Search Results (30 total)

Experiment 865 at the Brookhaven AGS has observed the decay K⁺→e⁺ν_eμ⁺μ^-. The branching ratio extracted is (1.72±0.37(stat)±0.17(syst)±0.19(model))×10^-8 where the third term in the error results from the use of a model to extrapolate into a kinematic region dominated by background.

Based on results of a search for the lepton-family-number-violating decay K⁺→π⁺μ⁺e^- with data collected by experiment E865 at the Alternating Gradient Synchrotron of Brookhaven National Laboratory, we place an upper limit on the branching ratio at 2.1×10^-11 (90% C.L.). Combining the results with earlier E865 data and those of a previous experiment, E777, an upper limit on the branching ratio of 1.3×10^-11 (90% C.L.) is obtained.

E865 at the Brookhaven National Laboratory AGS collected about 70 000 K_e3⁺ events to measure the K_e3⁺ branching ratio relative to the observed K⁺→π⁺π⁰, K⁺→π⁰μ⁺ν, and K⁺→π⁺π⁰π⁰ decays. The π⁰ in all the decays was detected using the e⁺e^- pair from π⁰→e⁺e^-γ decay and no photons were required. Using the 2002 Particle Data Group branching ratios for the normalization decays, we obtain BR(K_e3(γ)⁺)=(5.13±0.02_stat±0.09_syst±0.04_norm)%, where K_e3(γ)⁺ includes the effect of virtual and real photons. This result is ≈2.3σ higher than the current Particle Data Group value. Implications for the V_us element of the CKM matrix, and the matrix’s unitarity are discussed.

We report experimental details and results of a new measurement of the decay K⁺→π⁺π^-e⁺ν_e(K_e4). A sample of more than 400,000 K_e4 events with low background has been collected by Experiment 865 at the Brookhaven Alternate Gradient Synchrotron. From these data, the branching ratio (4.11±0.01±0.11)×10^-5 and the ππ invariant mass dependence of the form factors F, G, and H of the weak hadronic current as well as the phase shift difference δ₀⁰-δ₁¹ for ππ scattering were extracted. Using constraints based on analyticity and chiral symmetry, a new value with considerably improved accuracy for the s-wave ππ scattering length a₀⁰ has been obtained also: a₀⁰=0.216±0.013 (stat)±0.002 (syst)±0.002 (theor).

Experiment 865 at the Brookhaven Alternating Gradient Synchrotron obtained 410 K⁺→e⁺νe⁺e^- and 2679 K⁺→μ⁺νe⁺e^- events including 10% and 19% background. The branching ratios were measured to be [2.48±0.14(stat)±0.14(syst)]×10^-8 (m_ee>150   MeV) and (7.06±0.16±0.26)×10^-8 (m_ee>145   MeV), respectively. Results for the decay form factors are presented.

A sample of 4×10⁵ events from the decay K⁺→π⁺π^-e⁺ν_e  (K_e4) has been collected in experiment E865 at the Brookhaven Alternating Gradient Synchrotron. The analysis of these data yields new measurements of the K_e4 branching ratio [(4.11±0.01±0.11)×10^-5], the s-wave ππ scattering length [a₀⁰  =  0.216±0.013(stat)±0.004(syst)±0.005(theor)], and the form factors F, G, and H of the hadronic current and their dependence on the invariant ππ mass.

A search for lepton flavor violating decays, K⁺→μ⁺μ⁺π^-, K⁺→e⁺e⁺π^-, K⁺→π⁺e⁺μ^-, K⁺→μ⁺e⁺π^-, and π⁰→e⁺μ^-, was performed using the data collected in Experiment E865 at the Brookhaven Alternating Gradient Synchrotron. No signal was found in any of the decay modes. At the 90% confidence level, the branching ratios are less than 3.0×10^-9, 6.4×10^-10, 5.2×10^-10, 5.0×10^-10, and 3.4×10^-9, respectively.

We report results of a search for the lepton-family number violating decay K⁺→π⁺μ⁺e^- from data collected by experiment E865 in 1996 at the Alternating Gradient Synchroton of Brookhaven National Laboratory. We place an upper limit on the branching ratio at 3.9×10^-11 ( 90% C.L.). Together with results based on data collected in 1995 and an earlier experiment, E777, this result establishes a combined 90% confidence level upper limit on the branching ratio at 2.8×10^-11. We also report a new upper limit on the branching ratio for π⁰→μ⁺e^- of 3.8×10^-10 ( 90% C.L.).

