Electroproduction Experiments Research Articles

N* Experiments and what they tell us about Strong QCD Physics

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their in ternal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their de pendences on Q2, especially on some of the more promi nent resonances, especially Δ(1232)3/2+, N(1440)1/2+, and negative parity states N(1535)1/2-, and N(1675)5/2-.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.

Measurements of the γvp→p′π+π− cross section with the CLAS detector for 0.4GeV2<Q2<1.0GeV2 and 1.3GeV<W<1.825GeV

New results on the single-differential and fully-integrated cross sections for the process $\gamma_{v} p \rightarrow p' \pi^{+} \pi^{-}$ are presented. The experimental data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the kinematic region of the reaction invariant mass $W$ from 1.3 to 1.825 GeV and the photon virtuality $Q^2$ from 0.4 to 1.0 GeV$^2$. The cross sections were obtained in narrow $Q^{2}$ bins (0.05 GeV$^{2}$) with the smallest statistical uncertainties achieved in double-pion electroproduction experiments to date. The results were found to be in agreement with previously available data where they overlap. A preliminary interpretation of the extracted cross sections, which was based on a phenomenological meson-baryon reaction model, revealed substantial relative contributions from nucleon resonances. The data offer promising prospects to improve knowledge on the $Q^{2}$-evolution of the electrocouplings of most resonances with masses up to $\sim$1.8 GeV.

N $$^*$$ ∗ Experiments and Their Impact on Strong QCD Physics

I give a brief overview of experimental studies of the spectrum and the structure of the excited states of the nucleon and what we learn about their internal structure. The focus is on the effort to obtain a more complete picture of the light-quark baryon excitation spectrum employing electromagnetic beams, and on the study of the transition form factors and helicity amplitudes and their dependence on the magnitude of the photon virtuality $Q^2$, especially for some of the most prominent resonances. The results were obtained in pion and eta electroproduction experiments off proton targets. They strengthen the connection of experiment and new results from modeling sQCD in DSE and Light Cone SR approaches. They also point to the nature of these states as 3-quark excitations at the core.

A short-orbit spectrometer for low-energy pion detection in electroproduction experiments at MAMI

A new Short-Orbit Spectrometer (SOS) has been constructed and installed within the experimental facility of the A1 collaboration at Mainz Microtron (MAMI), with the goal to detect low-energy pions. It is equipped with a Browne–Buechner magnet and a detector system consisting of two helium–ethane based drift chambers and a scintillator telescope made of five layers. The detector system allows detection of pions in the momentum range of 50–147 MeV/c, which corresponds to 8.7–63 MeV kinetic energy. The spectrometer can be placed at a distance range of 54–66 cm from the target center. Two collimators are available for the measurements, one having 1.8 msr aperture and the other having 7 msr aperture. The Short-Orbit Spectrometer has been successfully calibrated and used in coincidence measurements together with the standard magnetic spectrometers of the A1 collaboration.

Amplitude reconstruction from complete electroproduction experiments and truncated partial-wave expansions

We compare the methods of amplitude reconstruction, for a complete experiment and a truncated partial-wave analysis, applied to the electroproduction of pseudoscalar mesons. We give examples which show, in detail, how the amplitude reconstruction (observables measured at a single energy and angle) is related to a truncated partial-wave analysis (observables measured at a single energy and a number of angles). A connection is made to existing data.

On the binding energy and the charge symmetry breaking in A ≤ 16 Λ-hypernuclei

In recent years, several experiments using magnetic spectrometers provided high precision results in the field of Hypernuclear Physics. In particular, the accurate determination of the Λ-binding energy, BΛ, contributed to stimulate considerably the discussion about the Charge Symmetry Breaking effect in Λ-hypernuclei isomultiplets.We have reorganized the results from the FINUDA experiment and we have obtained a series of BΛ values for Λ-hypernuclei with A≤ 16 by taking into account data only from magnetic spectrometers implementing an absolute calibration of the energy scale (FINUDA at DAΦNE and electroproduction experiments at JLab and at MaMi). We have then critically revisited the results obtained at KEK by the SKS Collaboration in order to make possible a direct comparison between data from experiments with and without such an absolute energy scale. A synopsis of recent spectrometric measurements of BΛ is presented, including also emulsion experiment results.Several interesting conclusions are drawn, among which the equality within the errors of BΛ for the A=7,12,16 isomultiplets, based only on recent spectrometric data. This observation is in nice agreement with a recent theoretical prediction.Ideas for possible new measurements which should improve the present experimental knowledge are finally put forward.

