Charged Pion Electroproduction Research Articles

Exclusive Single Pion Electroproduction off the Proton: Results from CLAS

Exclusive meson electroproduction off protons is a powerful tool to probe the effective degrees of freedom in excited nucleon states at the varying distance scale where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance. During the past decade, the CLAS collaboration has executed a broad experimental program to study the excited states of the proton using polarized electron beam and both polarized and unpolarized proton targets. The measurements covered a broad kinematic range in the invariant mass W and photon virtuality \(Q^2\) with nearly full coverage in polar and azimuthal angles in the hadronic CM system. As results, several low-lying nucleon resonance states in particular from pion threshold to \(W < 1.6\,\hbox {GeV}\) have been explored. These include \(\varDelta (1232)\frac{3}{2}^+\), \(N(1440)\frac{1}{2}^+\), \(N(1520)\frac{3}{2}^-\), and \(N(1535)\frac{1}{2}^-\) states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction \(\gamma ^*p\rightarrow n\pi ^+\) at higher W (1.6–2.0 GeV) which are the \(N(1675)\frac{5}{2}^-\), \(N(1680)\frac{5}{2}^+\), and \(N(1710)\frac{1}{2}^+\) states. These excited states with isospin 1/2 and with masses near 1.7 GeV can be accessed in single \(n\pi ^+\) production as there are no isospin 3/2 states present in this mass range with the same spin-parity assignments. I will briefly discuss these states from CLAS results of the single charged pion electroproduction data.

Recent HERMES results from inclusive and semi-inclusive hadron production

Hermes collected a wealth of data using the 27.6 GeV polarized Hera lepton beam and various pure, polarized and unpolarized, gaseous targets. This unique data set opens the door to various measurements sensi- tive to the multi-dimensional structure of the nucleon. Among them are two-hadron production in semi-inclusive deep-inelastic scattering on a transversely polarized target, providing access to different transverse-momentum- dependent distribution and fragmentation functions in a way complementary to single-hadron production. Also transverse-target single-spin asymmetries in inclusive electroproduction of charged pions and kaons have been recently extracted, complementing data from proton-proton collisions, where large, so far unexplained, asym- metries have been observed. From inclusive measurements on various nuclear targets, the Λ polarization is extracted, also here complementing measurements in proton-proton collisions.

K+Λelectroproduction above the resonance region

Background: In ${\ensuremath{\pi}}^{+}n$ and ${\ensuremath{\pi}}^{\ensuremath{-}}p$ electroproduction, conventional models cannot satisfactorily explain the data above the resonance region, in particular the transverse cross section. Although no high-energy l-t-separated cross-section data are available to date, a similar scenario can be inferred for ${K}^{+}\ensuremath{\Lambda}$ electroproduction.Purpose: Develop a phenomenological model for the $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ reaction at forward angles and high-energies. Propose a universal framework for interpreting charged-kaon and charged-pion electroproduction above the resonance region.Method: Guided by the recent model for charged-pion electroproduction, developed by the authors, a framework for ${K}^{+}\ensuremath{\Lambda}$ electroproduction at high energies and forward angles is constructed. To this end, a Reggeized background model for ${K}^{+}\ensuremath{\Lambda}$ photoproduction is first developed. This model is used as a starting base to set up an electroproduction framework.Results: The few available data of the unseparated $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ cross section are well explained by the model. Predictions for the l-t-separation experiment planned with the 12 GeV upgrade at Jefferson Lab are given. The newly proposed framework predicts an increased magnitude for the transverse structure function, similar to the situation in charged-pion electroproduction.Conclusions: Within a hadronic framework featuring Reggeized background amplitudes, $s$-channel resonance-parton effects can explain the observed magnitude of the unseparated $p({\ensuremath{\gamma}}^{*},{K}^{+})\ensuremath{\Lambda}$ cross section at high energies and forward angles. Thereby, no hardening of the kaon electromagnetic form factor is required.

