Search Results (29 total)

We present results of a two-pion correlation analysis performed with the Pb+Au collision data collected by the upgraded CERES experiment in the fall of 2000. The analysis was done in bins of the reaction centrality and the pion azimuthal emission angle with respect to the reaction plane. The pion source, deduced from the data, is slightly elongated in the direction perpendicular to the reaction plane, similarly as was observed at the Brookhaven National Laboratory Alternating Gradient Synchrotron and Relativistic Heavy Ion Collider.

We report on results of a measurement of ϕ meson production in central Pb-Au collisions at E_lab=158A  GeV. For the first time in the history of high energy heavy-ion collisions, ϕ mesons were reconstructed both in the K⁺K^- and the dilepton decay channels in the same experiment. This measurement yields rapidity densities near midrapidity, from the two decay channels, of 2.05±0.14(stat)±0.25(syst) and 2.04±0.49(stat)±0.32(syst), respectively. The shape of the measured transverse momentum spectrum is also in close agreement in both decay channels. The data rule out a possible enhancement of the ϕ yield in the leptonic over the hadronic decay channel of a factor 1.6 or larger at the 95% C.L. This rules out the discrepancy reported in the literature between measurements of the hadronic and dimuon decay channels by two different experiments.

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.

Elliptic flow and two-particle azimuthal correlations of charged hadrons and high-p_T pions (p_T>1  GeV/c) have been measured close to midrapidity in 158A  GeV/c Pb+Au collisions by the CERES experiment. Elliptic flow (v₂) rises linearly with p_T to a value of about 10% at 2  GeV/c. Beyond p_T≈1.5  GeV/c, the slope decreases considerably, possibly indicating a saturation of v₂ at high p_T. Two-pion azimuthal anisotropies for p_T>1.2  GeV/c exceed the elliptic flow values by about 60% in midcentral collisions. These nonflow contributions are attributed to nearside and back-to-back jetlike correlations, the latter exhibiting centrality dependent broadening.

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 describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We report on first measurements of low-mass electron-positron pairs in Pb-Au collisions at the CERN SPS beam energy of 40  AGeV. The observed pair yield integrated over the range of invariant masses 0.2<m≤1  GeV/c² is enhanced over the expectation from neutral meson decays by a factor of 5.9±1.5(stat)±1.2(syst   data)±1.8(syst  meson   decays), somewhat larger than previously observed at the higher energy of 158  AGeV. The results are discussed with reference to model calculations based on π⁺π^-→e⁺e^- annihilation with a modified ρ propagator. They may be linked to chiral symmetry restoration and support the notion that the in-medium modifications of the ρ are more driven by baryon density than by temperature.

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).

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions λ_f reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.

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.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

We report on measurements of low-mass electron pairs in 450 GeV p-Be, p-Au, and 200 GeV/nucleon S-Au collisions at central rapidities. For the proton induced interactions, the low-mass spectra are, within the systematic errors, satisfactorily explained by electron pairs from hadron decays, whereas in the S-Au system an enhancement over the hadronic contributions by a factor of 5.0±0.7(stat)±2.0(syst) in the invariant mass range 0.2<m<1.5  GeV/c² is observed. The properties of the excess suggest that it arises from two-pion annihilation ππ→e⁺e^-.

Employing the decay chain K⁺→π⁺π⁰, π⁰→e⁺e^- we have observed the decay π⁰→e⁺e^-. With a signal sample of ≊21 events we measure the branching ratio for π⁰→e⁺e^- to be (6.9±2.3±0.6)×10^-8, normalized to the decay K⁺→π⁺π⁰, π⁰→e⁺e^-γ. This result is consistent with the unitarity lower bound and the published 90% confidence level upper limit of 13×10^-8.

We present results of an experiment in which approximately 500 events of the decay K⁺→π⁺e⁺e^- were observed. We have measured the branching ratio for K_πee⁺ to be (2.75±0.23±0.13)×10^-7 assuming a vector interaction with a form factor of λ=0.105±0.035±0.015. We have also found the branching ratio times decay probability for K⁺→π⁺X⁰, X⁰→e⁺e^- to be less than 1.5×10^-8 (99% C.L.) over an X⁰ invariant mass range from 150 to 340 MeV/c².

A search for the lepton-flavor number-violating decay K⁺→π⁺μ⁺e^- has been conducted at the Brookhaven Alternating Gradient Synchrotron. No candidates have been found which would be consistent with this decay mode. An upper limit on the branching ratio is placed at 2.6×10^-10, which can be combined with our previous result to yield an upper limit of 2.1×10^-10 at the 90% confidence level. A limit on the branching ratio for π⁰→μ⁺e^- of 1.6×10^-8 (90% C.L.) is also established.

We report here on a search for the lepton-family-number-violating decay K⁺→π⁺μ⁺e^-. We found no evidence for this decay mode and we establish an upper limit on its branching ratio of 1.1×10^-9 at the 90% confidence level (C.L.) for a uniform phase-space decay distribution. We also use this result to establish an upper limit for the branching ratio of the decay π⁰→μ⁺e^- of 7.8×10^-8 (90% C.L.).

We have searched for the decay process K⁺→π⁺A⁰, where A⁰ is any particle of mass less than 100 MeV/c² decaying into e⁺e^-. Upper limits for the branching ratio are given as a function of mass and lifetime of the A⁰. For lifetimes shorter than 10^-13 sec, a limit of 4.5×10^-7 at the 90% confidence level is obtained.

We have measured inclusive e⁺e^- production in 17-GeV/c π^-p collisions and find anomalously large cross sections for pair masses below 0.6 GeV/c² and Feynman x<0.5. Charged particles and photons produced in association with e⁺e^- were measured. Decays of known resonances are not capable of explaining the effect. No excess of photons or charged particles associated with the anomalous pairs is detected. Implications of our results for various models and for direct single-electron and photon production are discussed.