately toroidal magnetic field. The six sectors are instrumented individually to form six independent spectrometers. The forward region (8deg < (theta) < 45deg) of each sector is equipped with a lead-scintillator electromagnetic sampling calorimeter (EC), 16 radiation lengths thick, using a novel triangular geometry with stereo readout. With its good energy and position resolution, the EC is used to provide the primary electron trigger for CLAS. It is also used to reject pions, reconstruct pi-0 and eta decays and detect neutrons, This paper treats the design, construction and performance of the calorimeter.
Date: May 1, 2001
Creator: Amarian, M.; Asryan, Geram; Beard, Kevin; Brooks, Will; Burkert, Volker; Carstens, Tom et al.
Partner: UNT Libraries Government Documents Department

QMP: LQCD Message Passing API

Description: Recent changes are: (1) There is no longer a logical node number, only a node number which does not change as the logical machine is define. Thus there are two styles of messaging: messages are sent to a node by node number, or messages are sent to a relative (logical) node. (2) Methods related to node numbers have been changed (some dropped, some added). This note presents: (1) the requirements for message passing within Lattice QCD applications; (2) a draft message API for both C and C++; and (3) implementation design ideas. The API is intended to be sufficiently flexible to be used by all Lattice QCD applications, and execute efficiently on all existing and anticipated platforms, so that there is no need to directly call non-portable message passing routines. Because of the highly regular grid communications with LQCD, MPI calls (which are more general) impose some additional overhead that is predicted to be non-negligible for large machines. Depending upon demand, a subset of MPI could be implemented above this new API so that legacy codes which use MPI could function on the new architectures which implement (only) the new API. Further, the new API has been implemented atop MPI so that new applications using this new API can still be run on older machines for which only MPI is available. Interspersed with the API description are some descriptions for how the API could be implemented for myrinet clusters and the QCDOC machine. These are meant to more fully illustrate the functionality, and are not intended as the final design. At the time of writing, the following implementations exist: (1) QMP-GM -- Uses GM; (2) QMP-MPI -- Uses MPI; tested above MPICH-GM, MPICH-SM (shared memory), and MPICH-P4 (sockets).
Date: March 1, 2003
Creator: Chen, Jie; Edwards, Robert & Watson, William
Partner: UNT Libraries Government Documents Department

A Polarized Target for the CLAS detector

Description: We describe the design, construction, and performance of a polarized solid target for use in electron scattering experiments with the CEBAF Large Acceptance Spectrometer (CLAS). Protons and deuterons are continuously polarized by microwave-induced spin flip transitions at 1 K and 5 T. The target operated successfully during two cycles in 1998 and 2000, providing proton and deuteron polarizations as high as 96% and 46%, respectively. The unique features of the target which permit its use inside a 4pi spectrometer are stressed. Comparison is made between the target polarization measured by the traditional method of NMR and by electron elastic scattering.
Date: September 1, 2002
Creator: Keith, C.D.; Anghinolfi, M.; Battaglieri, M.; Branford, D.; Bultmann, S.; Burkert, V.D. et al.
Partner: UNT Libraries Government Documents Department

Hunting for glueballs in electron-positron annihilation

Description: We calculate the cross section for the exclusive production of J{sup PC} = 0{sup ++} glueballs G{sub 0} in association with the J/{psi} in e{sup +}e{sup -} annihilation using the pQCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative {Upsilon} decay. The cross section for e{sup +}e{sup -} {yields} J/{psi} + G{sub 0} at {radical}s = 10.6 GeV is similar to exclusive charmonium-pair production e{sup +}e{sup -} {yields} J/{psi} + h for h = {eta}{sub c} and {chi}{sub c0}, and is larger by a factor 2 than that for h = {eta}{sub c}(2S). As the subprocesses {gamma}* {yields} (c {bar c}) (c {bar c}) and {gamma}* {yields} (c {bar c}) (g g) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e{sup +}e{sup -} {yields} J/{psi} X may actually be due to the production of charmonium-glueball J/{psi} G{sub J} pairs.
Date: May 1, 2003
Creator: Brodsky, Stanley; Goldhaber, Alfred Scharff & Lee, Jungil
Partner: UNT Libraries Government Documents Department

