197 research outputs found
Photodisintegration of \u3csup\u3e4\u3c/sup\u3eHE into \u3ci\u3ep\u3c/i\u3e+\u3ci\u3et\u3c/i\u3e
The two-body photodisintegration of 4He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid 4He target. This is the first measurement of the photodisintegration of 4He above 0.4 GeV. The differential cross sections for the Îł 4He âpt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6â1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of 3He that demonstrated the great importance of three-body mechanisms in the energy region 0.5â0.8 GeV
The HPS electromagnetic calorimeter
The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called âheavy photon.â Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015â2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier
Cross Section Measurements of Charged Pion Photoproduction in Hydrogen and Deuterium from 1.1 to 5.5 GeV
The differential cross section for the gamma +n --> pi- + p and the gamma + p
--> pi+ n processes were measured at Jefferson Lab. The photon energies ranged
from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4
GeV. The pion center-of-mass angles varied from 50 degree to 110 degree. The
pi- and pi+ photoproduction data both exhibit a global scaling behavior at high
energies and high transverse momenta, consistent with the constituent counting
rule prediction and the existing pi+ data. The data suggest possible
substructure of the scaling behavior, which might be oscillations around the
scaling value. The data show an enhancement in the scaled cross section at
center-of-mass energy near 2.2 GeV. The differential cross section ratios at
high energies and high transverse momenta can be described by calculations
based on one-hard-gluon-exchange diagrams.Comment: 18 pages, 19 figure
Probing the high momentum component of the deuteron at high Q^2
The d(e,e'p) cross section at a momentum transfer of 3.5 (GeV/c)^2 was
measured over a kinematical range that made it possible to study this reaction
for a set of fixed missing momenta as a function of the neutron recoil angle
theta_nq and to extract missing momentum distributions for fixed values of
theta_nq up to 0.55 GeV/c. In the region of 35 (deg) <= theta_nq <= 45 (deg)
recent calculations, which predict that final state interactions are small,
agree reasonably well with the experimental data. Therefore these experimental
reduced cross sections provide direct access to the high momentum component of
the deuteron momentum distribution in exclusive deuteron
electro-disintegration.Comment: 5 pages, 2 figure
The Heavy Photon Search test detector
The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experimentŚłs technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e+eâ invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW04 crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e+eâ pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab
Scaling Tests of the Cross Section for Deeply Virtual Compton Scattering
We present the first measurements of the \vec{e}p->epg cross section in the
deeply virtual Compton scattering (DVCS) regime and the valence quark region.
The Q^2 dependence (from 1.5 to 2.3 GeV^2) of the helicity-dependent cross
section indicates the twist-2 dominance of DVCS, proving that generalized
parton distributions (GPDs) are accessible to experiment at moderate Q^2. The
helicity-independent cross section is also measured at Q^2=2.3 GeV^2. We
present the first model-independent measurement of linear combinations of GPDs
and GPD integrals up to the twist-3 approximation.Comment: 5 pages, 4 figures, 2 tables. Text shortened for publication.
References added. One figure remove
Generalized Contact Formalism Analysis of the He Reaction
Measurements of short-range correlations in exclusive He
reactions are analyzed using the Generalized Contact Formalism (GCF). We
consider both instant-form and light-cone formulations with both the AV18 and
local N2LO(1.0) nucleon-nucleon () potentials. We find that kinematic
distributions, such as the reconstructed pair opening angle, recoil neutron
momentum distribution, and pair center of mass motion, as well as the measured
missing energy, missing mass distributions, are all well reproduced by GCF
calculations. The missing momentum dependence of the measured He
/ He cross-section ratios, sensitive to nature of the
interaction at short-distacnes, are also well reproduced by GCF calculations
using either interaction and formulation. This gives credence to the GCF
scale-separated factorized description of the short-distance many-body nuclear
wave-function.Comment: Accepted for publication in Physics Letters B. 8 pages, 4 figures and
online supplementary material
Calibration of a Proton Polarimeter
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
Exclusive Neutral Pion Electroproduction in the Deeply Virtual Regime
We present measurements of the ep->ep pi^0 cross section extracted at two
values of four-momentum transfer Q^2=1.9 GeV^2 and Q^2=2.3 GeV^2 at Jefferson
Lab Hall A. The kinematic range allows to study the evolution of the extracted
hadronic tensor as a function of Q^2 and W. Results will be confronted with
Regge inspired calculations and GPD predictions. An intepretation of our data
within the framework of semi-inclusive deep inelastic scattering has also been
attempted
Physics Opportunities with the 12 GeV Upgrade at Jefferson Lab
This white paper summarizes the scientific opportunities for utilization of
the upgraded 12 GeV Continuous Electron Beam Accelerator Facility (CEBAF) and
associated experimental equipment at Jefferson Lab. It is based on the 52
proposals recommended for approval by the Jefferson Lab Program Advisory
Committee.The upgraded facility will enable a new experimental program with
substantial discovery potential to address important topics in nuclear,
hadronic, and electroweak physics.Comment: 64 page
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