Nucleon and Roper electromagnetic elastic and transition form factors

We compute nucleon and Roper e.m. elastic and transition form factors using a symmetry-preserving treatment of a contact-interaction. Obtained thereby, the e.m. interactions of baryons are typically described by hard form factors. In contrasting this behaviour with that produced by a momentum-dependent interaction, one achieves comparisons which highlight that elastic scattering and resonance electroproduction experiments probe the infrared evolution of QCD's running masses; e.g., the existence, and location if so, of a zero in the ratio of nucleon Sachs form factors are strongly influenced by the running of the dressed-quark mass. In our description of baryons, diquark correlations are important. These correlations are instrumental in producing a zero in the Dirac form factor of the proton's d-quark; and in determining d_v/u_v(x=1), as we show via a formula that expresses d_v/u_v(x=1) in terms of the nucleon's diquark content. The contact interaction produces a first excitation of the nucleon that is constituted predominantly from axial-vector diquark correlations. This impacts greatly on the gamma*p->P_{11}(1440) form factors. Notably, our quark core contribution to F_2*(Q^2) exhibits a zero at Q^2~0.5mN^2. Faddeev equation treatments of a hadron's quark core usually underestimate its magnetic properties, hence we consider the effect produced by a dressed-quark anomalous e.m. moment. Its inclusion much improves agreement with experiment. On the domain 0<Q^2<2GeV^2, meson-cloud effects are important in making a realistic comparison between experiment and hadron structure calculations. Our computed helicity amplitudes are similar to the bare amplitudes in coupled-channels analyses of the electroproduction process. Thus supports a view that extant structure calculations should directly be compared with the bare-couplings, etc., determined via coupled-channels analyses.

Strange hadronic physics in electroproduction experiments at the Mainz Microtron

Present and future research into the electroproduction of strangeness plays an important role at Mainz Microtron MAMI. With the Kaos spectrometer for kaon detection operated in the multi-spectrometer facility first cross section measurements of the exclusive p ( e , e ′ K + ) Λ , Σ 0 reactions at low-momentum transfers have been performed. These measurements have clearly discriminated between effective Lagrangian models for photo- and electroproduction of strangeness. Recently, the Kaos spectrometer was upgraded to a double-arm spectrometer for the measurement of elementary cross sections at very forward scattering angles and for the missing mass spectroscopy of hypernuclear states. In parallel, pioneering experiments on decay-pion spectroscopy of electroproduced hypernuclei were carried out at MAMI. Future experiments will on the one side address the cross section of the elementary process with polarized beam and on the other side continue the hypernuclear spectroscopy with different light nuclear targets.

One pion production in neutrino reactions: Including nonresonant background

We investigate neutrino induced one pion production on nucleons. The elementary neutrino--nucleon cross section is calculated as the sum of the leading Delta pole diagram and several background diagrams obtained within the non-linear sigma model. This approach does not introduce any new adjustable parameters, which allows unambiguous predictions for the observables. Considering electroproduction experiments as benchmark, the model is shown to be applicable up to pion-nucleon invariant mass $W<1.4\GeV$ and provides a good accuracy. With respect to the total one pion cross section, the model predicts the background at the level of $=10% for the $p\pi^+$, 30% for $p\pi^0$ and 50% for $n\pi^+$ final states. The results are compared with experimental data for various differential cross sections. Distributions with respect to muon-nucleon and muon-pion invariant masses are presented for the first time. The model describes the data quite well, with the discrepancies being of the same order as those between different data sets.

Baryon spectroscopy

About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation. In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood. Heavy-quark baryons suffer from a lack of known spin-parities. In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood. At high masses, above 1.8 GeV, quark models predict a very high density of resonances per mass interval which is not observed. In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories.

Spectroscopy of Electro- and Photo-Productions of Hypernuclei

Recently hypernuclear electroproduction experiments have been successfully carried out at the Thomas Jefferson Laboratory, providing high-resolution spectroscopic data. This article discusses several facets inherent to this reaction process: First, numerous theoretical attempts in describing the elementary kaon photoproduction are summarized so as to choose appropriate elementary amplitudes to be used for hypernuclear production estimates. The reaction process is characterized by both the sizable momentum transfer and the spin-flip dominace involved in the transition operator. The DWIA theoretical excitation spectra of producing p-shell hypernucli are discussed together with the experimental ones obtained recently. Encouraged with the successful theory-experiment comparison, the calculational framework has been applied beyond p-shell to typical medium-heavy hypernuclear production. The hypernuclear photo-production spectra predicted with the F, Si, and Ca targets suggest that the unique characters of the electroproduction are more clearly appeared in the medium-mass region, providing interesting opportunities of spectroscopic study beyond p-shell.

