A new beam asymmetry measurement from pion photoproduction on the neutron using CLAS

We present a preliminary analysis of the photon beam asymmetry observable （Z） from the pho- toproduction reaction channel γn → pπ in the invariant mass range 1.6--2.3 GeV. The measurement was obtained using the near-4π CEBAF Large Acceptance Spectrometer （CLAS） at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1 2.3 GeV, incident on a liquid deuterium target. The measurement will provide new data to address the poorly established neutron excitation spectrum and will greatly expand the sparse world data-set both in energy and angle.     

Measurement of R at VEPP-2000

The new e^＋e^- storage ring, VEPP-2000, is being commissioned at Budker Institute of Nuclear Physics, Novosibirsk. Measurement of the total cross section e^＋e^- → hadrons in the whole VEPP-2000 energy range is one of the main goals of the new experiments. We discuss the goals of the measurement and the expected systematic errors.     

Baryon resonance analysis from SAID

We discuss the analysis of data from πN elastic scattering and single pion photo- and electroproduction. The main focus is a study of low-lying non-strange baryon resonances. Here we concentrate on some difficulties associated with resonance identification, in particular the Roper and higher P11 states.     

πN Elastic Scattering and Resonances in Quark Potential Model

The quark potential model is used to investigate the low-energy elastic scattering of πN system. The model potential consists of the t-channel and s-channel one-gluon exchange potentials and the harmonic oscillator confining potential. By means of the resonating group method, a nonlocal effective potential for the πN system is derived from the interquark potentials and used to calculate the πN elastic scattering phase shifts. By considering the effect of QCD renormalization, the suppression of the spin-orbital coupling and the contribution of the color octet of the clusters （qq） and （qqq）, the numerical results are in fairly good agreement with the experimental data. The same model and method are employed to investigate the possible πN resonances. For this purpose, the resonating group equation is transformed into a standard Schrodinger equation in which the nonlocal effective πN interaction potential is included. Solving the Schrodinger equation by the variational method, we are able to reproduce the masses of some currently concerned πN resonances.     

Measurement of the pion form factor for Mππ^2 between 0.1 and 0.85 GeV2 with the KLOE detector

The KLOE experiment at the φ-factory DAФPNE has measured the pion form factor in the range between 0.1 〈 Mππ^2 〈 0.85 GeV^2 using events taken at √s = 1 GeV with a photon emitted at large polar angles in the initial state. This measurement extends the Mππ^2 region covered by KLOE ISR measurements of the pion form factor down to the two pion production threshold. The value obtained in this measurement of the dipion contribution to the muon anomalous magnetic moment of △αμ^ππ= （478.5±2-0stat±4.8syst±2.9theo）·10^-10 further confirms the discrepancy between the Standard Model evaluation for αμ and the experimental value measured by the （g-2） collaboration at BNL.     

Energy calibration of tagged photons by the d（γ,π~-pp） reaction

The energy of tagged photons, which were provided from the internal photon tagging system of the Laboratory of Nuclear Science, Tohoku University, has been calibrated using the d（γ,π-pp） reaction. Charged pions and protons in the final state were detected with the Neutral Kaon Spectrometer （NKS2）. Photon energies were obtained from the reaction of d（γ,π-pp）. The derived photon energy was consistent with the design of the tagger system and the previous measurement using electron-positron pair production. The consistency demonstrates the performance of NKS2 and the capability of the photon energy calibration using d（γ,π-pp）.     

Experiment of constraining symmetry energy at supra-saturation density with π^-/π^＋ at HIRFL-CSR

The possibility of the experiment for constraining the symmetry energy Esym（ρ） at supra-densities via π^-/π^＋ probe on the external target experiment of phase I （ ETE（I） ） with part coverage at forward angle at HIRFL-CSR is studied for the first time by using the isospin and momentum dependent hadronic transport model IBUU04. Based on the transport simulation with Au＋Au collisions at 400 MeV/u, it is found that the differential π^-/π^＋ ratios are more sensitive to Esym（ρ） at forward angles in laboratory reference, compared with the total yield ratio widely proposed. The insufficient coverage at lower transverse momentum maintains the sensitivity of the dependence of π^-/π^＋ ratio on the Esym（ρ） at high density, indicating that the ETF （I） under construction in Lanzhou provides the possibility of performing the experiment for probing the asymmetric nuclear equation of state.     

Precise measurement of the e^＋e^- →π^＋π^-（γ） cross section with the initial state radiation method at BABAR

We present a precise BABAR measurement on the cross section of the process e^＋e^- →π^＋π^-（γ） from threshold to an energy of 3 GeV with the initial state radiation （ISR） technique, using 232 fb^-1 of data collected with the BABAR detector at e^＋e^- center-of-mass energies near 10.58 GeV. The ISR luminosity is determined from a study of the leptonic process e^＋e^- →μ^＋μ^-γ（γ）. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the ππ cross section measured from threshold to 1.8 GeV is （514.1± 2.2（stat） ± 3.1（syst）） × 10^-10.     

Baryon resonance analysis from MAID

The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W = 2 GeV and both single-Q2 and Q2 dependent transition form factors could be obtained for the Delta, Roper, D13（1520）, S11 （1535）, S31 （1620）, $11（1650）, D15（1675）, F15（1680） and P13（1720）. From the complete world data base, including also π data on the neutron, also Q2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S11 and D13 resonances.     