The Pion Form-Factor

Description: The experimental situation with regard to measurements of the pion charge form factor is reviewed. Both existing data and planned experiments are discussed.
Date: May 1, 2002
Creator: Blok, H.P.; Huber, G.M. & Mack, D.J.
Partner: UNT Libraries Government Documents Department

Exclusive Annihilation p{bar p}->gamma gamma in a generalized parton picture

Description: Exclusive proton-antiproton annihilation into two photons at large s ({approx}10 GeV{sup 2}) and |t|,|u| {approx} s can be described by a generalized parton picture analogous to the 'soft mechanism' in wide-angle real Compton scattering. The two photons are emitted in the annihilation of a single fast quark and antiquark. The matrix element describing the transition of the p-pbar system to a q-qbar pair can be related to the timelike proton elastic form factors as well as to the quark/antiquark distributions measured in inclusive deep-inelastic scattering. The reaction could be studied with the proposed 1.5-15 GeV high-luminosity antiproton storage ring (HESR) at GSI.
Date: August 1, 2002
Creator: Freund, A.; Radyushkin, A.; Schafer, A. & Weiss, C.
Partner: UNT Libraries Government Documents Department

Measurements of the electromagnetic form factor of the Proton at Jlab

Description: The ratio of the proton's elastic electromagnetic form factors G{sub E{sub p}}/G{sub M{sub p}} was obtained by measuring P{sub t} and P{sub {ell}}, the transverse and longitudinal recoil proton polarization, respectively. For the elastic reaction {rvec e}p {yields} e{rvec p}, G{sub E{sub p}}/G{sub M{sub p}} is proportional to P{sub t}/P{sub {ell}}. The simultaneous measurement of P{sub t} and P{sub {ell}} in a polarimeter reduces systematic uncertainties. The results for the ratio G{sub E{sub p}}/G{sub M{sub p}} measured in Hall A so far show a systematic decrease with increasing Q{sup 2}, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. Together these experiments cover the Q{sub 2}- range of 0.5 to 5.6 GeV{sup 2}. A new experiment is currently being prepared, to extend the Q{sup 2}-range to 9 GeV{sup 2} in Hall C.
Date: April 1, 2003
Creator: Punjabi, Vina & Perdrisat, Charles
Partner: UNT Libraries Government Documents Department

Excited Baryons from the FLIC Fermion Action

Description: Masses of positive and negative parity excited nucleons and hyperons are calculated in quenched lattice QCD using an order (a{sup 2}) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier N* simulations with improved actions, and exhibit a clear mass splitting between the nucleon and its parity partner, as well as a small mass splitting between the low-lying J{sup P}=1/2{sup -}N* states. Study of different Lambda interpolating fields suggests a similar splitting between the lowest two 1/2{sup -}Lambda* states, although the empirical mass suppression of the Lambda*(1405) is not seen.
Date: June 1, 2002
Creator: Melnitchouk, Wally; Hedditch, J.N.; Leinweber, D.B.; Williams, A.G.; Zanoti, J. & Zhang, J.B.
Partner: UNT Libraries Government Documents Department

tau-mu Flavor Violation as a Probe of the Scale of New Physics

Description: Motivated by recent experimental indications of large {nu}{sub {mu}} - {nu}{tau} mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving {tau}-{mu} mixing with typical Lorentz structures and discuss their relationship to the standard model gauge symmetry. We find the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of {tau}, hadrons or heavy quarks. For operators involving at least one light quark (u, d, s), these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at present, giving bounds of a few hundred GeV. A few scalar and pseudo-scalar operators are not subject to any current experimental constraints.
Date: April 1, 2002
Creator: Black, Deirdre; Han, Tao; He, Hong-Jian & Sher, Marc
Partner: UNT Libraries Government Documents Department

Nuclear Effects in Semi-inclusive Hadron Production

Description: Semi--inclusive production of charged hadrons ({pi}{sup +}, {pi}{sup -}, K{sup +}, K{sup -}, p, and {bar p}) in deep inelastic scattering has been studied by the HERMES experiment. Using the 27.5 GeV positron beam at DESY, the hadron multiplicity from {sup 14}N and {sup 84}Kr has been measured and compared to that from deuterium. Significant nuclear effects have been observed suggesting a modification of the quark fragmentation process in nuclei.
Date: December 1, 2002
Creator: Gaskell, David
Partner: UNT Libraries Government Documents Department