Extraction of the axial nucleon form factor from neutrino experiments on deuterium

We present new parameterizations of vector and axial nucleon form factors. We maintain an excellent descriptions of the form factors at low momentum transfers (Q2), where the spatial structure of the nucleon is important, and use the Nachtman scaling variable ξ to relate elastic and inelastic form factors and impose quark-hadron duality constraints at high Q2 where the quark structure dominates. We use the new vector form factors to re-extract updated values of the axial form factor from νμ experiments on deuterium. We obtain an updated world average value from νμ d, v̄μH and pion electroproduction experiments of MA = 1.014 ± 0.014 GeV/c2. Our parameterizations are useful in modeling v interactions at low energies (e.g. for vμ oscillations experiments). The predictions for high Q2 can be tested in the next generation electron and vμ scattering experiments. (Presented by A. Bodek at the European Physical Society Meeting, EPS2007, Manchester, UK, July 2007).

Vector and axial nucleon form factors: A duality constrained parameterization

We present new parameterizations of vector and axial nucleon form factors. We maintain an excellent descriptions of the form factors at low momentum transfers, where the spatial structure of the nucleon is important, and use the Nachtman scaling variable xi to relate elastic and inelastic form factors and impose quark-hadron duality constraints at high momentum transfers where the quark structure dominates. We use the new vector form factors to re-extract updated values of the axial form factor from neutrino experiments on deuterium. We obtain an updated world average value from neutrino-d and pion electroproduction experiments of M_A = 1.014 +- 0.014 GeV/c2. Our parameterizations are useful in modeling neutrino interactions at low energies (e.g. for neutrino oscillations experiments). The predictions for high momentum transfers can be tested in the next generation electron and neutrino scattering experiments.

Exclusive photoproduction ofJ/ψin proton-proton and proton-antiproton scattering

Protons and antiprotons at collider energies are a source of high-energy Weizsaecker-Williams photons. This may open a possibility to study exclusive photoproduction of heavy vector mesons at energies much larger than possible at the HERA accelerator. Here we present a detailed investigation of the exclusive J/{psi} photoproduction in proton-proton (Relativistic Heavy Ion Collider, CERN LHC) and proton-antiproton (Tevatron) collisions. We calculate several differential distributions in t{sub 1}, t{sub 2}, y, {phi}, as well as transverse momentum distributions of J/{psi}'s. We discuss correlations in the azimuthal angle between outgoing protons or proton and antiproton as well as in the (t{sub 1},t{sub 2}) space. Differently from electroproduction experiments, here both colliding beam particles can be a source of photons, and we find large interference terms in azimuthal-angle distributions in a broad range of rapidities of the produced meson. We also include the spin-flip parts in the electromagnetic vertices. We discuss the effect of absorptive corrections on various distributions. Interestingly, absorption corrections induce a charge asymmetry in rapidity distributions, and are larger for pp reactions than for the pp case. The reaction considered here constitutes an important nonreduceable background in recently proposed searches for odderon exchange.

Role of the ρ meson in the description of pion electroproduction experiments

We study the $p(e,{e}^{'}{\ensuremath{\pi}}^{+})n$ reaction in the framework of an effective Lagrangian approach including nucleon, \ensuremath{\pi} and \ensuremath{\rho} meson degrees of freedom and show the importance of the \ensuremath{\rho}-meson $t$-pole contribution to ${\ensuremath{\sigma}}_{T}$, the transverse part of cross section. We test two different field representations of the \ensuremath{\rho} meson, vector and tensor, and find that the tensor representation of the \ensuremath{\rho} meson is more reliable in the description of the existing data. In particular, we show that the \ensuremath{\rho}-meson $t$-pole contribution, including the interference with an effective nonlocal contact term, sufficiently improves the description of the recent JLab data at invariant mass $W\ensuremath{\lesssim}2.2$ GeV and ${Q}^{2}\ensuremath{\lesssim}2.5$ GeV${}^{2}/{c}^{2}$. A ``soft'' variant of the strong $\ensuremath{\pi}\mathit{NN}$ and $\ensuremath{\rho}\mathit{NN}$ form factors is also found to be compatible with these data. On the basis of the successful description of both the ${\ensuremath{\sigma}}_{L}$ and ${\ensuremath{\sigma}}_{T}$ parts of the cross section we discuss the importance of taking into account the ${\ensuremath{\sigma}}_{T}$ data when extracting the charge pion form factor ${F}_{\ensuremath{\pi}}$ from ${\ensuremath{\sigma}}_{L}$.

