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DOE OSTI2013

γvNN^* Transition Amplitudes and Excited Baryon Structure from CLAS

Studying excited nucleon structure through exclusive-meson electroproduction reactions is key for understanding the nature of the strong interaction in the non-perturbative regime. With its nearly complete coverage of the final-state phase space, the CLAS detector at JLab has provided the lion's share of the world's meson-electroproduction data for differential cross sections and the asymmetries arising from single- and double-polarization observables. Electrocouplings for most of the excited nucleon states (N*) in mass range of up to 1.8~GeV have been determined from several analyses of the CLAS data for photon virtualities (Q2) up to 5.0 GeV2 for the pi+n, pi0p, and eta p channels [1,3] as well as for the pi+pi-p reaction for Q2<1.5~GeV2 [2,3]. Physics analyses of these N* electrocouplings [2,3] have revealed that the structure of excited nucleon is formed of an internal core of dressed quarks with an external meson-baryon cloud. Our N*-electrocoupling results afford access to the non-perturbative strong interaction responsible for generating the different N* states and will also provide testing ground for the inspired by QCD quark model predictions. A dedicated experiment will run after the 12 GeV upgrade to JLab on the extraction of the N* electrocouplings in the yet unexplored region of high photon virtualities ranging from 5.0 to 12 GeV2. The anticipated results are of particular importance in providing a understanding of the nature of confinement and dynamical chiral symmetry breaking in baryons based upon the QCD [3].\\[4pt] [1] I.G. Aznauryan and V.D. Burkert, Prog. Part. Nucl. Phys. \textbf{67}, 1 (2012).\\[0pt] [2] V.I. Mokeev \textit{et~al.} (CLAS Collaboration), Phys. Rev. \textbf{C86}, 035203 (2012).\\[0pt] [3] I.G. Aznauryan \textit{et~al.}, ``Studies of Nucleon Resonance Structure in Exclusive Meson Electroproduction,'' arXiv:1212.4891[nucl-th].

Mokeev, Victor↗
DOE OSTI2017

Nucleon resonance structure in the finite volume of lattice QCD

An approach for relating the nucleon resonances extracted from πN reaction data to lattice QCD calculations has been developed by using the finite-volume Hamiltonian method. Within models of πN reactions, bare states are introduced to parametrize the intrinsic excitations of the nucleon. We show that the resonance can be related to the probability P N* (E) of finding the bare state, N*, in the πN scattering states in infinite volume. We further demonstrate that the probability P V N* (E) of finding the same bare states in the eigenfunctions of the underlying Hamiltonian in finite volume approaches P N* (E) as the volume increases. Our findings suggest that the comparison of P N* (E) and P V N* (E) can be used to examine whether the nucleon resonances extracted from the πN reaction data within the dynamical models are consistent with lattice QCD calculation. We also discuss the measurement of P V N* (E) directly from lattice QCD. Furthermore, the practical differences between our approach and the approach using the Lüscher formalism to relate LQCD calculations to the nucleon resonance poles embedded in the data are also discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

The structure of nucleons and the description of the electromagnetic form factors

The comparison of different sets of PDFs structure functions with the description of the whole sets of experimental data of electromagnetic form factors of the proton and neutron is made in the frame work of our model [1] of t -dependence of generalized parton distributions (GPDs) and some other models. It is shown that despite a small difference of the description of the inelastic processes by the different sets of PDF there is an essentially large difference in the description of electromagnetic form factors of the nucleons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Nucleon Form Factors on a Lattice: Overview

In this overview talk, I will present the current status of lattice QCD calculations of the nucleon form factors. Just recently, Lattice QCD has become capable of realistic calculations of nucleon structure with physical values of light quark masses. Values of nucleon vector (electromagnetic) form factors and nucleon radii have been successfully reproduced on a lattice. I will present this and other results, as well as discuss recent developments and challenges to further improving the systematic precision of nucleon structure calculations.

