Search Results (26 total)

New results on the rare, doubly radiative decay η→π⁰γγ have been obtained from a revised analysis of the Crystal Ball experiment performed at the AGS. The analysis yields the first information on the dependence of the decay width, Γ(η→π⁰γγ), on the two-photon invariant mass squared, m²(γγ). A re-evaluation of the branching ratio is also made, BR(η→π⁰γγ)=(2.21±0.24_stat±0.47_syst)×10^-4; it implies that the decay width is Γ(η→π⁰γγ)=0.285±0.031_stat±0.061_syst eV. These results are close to predictions based on chiral perturbation theory with vector-meson dominance.

The reaction K^-p→Σ⁰π⁰→ neutrals was studied with the Crystal Ball detector at the BNL Alternating Gradient Synchrotron for beam momenta between 514 and 750 MeV/c. The photons from Σ⁰ and π⁰ decays were detected, as well as the neutron in a small fraction of the events. The differential cross section and the Σ⁰ polarization are reported at eight momenta and nearly the full angular range. Total cross sections are derived from Legendre polynomial fits to the data. The measurements presented are of considerably higher precision than previous results.

We report angular distribution measurements of the differential cross section σ(θ) and the analyzing powers iT₁₁(θ),T₂₀(θ), and T₂₂(θ) for deuteron-deuteron elastic scattering at 231.8 MeV. These data are compared to calculations based on the lowest order terms in the Born series expansion of the Alt-Grassberger-Sandhas equation for four nucleons interacting through the CD Bonn potential.

A new upper limit is presented for the branching ratio (BR) of η→3γ. This decay is not allowed by charge-conjugation invariance. We have determined that BR(η→3γ)<4×10^-5. We also obtained BR(η→π⁰γ→3γ)<9×10^-5; this decay mode is absolutely forbidden, as it violates conservation of angular momentum. Finally, we have established that BR(η→π⁰π⁰γγ)<1.2×10^-3 in a restricted diphoton-energy region. All three upper limits are at the 90% confidence level. They were obtained by using the Crystal Ball multiphoton spectrometer at the AGS and are based on the analysis of 28 million η mesons produced in the π^-p→ηn reaction close to threshold.

The branching ratio (BR) for the rare decay η→π⁰γγ was measured with the Crystal Ball multiphoton spectrometer. The result, BR(η→π⁰γγ)=(3.5±0.7_stat±0.6_syst)×10^-4, is in agreement with calculations of chiral perturbation theory to third order. We have used a sample of 28 million η mesons produced at the BNL Alternating Gradient Synchrotron using the π^-p→ηn reaction close to threshold. We detail the intricate extraction of the signal, which has about 1.6 thousand η→π⁰γγ events, from the overwhelming background of η→3π⁰ decays and from the π^-p→π⁰π⁰n reaction.

We report a new measurement of the differential cross section for π^-p→π⁰n from p_π=649 to 752 MeV/c, which is around the opening of the η channel (685 MeV/c). Our data support the main features of the π^-p charge-exchange differential cross sections generated by the SAID πN partial-wave analysis. The opening of the η channel has a clear effect on the shape of the excitation function for dσ(π^-p→π⁰n), which is most noticeable in the backward direction.

The differential cross section for η production in reaction π^-p→ηn has been measured over the full angular range at seven incident π^- beam momenta from threshold to p_π^-=747 MeV/c using the Crystal Ball multiphoton spectrometer. The angular distributions are S wave dominated. At 10 MeV/c above threshold, a small D-wave contribution appears that interferes with the main S wave. The total η production cross section σ^tot is obtained by integration of dσ/dΩ. Starting at threshold, σ^tot rises rapidly, as expected for S-wave-dominated production. The features of the π^-p→ηn cross section are strikingly similar to those of the SU(3) flavor-related process K^-p→ηΛ. Comparison of the π^-p→ηn reaction is made with η photoproduction.

We report on the first determination of upper limits on the branching ratio (BR) of η decay to π⁰π⁰γ and to π⁰π⁰π⁰γ. Both decay modes are strictly forbidden by charge conjugation (C) invariance. Using the Crystal Ball multiphoton detector, we obtained BR(η→π⁰π⁰γ)<5×10^-4 at the 90% confidence level, in support of C invariance of isoscalar electromagnetic interactions of the light quarks. We have also measured BR(η→π⁰π⁰π⁰γ)<6×10^-5 at the 90% confidence level, in support of C invariance of isovector electromagnetic interactions.

Differential cross sections for the process π⁻p→γn have been measured at Brookhaven National Laboratory’s Alternating Gradient Synchrotron with the Crystal Ball multiphoton spectrometer. Measurements were made at 18 pion momenta from 238 to 748  MeV∕c, corresponding to E_γ for the inverse reaction from 285 to 769  MeV. The data have been used to evaluate the γn multipoles in the vicinity of the N(1440) resonance. We compare our data and multipoles to previous determinations.

