66 research outputs found
Measurement of Beam-Spin Asymmetries for Deep Inelastic Electroproduction
We report the first evidence for a non-zero beam-spin azimuthal asymmetry in
the electroproduction of positive pions in the deep-inelastic region. Data have
been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector
at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of
the modulation increases with the momentum of the pion relative to
the virtual photon, , with an average amplitude of for range.Comment: 5 pages, RevTEX4, 3 figures, 2 table
Two-Nucleon Momentum Distributions Measured in 3He(e,e'pp)n
We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic
range. The kinetic energy distribution for `fast' nucleons (p > 250 MeV/c)
peaks where two nucleons each have 20% or less, and the third nucleon has most
of the transferred energy. These fast pp and pn pairs are back-to-back with
little momentum along the three-momentum transfer, indicating that they are
spectators. Experimental and theoretical evidence indicates that we have
measured distorted two-nucleon momentum distributions by striking the third
nucleon and detecting the spectator correlated pair.Comment: 6 pages, 5 figures, submitted to PR
Single pi+ Electroproduction on the Proton in the First and Second Resonance Regions at 0.25GeV^2 < Q^2 < 0.65GeV^2 Using CLAS
The ep -> e'pi^+n reaction was studied in the first and second nucleon
resonance regions in the 0.25 GeV^2 < Q^2 < 0.65 GeV^2 range using the CLAS
detector at Thomas Jefferson National Accelerator Facility. For the first time
the absolute cross sections were measured covering nearly the full angular
range in the hadronic center-of-mass frame. The structure functions sigma_TL,
sigma_TT and the linear combination sigma_T+epsilon*sigma_L were extracted by
fitting the phi-dependence of the measured cross sections, and were compared to
the MAID and Sato-Lee models.Comment: Accepted for publication in PR
Observation of Nuclear Scaling in the Reaction at 1
The ratios of inclusive electron scattering cross sections of He,
C, and Fe to He have been measured for the first time. It is
shown that these ratios are independent of at Q1.4 (GeV/c) for
1.5 where the inclusive cross section depends primarily on the
high-momentum components of the nuclear wave function. The observed scaling
shows that the momentum distributions at high-momenta have the same shape for
all nuclei and differ only by a scale factor. The observed onset of the scaling
at Q1.4 and 1.5 is consistent with the kinematical expectation that
two nucleon short range correlations (SRC) are dominate the nuclear wave
function at 300 MeV/c. The values of these ratios in the scaling
region can be related to the relative probabilities of SRC in nuclei with
A3. Our data demonstrate that for nuclei with A12 these
probabilities are 5-5.5 times larger than in deuterium, while for He it is
larger by a factor of about 3.5.Comment: 11 pages, 10 figure
Exclusive electroproduction on the proton at CLAS
The reaction has been measured, using the 5.754
GeV electron beam of Jefferson Lab and the CLAS detector. This represents the
largest ever set of data for this reaction in the valence region. Integrated
and differential cross sections are presented. The , and
dependences of the cross section are compared to theoretical calculations based
on -channel meson-exchange Regge theory on the one hand and on quark handbag
diagrams related to Generalized Parton Distributions (GPDs) on the other hand.
The Regge approach can describe at the 30% level most of the features
of the present data while the two GPD calculations that are presented in this
article which succesfully reproduce the high energy data strongly underestimate
the present data. The question is then raised whether this discrepancy
originates from an incomplete or inexact way of modelling the GPDs or the
associated hard scattering amplitude or whether the GPD formalism is simply
inapplicable in this region due to higher-twists contributions, incalculable at
present.Comment: 29 pages, 29 figure
Evaluating model outputs using integrated global speleothem records of climate change since the last glacial
Although quantitative isotope data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to using the speleothem data for data–model comparisons. Here, we illustrate this using 456 globally distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates the process of procuring large numbers of records if data–model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotope values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model's ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on δ18O values, the optimum period for the modern observational baseline and the selection of an appropriate time window for creating means of the isotope data for palaeo-time-slices
First measurement of target and double spin asymmetries for polarized e- polarized p --> e p pi0 in the nucleon resonance region above the Delta(1232)
The exclusive channel polarized proton(polarized e,e prime p)pi0 was studied
in the first and second nucleon resonance regions in the Q2 range from 0.187 to
0.770 GeV2 at Jefferson Lab using the CEBAF Large Acceptance Spectrometer
(CLAS). Longitudinal target and beam-target asymmetries were extracted over a
large range of center-of-mass angles of the pi0 and compared to the unitary
isobar model MAID, the dynamic model by Sato and Lee, and the dynamic model
DMT. A strong sensitivity to individual models was observed, in particular for
the target asymmetry and in the higher invariant mass region. This data set,
once included in the global fits of the above models, is expected to place
strong constraints on the electrocoupling amplitudes A_{1/2} and S_{1/2} for
the Roper resonance N(1400)P11, and the N(1535)S11 and N(1520)D13 states.Comment: 13 pages, 13 figure
Late quaternary climate variability and change from Aotearoa New Zealand speleothems: Progress in age modelling, oxygen isotope master record construction and proxy-model comparisons
We re-evaluated speleothem isotope series from Aotearoa New Zealand that were recently contributed to the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database. COnstructing Proxy Records from Age Models (COPRA) software was used to produce Bayesian age models for those speleothems. The new age modelling helped us examine Late Quaternary temporal coverage for the national speleothem network, and also supported our exploration of three different isotope master record generation techniques using Holocene δ18O data from Waitomo. We then applied the output from one of the isotope master record techniques to test an application case of how climate transfer functions can be developed using climate model simulated temperatures. Our results suggest Holocene δ¹⁸O trends at Waitomo capture air temperature variations weighted toward the primary season of soil moisture (and epikarst) recharge during winter. This interpretation is consistent with the latest monitoring data from the Waitomo region. Holocene δ¹⁸O millennial-scale trends and centennial-scale variability at Waitomo likely reflect atmospheric circulation patterns that concomitantly vary with surface water temperature and the isotopic composition of the Tasman Sea. A climate model simulation context for the Holocene millennial-scale trends in the Waitomo δ¹⁸O isotope master record suggest that site is sensitive to changes in the subtropical front (STF) and the Tasman Front. Our comparison of isotope master record techniques using Waitomo δ¹⁸O data indicate that caution is needed prior to merging δ¹⁸O data series from different caves in order to avoid time series artefacts. Future work should incorporate more high-resolution cave monitoring and climate calibration studies, and develop new speleothem data from northern and eastern regions of the country
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