Syritsyn, Sergey↗
DOE OSTI2017

Overview of the SIDIS/TMD program at Jefferson Lab

The study of the single-hadron semi-inclusive deep inelastic scattering process (SIDIS) is a rapidly developing and expanding area of inquiry in hadronic physics, owing to the prospect of realizing a precision three-dimensional imaging of the quark structure of the nucleon in momentum space. Measurements of the azimuthal angular distributions of hadrons produced at large values of z = Eh=? and "small" transverse momentum pT ~ \Lambda_?QCD in deep inelastic lepton-nucleon scattering provides access to novel features of nucleon structure that are inaccessible in traditional inclusive DIS, including the e?ects of initial-state quark transverse momentum, transverse polarization of the parent nucleon and the struck quark, and "flavor tagging" due to the varying sensitivity of di?erent hadron species to di?fferent quark flavors. The Continuous Electron Beam Accelerator Facility (CEBAF) at Je?erson Lab has recently completed an upgrade to a maximum beam energy of 11 GeV at ?ve passes for electron scattering experiments. The energy upgrade of CEBAF, combined with its already unrivaled intensity, duty cycle and polarization, facilitates a three-dimensional nucleon imaging program of unprecedented precision in the valence region. In this talk,I will present a brief overview of the planned CEBAF SIDIS program in experimental Halls A, B and C.

Puckett, Andrew↗
DOE OSTI2014

Chiral dynamics and peripheral transverse densities

In the partonic (or light-front) description of relativistic systems the electromagnetic form factors are expressed in terms of frame-independent charge and magnetization densities in transverse space. This formulation allows one to identify the chiral components of nucleon structure as the peripheral densities at transverse distances b = O(M{sub {pi}}{sup -1}) and compute them in a parametrically controlled manner. A dispersion relation connects the large-distance behavior of the transverse charge and magnetization densities to the spectral functions of the Dirac and Pauli form factors near the two--pion threshold at timelike t = 4 M{ sub {pi}}{sup 2}, which can be computed in relativistic chiral effective field theory. Using the leading-order approximation we (a) derive the asymptotic behavior (Yukawa tail) of the isovector transverse densities in the "chiral" region b = O(M{sub {pi}}{sup -1}) and the "molecular" region b = O(M{sub N}{sup 2}/M{sub {pi}}{sup 3}); (b) perform the heavy-baryon expansion of the transverse densities; (c) explain the relative magnitude of the peripheral charge and magnetization densities in a simple mechanical picture; (d) include Delta isobar intermediate states and study the peripheral transverse densities in the large-N{ sub c} limit of QCD; (e) quantify the region of transverse distances where the chiral components of the densities are numerically dominant; (f) calculate the chiral divergences of the b{sup 2}-weighted moments of the isovector transverse densities (charge and anomalous magnetic radii) in the limit M{sub {pi}} -> 0 and determine their spatial support. Our approach provides a concise formulation of the spatial structure of the nucleon's chiral component and offers new insights into basic properties of the chiral expansion. It relates the information extracted from low-t elastic form factors to the generalized parton distributions probed in peripheral high-energy scattering processes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Polarised Drell-Yan measurements at COMPASS-II

The spin structure of the nucleon and its Parton Distribution Functions (PDFs) are important topics studied by the COMPASS experiment at CERN. So far, the transverse momentum dependent PDFs (TMD PDFs) of the proton and deuteron have been studied in Semi-Inclusive Deep Inelastic Scattering (SIDIS). The Drell-Yan (DY) process is a complementary way to access the TMD PDFs, using a transversely polarised target. Studying the angular distributions of dimuons from the DY events produced in the collisions of a π{sup −} beam with 190 GeV/c momentum off a transversely polarised proton target (NH{sub 3}) we are able to extract the azimuthal spin asymmetries, which are generated by 4 out of the 8 TMD PDFs needed to describe the nucleon structure at leading order QCD. The expected sign change in Sivers and Boer-Mulders functions when accessed from DY and SIDIS will be checked [1]. The opportunity to study, in the same experiment, the TMD PDFs from both SIDIS and DY processes is unique at COMPASS. The COMPASS II Proposal [2] was approved by CERN including one year for polarised DY measurements; the beginning of the DY data taking is scheduled for 2014. The feasibility of the measurement was proven by several beam tests performed so far.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