Reaction K⁻p→π⁰π⁰Σ⁰ was measured with the Crystal Ball multiphoton spectrometer at eight K⁻ momenta from 514 to 750  MeV∕c. Dynamics and the energy dependence of the reaction are shown by Dalitz plots, invariant mass spectra, production angular distributions, and total cross sections. The dipion invariant-mass spectra for K⁻p→π⁰π⁰Σ⁰ show an enhancement in the low mass region; the opposite is the case for the K⁻p→π⁰π⁰Λ and π⁻p→π⁰π⁰n reactions. There is no direct evidence for a f₀(600)-meson contribution to π⁰π⁰ production. Everywhere σ_t(K⁻p→π⁰π⁰Σ⁰)⪡σ_t(K⁻p→π⁰π⁰Λ).

Data for pion-single-charge exchange reactions at 750  MeV∕c on complex nuclei are analyzed for y-scaling, or single scattering quasifree, responses. The angular dependence of the data is used to separate the spin and nonspin isovector responses, with comparisons to electron scattering results.

Reaction K⁻p→π⁰π⁰Λ was measured at eight incident K⁻ momenta between 514 and 750  MeV∕c using the Crystal Ball multiphoton spectrometer. The reaction dynamics are displayed in total cross sections, Dalitz plots, invariant-mass spectra, production angular distributions, and the Λ polarization. The π⁰π⁰Λ production is dominated by the π⁰Σ⁰(1385) intermediate state; no trace of other light Σ^∗ states is observed, and the role of the f₀(600) meson appears to be insignificant. A striking similarity is seen between K⁻p→π⁰π⁰Λ and π⁻p→π⁰π⁰n; this can be understood as a consequence of dynamical flavor symmetry.

Reaction π⁻p→π⁰π⁰n has been measured with high statistics in the beam momentum range 270–750  MeV∕c. The data were obtained using the Crystal Ball multiphoton spectrometer, which has 93% of 4π solid angle coverage. The dynamics of the π⁻p→π⁰π⁰n reaction and the dependence on the beam energy are displayed in total cross sections, Dalitz plots, invariant-mass spectra, and production angular distributions. Special attention is paid to the evaluation of the acceptance that is needed for the precision determination of the total cross section σ_t(π⁻p→π⁰π⁰n). The energy dependence of σ_t(π⁻p→π⁰π⁰n) shows a shoulder at the Roper resonance [i.e., the N(1440)1 / 2⁺], and there is also a maximum near the N(1520)3 / 2⁻. It illustrates the importance of these two resonances to the π⁰π⁰ production process. The Dalitz plots are highly nonuniform; they indicate that the π⁰π⁰n final state is dominantly produced via the π⁰Δ⁰(1232) intermediate state. The invariant-mass spectra differ much from the phase-space distributions. The production angular distributions are also different from the isotropic distribution, and their structure depends on the beam energy. For beam momenta above 550  MeV∕c, the density distribution in the Dalitz plots strongly depends on the angle of the outgoing dipion system (or equivalently on the neutron angle). The role of the f₀(600) meson (also known as the σ) in π⁰π⁰n production remains controversial.

We report the first observation of the charge symmetry breaking d+d→⁴He+π⁰ reaction near threshold. Measurements using a magnetic channel (gated by two photons) of the ⁴He scattering angle and momentum (from time of flight) permitted reconstruction of the π⁰ “missing mass,” the quantity used to separate ⁴He+π⁰ events from the continuum of double radiative capture ⁴He+γ+γ events. We measured total cross sections for neutral pion production of 12.7±2.2   pb at 228.5 MeV and 15.1±3.1   pb at 231.8 MeV. The uncertainty is dominated by statistical errors. These cross sections arise fundamentally from the down-up quark mass difference and quark electromagnetic effects that contribute in part through meson mixing (e.g., π⁰-η) mechanisms.

Data are presented for the reaction π^-p→π⁰π⁰n in the range from threshold to p_π^-=750  MeV/c. The systematics of the data and multipole analyses are examined for sensitivity to a f₀(600) (“σ”) meson. A one-pion-exchange mechanism is found to be very weak, or absent. The reaction appears to become dominated by sequential π⁰ decays through the Δ(1232) resonance as the beam momentum increases, along with substantial interference effects from several competing mechanisms.

The results of a search for K^-p→π⁰π⁰π⁰Λ (where the 3π⁰’s are not from η-meson decay) are presented. The data were obtained with the Crystal Ball spectrometer at eight beam momenta from 514 to 750 MeV/c. For the six beam momenta below p_K^-=714 MeV/c, no signal was found; the 90% C.L. upper limit obtained for the K^-p→3π⁰Λ total cross section σ_t varies between 2 and 7 μb. This small upper limit is indicative that spontaneous π⁰ emission is insignificant, since the K^-p→π⁰π⁰π⁰Λ threshold is at p_K^-=397 MeV/c. A signal was observed only at p_K^-=750 MeV/c, with σ_t=25±7 μb. These results can be explained only if triple π⁰ production goes predominantly by hyperon resonance deexcitation, K^-p→Σ^*→π⁰Λ^*. There are several candidates for the Σ^* but only one for the Λ^*, namely, the Λ(1520)3 / 2^-, as the threshold for K^-p→π⁰Λ(1520) is at p_K^-=704 MeV/c.