Charged-pion electroproduction above the resonance region

[Background] Above the nucleon resonance region, the $N(e,e'\pi^\pm)N'$ data cannot be explained by conventional hadronic models. For example, the observed magnitude of the transverse cross section is significantly underestimated in a framework with Reggeized background amplitudes. [Purpose] Develop a phenomenological framework for the $N(e,e'\pi^\pm)N'$ reaction at high invariant mass $W$ and deep photon virtuality $Q^2$. [Method] Building on the work of Kaskulov and Mosel, a gauged pion-exchange current is introduced with a running cutoff energy for the proton electromagnetic transition form factor. A new transition form factor is proposed. It respects the correct on-shell limit, has a simple physical interpretation and reduces the number of free parameters by one. [Results] A study of the $W$ dependence of the $N(e,e'\pi^\pm)N'$ lends support for the newly proposed transition form factor. In addition, an improved description of the separated and unseparated cross sections at $-t \lesssim 0.5 \;\text{GeV}^2$ is obtained. The predictions overshoot the measured unseparated cross sections for $-t > 0.5 \;\text{GeV}^2$. Introducing a strong hadronic form factor in the Reggeized background amplitudes brings the calculations considerably closer to the high $-t$ data. [Conclusions] Hadronic models corrected for resonance/parton duality describe the separated pion electroproduction cross sections above the resonance region reasonably well at low $-t$. In order to validate the applicability of these models at high $-t$, separated cross sections are needed. These are expected to provide a more profound insight into the relevant reaction mechanisms.

CONTRIBUTION OF P11 (1440) RESONANCE TO THE d(e, e′ π+)nn REACTION INCLUDING NN FINAL STATE INTERACTION

The effect of P11(1440) (Roper) resonance on the response functions for the positivelly charged pion electroproduction from the deuteron is studied using the well known spectator model. The amplitude for the elementary reaction γ* N→π+ N is taken from MAID2003 model. The effect of the NN interaction subsystem in the final state interaction (FSI) is included. Two values for the four momentum transferred Q2 are used. A small effect of the Roper resonance on the structure functions is found.

Transverse target single-spin asymmetry in inclusive electroproduction of charged pions and kaons

Single-spin asymmetries were investigated in inclusive electroproduction of charged pions and kaons from transversely polarized protons at the Hermes experiment. The asymmetries were studied as a function of the azimuthal angle ψ about the beam direction between the target-spin direction and the hadron production plane, the transverse hadron momentum PT relative to the direction of the incident beam, and the Feynman variable xF. The sinψ amplitudes are positive for π+ and K+, slightly negative for π− and consistent with zero for K−, with particular PT but weak xF dependences. Especially large asymmetries are observed for two small subsamples of events, where also the scattered electron was recorded by the spectrometer.

Scalar and Pseudo-Scalar Form Factors in Electro- and Weak-Production of Pion Near Threshold

We investigated pion production near threshold by the weak current in terms of multipole amplitudes. By exploiting the chiral Ward identity based on the QCD Lagrangian, we derived relevant multipole amplitudes in closed forms and presented their numerical results. In the amplitudes, scalar and pseudo scalar (PS) form factors, which represent the scalar and the PS quark density distributions, manifest by themselves. We applied these amplitudes to the cross sections for the weak- and electro-production near threshold. Both pion and PS form factor contributions are shown to account for the t-channel contribution in the charged pion electro-production near threshold. The asymmetry on the pion production by the neutrino and anti-neutrino is also discussed with their longitudinal and transverse cross sections.

Beam-Target Double-Spin AsymmetryALTin Charged Pion Production from Deep Inelastic Scattering on a Transversely PolarizedHe3Target at1.4&lt;Q2&lt;2.7 GeV2

We report the first measurement of the double-spin asymmetry A{LT} for charged pion electroproduction in semi-inclusive deep-inelastic electron scattering on a transversely polarized {3}He target. The kinematics focused on the valence quark region, 0.16<x<0.35 with 1.4<Q{2}<2.7 GeV{2}. The corresponding neutron A{LT} asymmetries were extracted from the measured {3}He asymmetries and proton over {3}He cross section ratios using the effective polarization approximation. These new data probe the transverse momentum dependent parton distribution function g{1T}{q} and therefore provide access to quark spin-orbit correlations. Our results indicate a positive azimuthal asymmetry for π{-} production on {3}He and the neutron, while our π{+} asymmetries are consistent with zero.

