arXiv subjects

K. McCormick

Publications and source records attributed to K. McCormick.

At least 19 recordsLinked to original sources

arXiv2026

d(e,e'p) Studies of Exclusive Deuteron Electro-Disintegration

The d(e,e'p) cross section was measured at momentum transfers $Q^2 = $ 0.8, 2.1 and 3.5 $(GeV/c)^2$ covering a wide range of proton kinematics at each $Q^2$ setting that made it possible to study this reaction as a function of missing momentum as well as a function of the neutron laboratory recoil angle $\theta_{nq}$. Missing momentum distributions were determined for fixed values of $\theta_{nq}$ up to missing momenta of 0.65 $GeV/c$. For the two larger momentum transfer settings, the characteristics of the experimental momentum distributions confirm the theoretical prediction that final state interactions (FSI) contribute maximally around a $\theta_{nq} \sim 70^\circ$, while for $\theta_{nq} < 45^\circ$ FSI are significantly reduced. The data at reduced FSI settings were best reproduced by calculations using the CD-Bonn potential wave functions.

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arXiv2024

High precision measurements of the proton elastic electromagnetic form factors and their ratio at $Q^2$ = 0.50, 2.64, 3.20, and 4.10 GeV$^2$

The advent of high-intensity, high-polarization electron beams led to significantly improved measurements of the ratio of the proton's charge to electric form factors, GEp/GMp. However, high-$Q^2$ measurements yielded significant disagreement with extractions based on unpolarized scattering, raising questions about the reliability of the measurements and consistency of the techniques. Jefferson Lab experiment E01-001 was designed to provide a high-precision extraction of GEp/GMp from unpolarized cross section measurements using a modified version of the Rosenbluth technique to allow for a more precise comparison with polarization data. Conventional Rosenbluth separations detect the scattered electron which requires comparisons of measurements with very different detected electron energy and rate for electrons at different angles. Our Super-Rosenbluth measurement detected the struck proton, rather than the scattered electron, to extract the cross section. This yielded a fixed momentum for the detected particle and dramatically reduced cross section variation, reducing rate- and momentum-dependent corrections and uncertainties. We measure the cross section vs angle with high relative precision, allowing for extremely precise extractions of GEp/GMp at $Q^2$ = 2.64, 3.20, and 4.10 GeV$^2$. Our results are consistent with traditional extractions but with much smaller corrections and systematic uncertainties, comparable to the uncertainties from polarization measurements. Our data confirm the discrepancy between Rosenbluth and polarization extractions of the proton form factor ratio using an improved Rosenbluth extraction that yields smaller and less-correlated uncertainties than typical of previous Rosenbluth extractions. We compare our results to calculations of two-photon exchange effects and find that the observed discrepancy can be relatively well explained by such effects.

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arXiv2021

Measurement of the generalized spin polarizabilities of the neutron in the low $Q^2$ region

Understanding the nucleon spin structure in the regime where the strong interaction becomes truly strong poses a challenge to both experiment and theory. At energy scales below the nucleon mass of about 1 GeV, the intense interaction among the quarks and gluons inside the nucleon makes them highly correlated. Their coherent behaviour causes the emergence of effective degrees of freedom, requiring the application of non-perturbative techniques, such as chiral effective field theory. Here, we present measurements of the neutron's generalized spin-polarizabilities that quantify the neutron's spin precession under electromagnetic fields at very low energy-momentum transfer squared down to 0.035 GeV$^2$. In this regime, chiral effective field theory calculations are expected to be applicable. Our data, however, show a strong discrepancy with these predictions, presenting a challenge to the current description of the neutron's spin properties.

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arXiv2019

Measurement of the 3He Spin-Structure Functions and of Neutron (3He) Spin-Dependent Sum Rules at 0.035<Q^2<0.24 GeV^2

The spin-structure functions $g_1$ and $g_2$, and the spin-dependent partial cross-section $\sigma_\mathrm{TT}$ have been extracted from the polarized cross-sections differences, $\Delta \sigma_{\parallel}\hspace{-0.06cm}\left(\nu,Q^{2}\right)$ and $\Delta \sigma_{\perp}\hspace{-0.06cm}\left(\nu,Q^{2}\right)$ measured for the $\vec{^\textrm{3}\textrm{He}}(\vec{\textrm{e}},\textrm{e}')\textrm{X}$ reaction, in the E97-110 experiment at Jefferson Lab. Polarized electrons with energies from 1.147 to 4.404 GeV were scattered at angles of 6$^{\circ}$ and 9$^{\circ}$ from a longitudinally or transversely polarized $^{3}$He target. The data cover the kinematic regions of the quasi-elastic, resonance production and beyond. From the extracted spin-structure functions, the first moments $\overline{\Gamma_1}\hspace{-0.06cm}\left(Q^{2}\right)$, $\Gamma_2\hspace{-0.06cm}\left(Q^{2}\right)$ and $I_{\mathrm{TT}}\hspace{-0.06cm}\left(Q^{2}\right)$ are evaluated with high precision for the neutron in the $Q^2$ range from 0.035 to 0.24~GeV$^{2}$. The comparison of the data and the chiral effective field theory predictions reveals the importance of proper treatment of the $\Delta$ degree of freedom for spin observables.

