Strange quarks in the nucleon sea

The HAPPEX Collaboration measured parity-violating electron scattering from ⁴He$(e, e)$and H(e, e) in 2004 and 2005 for Q ²≤0.11GeV^2. Results for the strange-quark contributions to the electromagnetic form factors of the nucleon from the 2004 data will be reviewed. Preliminary results from the 2005 data, which have significantly greater statistical precision, are G _E ^s = 0.004±0.014_stat±0.013_syst for Q ² = 0.0772GeV^2 from the helium data and G _E ^s +0.088G _M ^s = 0.004±0.011_stat±0.005_syst±0.004_FF for Q ² = 0.1089GeV^2 from the hydrogen data.     

Extraction of G<Subscript>M</Subscript><Superscript>n</Superscript> from inclusive electron scattering on D, <Superscript>4</Superscript>He

We use the relation between Structure Functions (SFs) of nuclei A and nucleons N in order to fomulate a criterion which isolates the QE part out of the total inclusive cross-section. From data points around the QEP we extract the reduced neutron magnetic form factor 〈α _n = G _M ⁿ/μ _n G _d〉. The latter shows an unexpected decrease up to Q ² = 10GeV^2, the largest measured.     

Transverse beam asymmetries measured from <Superscript>4</Superscript>He and hydrogen targets

The HAPPEX Collaboration at Jefferson Lab has measured the transverse beam spin asymmetries (A_T) for elastic electron scattering from proton and ⁴He targets. The experiment was conducted using a vertically polarized electron beam of energy ∼ 3 GeV, at a Q ² ∼ 0.1 GeV^2 and a scattering angle θ_lab ∼ 6^° . The preliminary results are reported here. The ⁴He measurement is the first measurement of A_T from a nucleus. A_T for ⁴He is non-negligible; therefore, it will be necessary to make measurements of A_T for future parity-violating experiments using nuclear targets.     

The E158 experiment

We have carried out a precision measurement of the parity-violating asymmetry A _PV in the scattering of longitudinally polarized electrons off electrons in a liquid-hydrogen target. The measurement was performed with the 50GeV beam line at SLAC. The final result with the full data set collected in three production runs is A _PV = - 131±14 (stat) ±10 (syst) parts per billion. The result leads to new limits on possible contact interactions at the TeV scale. We discuss future prospects for more precise measurements.     

Extraction of the neutron charge form factor G<Superscript>n</Superscript><Subscript>E</Subscript>(Q<Superscript>2</Superscript>) from the charge form factor of deuteron G<Superscript>d</Superscript><Subscript>C</Subscript>(Q<Superscript>2</Superscript>)

We extract the neutron charge form factor G ⁿ _E(Q ²) from the charge form factor of deuteron G ^d _C(Q ²) obtained from T ₂₀(Q ²) data at 0≤Q ²≤ 1.717 (GeV/c)². The extraction is based on the relativistic impulse approximation in the instant form of the relativistic Hamiltonian dynamics. Our results (12 new points) are compatible with existing values of the neutron charge form factor of other authors. We propose a fit for the whole set (36 points) taking into account the data for the slope of the form factor at Q ² = 0. Received: 26 July 2002 / Accepted: 18 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: krutov@ssu.samara.ru RID="b" ID="b"e-mail: troitsky@theory.sinp.msu.ru Communicated by V.V. Anisovich     

Flavor content of nucleon form factors in a VMD approach

The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 Collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small 〈r ² _s〉_E = 0.005 fm^2 and to the magnetic moment it is positive μ_s = 0.315 μ_N .     

The pp→K<Superscript>+</Superscript><Emphasis Type="Italic">Σ</Emphasis><Superscript>+</Superscript>n cross-section from missing-mass spectra

We utilize existing inclusive data on K⁺-meson momentum spectra of the reaction pp→K ⁺ X at T _p = 2.3-2.85GeV to deduce total cross-sections for pp→K ⁺ Σ ⁺ n. The method used to extract those cross-sections is explained and discussed in detail. Our result for T _p = 2.85GeV is consistent with the data point from a direct measurement at the same beam energy. The cross-section obtained for T _p = 2.3GeV is with 13.7±2.3μb considerably smaller than the value found in a recent experiment by the COSY-11 Collaboration at a somewhat lower beam energy, indicating that the pp→K ⁺ Σ ⁺ n reaction cross-section could exhibit a rather unusual energy dependence.     

