The u- and d-quark contributions to the elastic nucleon electromagnetic form factors have been determined by using experimental data on G(E)(n), G(M)(n), G(E)(p), and G(M)(p). Such a flavor ...separation of the form factors became possible up to negative four-momentum transfer squared Q(2) = 3.4 GeV(2) with recent data on G(E)(n) from Hall A at Jefferson Lab. For Q(2) above 1 GeV(2), for both the u and the d quark, the ratio of the Pauli and Dirac form factors, F(2)/F(1), was found to be almost constant in sharp contrast to the behavior of F(2)/F(1) for the proton as a whole. Also, again for Q(2)>1 GeV(2), both F(2)(d) and F(1)(d) are roughly proportional to 1/Q(4), whereas the dropoff of F(2)(u) and F(1)(u) is more gradual.
We present a search at the Jefferson Laboratory for new forces mediated by sub-GeV vector bosons with weak coupling α' to electrons. Such a particle A' can be produced in electron-nucleus ...fixed-target scattering and then decay to an e + e- pair, producing a narrow resonance in the QED trident spectrum. Using APEX test run data, we searched in the mass range 175-250 MeV, found no evidence for an A'→ e+ e- reaction, and set an upper limit of α'/α ~/= 10(-6). Our findings demonstrate that fixed-target searches can explore a new, wide, and important range of masses and couplings for sub-GeV forces.
We report here the first experimental result for the anisotropy of the one-way maximum attainable speed of the electron, Δc→1,e, obtained via the study of a sidereal time dependence of the difference ...between momenta of the counterrotating electron and positron beams in the Cornell Electron Storage Ring at Cornell University. At 95% confidence, an upper limit for the component of Δc→1,e/c perpendicular to Earth's rotational axis is found to be 5.5×10−15.
The internal structure of nucleons (protons and neutrons) remains one of the greatest outstanding problems in modern nuclear physics. By scattering high-energy electrons off a proton we are able to ...resolve its fundamental constituents and probe their momenta and positions. Here we investigate the dynamics of quarks and gluons inside nucleons using deeply virtual Compton scattering (DVCS)-a highly virtual photon scatters off the proton, which subsequently radiates a photon. DVCS interferes with the Bethe-Heitler (BH) process, where the photon is emitted by the electron rather than the proton. We report herein the full determination of the BH-DVCS interference by exploiting the distinct energy dependences of the DVCS and BH amplitudes. In the regime where the scattering is expected to occur off a single quark, measurements show an intriguing sensitivity to gluons, the carriers of the strong interaction.
Probing Cold Dense Nuclear Matter Subedi, R; Shneor, R; Monaghan, P ...
Science (American Association for the Advancement of Science),
06/2008, Volume:
320, Issue:
5882
Journal Article
Peer reviewed
Open access
The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, in which a proton is knocked out of the nucleus with high-momentum transfer and high missing ...momentum, show that in carbon-12 the neutron-proton pairs are nearly 20 times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.
The Heavy Photon Search test detector Battaglieri, M.; Boyarinov, S.; Bueltmann, S. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2015, Volume:
777, Issue:
C
Journal Article
Peer reviewed
Open access
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 experiment׳s 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+e− 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+e− pairs requires the first layer of silicon sensors be placed only 10cm 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.
The status of the experiment to search for a new gauge boson, A′, in e+e− annihilation by means of a positron beam incident on a gas hydrogen target internal to the bypass at the VEPP-3 storage ring ...is reported. The search method is based on a missing mass spectra in reaction e+e− → γA′. It allows observation of the A′ signal independently of its decay modes and life time. The projected result of this experiment corresponds to an upper limit on the square of coupling constant ε2 = (0.2 ÷ 1.0) · 10−7 with a signal-to-noise ratio of two to one at A′ mass of 5–20 MeV.
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