Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2015
Abstract: (Elsevier)
The Jefferson Lab Qweak experiment determined the weak charge of the proton... more Abstract: (Elsevier)
The Jefferson Lab Qweak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise e→p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Møller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8° and 11.6° were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q 2 =0.025 GeV 2 was determined using dedicated low-current (~100pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
Note: 48 pages, 36 figures. Accepted by Nuclear Instruments and Methods A
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This White Paper presents the community inputs and scientific conclusions from the Hot and Cold QCD Town Meeting that took place September 23-25, 2022 at MIT, as part of the Nuclear Science Advisory Committee (NSAC) 2023 Long Range Planning process. A total of 424 physicists registered for the meeting. The meeting highlighted progress in Quantum Chromodynamics (QCD) nuclear physics since the 2015 LRP (LRP15) and identified key questions and plausible paths to obtaining answers to those questions, defining priorities for our research over the coming decade. In defining the priority of outstanding physics opportunities for the future, both prospects for the short (~ 5 years) and longer term (5-10 years and beyond) are identified together with the facilities, personnel and other resources needed to maximize the discovery potential and maintain United States leadership in QCD physics worldwide. This White Paper is organized as follows: In the Executive Summary, we detail the Recommendations and Initiatives that were presented and discussed at the Town Meeting, and their supporting rationales. Section 2 highlights major progress and accomplishments of the past seven years. It is followed, in Section 3, by an overview of the physics opportunities for the immediate future, and in relation with the next QCD frontier: the EIC. Section 4 provides an overview of the physics motivations and goals associated with the EIC. Section 5 is devoted to the workforce development and support of diversity, equity and inclusion. This is followed by a dedicated section on computing in Section 6. Section 7 describes the national need for nuclear data science and the relevance to QCD research.
Note: QCD Town Meeting White Paper, as submitted to 2023 NSAC LRP committee on Feb. 28, 2023
(e,e ′ K+ ) reaction at Jefferson Lab's experimental Hall A. The present experiment has higher sensitivity to the nnΛ-state investigation in terms of better precision by a factor of about three. The analysis shown in this article focuses on the derivation of the reaction cross-section.
We report the first measurement of the (e,e′p) reaction cross-section ratios for Helium-3 ( He3 ), Tritium ( H3 ), and Deuterium ( d ). The measurement covered a missing momentum range of 40≤pmiss≤550MeV/c , at large momentum transfer ( 〈Q2〉≈1.9 (GeV/c) 2 ) and xB>1 , which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with plane-wave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for He3 / d and H3 / d extend to just above the typical nucleon Fermi-momentum ( kF≈250 MeV/c) and differ from each other by ∼20% , while for He3 / H3 they agree within the measurement accuracy of about 3%. At momenta above kF , the measured He3 / H3 ratios differ from the calculation by 20%−50% . Final state interaction (FSI) calculations using the generalized Eikonal Approximation indicate that FSI should change the He3 / H3 cross-section ratio for this measurement by less than 5%. If these calculations are correct, then the differences at large missing momenta between the He3 / H3 experimental and calculated ratios could be due to the underlying NN interaction, and thus could provide new constraints on the previously loosely-constrained short-distance parts of the NN interaction.
performed in a photon energy range 1.45 < Eγ < 1.55 GeV. The fission probabilities are compared with an inclusive fission probabilities obtained with photons, protons and pions. The fission probability of Bi nuclei in coincidence with kaons is 0.18 ± 0.06 which is ∼3 times larger than the proton and pion associated fission probabilities and ∼2 times larger than inclusive ones. The kaon associated excess fission events are
explained in terms of bound Λ residual states and their weak nonmesonic decays.
The pion absorption reaction (π+,3p) on Ar was studied at pion energies of 70, 118, 162 and 239 MeV, and on N and Xe at 239 MeV. The 3p cross secti
Quasi-elastic scattering on 12C(e,e′p) was measured in Hall C at Jefferson Lab for space-like 4-momentum transfer squared Q2 in the range of 8--14.2\,(GeV/c)2 with proton momenta up to 8.3\,GeV/c. The experiment was carried out in the upgraded Hall C at Jefferson Lab. It used the existing high momentum spectrometer and the new super high momentum spectrometer to detect the scattered electrons and protons in coincidence. The nuclear transparency was extracted as the ratio of the measured yield to the yield calculated in the plane wave impulse approximation. Additionally, the transparency of the 1s1/2 and 1p3/2 shell protons in 12C was extracted, and the asymmetry of the missing momentum distribution was examined for hints of the quantum chromodynamics prediction of Color Transparency. All of these results were found to be consistent with traditional nuclear physics and inconsistent with the onset of Color Transparency.