Fast helium production in interactions of 3.7 A GeV <Superscript>24</Superscript>Mg with emulsion nuclei

We have studied the properties of the relativistic helium fragments emitted from the projectile in the interactions of ²⁴Mg ions accelerated at an energy of 3.7 A GeV with emulsion nuclei. The total, partial nuclear cross-sections and production rates of helium fragmentation channels in relativistic nucleus-nucleus collisions and their dependence on the mass and energy of the incident projectile nucleus are investigated. The yields of multiple helium projectile fragments disrupted from the interactions of ²⁴Mg projectile nuclei with hydrogen H, light CNO and heavy AgBr groups of target emulsion nuclei are discussed and they indicate that the breakup mechanism of the projectile seems to be independent of the target mass. Limiting fragmentation behavior of fast-moving helium fragments is observed in both the projectile and target nuclei. The multiplicity distributions of helium projectile fragments emitted in the interactions of ²⁴Mg projectile nuclei with the different target nuclei of the emulsion are well described by the KNO scaling presentation. The mean multiplicities of the different charged secondary particles, normally defined shower, grey and black ( n_s, n_g and n_b) emitted in the interactions of 3.7 A GeV ²⁴Mg with the different groups of emulsion nuclei at different ranges of projectile fragments are decreasing when the number of He fragments stripped from projectile increases. These values of n_i ( i = s, g, b and h particles) in the events where the emission of fast helium fragments were accompanied by heavy fragments having Z 3 seem to be constant as the He multiplicity increases, and exhibit a behavior independent of the He multiplicity.     

A study of the radiative KL→π±e?νγ decay and search for direct photon emission with the KLOE detector

We present a measurement of the ratio R=Γ(K⁰ _e3γ;E^* _γ > 30 MeV, θ^* _γ>20^°)/Γ(K⁰ _e3) and a first measurement of the direct emission contribution in semileptonic K_L decays. The measurement was performed at the DA NE φ factory by selecting φ→K_LK_S decays with the KLOE detector. We use 328 pb^-1 of data, corresponding to about 3.5 million K⁰ _e3 events and about 9000 K⁰ _e3γ radiative events. Our result is R=(924±23_stat±16_syst)×10^-5 for the branching ratio and 〈X〉=-2.3±1.3_stat±1.4_syst for the effective strength parameter describing direct emission. PACS  13.20.Eb     

Λ and K s 0 production in pC collisions at 10 GeV/c

The experimental data from the 2-m propane bubble chamber have been analyzed for pC → Λ(K _s ⁰ )X reactions at 10 GeV/c. The estimation of experimental inclusive cross sections for Λ and K _s ⁰ production in the p ¹²C collision is equal to σ_Λ = (13.3 ± 1.7) mb and σ _{K s} ⁰ = (4.6 ± 0.6) mb, respectively. The measured 〈Λ〉/〈π⁺〉 ratio from pC reaction is equal to (5.3 ± 0.8) × 10⁻², and it is approximately two times larger than the 〈Λ〉/〈π⁺〉 ratio simulated by the FRITIOF model and than that of experimental pp reactions at the same energy. The text was submitted by the authors in English.     

The total yields of K *(892)+, Σ+(1385), and Σ0 in neutrino-induced reactions at ? E v? ≈ 10 GeV

Using the data obtained with the SKAT bubble chamber, the total yields of K*(892)⁺, Σ⁺(1385), and Σ⁰ are estimated for the first time in neutrino-induced reactions at moderate energy 〈E _v〉 = 10.4 GeV. It is shown that the recently observed enhancement of the K ⁰ and Λ yields in vA interactions as compared to vN interactions is contributed only slightly by the K*(892)⁺ and Σ⁺(1385) production. The contribution of resonances to the K ⁰ and Λ yields is found to be in qualitative agreement with higher energy (〈E _v〉 ≳ 40 GeV) data. It is shown that the energy dependence of the K*(892)⁺ mean multiplicity in vN interactions is approximately linear in the range of 〈E _v〉 ≈ 10−60 GeV, while that for Σ⁰ in vA interactions (A = 20−21) is approximately logarithmic in the range of 〈E _v〉 ≈ = 10−150 GeV. The text was submitted by the authors in English.     

Scalar-isovector K¯ production close to threshold

The reaction pp → dK ⁺ˉ has been investigated at excess energies Q = 47.4 and 104.7MeV above the K ⁺ˉ threshold at COSY Jülich. Coincident dK⁺ pairs were detected with the ANKE spectrometer, and subsequently ∼ 2000 events with a missing ˉ invariant mass were identified, which fully populate the Dalitz plot. The joint analysis of invariant mass and angular distributions reveals s-wave dominance between the two kaons, in conjunction with a p-wave between the deuteron and the kaon pair, i.e. Kˉ production via the a ₀ ⁺(980) channel. Integration of the differential distributions yields total cross-sections of σ(pp → dK ⁺ˉ) = (38±2_stat±14_syst)nb and (190±4_stat±39_syst)nb for the low and high Q value, respectively.     