SoLID-SIDIS: Future Measurements of Transversity, TMDs and more

Over the past few decades, investigations of the nucleon structure mainly focused on the one-dimensional study of parton distributions and structure functions. New theoretical developments, including both transverse momentum distributions (TMDs) and generalized parton distributions (GPDs), provide a new way to understand the 3-dimensional structure of the nucleon. TMDs give access to the nucleon tomography in the momentum space, and also provide an opportunity to evaluate the contribution of quarks’ and gluons’ orbital angular momenta to the nucleon spin. The experimental study of TMDs requires a device with high luminosity, large kinematic coverage and great detection resolutions. With the Jefferson Lab (JLab) 12 GeV electron beam, we have proposed a Solenoidal Large Intensity Device (SoLID) in Hall A which is capable of performing such measurements. Several newly approved experiments will perform measurements of both the single and double spin asymmetries via semi-inclusive deep inelastic scattering (SIDIS) from polarized 3 He ("neutron") and proton targets. The new data will provide important information to extract TMDs with unprecedented precision. Besides, we are also able to use SoLID to explore many more important physics topics. Several experiments for the measurements of PVDIS and J=y production have been approved, and new proposals are under development. For example, with the similar SIDIS configuration, we are actively developing new measurements to study GPDs via deep virtual Compton scattering (DVCS) with polarized targets, doubly-DVCS, deep virtual meson production, time-like Compton scattering, and so on. Our collaboration has submitted the pre-conceptual design report to JLab and successfully passed the Director’s Review in early 2015. Our collaborators are focusing on optimizing the detector system, finalizing the detector designs and proceeding on the detector R&D. We are looking forward to having the DOE Science Review in the near future.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

The (Sea)Quest to map anti-up and anti-down quarks

The SeaQuest experiment at Fermilab continues a series of Drell-Yan measurements to explore the antiquark content of the nucleon and to study the modifications to nucleon structure when the nucleon is embedded into a nuclei. To extend existing measurements to larger values of Bjorken-x, a 120 GeV proton beam extracted from Fermilab's Main Injector is used, resulting in 50 times more luminosity than previous experiments and enabling access to values of x up to 0.9. One of the key physics goals of the SeaQuest collaboration is the exploration of the origin of the intrinsic nucleon sea. The scale dependence observed in high-energy scattering experiments reflects the quantum fluctuations predicted by QCD. A sea of virtual gluons arises in the nucleon; these gluons radiate other gluons or pairs of quarks and antiquarks. Gluon splitting, e.g. into an u anti-u quark pair or a d anti-d quark pair, is a perturbative QCD process and flavor symmetric. However, there is clear experimental evidence that the nucleon sea is not flavor symmetric. The distributions for anti-d quarks and anti-u quarks differ by up to 50{\%} and suggest a substantial role of non-perturbative QCD processes in the creation of the nucleon sea. Measuring the ratio of the anti-d quark and anti-u quark distributions with high accuracy and within a large x-range is the key measurement of the SeaQuest experiment. The SeaQuest results will help to identify effective theories that can describe the intrinsic nucleon sea and help to explore its origin.

Diefenthaler, Markus↗
DOE OSTI2013

Probing the Spin Structure of the Nucleon: New Experimental Results on d2 and A1 for both Neutron and Proton from JLab

The last several years of Jefferson Lab's 6-GeV physics program saw a wealth of nucleon spin structure measurements run across several Halls. In particular, E06-014 (dn2) in Hall A (polarized neutron), and E07-003 (SANE) in Hall C (polarized proton) took advantage of significant advances in target design and new/upgraded large solid angle detector packages to run precision measurements of neutron and proton spin structure functions over the region 0.2

Sawatzky, Bradley↗
DOE OSTI2023

Precision Measurement of the Neutron Asymmetry An 1 at Large Bjorken x at 12GeV Jefferson Lab