More than 400 K⁺→π⁺μ⁺μ^- events were observed in a rare K⁺ decay experiment at the Brookhaven Alternating Gradient Synchrotron. Normalized to the K⁺→π⁺π⁺π^- decay, the branching ratio is determined to be [9.22±0.60(stat)±0.49(syst)]×10^-8. This branching ratio and the μμ mass spectrum is in very good agreement with the measurement of the K⁺→π⁺e⁺e^- decay, but deviates significantly from the previous measurement.

A low-background sample of 10 300 events has been collected for the decay mode K⁺→π⁺e⁺e^- by experiment E865 at the Brookhaven Alternating Gradient Synchrotron. The branching ratio is measured to be [2.94±0.05(stat)±0.13(syst)±0.05(model)]×10^-7, the vector nature of this decay is firmly established, and the decay form factor f(z)  =  f₀(1+δz) (z  =  M_ee²/m_K²) is determined to have δ  =  2.14±0.13±0.15. The experiment is described and results discussed in the context of chiral perturbation theory.

We have conducted a search for bound states of a negative pion and a number of neutrons (pineuts) using the E814 spectrometer. A beam of ²⁸Si at a momentum of 14.6A GeV/c was used to bombard targets of Al, Cu, Sn, and Pb. We describe our experimental technique, present measured upper limits for pineut production, and discuss the significance of our results.

We present the transverse energy distributions, dσ/dE_T and dE_T/dη, and the charged particle multiplicity distributions, dσ/dN_c and dN_c/dη, produced in p+Al and p+Pb collisions at 14.6 GeV/c. The data exhibit a weak correlation between these global variables. While a significant increase of the mean multiplicity with the mass of the target is observed, the transverse energy distributions show little target dependence. The dE_T/dη distribution shifts backward as the mass of the target increases, indicating the presence of rescattering. The data are compared to the predictions of the relativistic quantum molecular dynamic model (RQMD) and Fritiof event generators. RQMD reproduces well the main features of the data while Fritiof predicts too forward peaked transverse energy and particle multiplicity pseudorapidity distributions.

An investigation of the production of neutron-rich isotopes from the fragmentation of ²⁸Si projectiles at p_lab=14.6 GeV/c per nucleon was performed using the BNL-AGS-E814 spectrometer. We have measured the inclusive production cross sections of neutron-rich fragments (⁶He, ⁸He, ⁸Li, ⁹Li, ¹⁰Be, ¹¹Be, and ¹³B). We have also measured the transverse momentum distributions for ⁶He and ⁹Li, and the forward and transverse energy distributions associated with ⁶He production. The momentum distributions were analyzed in the context of the Goldhaber model. The question of whether the fragments are produced in the decay of the projectile following its electromagnetic excitation was also investigated.

We have studied in detail the electromagnetic dissociation of ²⁸Si projectiles at 14.6 GeV/(c nucleon), interacting with Pb, Sn, Cu, and Al targets. Exclusive cross sections were measured for several decay channels, including final states involving the emission of protons, neutrons, and α particles. Excitation energy distributions for the 1n+²⁷Si and 2p+²⁶Mg decay channels were reconstructed with a resolution of 2 MeV, using a constrained kinematic fit. The energy distributions obtained for 1n+²⁷Si are in good agreement with the σ(γ,n) photoneutron cross sections multiplied by the virtual photon spectrum obtained in the Weizsäcker-Williams approximation. A search for the double photon excitation process, based on the dependence of the cross sections on the target atomic number, was performed.

Inclusive double differential multiplicities d²N/dy dp_t and related quantities have been measured for protons and deuterons produced in 14.6A GeV/c Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision ‘‘centrality’’ is determined by measuring N_c, the total charged particle multiplicity in the pseudorapidity range 0.85<η<3.8. For both systems Si + Al and Si + Pb, an increase in the proton rapidity distribution dN/dy at midrapidity and a corresponding decrease at higher rapidities are observed with increasing N_c. For Si+Pb, Boltzmann slope parameters T_B increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when σ/σ_geo≲10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution dN/dy is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with N_c. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.

We have measured cross sections for the synthesis of nuclei of mass A≤4 in collisions of 14.6A GeV/c ²⁸Si nuclei with targets of Pb, Cu, and Al. The data are measured at close to center-of-mass rapidities, and are unique in their exploration of the centrality dependence of nucleosynthesis. Simple coalescence models that were used to study nucleosynthesis at lower energies are inadequate for the description of our measurements. Our data and improved models are used to extract parameters related to the size of the interaction volume at freeze-out.