Super-allowed α decay above doubly-magic <Superscript>100</Superscript>Sn and properties of <Superscript>104</Superscript>Te = <Superscript>100</Superscript>Sn ⊗ α

α-decay half-lives for ^{104, 105, 106}Te and ^{108, 109, 110}Xe close above the doubly-magic ¹⁰⁰Sn are calculated from systematic double-folding potentials. The derived α preformation factors are compared to results for ^{212, 213, 214}Po and ^{216, 217, 218}Rn above the doubly-magic ²⁰⁸Pb. α-decay energies of Q _α = 5.42±0.07MeV and 4.65±0.15MeV are predicted for ¹⁰⁴Te and ¹⁰⁸Xe; the corresponding half-lives are T _1/2 ≈ 5ns for ¹⁰⁴Te and of the order of 60μs for ¹⁰⁸Xe. Additionally, the properties of rotational bands in ¹⁰⁴Te are analyzed, and the first excited 2⁺ state in ¹⁰⁴Te is predicted at E _x = 650±40keV; it decays preferentially by γ emission with a reduced transition strength of 10 Weisskopf units to the ground state of ¹⁰⁴Te and with a minor branch by α emission to the ground state of ¹⁰⁰Sn.     

Search for exotic baryons at HERMES

An experimental search for the Θ(1540)- and Λ(1520)-resonance was performed in quasi-real photoproduction on deuterium at the HERMES experiment. While evidence for Θ(1540) was found in the decay channel pK ⁰ _S→pπ ⁺ π ^-, no evidence for the corresponding anti-particle was found. In some models it is expected that the Θ(1540) and the Λ(1520) have similar production mechanisms. The photoproduction cross-sections for the Λ(1520) in the decay channel Λ(1520)→pK ^- and the corresponding anti-particle are determined. The partial photoproduction cross-sections for Λ(1520) and ˉ(1520) are obtained as σ _Λ(1520) = 65.3±8.8(stat)±6.9(syst)nb and σ _ˉ(1520) = 9.8±2.6(stat)±0.9(syst)nb, corresponding to a ratio R _Λ(1520) = σ _ˉ(1520)/σ _Λ(1520) = 0.15±0.05(stat)±0.02(syst).     

Leading-order 2πγ exchange NN interaction: Central potentials proportional to g<Subscript>A</Subscript><Superscript>0</Superscript> and g<Subscript>A</Subscript><Superscript>2</Superscript>

We calculate at two-loop order in chiral perturbation theory the electromagnetic corrections to the leading-order 2π exchange NN interaction proportional to g _A ⁰ and g _A ². The resulting 2πγ exchange potential contains isospin-breaking components which reach up to about -2% of the corresponding isovector 2π exchange potential. With a value of only -17keV at r = m _π ^-1 = 1.4fm the charge-independence breaking central potential obtained here is negligibly small in comparison to the one generated by the isoscalar c₃ contact vertex. Our calculation confirms that the largest long-range isospin-violating NN potentials arise from the 2πγ exchange diagrams involving the large low-energy constants c ₄ ≃ - c ₃ ≃ 3.3GeV^-1 representing the important Δ(1232) dynamics.     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999     

Analysis and correction of the measurement of the neutron lifetime

Corrections have been introduced into the result τ_β = 885.4 ± 0.9_stat ± 0.4_syst s of our measurements of the neutron lifetime. The corrected value is τ_β = 881.6 ± 0.8_stat ± 1.9_syst s.     

Polarization of leading Λ hyperons produced by neutrons on target nuclei

The polarization of the leading Λ hyperons produced on carbon and lead target nuclei by 4–10 GeV neutrons in the angle range Θ<8.5° with respect to the beam and with only neutral-particle accompaniment is measured: 〈Π〉=−0.096±0.018 for C and 〈Π〉=− 0.128±0.047 for Pb. The dependence of the polarization on the transverse momentum and the Feynman variable is measured. The normalized invariant cross section as a function of p _⊥ ² is found to be approximated by the function A exp(−Bp _⊥ ² ), where the parameter B is independent of the kind of nucleus (B=8.71±0.09 (GeV/c)⁻² for carbon and B=8.83±0.18 (GeV/c)⁻² for lead). Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 237–240 (25 August 1996)     

High-spin states in <Superscript>36</Superscript>Ar and their underlying shell model configurations

Eight high-spin states in ³⁶Ar below 10MeV excitation energy, among them a prospective J ^π = 8^- state at 9408keV and the J? 8 levels of the recently discovered superdeformed rotational band, have been observed by n-γ coincidence measurements with the ³³S(α, nγ) reaction at E _α = 14.4 and 13.4MeV. High-spin assignments of, respectively, J ^π = 6⁺ and 5^- were obtained for the E _p = 1209 and 1462keV (E _x = 9682 and 9927keV) resonances of the ³⁵Cl (p,γ) reaction by a measurement of γ-ray angular distributions. The spectrum of the high-spin and of the E _x? 7.4MeV levels is decomposed according to the underlying shell model configurations with n = 0, 1, 2, 4 particles excited from the N = 2 into the N = 3 major shell. The role of four-particle excitations, all connected with large prolate distortions, is elucidated for the entire A = 36-40 mass region. Received: 21 December 2001 / Accepted: 25 March 2002