The virtual photon asymmetry A1 is one of the fundamental quantities that provide information on the spin structure of the nucleon. The value of A1 at high xBj is of particular interest because valence quarks dominate in this region, which makes it a relatively clean region to study the nucleon structure. Several theoretical calculations, including naive SU(6) quark model, relativistic constituent quark model (RCQM), perturbative QCD (pQCD), predict the behavior for A1 and the quark polarization in the high xBj valence quark region. The An 1 experiment during the 6 GeV JLab era showed that An 1 turns positive at x ? 0.5, while up to the highest measured x value of 0.61 the down quark polarization ?d/d remains negative, in contrast to the pQCD prediction. Subsequent theoretical studies following the 6 GeV results claimed that quark orbital angular momentum could delay the upward turn of ?d/d to higher xBj or the non-perturbative nature of the strong interaction could keep it negative all the way to xBj = 1 as predicted in a Schwinger-Dyson approach with di-quark model assumptions. With the 12 GeV upgrade of JLab, a new experiment on An1 (E12-06-110)1 was carried out using a 10.4 GeV beam, a polarized 3He target, and the HMS and the Super-HMS (spectrometers) in Hall C. This measurement reached a deeper valence quark region, reaching up to x ? 0.75. This thesis reports on the 3He asymmetries A?, A?, A3He 1 , and A3He 2 without radiative corrections.

Chen, Mingyu↗
DOE OSTI2015

Studying the information content of TMDs using Monte Carlo generators

Theoretical advances in studies of the nucleon structure have been spurred by recent measurements of spin and/or azimuthal asymmetries worldwide. One of the main challenges still remaining is the extraction of the parton distribution functions, generalized to describe transverse momentum and spatial distributions of partons from these observables with no or minimal model dependence. In this topical review we present the latest developments in the field with emphasis on requirements for Monte Carlo event generators, indispensable for studies of the complex 3D nucleon structure, and discuss examples of possible applications.

3D nucleon structure↗
DOE OSTI2013

Drell-Yan studies in ppbar reactions at FAIR

The nucleonic structure is far to be completely understood. A transverse momentum dependent description of the nucleon structure is a crucial milestone for several forthcoming studies in a wide range of experimental scenarios. By mean of antiproton beams, eventually polarized, that will be available at the future FAIR facility with a beam momentum up to 15 GeV/ c, the non-perturbative region of the QCD could be accessed. One of the main goal of the forthcoming experiments at FAIR is the investigation of those Drell-Yan lepton pairs produced in proton-antiproton annihilations, taking advantage of the high expected luminosities. Drell-Yan studies are a unique tool to access the spin depending properties of the nucleon, and namely its transverse degrees of freedom. Transverse Momentum Dependent (TMD) Parton Distribution Functions (PDFs), in particular the Boer-Mulders, the Sivers and the Transversity distribution functions, could be widely investigated by mean of the corresponding experimental azimuthal asymmetries. In later stages of FAIR, single- and double-spin asymmetries could be investigated as well. The Drell-Yan physics program which could be accessed at FAIR will be discussed in details, with a particular focus on the PANDA experimental scenario.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Nucleon and nuclear structure functions with nonperturbative and higher order perturbative QCD effects