The proposed measurement is a dedicated study of the exclusive electroproduction process,1H(e,e'p)pi0, in the backward-angle regime (u-channel process) above the resonance region. The produced pi0 is emitted 180 degrees opposite to the virtual-photon momentum. This study also aims to apply the well-known Rosenbluth separation technique that provides the model-independent differential cross-sections at the never explored u-channel kinematics region. Currently, the "soft-hard transition" in u-channel meson production remains an interesting and unexplored subject. The available theoretical frameworks offer competing interpretations for the observed backward-angle cross section peaks. In a "soft" hadronic Regge exchange description, the backward meson production comes from the interference between nucleon exchange and the meson produced via re-scattering within the nucleon. Whereas in the "hard" GPD-like backward collinear factorization regime, the scattering amplitude factorizes into a hard subprocess amplitude and baryon to meson transition distribution amplitudes (TDAs), otherwise known as super skewed parton distributions (SuperSPDs). Both TDAs and SPDs are universal non-perturbative objects of nucleon structure accessible only through backward-angle kinematics. The separated cross sections:sigma_T,sigma_L and T/L ratio at Q2=2-6 GeV2, provide a direct test of two predictions from the TDA model. The magnitude and u-dependence of the separated cross sections also provide a direct connection to the re-scattering Regge picture. The extracted interaction radius (from u-dependence) at different Q2 can be used to study the soft-hard transition in the u-channel kinematics. The acquisition of these data will be an important step forward in validating the existence of a backward factorization scheme of the nucleon structure function and establishing its applicable kinematic range.
The parity-violating asymmetry arising from inelastic electron-nucleon scattering at backward angle (~95 degrees) near the Delta(1232) resonance has been measured using a hydrogen target. From this asymmetry, we extracted the axial transition form factor G^A_{N\\Delta}, a function of the axial Adler form factors C^A_i. Though G^A_{N\\Delta} has been previously studied using charged current reactions, this is the first measurement of the weak neutral current excitation of the Delta using a proton target. For Q^2 = 0.34 (GeV/c)^2 and W = 1.18 GeV, the asymmetry was measured to be -33.4 \\pm (5.3)_{stat} \\pm (5.1)_{sys} ppm. The value of G^A_{N\\Delta} determined from the hydrogen asymmetry was -0.05 \\pm (0.35)_{stat} \\pm (0.34)_{sys} \\pm (0.06)_{theory}. These findings agree within errors with theoretical predictions for both the total asymmetry and the form factor. In addition to the hydrogen measurement, the asymmetry was measured at the same kinematics using a deuterium target. The asymmetry for deuterium was determined to be -43.6 \\pm (14.6)_{stat} \\pm (6.2)_{sys} ppm.
of their charge and magnetization distributions. While measurements of the proton are relatively
straightforward, the lack of a free neutron target makes measurements of the neutron’s electromagnetic
structure more challenging and more sensitive to experimental or model-dependent uncertainties. Various
experiments have attempted to extract the neutron form factors from scattering from the neutron in
deuterium, with different techniques providing different, and sometimes large, systematic uncertainties. We
present results from a novel measurement of the neutron magnetic form factor using quasielastic scattering
from the mirror nuclei 3
H and 3
He, where the nuclear effects are larger than for deuterium but expected to
largely cancel in the cross-section ratios. We extracted values of the neutron magnetic form factor for lowto-modest momentum transfer, 0.6 < Q2 < 2.9 GeV2, where existing measurements give inconsistent
results. The precision and Q2 range of these data allow for a better understanding of the current world’s data
and suggest a path toward further improvement of our overall understanding of the neutron’s magnetic form
factor.
H3 and He3 nuclei. The experiment was performed in the Hall A Facility of Jefferson Lab using two high-resolution spectrometers for electron detection, and a cryogenic target system which included a low-activity tritium cell. The data analysis used a novel technique exploiting the mirror symmetry of the two nuclei, which essentially eliminates many theoretical uncertainties in the extraction of the ratio.
