JLab Data Research Articles

Towards EPPS21 nuclear PDFs

We report on the progress in updating our global analysis of nuclear PDFs. In particular, we will discuss the inclusion of double differential 5.02~TeV dijet and D-meson measurements, as well as 8.16~TeV W-production data from p-Pb collisions at the LHC. The new EPPS21 analysis will also involve recent JLab data for deep-inelastic scattering. As a novel aspect within our approach, we now also quantify the impact of free-proton PDF uncertainties on our extraction of nuclear PDFs.

A novel determination of non-perturbative contributions to Bjorken sum rule

Based on the operator product expansion, the perturbative and nonperturbative contributions to the polarized Bjorken sum rule (BSR) can be separated conveniently, and the nonperturbative one can be fitted via a proper comparison with the experimental data. In the paper, we first give a detailed study on the pQCD corrections to the leading-twist part of BSR. Basing on the accurate pQCD prediction of BSR, we then give a novel fit of the non-perturbative high-twist contributions by comparing with JLab data. Previous pQCD corrections to the leading-twist part derived under conventional scale-setting approach still show strong renormalization scale dependence. The principle of maximum conformality (PMC) provides a systematic and strict way to eliminate conventional renormalization scale-setting ambiguity by determining the accurate alpha _s-running behavior of the process with the help of renormalization group equation. Our calculation confirms the PMC prediction satisfies the standard renormalization group invariance, e.g. its fixed-order prediction does scheme-and-scale independent. In low Q^2-region, the effective momentum of the process is small and in order to derive a reliable prediction, we adopt four low-energy alpha _s models to do the analysis, i.e. the model based on the analytic perturbative theory (APT), the Webber model (WEB), the massive pQCD model (MPT) and the model under continuum QCD theory (CON). Our predictions show that even though the high-twist terms are generally power suppressed in high Q^2-region, they shall have sizable contributions in low and intermediate Q^2 domain. Based on the more accurate scheme-and-scale independent pQCD prediction, our newly fitted results for the high-twist corrections at Q^2=1;mathrm{GeV}^2 are, f_2^{p-n}|_{mathrm{APT}}=-0.120pm 0.013, f_2^{p-n}|_mathrm{WEB}=-0.081pm 0.013, f_2^{p-n}|_{mathrm{MPT}}=-0.128pm 0.013 and f_2^{p-n}|_{mathrm{CON}}=-0.139pm 0.013; mu _6|_mathrm{APT}=0.003pm 0.000, mu _6|_{mathrm{WEB}}=0.001pm 0.000, mu _6|_mathrm{MPT}=0.003pm 0.000 and mu _6|_{mathrm{CON}}=0.002pm 0.000, respectively, where the errors are squared averages of those from the statistical and systematic errors from the measured data.

Extending nuclear PDF analyses into the high- x , low- Q2 region

We use the nCTEQ analysis framework to investigate nuclear Parton Distribution Functions (nPDFs) in the region of large x and intermediate-to-low $Q$, with special attention to recent JLab Deep Inelastic Scattering data on nuclear targets. This data lies in a region which is often excluded by $W$ and $Q$ cuts in global nPDF analyses. As we relax these cuts, we enter a new kinematic region, which requires new phenomenology. In particular, we study the impact of i) target mass corrections, ii) higher twist corrections, iii) deuteron corrections, and iv) the shape of the nuclear PDF parametrization at large-$x$ close to one. Using the above tools, we produce a new nPDF set (named nCTEQ15HIX) which yields a good description of the new JLab data in this challenging kinematic region, and displays reduced uncertainties at large $x$, in particular for up and down quark flavors.