Elasto-Plastic Behavior of High RRR Niobium: Effects of Crystallographic Texture, Microstructure and Hydrogen Concentration

Description: Conventional assessments of the mechanical properties of polycrystalline high RRR niobium via tensile testing have revealed unusually low apparent Young's moduli and yield strength in annealed samples. These observations motivated the current investigation of a variety of possible contributors: crystallographic texture, grain size, and impurity concentration. It is shown that the crystallographic textures of a single lot of niobium are essentially unchanged by post-recrystallization anneals at temperatures up to 800 C. Ultrasonic measurements reveal that the elastic response is not degraded by annealing. Rather, the material's extremely low yield point gives the impression of a low elastic modulus during tensile testing.
Date: November 1, 2002
Creator: Myneni, G.R. & Agnew, S.R.
Partner: UNT Libraries Government Documents Department

Excited Baryons in Lattice QCD

Description: We present first results of calculations of masses of positive and negative parity excited baryons in lattice QCD using an O(a{sup 2}) improved gluon action and a fat-link clover fermion action in which only the irrelevant operators are constructed with fat links. The results are in agreement with earlier calculations of N* resonances using improved actions and exhibit a clear mass splitting between the nucleon and its chiral partner. The results also indicate a splitting between the lowest J{sup P}=(1/2){sup -} states for the two standard nucleon interpolating fields. The study of different Lambda interpolating fields suggests a similar splitting between the lowest two Lambda{sup 1/2{sup -}} octet states. However, the empirical mass suppression of the Lambda*(1405) is not evident in these quenched QCD simulations, suggesting an important role for the meson cloud of the Lambda*(1405) or a need for more exotic interpolating fields.
Date: March 1, 2002
Creator: Melnitchouk, W.; Bilson-Thompson, S.; Bonnet, F.; F.X.Lee; D.B.Leinweber; A.G.Williams et al.
Partner: UNT Libraries Government Documents Department

The CEBAF Large Acceptance Spectrometer (CLAS)

Description: The CEBAF Large Acceptance Spectrometer (CLAS) is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. A collaboration of about thirty institutions has designed, assembled, and commissioned CLAS in Hall B at the Thomas Jefferson National Accelerator Facility. The CLAS detector is based on a novel six-coil toroidal magnet which provides a largely azimuthal field distribution. Trajectory reconstruction using drift chambers results in a momentum resolution of 0.5% at forward angles. Cerenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data acquisition rates allow operation at a luminosity of 10{sup 34} nucleon cm {sup -2}s{sup -1}. These capabilities are being used in a broad experimental program to study the structure and interactions of mesons, nucleons, and nuclei using polarized and unpolarized electron and photon beams and targets. This paper is a comprehensive and general description of the design, construction and performance of CLAS.
Date: January 1, 2003
Creator: Mecking, Bernhard & Collaboration, CLAS
Partner: UNT Libraries Government Documents Department

Record High Power Terahertz Radiation from Relativistic Electrons

Description: Calculations and measurements confirm the production of coherent broadband THz radiation from relativistic electrons with an average power of nearly 20 watts. The radiation has qualities closely analogous to the THz radiation produced by ultrafast laser techniques (spatially coherent, short duration pulses with transform-limited spectral content). But in contrast to conventional THz radiation, the intensity is many orders of magnitude greater due to a relativistic enhancement. The absorption and dispersive properties of materials in this spectral range provide contrast for a unique type of imaging [1,2]. The striking improvement in power reported here could revolutionize this application by allowing full-field, real-time image capture. High peak and average power THz sources are also critical in driving new non-linear phenomena with excellent signal to noise, and for pump-probe studies of dynamical properties of novel materials, both of which are central to future high-speed electronic devices [3,4]. It should also be useful for studies of molecular vibrations and rotations, low frequency protein motions, phonons, superconductor bandgaps, electronic scattering and collective electronic excitations (e.g., charge density waves).
Date: March 1, 2002
Creator: Carr, G.L.; Martin, Michael C.; McKinney, Wayne R.; Jordan, Kevin; Neil, George R. & Williams, Gwyn P.
Partner: UNT Libraries Government Documents Department