Semi-inclusive charged-pion electroproduction off protons and deuterons: Cross sections, ratios, and access to the quark-parton model at low energies

A large set of cross sections for semi-inclusive electroproduction of charged pions (π±) from both proton and deuteron targets was measured. The data are in the deep-inelastic scattering region with invariant mass squared W2>4 GeV2 (up to ≈7 GeV2) and range in four-momentum transfer squared 2<Q2<4 (GeV/c)2, and cover a range in the Bjorken scaling variable 0.2<x<0.6. The fractional energy of the pions spans a range 0.3<z<1, with small transverse momenta with respect to the virtual-photon direction, Pt2<0.2 (GeV/c)2. The invariant mass that goes undetected, Mx or W′, is in the nucleon resonance region, W′<2 GeV. The new data conclusively show the onset of quark-hadron duality in this process, and the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms. The x, z, and Pt2 dependences of several ratios (the ratios of favored-unfavored fragmentation functions, charged pion ratios, deuteron-hydrogen and aluminum-deuteron ratios for π+ and π−) have been studied. The ratios are found to be in good agreement with expectations based upon a high-energy quark-parton model description. We find the azimuthal dependences to be small, as compared to exclusive pion electroproduction, and consistent with theoretical expectations based on tree-level factorization in terms of transverse-momentum-dependent parton distribution and fragmentation functions. In the context of a simple model, the initial transverse momenta of d quarks are found to be slightly smaller than for u quarks, while the transverse momentum width of the favored fragmentation function is about the same as for the unfavored one, and both fragmentation widths are larger than the quark widths.19 MoreReceived 7 March 2011DOI:https://doi.org/10.1103/PhysRevC.85.015202©2012 American Physical Society

Beam spin asymmetry in deeply virtual π production

An interpretation of the beam spin azimuthal asymmetries measured at JLAB in deep exclusive electroproduction of charged and neutral pions is presented. The model combines a Regge pole approach with the effect of nucleon resonances. The s- and u-channel contributions are described using a dual Bloom-Gilman connection between the exclusive form factors and inclusive deep inelastic structure functions. The results are in agreement with data provided the excitations of nucleon resonances are taken into account.

Transverse single-spin asymmetries in inclusive hadron electroproduction at HERMES

Single-spin asymmetries were investigated in inclusive electroproduction of charged pions and kaons from a transversely polarized hydrogen target at the Hermes experiment. In the kinematic range pT < 3.0 GeV and −0.01 < xF < 1, positive asymmetries were measured for positive hadrons, while for negative hadrons they were found to be of smaller magnitude and with significantly different pT dependence for different bins in xF.

Transverse spin structure of the nucleon through target single-spin asymmetry in semi-inclusive deep-inelastic (e, e’ $ \pi^{\pm}_{}$ ) reaction at Jefferson Lab

Jefferson Lab (JLab) 12 GeV energy upgrade provides a golden opportunity to perform precision studies of the transverse spin and transverse-momentum-dependent structure in the valence quark region for both the proton and the neutron. In this paper, we focus our discussion on a recently approved experiment on the neutron as an example of the precision studies planned at JLab. The new experiment will perform precision measurements of target Single Spin Asymmetries (SSA) from semi-inclusive electro-production of charged pions from a 40-cm long transversely polarized $^3$He target in Deep-Inelastic-Scattering kinematics using 11 and 8.8 GeV electron beams. This new coincidence experiment in Hall A will employ a newly proposed solenoid spectrometer (SoLID). The large acceptance spectrometer and the high polarized luminosity will provide precise 4-D ($x$, $z$, $P_T$ and $Q^2$) data on the Collins, Sivers, and pretzelocity asymmetries for the neutron through the azimuthal angular dependence. The full 2$\pi$ azimuthal angular coverage in the lab is essential in controlling the systematic uncertainties. The results from this experiment, when combined with the proton Collins asymmetry measurement and the Collins fragmentation function determined from the e$^+$e$^-$ collision data, will allow for a quark flavor separation in order to achieve a determination of the tensor charge of the d quark to a 10% accuracy. The extracted Sivers and pretzelocity asymmetries will provide important information to understand the correlations between the quark orbital angular momentum and the nucleon spin and between the quark spin and nucleon spin.

Nucleon transversity study using a polarized 3He target in Hall A at JLab

A first measurement of Single target Spin Asymmetry (SSA) from semi-inclusive electro-production of charged pions from a transversely polarized 3He target in Deep-Inelastic-Scattering kinematics was completed in Hall A at Jefferson Lab in early 2009. Such SSA will allow for an extraction of the much desired information on the Collins and Sivers asymmetry from the “neutron” in order to probe the quark transversity distributions. Future 11 GeV measurements at Jefferson Lab in Hall A using a solenoid magnetic detector system is being planned.