Nucleon\u2019s energy\u2013momentum tensor form factors in light-cone QCD

We use the energy–momentum tensor (EMT) current to compute the EMT form factors of the nucleon in the framework of the light cone QCD sum rule formalism. In the calculations, we employ the most general form of the nucleon’s interpolating field and use the distribution amplitudes (DAs) of the nucleon with two sets of the numerical values of the main input parameters entering the expressions of the DAs. The directly obtained results from the sum rules for the form factors are reliable at Q^2ge 1 GeV^2 : to extrapolate the results to include the zero momentum transfer squared with the aim of estimation of the related static physical quantities, we use some fit functions for the form factors. The numerical computations show that the energy–momentum tensor form factors of the nucleon can be well fitted to the multipole fit form. We compare the results obtained for the form factors at Q^2=0 with the existing theoretical predictions as well as experimental data on the gravitational form factor d_1^q(0). For the form factors M_2^q (0) and J^q(0) a consistency among the theoretical predictions is seen within the errors: our results are nicely consistent with the Lattice QCD and chiral perturbation theory predictions. However, there are large discrepancies among the theoretical predictions on d_1^q(0). Nevertheless, our prediction is in accord with the JLab data as well as with the results of the Lattice QCD, chiral perturbation theory and KM15-fit. Our fit functions well define most of the JLab data in the interval Q^2in [0,0.4] GeV^2 , while the Lattice results suffer from large uncertainties in this region. As a by-product, some mechanical properties of the nucleon like the pressure and energy density at the center of nucleon as well as its mechanical radius are also calculated and their results are compared with other existing theoretical predictions.

Constraints in modeling the quasielastic response in inclusive lepton-nucleus scattering

We show that the quasielastic (QE) response calculated with the SuSAv2 (superscaling approach) model, that relies on the scaling phenomenon observed in the analysis of (e,e') data and on the relativistic mean-field theory, is very similar to that from a relativistic distorted wave impulse approximation model when only the real part of the optical potentials is employed. The coincidence between the results from these two completely independent approaches, which satisfactorily agree with the inclusive data, reinforces the reliability of the quasielastic predictions stemming from both models and sets constraints for the QE response. We also study the low energy and momentum transfer region of the inclusive response by confronting the results of the relativistic mean-field model with those of the Hartree-Fock continuum random-phase approximation model, which accounts for nuclear long-range correlations. Finally, we present a comparison of our results with the recent JLab (e,e') data for argon, titanium and carbon, finding good agreement with the three data sets.

Mean-field and two-body nuclear effects in inclusive electron scattering on argon, carbon, and titanium: The superscaling approach

We compare the predictions of the SuSAv2 model including two-particle two-hole meson-exchange currents with the recent JLab data for inclusive electron scattering on three different targets (C, Ar and Ti). The agreement is very good over the full energy spectrum, with some discrepancy seen only in the deep inelastic region. The 2p2h response, peaked in the dip region between the quasielastic and $\Delta$-resonance peak, is essential to reproduce the data. We also analyze the $k_F$ (Fermi momentum) dependence of the data in terms of scaling of second kind, showing that the 2p2h response scales very differently from the quasielastic one, in full accord with what is predicted by the model. The results represent a valuable test of the applicability of the model to neutrino scattering processes on different nuclei.

Studying the $$P_c(4450)$$ Resonance in $$J/\psi $$ Photoproduction Off Protons

The LHCb has reported the observation of a resonancelike structure, the Pc(4450), in the J/psi p invariant masses. In our work, we discuss the feasibility of detecting this structure in J/psi photoproduction, e.g. in the measurements that have been approved for the experiments in Hall A/C and in Hall B with CLAS12 at JLab. Also the GlueX Collaboration has already reported preliminary results. We take into account the experimental resolution effects, and perform a global fit to world J/psi photoproduction data in order to study the possibility of observing the Pc(4450) signal in future JLab data. We present a first estimate of the upper limit for the branching ratio of the Pc(4450) into the J/psi p channel, and we study the angular distributions of the differential cross sections. This will shed light on the nature and couplings of the Pc(4450) structure in the future photoproduction experiments.