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arXiv2018

High-resolution hypernuclear spectroscopy at Jefferson Lab, Hall A

The experiment E94-107 in Hall A at Jefferson Lab started a systematic study of high resolution hypernuclear spectroscopy in the 0p-shell region of nuclei such as the hypernuclei produced in electroproduction on 9Be, 12C and 16O targets. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring-imaging Cherenkov detector were added to the Hall A standard equipment. The high-quality beam, the good spectrometers and the new experimental devices allowed us to obtain very good results. For the first time, measurable strength with sub-MeV energy resolution was observed for the core-excited states of Lambda 12B. A high-quality Lambda 16N hypernuclear spectrum was likewise obtained. A first measurement of the Lambda binding energy for Lambda 16N, calibrated against the elementary reaction on hydrogen, was obtained with high precision, 13.76 +/- 0.16 MeV. Similarly, the first Lambda 9Li hypernuclear spectrum shows general agreement with theory (distorted-wave impulse approximation with the SLA and BS3 electroproduction models and shell-model wave functions). Some disagreement exists with respect to the relative strength of the states making up the first multiplet. A Lambda separation energy of 8.36 MeV was obtained, in agreement with previous results. It has been shown that the electroproduction of hypernuclei can provide information complementary to that obtained with hadronic probes and the gamma-ray spectroscopy technique.

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arXiv2015

Moments of the neutron $g_2$ structure function at intermediate $Q^2$

We present new experimental results of the $^3$He spin structure function $g_2$ in the resonance region at $Q^2$ values between 1.2 and 3.0 (GeV/c)$^2$. Spin dependent moments of the neutron were then extracted. Our main result, the resonance contribution to the neutron $d_2$ matrix element, was found to be small at $ $=2.4 (GeV/c)$^2$ and in agreement with the Lattice QCD calculation. The Burkhardt-Cottingham sum rule for $^3$He and the neutron was tested with the measured data and using the Wandzura-Wilczek relation for the low $x$ unmeasured region. A small deviation was observed at $Q^2$ values between 0.5 and 1.2 (GeV/c)$^2$ for the neutron.

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arXiv2015

The E00-110 experiment in Jefferson Lab's Hall A: Deeply Virtual Compton Scattering off the Proton at 6 GeV

We present final results on the photon electroproduction ($\vec{e}p\rightarrow epγ$) cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region from Jefferson Lab experiment E00-110. Results from an analysis of a subset of these data were published before, but the analysis has been improved which is described here at length, together with details on the experimental setup. Furthermore, additional data have been analyzed resulting in photon electroproduction cross sections at new kinematic settings, for a total of 588 experimental bins. Results of the $Q^2$- and $x_B$-dependences of both the helicity-dependent and helicity-independent cross sections are discussed. The $Q^2$-dependence illustrates the dominance of the twist-2 handbag amplitude in the kinematics of the experiment, as previously noted. Thanks to the excellent accuracy of this high luminosity experiment, it becomes clear that the unpolarized cross section shows a significant deviation from the Bethe-Heitler process in our kinematics, compatible with a large contribution from the leading twist-2 DVCS$^2$ term to the photon electroproduction cross section. The necessity to include higher-twist corrections in order to fully reproduce the shape of the data is also discussed. The DVCS cross sections in this paper represent the final set of experimental results from E00-110, superseding the previous publication.

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arXiv2014

Spectroscopy of Lambda-9Li by electroproduction

In the absence of accurate data on the free two-body hyperon-nucleon interaction, the spectra of hypernuclei can provide information on the details of the effective hyperon-nucleon interaction. Electroproduction of the hypernucleus Lambda-9Li has been studied for the first time with sub-MeV energy resolution in Hall A at Jefferson Lab on a 9Be target. In order to increase the counting rate and to provide unambiguous kaon identification, two superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH) were added to the Hall A standard equipment. The cross section to low-lying states of Lambda-9Li is concentrated within 3 MeV of the ground state and can be fitted with four peaks. The positions of the doublets agree with theory while a disagreement could exist with respect to the relative strengths of the peaks in the doublets. A Lambda separation energy of 8.36 +- 0.08 (stat.) +- 0.08 (syst.) MeV was measured, in agreement with an earlier experiment.

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arXiv2012

Final Analysis of Proton Form Factor Ratio Data at $\mathbf{Q^2 = 4.0}$, 4.8 and 5.6 GeV$\mathbf{^2}$

Precise measurements of the proton electromagnetic form factor ratio $R = μ_p G_E^p/G_M^p$ using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of $R$ with momentum transfer $Q^2$ for $Q^2 \gtrsim 1$ GeV$^2$, in strong disagreement with previous extractions of $R$ from cross section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Lab's Hall A measured $R$ at four $Q^2$ values in the range 3.5 GeV$^2 \le Q^2 \le 5.6$ GeV$^2$. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher $Q^2$ motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for $R$, improving the consistency of the polarization transfer data in the high-$Q^2$ region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis.