We report a new measurement of the π^-p→3π⁰n total cross section from threshold to p_π=0.75 GeV/c. The cross section near the N(1535)1 / 2^- resonance is only a few μb after subtracting the large η→3π⁰ background associated with π^-p→ηn. A simple analysis of our data results in the estimated branching fraction B[S₁₁→πN(1440)1 / 2⁺]=(8±2)%. This is the first such estimate obtained with a three-pion production reaction.

The analyzing powers of π⁺ and π^- were measured using an incident 22-GeV/c transversely polarized proton beam at the Brookhaven Alternating Gradient Synchrotron. A magnetic spectrometer measured π^± inclusive asymmetries on a hydrogen and a carbon target. An elastic polarimeter with a CH₂ target measured pp elastic-scattering asymmetries to determine the beam polarization using published data for the pp elastic analyzing power. Using the beam polarization determined from the elastic polarimeter and asymmetries from the inclusive spectrometer, analyzing powers A_N for π^± were determined in the x_F and p_T ranges (0.45–0.8) and (0.3–1.2 GeV/c), respectively. The analyzing power results are similar in both sign and character to other measurements at 200 and 11.7 GeV/c, confirming the expectation that high-energy pion inclusive analyzing powers remain large and relatively energy independent. This suggests that pion inclusive polarimetry may be a suitable method for measuring future beam polarizations at BNL RHIC or DESY HERA. Analyzing powers of π⁺ and π^- produced on hydrogen and carbon targets are the same. Various models to explain inclusive analyzing powers are also discussed.

Recently the Crystal Ball Collaboration measured precise new data for the near-threshold reaction K^-p→ηΛ, which is dominated by formation of the Λ(1670)1 / 2^-. In this Letter, we present results of a unitary, multichannel analysis that incorporates the new Crystal Ball data. For our preferred fit, we obtain mass M  =  1673±2 MeV, width Γ  =  23±6 MeV, and elasticity x  =  0.37±0.07. This elasticity is significantly larger than previously recognized. Resonance parameters of our preferred fit are in striking agreement with the quark-model predictions of Koniuk and Isgur.

We have determined the quadratic slope parameter α for η→3π⁰ to be α  =  -0.031(4) from a 99% pure sample of 10⁶η→3π⁰ decays produced in the reaction π^-p→nη close to the η threshold using the Crystal Ball detector at the AGS. The result is four times more precise than the present world data and disagrees with current chiral perturbation theory calculations by about four standard deviations.

We present measurements of the differential and total cross sections and the Λ polarization for the reaction K^-p→ηΛ from threshold to p_K^-=770 MeV/c, with much better precision than previous measurements. Our cross-section data show a remarkable similarity to the SU(3) flavor-related π^-p→ηn cross-section results. The reaction K^-p→ηΛ at threshold is dominated by formation of the intermediate Λ(1670)1 / 2^- state.

Measurements at 18 beam kinetic energies between 1975 and 2795 MeV and at 795 MeV are reported for the pp elastic scattering spin correlation parameter A_00nn=(N,N;0,0)=C_NN=A_NN. The c.m. angular range is typically 60°-100°. These results are compared to previous data from Saturne II and other accelerators. A search for energy-dependent structure at fixed c.m. angles is performed. Comparisons are made to phase shift analysis and theoretical model predictions of this spin observable.

We report on an investigation of the (π^-,π⁰π⁰) reaction by means of measurements of the π⁰π⁰ invariant mass distributions from π^- interactions on H, D, C, Al, and Cu targets at p_π^-  =  0.408 GeV/c. The sharp, strong peak in the π⁺π^- invariant mass near 2m_π reported by the CHAOS Collaboration is not seen in our π⁰π⁰ data. However, we do observe a change in the shape of the π⁰π⁰ invariant mass spectrum for the different targets, indicating that the π⁰π⁰ interaction diminishes in the nuclear medium as represented by nuclei D, C, Al, and Cu, compared to hydrogen.

Measurements at 19 beam kinetic energies between 1795 and 2235 MeV are reported for the pp elastic scattering spin correlation parameter A_00nn=A_NN=C_NN. The c.m. angular range is typically 60–100°. The measurements were performed at Saturne II with a vertically polarized beam and target (transverse to the beam direction and scattering plane), a magnetic spectrometer and a recoil detector, both instrumented with multiwire proportional chambers, and beam polarimeters. These results are compared to previous data from Saturne II and elsewhere.

Measurements at 18 beam kinetic energies between 1975 and 2795 MeV and at 795 MeV are reported for the pp elastic-scattering single spin parameter A_ooon=A_oono=A_N=P. The c.m. angular range is typically 60–100°. These results are compared to previous data from Saturne II and other accelerators. A search for energy-dependent structure at fixed c.m. angles is performed, but no rapid changes are observed.