Separated kaon electroproduction cross section and the kaon form factor from 6 GeV JLab data

The $^{1}H$($e,e^\prime K^+$)$\Lambda$ reaction was studied as a function of the Mandelstam variable $-t$ using data from the E01-004 (FPI-2) and E93-018 experiments that were carried out in Hall C at the 6 GeV Jefferson Lab. The cross section was fully separated into longitudinal and transverse components, and two interference terms at four-momentum transfers $Q^2$ of 1.00, 1.36 and 2.07 GeV$^2$. The kaon form factor was extracted from the longitudinal cross section using the Regge model by Vanderhaeghen, Guidal, and Laget. The results establish the method, previously used successfully for pion analyses, for extracting the kaon form factor. Data from 12 GeV Jefferson Lab experiments are expected to have sufficient precision to distinguish between theoretical predictions, for example recent perturbative QCD calculations with modern parton distribution amplitudes. The leading-twist behavior for light mesons is predicted to set in for values of $Q^2$ between 5-10 GeV$^2$, which makes data in the few GeV regime particularly interesting. The $Q^2$ dependence at fixed $x$ and $-t$ of the longitudinal cross section we extracted seems consistent with the QCD factorization prediction within the experimental uncertainty.

Role ofω-meson exchange in scaling of theγp→π0pprocess from a Regge-type model with resonances

The scaling of photoproduction $\gamma p\to\pi^0 p$ is investigated in the Reggeized model with $N^*$ and $\Delta$ resonances included to describe resonance peaks up to photon energy $E_\gamma$= 3 GeV. Given the $t$-channel exchanges $\rho(770)+\omega(780)+b_1(1235)+h_1(1170)$ Reggeized for the background contribution, the resonances of the Breit-Wigner form are introduced to agree with cross sections for total, differential and beam asymmetry in the low energy region. The scaled differential-cross sections $s^7{d\sigma/ dt}$ are reproduced to agree with the recent JLab data, revealing the production mechanism of the big bump structure around $W\approx 2.2$ GeV by the deep-dip pattern of the $\omega$ exchange that originates from the zeros of the trajectory $\alpha_\omega(t)=0$ in the canonical phase, ${1\over2}(-1+e^{-i\pi \alpha_{\omega}(t)})$.

Pion and proton form factors in the regge description of electroproduction p(e, e′π +)n

Electroproduction of $\pi^+$ above the resonance region is analyzed in the Regge model for $\pi+\rho$ exchanges. Importance of the roles of pion and proton form factors in the process is discussed in comparison with existing models of Kaskulov and Mosel, and of Vrancx and Rychebusch. It is shown that the present model with proton form factor of a simple dipole-type yields a better description of DESY and JLab data over those models for high $Q^2$ and $-t$ region up to 5 GeV$^2$.

Unpolarised transverse momentum dependent distribution and fragmentation functions from SIDIS multiplicities

The unpolarised transverse momentum dependent distribution and fragmentation functions are extracted from HERMES and COMPASS experimental measurements of SIDIS multiplicities for charged hadron production. The data are grouped into independent bins of the kinematical variables, in which the TMD factorisation is expected to hold. A simple factorised functional form of the TMDs is adopted, with a Gaussian dependence on the intrinsic transverse momentum, which turns out to be quite adequate in shape. HERMES data do not need any normalisation correction, while fits of the COMPASS data much improve with a $y$-dependent overall normalisation factor. A comparison of the extracted TMDs with previous EMC and JLab data confirms the adequacy of the simple Gaussian distributions. The possible role of the TMD evolution is briefly considered.

Omega Electroproduction

The differential cross section for p(e, e′ω)p has been studied at Q2 ~ 5.5 (GeV/c)2. Here Q2 represents the four momentum squared of the virtual photon in the excitation of baryonic resonances by an electron projectile. In order to extract the ω-meson differential cross section from the JLAB data, the data was compared to a full Monte Carlo simulation of the detector based on events generated for omega production in a way that the production cross section was varied to achieve a match to the data. The bin selected for this procedure takes into account the measure of robustness of the stripping of the ω peak from the multi-pion background as well as the statistics in the measured data and the Monte Carlo simulation of the signal and background physics. An error estimation technique for the cross section was based on determining the dependence of the extracted cross section parameters on the experimental set-up (including parameters for the spectrometer, target beam geometeries and performance). We compare our results with a Regge-based model for hadronic content in the t-channel exchange of a photon in Q2 region of overlap. There is an extension of this data into a completely new region, which is the highest yet measured.

