SoLID in Hall A

With the 12 GeV energy upgrade at Jefferson Lab, a newly proposed SoLID (Solenoidal Large Intensity Device) spectrometer in Hall A will exploit the multiple physics programs with its great advantage of large acceptance and high luminosity. SoLID will allow us to perform multi-dimensional measurements of transverse momentum dependent parton distribution functions (TMDs) with unprecedented precision via semi-inclusive deep inelastic scattering (SIDIS). It can also enable us a precise test of the standard model with the parity violation deep inelastic scattering on proton and deuteron. Last but not least, the exclusive measurement of the electroproduction of J/$\psi$ mesons near threshold will provide an opportunity to use the charmonium to study the non-perturbative QCD. More physics topics can be explored with this advanced device and new physics programs are under development.

TMDs and Transverse Spin Study SIDIS using SoLID in Hall A

The precision measurements of both the single and double spin asymmetries (SSA/DSA) in SIDIS, from polarized $^{3}He$ ("neutron") and proton targets, enable the extraction of TMDs which give the description of the parton distribution beyond the collinear approximation. The transversely polarized quark distribution function (transversity) will enable a model-dependent extraction of the u- and d-quark tensor charge, which will provide an excellent testing ground for the lattice QCD predictions. The Pretzelosity and Sivers asymmetry can also be measured and will allow us to obtain the quantitative knowledge of quark orbital angular momentum in a model-dependent way. In addition, the measurements will help us to investigate the prediction of the Sivers function sign change between the SIDIS process and Drell-Yan process.

Several highly rated experiments will use SoLID to exploit the multi-dimensional mapping of SSA/DSA in all relevant kinematic quantities with the combination of high precision and large kinematic coverage. More useful physics quantities can be extracted from these new data without costing additional beam time, e.g., two ``bonus-runs'' experiments have been approved and more are coming. Meanwhile, we are also working on adding the kaon detection capability which will can help us to study TMDs with much abundant information.

GPD Study via DVCS using SoLID in Hall A

The fundamental blocks to form hadrons are well known to be quarks and gluons whose interactions are described by the quantum chromodynamics (QCD). However, the mechanism of hadron formation from the quark and gluon degrees of freedom is difficult to be fully quantitatively calculated starting from the first principle. An alternative approach to prove how QCD works is to use certain phenomenological functions, such as the form factors, parton densities, distribution amplitudes and so on. The development of generalized parton distributions (GPDs) provides a much powerful way to describe the microscopic structure of the hadrons in terms of quarks and gluons. GPDs are analogous to the phase-space Wigner function which encodes full information of a quantum-mechanical system and they combine the features of these phenomenological functions. The study of GPDs can explore the multi-dimensional imaging of the nucleon and is able to quantify how the orbital motion of quarks contributes to the nucleon spin.

    Deeply virtual Compton scattering (DVCS) is the simplest reaction to provide direct measurement of GPDs. The DVCS process, $eN\rightarrow e'N'\gamma$, depends on multiple kinematic variables (such as $Q^{2}$, $t$, $x$ and $\phi$). Exclusive measurements of these dependencies probe an unknown soft matrix element describing the nucleon structure. The Bethe-Heitler (BH) process interferes with the DVCS and severs as the reference amplitude, while the amplitude of the BH process can be precisely determined by the well-measured nucleon form factors. With the polarized electron beam and the polarized targets, one can measure the SSA and DSA distributions and isolate the imaginary part of the BH+DVCS amplitudes. Hence the DVCS measurements directly access a linear combination of different GPDs terms.

    One of the biggest challenges to perform the DVCS measurements is the low production rate. The high luminosity and large acceptance feature of SoLID provides a great opportunity to study GPDs via the DVCS. I am leading the investigation of using the SoLID-SIDIS configuration to perform the DVCS measurements with the polarized $^{3}He$ target. At the same time, I also involve in the Doubly-DVCS project that will also run with SoLID. Two letters of intents (LOIs) for these projects will be submitted to the PAC 2015, and new proposals are under developing and will be available for submission in the PAC 2016.

