A glimpse of gluons through deeply virtual compton scattering on the proton

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

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 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 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

Rosenbluth Separation of the pi(0) Electroproduction Cross Section Off the Neutron

We report the first longitudinal-transverse separation of the deeply virtual exclusive pi(0) electroproduction cross section off the neutron and coherent deuteron. The corresponding four structure functions d sigma(L)/dt, d sigma(T)/dt, d sigma(LT)/dt, and d sigma(TT)/dt are extracted as a function of the momentum transfer to the recoil system at Q(2) = 1.75 GeV2 and x(B) = 0.36. The ed -\u3e ed pi(0) cross sections are found compatible with the small values expected from theoretical models. The en -\u3e en pi(0) cross sections show a dominance from the response to transversely polarized photons, and are in good agreement with calculations based on the transversity generalized parton distributions of the nucleon. By combining these results with previous measurements of pi(0) electroproduction off the proton, we present a flavor decomposition of the u and d quark contributions to the cross section

Rosenbluth Separation of the pi(0) Electroproduction Cross Section

We present deeply virtual pi(0) electroproduction cross-section measurements at x(B) = 0.36 and three different Q(2) values ranging from 1.5 to 2 GeV2, obtained from Jefferson Lab Hall A experiment E07-007. The Rosenbluth technique is used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component and, thus, is far from the asymptotic limit predicted by perturbative quantum chromodynamics. Nonetheless, an indication of a nonzero longitudinal contribution is provided by the measured interference term sigma(LT). Results are compared with several models based on the leading-twist approach of generalized parton distributions (GPDs). In particular, a fair agreement is obtained with models in which the scattering amplitude includes convolution terms of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. This experiment, together with previous extensive unseparated measurements, provides strong support to the exciting idea that transversity GPDs can be accessed via neutral pion electroproduction in the high-Q(2) regime

Deeply Virtual Compton Scattering Cross Section at High Bjorken x_{B}.

We report high-precision measurements of the deeply virtual Compton scattering (DVCS) cross section at high values of the Bjorken variable x_{B}. DVCS is sensitive to the generalized parton distributions of the nucleon, which provide a three-dimensional description of its internal constituents. Using the exact analytic expression of the DVCS cross section for all possible polarization states of the initial and final electron and nucleon, and final state photon, we present the first experimental extraction of all four helicity-conserving Compton form factors (CFFs) of the nucleon as a function of x_{B}, while systematically including helicity flip amplitudes. In particular, the high accuracy of the present data demonstrates sensitivity to some very poorly known CFFs

Deep Exclusive Electroproduction of π^{0} at High Q^{2} in the Quark Valence Regime.

We report measurements of the exclusive neutral pion electroproduction cross section off protons at large values of x_{B} (0.36, 0.48, and 0.60) and Q^{2} (3.1 to 8.4  GeV^{2}) obtained from Jefferson Lab Hall A experiment E12-06-014. The corresponding structure functions dσ_{T}/dt+εdσ_{L}/dt, dσ_{TT}/dt, dσ_{LT}/dt, and dσ_{LT^{'}}/dt are extracted as a function of the proton momentum transfer t-t_{min}. The results suggest the amplitude for transversely polarized virtual photons continues to dominate the cross section throughout this kinematic range. The data are well described by calculations based on transversity generalized parton distributions coupled to a helicity flip distribution amplitude of the pion, thus providing a unique way to probe the structure of the nucleon

Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering.

We present new precision measurements of the elastic electron-proton scattering cross section for momentum transfer (Q^{2}) up to 15.75  (GeV/c)^{2}. Combined with existing data, these provide an improved extraction of the proton magnetic form factor at high Q^{2} and double the range over which a longitudinal or transverse separation of the cross section can be performed. The difference between our results and polarization data agrees with that observed at lower Q^{2} and attributed to hard two-photon exchange (TPE) effects, extending to 8  (GeV/c)^{2} the range of Q^{2} for which a discrepancy is established at >95% confidence. We use the discrepancy to quantify the size of TPE contributions needed to explain the cross section at high Q^{2}

A glimpse of gluons through deeply virtual compton scattering on the proton

It remains a challenge to find the structure and the distribution of the constituents of nucleons. Here the authors use a scattering method to get information about the gluons and quarks inside a proton and separate the contribution of Bethe-Heitler from the deeply virtual Compton scattering process

Rosenbluth separation of the π0\pi^0 Electroproduction Cross Section off the Neutron

International audienceWe report the first longitudinal-transverse separation of the deeply virtual exclusive π0 electroproduction cross section off the neutron and coherent deuteron. The corresponding four structure functions dσL/dt, dσT/dt, dσLT/dt, and dσTT/dt are extracted as a function of the momentum transfer to the recoil system at Q2=1.75  GeV2 and xB=0.36. The ed→edπ0 cross sections are found compatible with the small values expected from theoretical models. The en→enπ0 cross sections show a dominance from the response to transversely polarized photons, and are in good agreement with calculations based on the transversity generalized parton distributions of the nucleon. By combining these results with previous measurements of π0 electroproduction off the proton, we present a flavor decomposition of the u and d quark contributions to the cross section