.
beam (p= 682 MeV/c) stops in a carbon target (2.2 g/cm’). The stopped K- are sub-
sequently captured on a nucleus and may then react to produce a 7r” and a recoiling
hypernucleus. The x0 immediately decays into two y rays (BR=lOO %) so that to observe
the 12C(K&,~,d,~o)j,zB reacti on, one must measure the energy and direction of the two
photons.
Encouraged by the success of the first hypernuclear spectroscopy through the ( e , e ′ K + ) reaction (JLab E89-009), a new improved experiment with a newly developed High resolution Kaon Spectrometer (HKS) and a new configuration of the electron spectrometer is planned at the JLab Hall C. The introduction of the HKS will improve by a factor of two, the energy resolution which was limited by the previous kaon spectrometer. The hypernuclear yield and the signal to noise ratio will be also improved by a factor of 50 and 10, respectively.
The HKS (Jlab E01-011) experiment on spectroscopy of Λ-hypernuclei using (e,e'K + ) reaction was successfully carried out in 2005. This paper gives a brief description of the experiment and its technique and shows some of the preliminary spectra that are still under analysis.
Hypernuclear production by the ( e , e ′ K + ) reaction has unique advantages in hypernuclear spectroscopy of the S = − 1 regime. The second-generation spectroscopy experiment on 12 C, 7 Li and 28 Si targets has been recently carried out at JLab Hall C with a new experimental configuration (Tilt method) and also using a new high-resolution kaon spectrometer (HKS). The experiment is described and preliminary results are presented together with the empasis of significance of the ( e , e ′ K + ) reaction for Λ hypernuclear spectroscopy and its future prospects.
Since the 1st generation experiment, E89-009, which was successfully carried out as a pilot experiment of (e,e'K+) hypernuclear spectroscopy at JLab Hall C in 2000, precision hypernuclear spectroscopy by the (e,e'K+) reactions made considerable pr ogress. It has evolved to the 2nd generation experiment, E01-011, in which a newly constructed high resolution kaon spectrometer (HKS) was installed and the ''Tilt method'' was adopted in order to suppress large electromagnetic background and to run with high luminosity. Preliminary high-resolution spectra of ^7_@LHe and ^2^8_@LAl together with that of ^1^2_@LB that achieved resolution better than 500 keV(FWHM) were obtained. The third generation experiment, E05-115, has completed data taking with an expe rimental setup combining a new splitter magnet, high resolution electron spectrometer (HES) and the HKS used in the 2nd generation experiment. The data were accumulated with targets of ^7Li, ^9Be, ^1^0B, ^1^2C and ^5^2Cr as well as with those of CH_2 and H_2O for calibration. The analysis is under way with particular emphasis of determining precision absolute hypernuclear masses. In this article, hypernuclear spectroscopy program in the wide mass range at JLab Hall C that has undergone three generation is described.
The lifetime of a Λ particle embedded in a nucleus (hypernucleus) decreases from that of free Λ decay mainly due to the opening of the ΛN→NN weak decay channel. However, it is generally believed that the lifetime of a hypernucleus attains a constant value (saturation) for medium to heavy hypernuclear masses, yet this hypothesis has been difficult to verify. This paper presents a direct measurement of the lifetime of medium-heavy hypernuclei that were hyper-fragments produced by fission or break-up from heavy hypernuclei initially produced with a 2.34 GeV photon-beam incident on thin Fe, Cu, Ag, and Bi target foils. For each event, fragments were detected in coincident pairs by a low-pressure multi-wire proportional chamber system. The lifetime was extracted from decay time spectrum formed by the difference of the time zeros between the pairs. The measured lifetime from each target is actually a statistical average over a range of mass with mean about 1/2 of the target mass and appears to be a constant of about 200 ps. Although this result cannot exclude unexpected shorter or longer lifetimes for some specific hypernuclei or hypernuclear states, it shows that a systematic decrease in lifetime as hypernuclear mass increases is not a general feature for hypernuclei with mean mass up to A≈130 . On the other hand, the success of this experiment and its technique shows that the time delayed fissions observed and used by all the lifetime measurements done so far on heavy hypernuclei could likely have originated from hyper-fragments lighter than the assumed masses.