Forward-angle incoherent photoproduction of pseudoscalar mesons off nuclei

The mechanism of forward angle incoherent photoproduction of pseudoscalar mesons off nuclei is revisited via the time-dependent multicollisional Monte Carlo (MCMC) intranuclear cascade model. Our results---combined with recent developments to address coherent photoproduction---reproduce with good accuracy recent JLab data of ${\ensuremath{\pi}}^{0}$ photoproduction from carbon and lead at an average photon energy $k\ensuremath{\sim}5.2$ GeV. For the case of $\ensuremath{\eta}$ photoproduction, our results for $k=9$ GeV suggest that future measurements to extract the $\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ decay width via the Primakoff method should be focused on light nuclei, where the disentanglement between the Coulomb and strong amplitudes is more easily achieved. The prospects to use heavy nuclei data to access the unknown $\ensuremath{\eta}N$ cross section in cold nuclear matter are also presented.

A Few Lessons from pQCD Analysis at Low Energies

Motivated by the recent 4-loop analysis of the JLab data on Bjorken Sum Rule, where the pQCD series seems to blow up at $|Q|\lesssim 1.5\,\GeV, \alpha_s \gtrsim 0.33\,,$ we overview the general origin of the divergency of common perturbation expansion over powers of a small coupling parameter in QFT and consider in detail the {\it blowing-up phenomenon} and accuracy of finite sums for simple alternating and non-alternating examples of divergent series.

Four-loop QCD analysis of the Bjorken sum rule

We study the polarized Bjorken sum rule (BSR) at low momentum transfers in the range 0.22<Q<1.73 GeV with the four-loop N3LO expression for the coefficient function CBj(αs) in the framework of the common QCD perturbation theory (PT) and the singularity-free analytic perturbation theory (APT). The analysis of the PT series for CBj(αs) gives a hint to its asymptotic nature manifesting itself in the region Q<1 GeV. It relates to the observation that the accuracy of both the three- and four-loop PT predictions happens to be at the same 10% level. On the other hand, the usage of the two-loop APT allows one to describe the precise low energy JLab data down to Q∼300 MeV and gives a possibility for reliable extraction of the higher twist (HT) corrections. At the same time, above Q∼700 MeV the APT two-loop order with HT is equivalent to the four-loop PT with HT compatible to zero and is adequate to current accuracy of the data.

Short Range Correlations and Wave Function Factorization in Light and Finite Nuclei

Recent BNL and Jlab data provided new evidence on two nucleon correlations (2NC) in nuclei. The data confirm the validity of the convolution model, describing the spectral function (SF) of a correlated pair moving in the mean field with high and low relative and center-of-mass (cm) momenta, respectively. The model is built assuming that the wave function (WF) of a nucleus A, describing a configuration where the cm momentum of a correlated pair is low and its relative momentum is high, factorizes into the product of the two-body WF and that of the A-2 system. Such a factorization has been shown to occur in nuclear matter (NM). Here it is shown that few-body systems exhibit factorization, which seems to be therefore a general property, to be reproduced also in studies of the WF of finite nuclei.

γZcorrections to forward-angle parity-violatingepscattering

We use dispersion relations to evaluate the $\ensuremath{\gamma}Z$ box contribution to parity-violating electron scattering in the forward limit arising from the axial-vector coupling at the electron vertex. The calculation makes full use of the critical constraints from recent JLab data on electroproduction in the resonance region as well as high-energy data from HERA. At the kinematics of the ${Q}_{\mathrm{weak}}$ experiment, this gives a correction of ${0.0047}_{\ensuremath{-}0.0004}^{+0.0011}$ to the standard model value 0.0713(8) of the proton weak charge. While the magnitude of the correction is highly significant, the uncertainty is within the anticipated experimental uncertainty of $\ifmmode\pm\else\textpm\fi{}0.003$.

