Search Results (51 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 tagged medium-energy neutron beam was used in a precise measurement of the absolute differential cross section for np backscattering. The results resolve significant discrepancies within the np database concerning the angular dependence in this regime. The experiment has determined the absolute normalization with ±1.5% uncertainty, suitable to verify constraints of supposedly comparable precision that arise from the rest of the database in partial wave analyses. The analysis procedures, especially those associated with the evaluation of systematic errors in the experiment, are described in detail so that systematic uncertainties may be included in a reasonable way in subsequent partial wave analysis fits incorporating the present results.

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 describe a double-scattering experiment with a novel tagged neutron beam to measure differential cross sections for np backscattering to better than ±2% absolute precision. The measurement focuses on angles and energies where the cross section magnitude and angle dependence constrain the charged pion-nucleon coupling constant, but existing data show serious discrepancies among themselves and with energy-dependent partial-wave analyses. The present results are in good accord with the partial-wave analyses, but deviate systematically from other recent measurements.

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.

Measured values of the differential cross section for pion-nucleon charge exchange, π⁻p→π⁰n, are presented at π⁻ momenta of 148, 174, 188, 212, 238, 271, 298, and 323  MeV∕c, a region dominated by the Δ(1232) resonance. Complete angular distributions were obtained using the Crystal Ball detector at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL). Statistical uncertainties of the differential cross sections are typically 2–6  %, exceptions being the results at the lowest momentum and at the most forward measurements at the five lowest momenta. We estimate the systematic uncertainties to be 3–6  %.

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.

Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, Au+Au collisions at sqrt[s_NN]=130   GeV, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freeze-out. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.

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 balance function is a new observable based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for charged particle pairs and identified charged pion pairs in Au+Au collisions at sqrt[s_NN]=130  GeV at the Relativistic Heavy Ion Collider using STAR. Balance functions for peripheral collisions have widths consistent with model predictions based on a superposition of nucleon-nucleon scattering. Widths in central collisions are smaller, consistent with trends predicted by models incorporating late hadronization.

Azimuthal correlations for large transverse momentum charged hadrons have been measured over a wide pseudorapidity range and full azimuth in Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. The small-angle correlations observed in p+p collisions and at all centralities of Au+Au collisions are characteristic of hard-scattering processes previously observed in high-energy collisions. A strong back-to-back correlation exists for p+p and peripheral Au+Au. In contrast, the back-to-back correlations are reduced considerably in the most central Au+Au collisions, indicating substantial interaction as the hard-scattered partons or their fragmentation products traverse the medium.

Azimuthal anisotropy (v₂) and two-particle angular correlations of high p_T charged hadrons have been measured in Au+Au collisions at sqrt[s_NN]=130  GeV for transverse momenta up to 6  GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high p_T partons. The monotonic rise of v₂(p_T) for p_T<2   GeV/c is consistent with collective hydrodynamical flow calculations. At p_T>3   GeV/c, a saturation of v₂ is observed which persists up to p_T=6   GeV/c.