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arXiv2010

Measurements of the Electric Form Factor of the Neutron up to Q2=3.4 GeV2 using the Reaction He3(e,e'n)pp

The electric form factor of the neutron was determined from studies of the reaction He3(e,e'n)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2-range over which it is known, we find GEn = 0.0225 +/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142 +/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.

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arXiv2010

Towards a liquid Argon TPC without evacuation: filling of a 6 m^3 vessel with argon gas from air to ppm impurities concentration through flushing

In this paper we present a successful experimental test of filling a volume of 6 m$^3$ with argon gas, starting from normal ambient air and reducing the impurities content down to few parts per million (ppm) oxygen equivalent. This level of contamination was directly monitored measuring the slow component of the scintillation light of the Ar gas, which is sensitive to {\it all} sources of impurities affecting directly the argon scintillation.

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arXiv2009

Cross sections and Rosenbluth separations in 1H(e, e'K+)Lambda up to Q2=2.35 GeV2

The kaon electroproduction reaction 1H(e,e'K+)Lambda was studied as a function of the virtual-photon four-momentum, Q2, total energy, W, and momentum transfer, t, for different values of the virtual- photon polarization parameter. Data were taken at electron beam energies ranging from 3.40 to 5.75 GeV. The center of mass cross section was determined for 21 kinematics corresponding to Q2 of 1.90 and 2.35 GeV2 and the longitudinal, sigmaL, and transverse, sigmaT, cross sections were separated using the Rosenbluth technique at fixed W and t. The separated cross sections reveal a flat energy dependence at forward kaon angles not satisfactorily described by existing electroproduction models. Influence of the kaon pole on the cross sections was investigated by adopting an off-shell form factor in the Regge model which better describes the observed energy dependence of sigmaT and sigmaL.

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arXiv2009

High Resolution Spectroscopy of 16N_Lambda by Electroproduction

An experimental study of the 16O(e,e'K^+)16N_Lambda reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e,e'K^+)Lambda,Sigma_0 exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76 +/- 0.16 MeV was obtained for 16N_Lambda with better precision than previous measurements on the mirror hypernucleus 16O_Lambda. Precise energies have been determined for peaks arising from a Lambda in s and p orbits coupled to the p_{1/2} and p_{3/2} hole states of the 15N core nucleus.

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arXiv2008

Quark-Hadron Duality in Neutron (3He) Spin Structure

We present experimental results of the first high-precision test of quark-hadron duality in the spin-structure function g_1 of the neutron and $^3$He using a polarized 3He target in the four-momentum-transfer-squared range from 0.7 to 4.0 (GeV/c)^2. Global duality is observed for the spin-structure function g_1 down to at least Q^2 = 1.8 (GeV/c)^2 in both targets. We have also formed the photon-nucleon asymmetry A_1 in the resonance region for 3He and found no strong Q^2-dependence above 2.2 (GeV/c)^2.

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arXiv2008

3He Spin-Dependent Cross Sections and Sum Rules

We present a measurement of the spin-dependent cross sections for the \vec{^3He}(\vec{e},e')X} reaction in the quasielastic and resonance regions at four-momentum transfer 0.1 < Q^2< 0.9 GeV^2. The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt--Cottingham and extended GDH sum rules for the first time. Impulse approximation and exact three-body Faddeev calculations are also compared to the data in the quasielastic region.

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arXiv2007

High Resolution Spectroscopy of 12B_Lambda by Electroproduction

An experiment measuring electroproduction of hypernuclei has been performed in Hall A at Jefferson Lab on a $^{12}$C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH) were added to the Hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed \lam{12}{B} spectrum shows for the first time identifiable strength in the core-excited region between the ground-state {\it s}-wave $Λ$ peak and the 11 MeV {\it p}-wave $Λ$ peak.

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arXiv2007

Angular dependence of recoil proton polarization in high-energy γd \to p n

We measured the angular dependence of the three recoil proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily.. Transverse polarizations are not well described, but suggest isovector dominance.

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arXiv2004

Precision Rosenbluth measurement of the proton elastic form factors

We report the results of a new Rosenbluth measurement of the proton form factors at Q^2 values of 2.64, 3.20 and 4.10 GeV^2. Cross sections were determined by detecting the recoiling proton in contrast to previous measurements in which the scattered electron was detected. At each Q^2, relative cross sections were determined to better than 1%. The measurement focussed on the extraction of G_E/G_M which was determined to 4-8% and found to approximate form factor scaling, i.e. μ_p G_E \approx G_M. These results are consistent with and much more precise than previous Rosenbluth extractions. However, they are inconsistent with recent polarization transfer measurements of comparable precision, implying a systematic difference between the two techniques.

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