We present the results of an analysis of two charged particle and transverse energy correlations in Si+Pb collisions at BNL AGS at 14.6 GeV/c per nucleon. The measured semi-inclusive normalized two particle pseudorapidity corrleation function exhibits short range correlations similar to the correlations observed in hadron-hadron and hadron-nucleus collisions at higher energies, although the observed correlations are smaller than the values scaled from hp and hA data. Estimates, provided by the observed correlations, of the intermittency indices as well as of the parameters of the cluster model are presented. Predictions using the fritiof event generator, which at this level of statistical accuracy show no pseudorapidity correlations, are not in agreement with our data. Azimuthal angle two particle correlations show nonzero back-to-back correlations in the central region (consistent with fritiof predictions) and are almost flat in the projectile fragmentation region. We also present results on the transverse energy azimuthal correlation function, which are similar to those from the two particle correlation function.

We present a systematic study of transverse energy (E_T) production in collisions of 11.4A GeV/c Au and 14.6A GeV/c Si ions with targets of Al, Au, and Pb. Comparison of data for Au+Au and Si+Al indicates that, for the heavier system, there is an increase in the amount of stopping which is accompanied by a decrease in the width of the dE_T/dη distribution. The ratio of the maximum E_T observed for the two systems is significantly greater than the ratio of the total energy available in the center of mass frame.

We have measured antiproton production cross sections as functions of centrality in collisions of 14.6 GeV/c per nucleon ²⁸Si ions with targets of Al, Cu, and Pb. For all targets, the antiproton yields increase linearly with the number of projectile nucleons that have interacted, and show little target dependence. We discuss the implications of this result on the production and absorption of antiprotons within the nuclear medium.

Collisions of ²⁸Si+Al, Cu, and Pb at E_lab=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ²⁸Si into p+ ²⁷Al at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-²⁷Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ²⁸Si and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ²⁸Si(γ,p) ²⁷Al, and slight differences are observed only at low final-state energy.

We present the rapidity and transverse momentum distributions of protons and neutrons from collisions between 14.6 GeV/nucleon beams of ²⁸Si and targets of Al, Cu, and Pb. The data were measured in the forward spectrometer/target calorimeter detectors of the E814 apparatus. The results indicate the existence of two distinct domains, one of beam rapidity projectilelike nucleons, and the second of participant nucleons. From the former, the in-medium inelastic nucleon-nucleon cross section is deduced. It is found to agree, within 10%, with the ‘‘free’’ value of 30 mb although under present conditions one of the two colliding nucleons has been struck before with a high probability. We compare with the present data the predictions of a fragmentation model as well as of models dealing explicitly with the heavy-ion collision and particle creation and emission.

Electromagnetic dissociation cross sections for one, two, and three nucleon removal were measured for an E_lab/A=14.6 GeV ²⁸Si beam impinging on ²⁷Al, ¹²⁰Sn, and ²⁰⁸Pb targets. The measured cross sections exhibit an approximate Z_T² dependence and are well reproduced by calculations convoluting the virtual photon spectrum with the experimental σ(γ,N) photonuclear cross sections. Comparison with other experimental results and energy dependence of the cross sections are discussed.

Collisions of ²⁸Si+Al, Cu, Pb at E_lab/A=14.6 GeV were studied in a calorimetry-based experiment at the BNL Alternating Gradient Synchrotron. Transverse-energy production was measured for pseudorapidities -0.5<η<0.8. Correlations with the spectra and multiplicity of neutrons and protons emitted into a forward 0.8° cone demonstrate quantitatively the large amount of nuclear stopping observed in these reactions. Calculations in hadronic-fireball or nucleon-nucleon based models underpredict the measured transverse-energy production for Si+Pb and indicate the need to include rescattering of secondaries and /or contributions from target fragmentation.

The decays Σ^-→Λe^-ν and Ξ^-→Λe^-ν are studied in a counter—spark-chamber experiment using unpolarized hyperons from the BNL hyperon beam. A threshold Čerenkov counter identifies the electron. From a sample of 119 reconstructed Σ^-→Λe^-ν events (including ∼ 5 background events), we find (Σ^-→Λe^-ν) / (Σ^-→nπ)=(0.63±0.10)×10^-4 and g_V / g_A=-0.32±0.30, g_WM=-0.6±3.6 (with sign convention of the Particle Data Group tables). We find Im(g_V / g_A)=0.2±0.7, consistent with time-reversal invariance. From a sample of 14 Ξ^-→Λe^-ν events (including ≈ 3.4 background events) we find (Ξ^-→Λe^-ν) / (Ξ^-→Λπ)=(0.30±0.14)×10^-3. We also searched for Ξ^-→Λμν and Ξ^-→Σ⁰eν and set upper limits on these decays from our data.