We have studied the nucleon structure functions F EM iN (x,Q 2 ), i = 1, 2, by including contributions due to the higher order perturbative QCD effect up to next-to-next-to-leading order (NNLO) and the nonperturbative effects due to the kinematical and dynamical higher twist (HT) effects. The numerical results for F EM iN (x,Q 2 ) are obtained using Martin, Motylinski, Harland-Lang, Thorne 2014 NLO and NNLO nucleon parton distribution functions (PDFs). The dynamical HT correction has been included following the renormalon approach as well as the phenomenological approach, and the kinematical HT effect is incorporated using the works of Schienbein et al. These nucleon structure functions have been used as an input to calculate the nuclear structure functions F EM iN (x,Q 2 ). In a nucleus, the nuclear corrections arise because of the Fermi motion, binding energy, nucleon correlations, mesonic contribution, and shadowing and antishadowing effects. These nuclear corrections are taken into account in the numerical calculations to obtain the nuclear structure functions F EM iN (x,Q 2 ), for the various nuclear targets such as 12 C, 27 Al, 56 Fe, 64 Cu, 118 Sn, 197 Au, and 208 Pb which are of experimental interest. The effect of isoscalarity correction for nonisoscalar nuclear targets has also been studied. The results for the F EM iN (x,Q 2 ) are compared with nCTEQ nuclear PDFs parametrization as well as with the experimental results from JLab, SLAC, and NMC in the kinematic region of 0.1 ≤ x ≤ 0.8 for several nuclei. We have also calculated the ratio R A (x,Q 2 ) = F 2A (x,Q 2 )/2x F 1A (x,Q 2 ) in the moderate Q 2 region for various nuclei and compared the results with the available experimental data from JLab to examine the validity of the Callan-Gross relation in the nuclei. We also make predictions for the nuclear structure functions in 12 C, 64 Cu, and 197 Au in the kinematic region of the proposed experiment at JLab.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Measurement of Hard Exclusive Electroproduction of π 0 Meson Cross Section in Hall A of JLab with CEBAF at 12 GeV

A complete description of nucleon structure requires the simultaneous knowledge of both the spatial and momentum information of the ultimate constituents of the nucleon, the quarks and gluons. Generalized Parton Distributions (GPDs) provide such tools to describe nucleon structure. GPDs are measurable through hard exclusive processes like Deeply Virtual Compton Scattering (DVCS) and Deeply Virtual Meson Production (DVMP). GPDs can describe hard exclusive processes only if Bjorken factorization is achieved during the hard scattering process. While DVCS data have given hints of the factorization regime being attained, such hints have not been observed for DVMP data. Testing for factorization in DVMP processes is the topic of this thesis. Exclusive π 0 electroproduction has been measured by experiment E12-06-114 in Hall A of JLab. Cross sections have been measured at three fixed Bjorken- x ( x B ): 0.35, 0.48 and 0.6 in the Q 2 range 3 to 9 GeV 2 . In this document we present an analysis of a subset of the data: x B = 0.35 in the Q 2 range 3.1 to 4.5 GeV 2 . The different structure functions: unseparated cross section (σ T + ϵσ L ), longitudinal-transverse interference (σ LT ), transverse-transverse interference (σ TT ), and the polarized response (σ LT' ) terms were extracted. The data was compared to a transversity GPD model. The model fails to reproduce the data even though the order of magnitude is in agreement for both model and data. We observe a strong disagreement between the data and the model for σ LT in terms of both magnitude and the cross section sign. Our results are in a larger and wider Q 2 domain but they are in agreement with existing measurements.

Dlamini, Mongi↗
DOE OSTI2019

Hadronization of quarks and correlated di-hadron production in hard scattering

Understanding the dynamics of partons in general, and non perturbative sea, in particular, will be crucial for the understanding of strong interactions. Correlations of the spin of the target or/and the momentum and the spin of quarks, combined with final state interactions define the azimuthal distributions of produced particles. Production of correlated hadron pairs, in addition, will play an increasingly important role in the interpretation of pion electroproduction data in general, and hadronization process of quarks, in particular. More significant, than originaly anticipated, fraction of pions coming from correlated di-hadrons, indicated by recent CLAS12 measurements, and supported by various realistic models describing the hadronization process, may have a significant impact on various aspects of data analysis, including the modeling, composition, and interpretation of semi-inclusive DIS data, as well as calculations of radiative corrections. Studies of the nucleon structure beyond the traditional leading twist and current fragmentation of inclusive hadrons, provide qualitatively new tools to study the nucleon structure and extension of large x region well covered by JLab, to large Q2 at the EIC where it will be very important in interpretation of the data from JLab.

Avakian, Harut↗
DOE OSTI2013

Physics updates from HERMES

The HERMES collaboration presents two sets of recent results: the first related to an extraction of the g{sub 2} nucleon structure function determined for DIS on a polarised target and the second related to the measurement of asymmetries related to the deeply virtual Compton scattering process that can be used to discover information on generalised parton distributions and hence nucleon structure.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