Narrow Results

High energy elastic pp scattering at the Large Hadron Collider (LHC) at c.m. energy 14 TeV is predicted using the asymptotic behavior of σ_tot(s) and ρ(s) known from dispersion relation calculations and the measured elastic differential cross-section at . The effective field theory model underlying the phenomenological analysis describes the nucleon as having an outer cloud of quark–antiquark condensed ground state, an inner core of topological baryonic charge of radius ≃ 0.44 F and a still smaller valence quark-bag of radius ≲ 0.1 F. The LHC experiment TOTEM (Total and Elastic Measurement), if carried out with sufficient precision from |t| = 0 to |t| > 10 GeV², will be able to test this structure of the nucleon.

The nucleon is naturally viewed as a bipartite system of valence spin — defined by its non-vanishing chiral charge — and non-valence or sea spin. The sea spin can be traced over to give a reduced density matrix, and it is shown that the resulting entanglement entropy acts as an order parameter of chiral symmetry breaking in the nucleon. In the large-$N_c$ limit, the entanglement entropy vanishes and the valence spin accounts for all of the nucleon spin, while in the limit of maximal entanglement entropy, the nucleon loses memory of the valence spin and consequently has spin dominated by the sea. The nucleon state vector in the chiral basis, fit to low-energy data, gives a valence spin content consistent with experiment and lattice QCD determinations, and has large entanglement entropy.

The nucleon electromagnetic form factors have been studied in the past extensively from unpolarized electron scattering experiments. With the development in polarized beam, recoil polarimetry, and polarized target technologies, polarization experiments have provided more precise data on these quantities. In this talk, I review recent experimental progress on this subject.

The measurement of spin observables in the Λ hyperon production has shown how poor is our understanding of the spin effects in the nucleon structure and in the hadronisation processes. New possibilities are offered by future facilities.

I present some recent progress in lattice computations of moments of structure functions using domain wall fermions.

High energy pp and elastic scattering carried out at CERN ISR and SPS Collider and at Fermilab Tevatron are studied first in a model where the nucleon has an outer cloud and an inner core. Elastic scattering is viewed as primarily due to two processes: (a) diffraction scattering originating from cloud–cloud interaction; (b) a hard or large |t| scattering originating from one nucleon core scattering off the other via vector meson ω exchange, while their outer clouds interact independently. For small |t| diffraction dominates, but the hard scattering takes over as |t| increases. The ω-exchange amplitude shows that ω behaves like an elementary vector meson at high energy, contrary to a regge pole behavior. This behavior, however, can be understood in the nonlinear σ-model where ω couples to a topological baryonic current like a gauge boson, and the nucleon is described as a topological soliton. Further investigation shows that the underlying effective field theory model is a gauged Gell-Mann–Levy type linear σ-model that has not only the pion sector and the Wess–Zumino–Witten action of the nonlinear σ-model, but also a quark sector where quarks and antiquarks interact via a scalar field. The scalar field vanishes near the center of the nucleon, but rises sharply at some critical distance to its vacuum value f_π leading to a condensate analogous to a BCS condensate in superconductivity. The nucleon structure that emerges then is that the nucleon has an outer cloud of condensed ground state, an inner core of topological baryonic charge probed by ω, and a still smaller quark-bag containing massless valence quarks. Large |t|pp elastic scattering is attributed to valence quark–quark elastic scattering, which is taken to be due to the hard pomeron (BFKL pomeron with next to leading order corrections). The parameters in the model are determined by requiring that they satisfactorily describe the known asymptotic behavior of σ_tot(s) and ρ(s), and the measured at , 630 GeV, and 1.8 TeV. The model is then used to predict pp elastic differential cross-section at LHC at and |t| = 0–10 GeV². Our predicted dσ/dt at 14 TeV is found to be very different from those predicted by the impact-picture model and the eikonalized pomeron–reggeon model. Precise measurement of dσ/dt at LHC by the TOTEM group will be able to distinguish between these models. If our predicted dσ/dt is quantitatively confirmed, then it will indicate that various novel ideas developed to describe the nucleon combine and lead to a unique physical description of its structure.