A Large Acceptance Detector System (LADS) has been designed and built at the Paul Scherrer Institute to study multiparticle final states following pion-nucleus absorption. It consists of a 28-sector cylinder of plastic scintillators of 1.6 m active length and 1.4 m diameter, two cylindrical wire chambers, and two 14-sector plastic scintillator end-caps which close each end. The nearly 4π solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The design and the performance of the LADS detector are presented.
The design, construction, and performance of a segmented-target, cathode-strip, tracking-detector is discussed. The chamber was made of low- Z materials in order to allow photons to leave the target region. It was used to determine the reaction vertex of stopping kaons, and was successfully operated in a high-intensity kaon beamline at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The vertical and horizontal resolutions of the stopping kaon reaction positions were σ X ∼0.454 mm and σ Y ∼1.180 mm , respectively. The uncertainty in the longitudinal ( Z ) direction is given by one-half the thickness of a target segment.
The Jefferson Lab Qweak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise e→p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Møller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8° and 11.6° were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q 2 =0.025 GeV 2 was determined using dedicated low-current (~100pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
Note: 48 pages, 36 figures. Accepted by Nuclear Instruments and Methods A
An experimental technique for single‐Λ spectroscopy of the light and medium‐heavy mass Λ‐hypernuclei developed at the Jlab by the E01‐011 Collaboration has been described. The technique is based on the electroproduction of Λ‐hypernuclei by the (e,e′K +) reaction and newly constructed 2nd generation high resolution large solid angle kaon spectrometer (HKS).
The first experiment in Λ-hypernuclear spectroscopy using the high-precision electron beam at Jefferson laboratory (JLab) has been carried out. The hypernuclear spectrometer system (HNSS) was used to measure spectra from the 12 C ( e , e ′ K + ) Λ 12 B reaction with sub-1-MeV resolution, the best energy resolution obtained thus far in hypernuclear spectroscopy with magnetic spectrometers. This paper describes the HNSS and the preliminary results for the Λ 12 B system. The experimental spectrum is consistent with the expected strong spin-flip excitations of unnatural parity states. A program of hypernuclear physics experiments is planned for the future with much higher yield and even better energy resolution.
A Λ hyperon which has a strangeness can be bound in deep inside of a nucleus since a Λ does not suffer from the Pauli exclusion principle from nucleons. Thus, a Λ could be a useful tool to investigate inside of a nucleus. Since 2000, Λ hypernuclear spectroscopic experiments by the (e,e′k) reaction have been performed at the experimental hall C in Thomas Jefferson National Accelerator Facility (JLab Hall C). An experiment, JLab E05-115 was carried out to investigate Λ hypernuclei with a wide mass range (the mass number, A = 7, 9, 10, 12, 52). The latest analysis status of JLab E05-115 experiment is discussed in the present article.
JLab E05-115 which is an experiment for Λ hypernuclear spectroscopy with electron beams was carried out at Jefferson Lab (JLab) in 2009. In the experiment, 7ΛHe, 9ΛLi, 10ΛBe, 12ΛB and 52ΛV were measured by new magnetic spectrometer systems (SPL+HES+HKS) which were necessary for spectroscopy with high energy resolution of sub-MeV (FWHM). This is the first attempt to measure a Λ hypernucleus with up to medium–heavy mass region by the (e,e′K+) reaction, overcoming high rate and high multiplicity conditions due to electromagnetic background particles. An overview of the hypernuclear experiments at JLab Hall-C and preliminary binding energy spectrum of 10ΛBe are shown.
The third generation spectroscopic study of hypernuclei using (e,e'K(+)) reaction (JLab E05-115) was performed at JLab Hall-C in 2009. The experiment introduced the newly developed high-resolution electron spectrometer (HES) with the existing h igh-resolution kaon spectrometer (HKS). Experimental configuration, conditions, spectrometer designs and current status of analysis are presented.
The binding energy of7ΛHe has been obtained for the first time with reaction spectroscopy using the (e,e'K+) reaction at Jefferson Lab's Hall C. A comparison among the binding energies of the A = 7 T = l iso-triplet hypernuclei,7ΛHe,7ΛLi*and7ΛBe, is made and possible charge symmetry breaking (CSB) in the ΛN potential is discussed. For7ΛHe and7ΛBe, the shifts in binding energies are opposite to those predicted by a recent cluster model calculation, which assumes that the unexplained part of the binding energy difference between4ΛH and4ΛHe, is due to the CSB of the ΛN potential. Further examination of CSB in light hypernuclear systems is required both experimentally and theoretically.