Search Results (29 total)

Measured values of the differential cross sections for pion-nucleon charge exchange, π^-p→π⁰n, are presented for π^- momenta of 103,112,120,130,139,152, and 178 MeV/c. Complete angular distributions were obtained by using the Crystal Ball detector at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. Statistical uncertainties of the differential cross sections vary from 3 to 6% in the backward angle region and from 6 to about 20% in the forward region with the exception of the two most forward angles. The systematic uncertainties are estimated to be about 3% for all momenta.

Differential cross sections and hyperon polarizations have been measured for K̅ ⁰n, π⁰Λ, and π⁰Σ⁰ production in K^-p interactions at eight K^- momenta between 514 and 750 MeV/c. The experiment detected the multiphoton final states with the Crystal Ball spectrometer using a K^- beam from the BNL Alternating Gradient Synchrotron. The results provide significantly greater precision than the existing data, allowing a detailed reexamination of the excited hyperon states in our energy range.

We report the first measurements of the total cross section of the reaction K^-p→Σ⁰γ at eight beam momenta from 514 to 750 MeV/c. The data were obtained at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory using the Crystal Ball detector consisting of 672 NaI crystals. All charged particles were vetoed by a barrel of plastic scintillators, resulting in a study of only the neutral decays of the Σ⁰. The prompt photon and the photons from the decay products of Σ⁰ were detected for each reconstructed event; the neutron was also detected in a small fraction of the events.

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.

We present a new technique to continuously measure and compensate the global difference coupling coefficient through the continuous measurements of eigenmode projection parameters, using a high resolution phase-locked-loop tune meter. First, four eigenmode projection parameters are defined as the observables for weak difference coupling. Then, their analytical expressions are obtained using the strict matrix treatment and the Hamiltonian perturbation theory of linear coupling. From these parameters, the complex global coupling coefficient can be fully determined and compensated. This method was successfully demonstrated in the Relativistic Heavy Ion Collider (RHIC) 2006 run.

Simultaneous tune and coupling feedback were successfully implemented during RHIC run 6. In this paper we describe the measurement and control hardware and software used to accomplish this, present some of the results, discuss areas that require further investigation, and finally offer a few comments on possible implications of these results for LHC commissioning.

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.

We describe a fast and robust global decoupling scheme, coupling angle modulation. This novel technique introduces an extra rotating coupling coefficient into the coupled optics to determine the global decoupling strengths. The eigentune split is used as the observable during the modulation. The two eigentunes are tracked with a high-resolution phase locked loop tune measurement system. In the article, the principle of coupling angle modulation is presented, followed by its application to the Relativistic Heavy Ion Collider (RHIC). Coupling angle modulation coupling correction has been used for the global coupling correction on the nonstop RHIC ramp.

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.

Measurement of the residual betatron coupling with skew quadrupole modulation is a new diagnostics technique. It was developed and tested at the Relativistic Heavy Ion Collider (RHIC) as a promising method for measuring coupling on the ramp. By modulating the strengths of skew quadrupole families, the two tunes' responses are precisely measured with the phase lock loop system. The projection ratio of the residual coupling coefficient onto the coupling modulation direction can be determined. In this article, the analytical solution to the skew quadrupole modulation is given. Dedicated beam studies were carried out in RHIC Run'04 and the results are presented. The ability to measure coupling on the ramp opens the possibility of continuously correcting coupling during acceleration.

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 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.

We present new data on 2π⁰ production by 750 MeV/c π^- on hydrogen and the complex nuclei C, Al, and Cu, in search of the proposed medium modifications of the f₀(400-1200), or σ, state. We have found that the π⁰π⁰ invariant mass spectra for the four targets are different. The data indicate that nuclear absorption is mainly responsible for the change in the shape of the 2π⁰ invariant mass spectra for the complex targets. There is no hint of a narrow peak in the ππ invariant mass around 2m_π. We have also measured single π⁰ and η production on the above nuclei, in support of our investigation of nuclear final-state interaction processes. We observed strong η absorption in nuclei.

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.