An attempt to extend simple Parton Distribution Functions based on the quantum statistical model to consider more active flavors is presented here. Considering partons as clusters, inspiring from the static picture of proton composed of three valence quarks, such a reasonable extension is surveyed to improve the results. Analytic expressions for the longitudinal Quantum Statistical Parton Distribution Functions are obtained by applying the maximum entropy principle beside theoretical proton sum rules. Furthermore, the main purpose of this study is to predict a minimum energy scale to probe the proton with a defined condition, theoretically. An interesting approach to determine the statistical variables exactly without fitting and fixing is employed and the consistency of computed distributions with the experimental observations gives a robust confirm of presented simple statistical model.

In this paper, I review recent progress in lattice-QCD calculations of hadron structure with an emphasis on nucleon structure. A wide range of nucleon observables are being studied in modern lattice calculations, and important progress has been made at physical pion mass, including the spin decomposition of the nucleon and the Bjorken-$x$ dependence of hadron structure. Challenges and perspectives for future lattice hadron-structure calculations will be discussed.

In this paper, we present our QCD analysis of the Nucleon Form Factors presented by JLab to extract a new parameterization for the valence Generalized Parton Distributions (GPDs), $H_v^q(x,\xi =0,t)$ and $E_v^q(x,\xi =0,t)$. We choose simple parameterizations for $x-$ and $t-$ dependence of GPDs at zero skewness. These free parameters of the model are then obtained using old and new data of nucleon’s elastic electromagnetic form factors (FFs) from JLAB. We have also used the “Hessian method” to perform a careful estimation of the uncertainties for the nucleon GPDs and corresponding observables originate from experimental errors. We also calculate the parton distribution functions (PDFs) in Impact parameter space to show the 3D structure of the Nucleon. We also calculated the proton and the neutron charge densities ${\rho }_{\mathrm{\text{ch}}}^{(p,n)}$. We show that there is a highly localized negative charge near the center of the neutron and it is because of the negatively charged down quarks dominating the high momentum quarks in the neutron. Finally using the obtained Dirac’s FF, we had an estimation for the proton charge radius. The predictions based on the obtained nucleon GPDs are in good agreement with all available experimental data and other phenomenological models.

There are different quark and gluon momentum, spin and orbital angular momentum operators used in the study of nucleon structure. We analyze the physical contents of these operators and propose a new set of operators based on gauge invariance principle, canonical quantization rule and Poincaré covariance. Atomic structure is a simpler testing ground of these operators and has been analyzed together. These new operators are the gauge invariant version of the gauge non-invariant canonical version used in physics since the establishment of quantum mechanics and reduce to the familiar canonical ones in Coulomb gauge.

We present preliminary COMPASS results on pion and kaon multiplicities produced in semi inclusive deep inelastic scattering of 160GeV muons off an isoscalar (${}^{\mathrm{\text{6}}}$LiD) target. The results constitute an impressive data set of more than 400 points in p and 400 in K, covering a large x,Q${}^{\mathrm{\text{2}}}$ and z domain in a fine binning, which will be used in future QCD fits at next to leading order to extract quark fragmentation functions. We show results of a first leading order fit performed to extract the favored and unfavored quark fragmentation functions into pions D${}_{\mathrm{\text{fav}}}^{\pi }$ and D${}_{\mathrm{\text{unfav}}}^{\pi }$.

A selection of recent key results obtained in semi-inclusive deeply inelastic scattering (SIDIS) experiments is presented. The observations strongly support the description of the nucleon structure in terms of transverse momentum dependent parton distribution functions, which represent the various correlations between the quarks spins, the quarks transverse momenta and the nucleon spin which give rise to specific spin-dependent azimuthal asymmetries.