π-electroproduction on hydrogen near threshold at four-momentum transfers of 0.2, 0.4 and 0.6 GeV

The reaction e + p → e + π⁰ + p was measured near the one pion threshold, detecting the final electron and proton in coincidence for values of q² = 0.2, 0.4 and 0.6 GeV². The slope of the cross section at threshold is determined. The data are compared with those of the e + p → e + π⁺ + n reaction, measured simultaneously, and with the results of pseudovector Born approximation and with dispersion theoretical calculations.     

Suppression factors for vector-meson-dominance couplings implied by duality

It is noted that recent experiments on φ decays indicate a suppression of the radiative decay by about a factor of 3 less than the vector-meson-dominance model predictions. FESR's on pion compton scattering amplitudes with identical t-channels but different direct channel resonances are shown to relate radiative decays of A₂ and ω. This again leads to a suppression factor of 3 to 4 for the A₂ radiative decay relative to the VMD prediction.     

The pion form factor at a timelike momentum transfer of approximately 1 fm from π electroproduction at threshold

From an extrapolation of threshold π⁺ electroproduction data combined with a kinematical constraint at an unphysical point, we derive the value of the pion form factor at a time-like value of the momentum transfer k²_π 1 fm⁻². The result is F_π(k²_π) = 1.19 ± 0.1. From this value, using a twice subtracted dispersion relation, we obtain r_π 0.98 ± 0.24 fm.     

Deeply virtual compton scattering at HERMES

Generalized Parton Distributions （GPDs） provide a way to access total angular momenta of partons and give a multidimensional picture of the nucleon structure.Deeply Virtual Compton Scattering （DVCS） is the most direct exclusive process to study GPDs.Different azimuthal cross-section asymmetries with respect to beam helicity,beam charge,and target polarization have been measured in the HERMES experiment.A recoil detector was installed at HERMES to directly detect the recoil proton.     

Rigorous pion-pion scattering lengths from threshold πN → ππN data

A new evaluation of the universal ππ scattering length relation is used to extract the ππ s-wave scattering lengths from threshold pion production data. Previous work has shown that the chiral perturbation series relating threshold pion production to ππ scattering lengths appears to converge well only for the isospin-2 case, giving a₂ = −0.031 ± 0.007m_π⁻¹. A model-independent and data-insensitive universal curve then implies a₀ = 0.235 ± 0.03m_π⁻¹ for the isospin-0 scattering length.     

Self energies of the pion and the Δ isobar from the He(e,e′π)H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the ³He(e,e′π⁺)³H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π⁺ mass of MeV/c² in the neutron-rich nuclear medium at a density of fm⁻³. Our data are consistent with the Δ self energy determined from measurements of π⁰ photoproduction from ⁴He and heavier nuclei.     

Charged Pion Form Factor Determination in the Range of Q^2= 0.6-1.6 （GeV／c）^2

Using the most recent differential cross section data for e-p quasi-elastic scattering, the charged pion formation and its form factor Fπ is calculated in the energy range of 2.4-4 GeV at Q^2 = 0.6-1.6 (GeV/c)^2. The functional dependence of the charged pion form factor to the separated cross section aL is investigated and compared to the previously determined result.     

Study of Pion-Source Size in Relativistic Nuclear Collisions

The BoseEinstein correlation of pion pairs is calculated using the space-time and four-momentum distributions Gom an event generator URASiMA for 200 AGeV/c Pb + Pb and compared to pion correlation measurement from CERN experiment of NA44. The spacetime distributions of the simulated pion-emitting source are also analyzed.     

Update on pion weak decay constants in nuclear matter

The QCD sum rule calculation of the in-medium pion decay constants using pseudoscalar–axial-vector correlation function,

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is revisited. In particular, we argue that the dimension 5 condensate, , which is crucial for splitting the time (f_t) and space (f_s) components of the decay constant, is not necessarily restricted to be positive. Its positive value is found to yield a tachyonic pion mass. Using the in-medium pion mass as an input, we fix the dimension 5 condensate to be around −0.025 GeV²−0.019 GeV². The role of the N and Δ intermediate states in the correlation function is also investigated. The N intermediate state is found not to contribute to the sum rules. For the Δ intermediate state, we either treat it as a part of the continuum or propose a way to subtract explicitly from the sum rules. With (and without) explicit Δ subtraction while allowing the in-medium pion mass to vary within 139 MeVm_π^*159 MeV, we obtain f_s/f_π=0.370.78 and f_t/f_π=0.630.79.     

Effects of meson exchange currents on pion absorption

The effect of the π exchange current on the pion absorption process π⁺ + d → p + p has been investigated. The absorption cross section is significantly enhanced particularly in the energy region above the 33 resonance.     

Source breakup dynamics in Au + Au collisions at sqrt[s(NN)]=200 GeV via three-dimensional two-pion source imaging

A three-dimensional correlation function obtained from midrapidity, low p(T), pion pairs in central Au+Au collisions at sqrt[s(NN)]=200 GeV is studied. The extracted model-independent source function indicates a long range tail in the directions of the pion pair transverse momentum (out) and the beam (long). A proper breakup time tau(0) ~ 9 fm/c and a mean proper emission duration Delta tau ~ 2 fm/c, leading to sizable emission time differences ({|Delta t(LCM)|} approximately 12 fm/c), are required to allow models to be successfully matched to these tails. The model comparisons also suggest an outside-in "burning" of the emission source reminiscent of many hydrodynamical models.