Deep exclusive chargedπelectroproduction above the resonance region

A description of exclusive charged-pion electroproduction $(e,{e}^{\ensuremath{'}}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$ off nucleons at high energies is proposed. The model combines a Regge pole approach with the residual effect of nucleon resonances. The exchanges of $\ensuremath{\pi}(140)$, vector $\ensuremath{\rho}(770)$, and axial-vector ${a}_{1}(1260)$ and ${b}_{1}(1235)$ Regge trajectories are considered. The contribution of nucleon resonances is described using a dual connection between the exclusive hadronic form factors and inclusive deep inelastic structure functions. The model describes the measured longitudinal, transverse, and interference cross sections at the Thomas Jefferson Lab National Accelerator Facility (JLAB) and the Deutsches Elektronen Synchrotron. The scaling behavior of the cross sections is in agreement with JLAB and deeply virtual HERMES data. The results for a polarized beam-spin azimuthal asymmetry in $(\mathrm{eP\vec},{e}^{\ensuremath{'}}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$ are presented. Model predictions for JLAB at $12$ GeV are given.

CAN WE EXTRACT THE PION ELECTROMAGNETIC FORM FACTOR FROM A t-CHANNEL DIAGRAM ONLY?

We show that we are able to extract the pion electromagnetic form factor by using the recent charged pion electroproduction data from JLab and a simple t-channel diagram. For this purpose we have performed the Q2-independent and dependent analyses. The result of the first analysis is in good agreement with those of previous works and fit the Maris and Tandy model as well as the monopole parametrization which describes a pion radius of 0.672 fm. The result of the second analysis corroborates the findings in the first analysis. Our findings therefore provide a direct proof that at the given kinematics the t-channel diagram really dominates the process. This could also set a new constraint to the phenomenological models that try to describe the process.

Transverse momentum dependence of semi-inclusive pion production

Cross sections for semi-inclusive electroproduction of charged pions (π±) from both proton and deuteron targets were measured for 0.2<x<0.5, 2<Q2<4 GeV2, 0.3<z<1, and Pt2<0.2 GeV2. For Pt<0.1 GeV, we find the azimuthal dependence to be small, as expected theoretically. For both π+ and π−, the Pt dependence from the deuteron is found to be slightly weaker than from the proton. In the context of a simple model, this implies that the initial transverse momenta width of d quarks is larger than for u quarks and, contrary to expectations, the transverse momentum width of the favored fragmentation function is larger than the unfavored one.

Neutron transversity measurement at Jefferson Lab with a polarized 3He target

A first measurement of single target spin asymmetry (SSA) from semi-inclusive electroproduction of charged pions from a transversely polarized 3He target in deep-inelastic-scattering kinematics will take data in Hall A at Jefferson Lab in 2008. Such SSA will allow for an extraction of the much desired information on the Collins and Sivers asymmetry from the “neutron” in order to probe the quark transversity distributions. The experiment will be a coincidence measurement with the BigBite spectrometer detecting the scattered electrons and the left-arm HRS spectrometer detecting the charged pions. The prospect of future 11 GeV measurements at Jefferson Lab in Hall A using a solenoid magnetic detector system is also discussed in this talk.

Phenomenological studies of double charged pion electroproduction from the CLAS data

Recent results from phenomenological studies of the CLAS data on 2π electroproduction off proton are presented The analysis is focused on extracting of N → N* electromagnetic transition amplitudes from the full data set on unpolarized 1-differential 2π cross-sections.

From Inclusive Scattering to Exclusive Reactions – recent results from HERMES

Single-spin asymmetries for semi-inclusive electroproduction of charged pions in deep-inelastic scattering of positrons are measured for the first time with transverse target polarization. They provide access to the transversity distribution via the Collins mechanism and the previously unmeasured Sivers function. The comparison of this data to similar data taken with longitudinally polarized protons provides insight into subleading-twist contributions. A second class of reaction promising new insights into the structure of the nucleon, hard exclusive electroproduction of real photons (named Deeply Virtual Compton Scattering) or mesons closely linked to the novel formalism of Generalized Parton Distributions is studied. The data were accumulated by the HERMES experiment at DESY scattering the HERA 27.6 GeV electron or positron beam off longitudinally or transversely polarized hydrogen targets.

The pseudo-scalar form factor of the nucleon, the sigma-like term, and theL0+amplitude for charged pion electro-production near threshold

Abstract The pseudo-scalar form factor, which represents the pseudo-scalar quark density distribution due to finite quark masses on the nucleon, is shown to manifest itself with the induced pseudo-scalar form factor in the L 0 + amplitude for the charged pion electro-production. Both form factors show their own peculiar momentum dependence. Under the approximation on which the Goldberger–Treiman relation holds, a sum of both form factors' contributions accounts for the t-channel contribution in the charged pion electro-production near threshold.