Search for Pentaquarks at BaBar

The results of searches for the strange pentaquark states, Θ5(1540)+, Ξ5(1860)−− and Ξ5(1860)0 in data recorded by the BaBar experiment are presented. We search for these three states inclusively in 123.4 fb−1 of e+e− annihilation data produced at the PEP-II asymmetric storage rings; we find no evidence for their production in any physics process, and set limits on their production rates that are well below the measured rates for conventional baryons. We also search for Θ5(1540)+ produced in interactions of electrons or hadrons in the material of the inner part of the detector. No evidence for this state is found in a sample with much higher statistics than similar electroproduction experiments that claim a signal.

The Bonn Electron Stretcher Accelerator ELSA: Past and future

In 1953, it was decided to build a 500MeV electron synchrotron in Bonn. It came into operation 1958, being the first alternating gradient synchrotron in Europe. After five years of performing photoproduction experiments at this accelerator, a larger 2.5GeV electron synchrotron was built and set into operation in 1967. Both synchrotrons were running for particle physics experiments, until from 1982 to 1987 a third accelerator, the electron stretcher ring ELSA, was constructed and set up in a separate ring tunnel below the physics institute. ELSA came into operation in 1987, using the pulsed 2.5GeV synchrotron as pre-accelerator. ELSA serves either as storage ring producing synchrotron radiation, or as post-accelerator and pulse stretcher. Applying a slow extraction close to a third integer resonance, external electron beams with energies up to 3.5GeV and high duty factors are delivered to hadron physics experiments. Various photo- and electroproduction experiments, utilising the experimental set-ups PHOENICS, ELAN, SAPHIR, GDH and Crystal Barrel have been carried out. During the late 90's, a pulsed GaAs source of polarised electrons was constructed and set up at the accelerator. ELSA was upgraded in order to accelerate polarised electrons, compensating for depolarising resonances by applying the methods of fast tune jumping and harmonic closed orbit correction. With the experimental investigation of the GDH sum rule, the first experiment requiring a polarised beam and a polarised target was successfully performed at the accelerator. In the near future, the stretcher ring will be further upgraded to increase polarisation and current of the external electron beams. In addition, the aspects of an increase of the maximum energy to 5GeV using superconducting resonators will be investigated.

Future hypernuclear physics at MAMI-C and PANDA-GSI

Hypernuclei represent the first steps towards an extension of the periodic system into the sector of strangeness and thus add a third dimension to our evolving picture of nuclei. They provide a large variety of new and exciting perspectives ranging from new dynamical symmetries in hypernuclei spectra, non-mesonic weak decays, and the interplay of the quark-exchange and meson-exchange aspects of strong baryon–baryon forces in the flavor SU ( 3 ) world. Furthermore, double hypernuclei may provide a doorway towards exotic quark states. In the near future, electroproduction experiments at TJNAF and MAMI-C will add more detailed information on the structure of single hypernuclei states. On the long term, experiments at the Japanese hadron facility and the high energy storage ring for antiprotons at the future GSI accelerator facility will allow us to extend high resolution γ-ray studies also into the domain of double hypernuclei.

Deep electroproduction of exotic hybrid mesons

We evaluate the leading order amplitude for the deep exclusive electroproduction of an exotic hybrid meson in the Bjorken regime. We show that, contrary to naive expectation, this amplitude factorizes at the twist 2 level and thus scales like usual meson electroproduction when the virtual photon and the hybrid meson are longitidinally polarized. Exotic hybrid mesons may thus be studied in electroproduction experiments at JLAB, HERA (HERMES) or CERN (COMPASS).

Perturbative color transparency in electroproduction experiments

We calculate quasi-exclusive scattering of a virtual photon and a proton or pion in nuclear targets. This is the first complete calculation of ``color transparency'' and ``nuclear filtering'' in perturbative QCD. The calculation includes full integrations over hard interaction kernels and distribution amplitudes in Feynman-$x$ fractions and transverse spatial separation space b. Sudakov effects depending on b and the momentum transfer ${Q}^{2}$ are included. Attenuation of the hadronic states propagating through the medium is calculated using an eikonal Glauber formalism. Nuclear correlations are included explicitly. We find that the color transparency ratio is comparatively insensitive to theoretical uncertainties inherent in perturbative formalism, such as choice of infrared cutoff scales. However, the ${Q}^{2}$ dependence of the transparency ratio is found to depend sensitively on the model of the distribution amplitude, with end-point-dominated models failing to be dominated by short distance. Color transparency experiments should provide an excellent test of the underlying theoretical assumptions used in the PQCD calculations.