Neutron observables from inclusive lepton scattering on nuclei

We analyze new JLAB data for inclusive electron scattering on various targets. Computed and measured total inclusive cross sections in the range $0.3\lesssim x\lesssim 0.95$ show on a logarithmic scale reasonable agreement for all targets. However, closer inspection of the Quasi-Elastic components bares serious discrepancies. EMC ratios which may contain less systematic errors fare the same. The above observations for the new data do not enable the extraction of the magnetic form factor (FF) $G_M^n$ and the Structure Function (SFs) $F_2^n$ of the neutron, although the application of exactly the same analysis to older data had been successful. We add to the above analysis older CLAS collaboration on $F_2^D$. Removing some scattered points, it appears possible to obtain the above mentioned neutron information. We compare our results with others from alternative sources. Particular attention is paid to the A=3 iso-doublet. Present data exist only for $^3$He, but the available input and charge symmetry also enable computations for $^3$H. Their average is the computed iso-scalar part and is compared with the empirical modification of $^3$He towards a fictitious A=3 iso-singlet.

Determination of gluon polarization from deep inelastic scattering and collider data

We investigate impact of π 0 -production data at Relativistic Heavy Ion Collider (RHIC) and future E07-011 experiment for the structure function g 1 of the deuteron at the Thomas Jefferson National Accelerator Facility (JLab) on studies of nucleonic spin structure, especially on the polarized gluon distribution function. By global analyses of polarized lepton–nucleon scattering and the π 0 -production data, polarized parton distribution functions are determined and their uncertainties are estimated by the Hessian method. Two types of the gluon distribution function are investigated. One is a positive distribution and the other is a node-type distribution which changes sign at x ∼ 0.1 . Although the RHIC π 0 data seem to favor the node type for Δ g ( x ) , it is difficult to determine a precise functional form from the current data. However, it is interesting to find that the gluon distribution Δ g ( x ) is positive at large x (>0.2) due to constraints from the scaling violation in g 1 and RHIC π 0 data. The JLab-E07-011 measurements for g 1 d should be also able to reduce the gluon uncertainty, and the reduction is comparable to the one by RUN-5 π 0 -production data at RHIC. The reduction is caused mainly by the error correlation between polarized antiquark and gluon distributions and by a next-to-leading-order (NLO) gluonic effect in the structure function g 1 d . We find that the JLab-E07-011 data are accurate enough to probe the NLO gluonic term in g 1 . Both RHIC and JLab data contribute to better determination of the polarized gluon distribution in addition to improvement on polarized quark and antiquark distributions.

Bjorken sum rule and perturbative QCD frontier on the move

The reasonableness of the use of perturbative QCD notions in the region close to the scale of hadronization, i.e., below $\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{GeV}$ is under study. First, the interplay between higher orders of pQCD expansion and higher-twist contributions in the analysis of recent Jefferson Lab (JLab) data on the generalized Bjorken sum rule function ${\ensuremath{\Gamma}}_{1}^{p\ensuremath{-}n}({Q}^{2})$ at $0.1l{Q}^{2}l3\text{ }\text{ }{\mathrm{GeV}}^{2}$ is studied. It is shown that the inclusion of the higher-order pQCD corrections could be absorbed, with good numerical accuracy, by change of the normalization of the higher-twist terms. Second, to avoid the issue of unphysical singularity (Landau pole at $Q=\ensuremath{\Lambda}\ensuremath{\sim}400\text{ }\text{ }\mathrm{MeV}$), we deal with the ghost-free analytic perturbation theory (APT) that recently proved to be an intriguing candidate for a quantitative description of light quarkonia spectra within the Bethe-Salpeter approach. The values of the twist coefficients ${\ensuremath{\mu}}_{2k}$ extracted from the mentioned data by using the APT approach provide a better convergence of the higher-twist series than with the common pQCD. As the main result, a good quantitative description of the JLab data down to $Q\ensuremath{\simeq}350\text{ }\text{ }\mathrm{MeV}$ is achieved.