The next generation of Compton scattering experiments is underway at Mainz and other laboratories, where the goal is precision measurements of the scalar and vector polarizabilities of the nucleon using polarized photons and polarized targets. Results are presented for the first double polarized Compton scattering experiment utilizing a polarized proton target. Preliminary results are presented for the four spin polarizabilities of the proton.

Baryon spectroscopy and structure studies are the key to a deeper understanding of the processes which happened during the first few microseconds of the universe. The understanding of the fundamental degrees of freedom inside a proton or a neutron and how they change with varying quark masses can help us to obtain a deeper understanding on the origin of confinement and its connection to chiral symmetry breaking. While photoproduction experiments are have established as a very effective tool to search and identify new resonances, a hard scale, provided by electroproduction is needed to probe the internal structure of the N* states and to reveal their true nature. The paper will give an overview on the spectroscopy of light baryon states, with a special focus on recent progress in hadron structure studies with CLAS at JLAB. The CLAS spectrometer offers a unique dataset to determine the electrocouplings of N* and Δ resonances using both single and double pion electroproduction. As an outlook, an overview of the planed baryon resonance studies with CLAS12 at the recently upgraded 12 GeV CEBAF electron accelerator will be presented.

New measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets 〈θ_lab〉 ≈ 6.0° are reported. The ⁴He result is A_PV = (+6.40 ± 0.23 (stat) ± 0.12 (syst)) × 10^-6. The hydrogen result is A_PV = (−1.58 ± 0.12 (stat) ± 0.04 (syst)) × 10⁻⁶. The asymmetry for hydrogen is a function of a linear combination of and , the strange quark contributions to the electric and magnetic form factors of the nucleon respectively, and that for ⁴He is a function solely of . The combination of the two measurements separates and and provide new limits on the role of strange quarks in the nucleon charge and magnetization distributions. We extract at 〈Q²〉 = 0.077 GeV², and at 〈Q²〉 = 0.109 GeV².

Research on the nucleon electromagnetic form factors has been revitalized by new class of experiments exploiting spin degrees of freedom. A summary of recent progress at both high and low Q² is presented.

Seven years of experimental efforts at Jefferson Lab on Deeply Virtual Compton Scattering yielded very significant results such as a strong indication for the handbag dominance (i.e. scaling) even et low energy, the first comparisons to models and a large data sample to constrain their free parameters and even a first model-dependent extraction of the total angular momentum carried by quarks. These proceedings try to summarize the information collected so far without going into details, and in a semi-chronological fashion.

We discuss the interpretation of the ep → epρ⁰ process in terms of, on the one hand, Generalized Parton Distributions and, on the other hand, an effective hadronic model based on Regge theory.

Generalized parton distributions (GPDs) provide a new and powerful framework for a complete description of the nucleon structure. They can provide a three-dimensional picture of how the quarks and gluons form a nucleon. GPDs can be probed experimentally in hard exclusive meson production or deeply virtual Compton scattering (DVCS). The COMPASS experiment at CERN is a unique place to study these reactions. At COMPASS, a high energy polarized positive or negative muon beam is scattered off a polarized or unpolarized fixed target. First results for exclusive ρ⁰ meson production are shown. The transverse target spin asymmetry for exclusively produced ρ⁰ on a transversely polarized deuteron target has been measured. Prospects for future measurements of DVCS and exclusive meson production at COMPASS will be shown. The experiment will use the existing COMPASS spectrometer with a new target, a new recoil detector and extended calorimetry. Simulations for different models and a test of the recoil detector have been performed.

The study of exclusive reactions like Deeply Virtual Compton Scattering (DVCS) and Meson Production is one major part of the future COMPASS program in order to investigate nucleon structure through Generalised Parton Distributions (GPD). The high energy of the muon beam allows to measure the x_B-dependence of the t-slope of the pure DVCS cross section and to study nucleon tomography. The use of positive and negative polarised muon beams allows to determine the Beam Charge and Spin Difference of the DVCS cross sections to access the real part of the Compton form factor related to the dominant GPD H.