Short Range Correlations and EMC effect Study

Short-Range Correlations (SRC) in nuclei raise from the interaction of two or more nucleons with momenta much higher than the Fermi momentum because of the strong repulsive core at very close distance. When measuring the break-up nucleons from these correlations with momenta above the Fermi momentum, one can observe great enhancement in the momentum distribution. The SRC theory provides a plausible explanation to describe the discrepancy of nuclear strength predicted by the Mean Field Theory which overestimates the nuclear strength by 30\%$\sim$ 40\% compared with the experimental results. The inclusive electron scattering on nuclei in the quasi-elastic (QE) region provides an outstanding probe to investigate two-nucleon SRC (2N-SRC) and is currently the only process to study three-nucleon SRC (3N-SRC). My Ph.D thesis experiment, \emph{Study of Three-Nucleon Short Range Correlations (SRC) through Inclusive Electron Scattering at large $x$} (E08-014), aimed to study both 2N-SRC and 3N-SRC by measuring the cross section ratio between two different nuclei. We have obtained high quality data in 2011 with exclusive precision thanks to the high resolution features of Hall-A HRSs. Our new data will resolve the discrepancy of two experimental results in Hall-B and Hall-C which showed different approaches of the 3N-SRC plateau with the $^{4}He/^{3}He$ ratio. The scaling behavior of 2N- and 3N-SRC will be also carefully studied with the $^{12}C/^{3}He$ and $^{12}C/^{4}He$ ratios.

The experimental investigations also suggest that the np pairs dominate in (2N-SRC). The triple-coincidence SRC experiment in Hall A revealed that the number of np pairs in $^{12}C$ is $18\pm 5$ more than for pp pairs in the high momentum region. Such an isospin-dependent effect can also be studied by the inclusive cross section measurements of isotopes. My thesis experiment measured the cross sections of $^{40}Ca$ and $^{48}Ca$, and the result will also be published very soon. With the more asymmetric nuclei $^{3}H$ and $^{3}He$ which have less mass difference, an approved Hall A experiment, E12-11-112, will perform more precise studies of the isospin structure in 2N-SRC and the transition to the 3N-SRC. This experiment will run in late 2016.

The feature of the SRC has been recently linked to the EMC effect which shows the medium modification of the nucleon structure in different nuclei. The connection between SRC and EMC indicates that the medium modification effect on nucleons could be due to the local high density cluster inside the nucleus. It is crucial to systematically study the SRC and EMC effects and their connection. Therefore, one can better understand both their individual phenomena and the transition from the nucleon degree of freedom to the nucleus degree of freedom. Several approved experiments in Hall A and C will fully study these two effects.

Proton Charged Radius and PRad in Hall B

The proton charge radius experiment (PRad) in Hall B is another important project to which I have been largely contributing. The proton is one of the fundamental blocks of the universe and its charged radius provides a vital input to the high precision test of QED. The value has been precisely measured by hydrogen Lamb shift measurements. An alternative method to extract the proton charged radius is through the electron-proton (ep) elastic scattering measurements, where one can fit the distribution of $G_{E}^{p}$ as a function of $\mathrm{Q^{2}}$ and extrapolate the proton charged radius, given as $_{E}^{p}=-dG_{E}^{p}/6dQ^{2}$. The ep scattering measurements have been performed in many facilities with different experimental techniques and the extracted values of the proton charged radius agree nicely with the ones from hydrogen Lamb shift measurements.

    However, the "proton charged radius crisis" arises when recent Lamb shift measurements reported a much smaller value ($7\sigma$ away) of the proton charged radius. The situation becomes more interesting when the newly Lamb shift measurement in muonic Helium atoms gave a preliminary result that agreed with the results from scattering experiments. While new theories to explain the discrepancy, e.g. the contribution of two-photon-exchange and even potentially new physics, are under discussion, several new experiments, such as the PRad, the MUSE experiment at PSI and more precise Lamb shift measurements, have been proposed and many new results are expected to be available in the next few years. In the PRad experiment, we will measure the ep elastic cross sections and extract the $G_{E}^{p}$ values at low $\mathrm{Q^{2}}$ range (from $\mathrm{2 \cdot 10^{-2}}$ to $\mathrm{2 \cdot 10^{-4}~GeV^{2}}$) with high resolution. If the experiment can be successfully carried out next year, we will be able to present the most updated proton charged radius values from the scattering process.

Scintillating Fiber Tracker

With the support from the 2014 JSA postdoc research grant, I am investigating the first large-area (1.2 $m^{2}$) SciFi Tracker at JLab to serve the PRad experiment. The new tracker composed of high performance SciFis can provide both the precise timing information and accurate position resolution for charged particles. Its application on the PRad experiment can largely improve the angular resolution at every low $\mathrm{Q^{2}~(\sim 10^{-4}~GeV^{2})}$ and it can also be very useful for many other experiments thanks to its easy-handling features.

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