Search Results (13 total)

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

Precise measurements of the spin structure functions of the proton g₁^p(x,Q²) and deuteron g₁^d(x,Q²) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18  GeV²≤Q²≤20  GeV². The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g₁ⁿ is extracted by combining proton and deuteron data. The integrals of g₁^p,d at Q²=5  GeV² are evaluated over the measured x range. Neglecting any possible contribution to the g₁^d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a₀ in a leading-twist next-to-next-to-leading-order analysis.

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We report evidence for direct CP violation in the decay B⁰→K⁺π^- with 253  fb^-1 of data collected with the Belle detector at the KEKB e⁺e^- collider. Using 275×10⁶ BB̅ pairs we observe a B→K^±π^∓ signal with 2140±53 events. The measured CP violating asymmetry is A_CP(K⁺π^-)=-0.101±0.025(stat)±0.005(syst), corresponding to a significance of 3.9σ including systematics. We also search for CP violation in the decays B⁺→K⁺π⁰ and B⁺→π⁺π⁰. The measured CP violating asymmetries are A_CP(K⁺π⁰)=0.04±0.05(stat)±0.02(syst) and A_CP(π⁺π⁰)=-0.02±0.10(stat)±0.01(syst), corresponding to the intervals -0.05<A_CP(K⁺π⁰)<0.13 and -0.18<A_CP(π⁺π⁰)<0.14 at 90% confidence level.

We report the observation of a narrow charmoniumlike state produced in the exclusive decay process B^±→K^±π⁺π^-J/ψ. This state, which decays into π⁺π^-J/ψ, has a mass of 3872.0±0.6(stat)±0.5(syst)  MeV, a value that is very near the M_D⁰+M_D^*0 mass threshold. The results are based on an analysis of 152M B-B̅ events collected at the Υ(4S) resonance in the Belle detector at the KEKB collider. The signal has a statistical significance that is in excess of 10σ.

This article describes an observation of mixing-induced CP violation and a measurement of the CP violation parameter, sin 2φ₁, with the Belle detector at the KEKB asymmetric e⁺e^- collider. Using a data sample of 29.1 fb^-1 recorded on the Υ(4S) resonance that contains 31.3 million BB̅ pairs, we reconstruct decays of neutral B mesons to the following CP eigenstates: J/ψK_S⁰, ψ(2S)K_S⁰, χ_c1K_S⁰, η_cK_S⁰, J/ψK_L⁰ and J/ψK^*0. The flavor of the accompanying B meson is identified by combining information from primary and secondary leptons, K^± mesons, Λ baryons, slow and fast pions. The proper-time interval between the two B meson decays is determined from the distance between the two decay vertices measured with a silicon vertex detector. The result sin 2φ₁=0.99±0.14(stat)±0.06(syst) is obtained by applying a maximum likelihood fit to the 1137 candidate events. We conclude that there is large CP violation in the neutral B meson system. A zero value for sin 2φ₁ is ruled out by more than six standard deviations.

The lifetimes of the B̅ ⁰ and B^- mesons are extracted from 29.1 fb^-1 of data collected with the Belle detector at the KEK B factory. A fit to the decay length differences of neutral and charged B meson pairs, measured in events where one of the B mesons is fully reconstructed in several hadronic modes, yields τ_{B̅ ⁰}  =  1.554±0.030(stat)±0.019(syst) ps, τ_B^-  =  1.695±0.026(stat)±0.015(syst) ps, and τ_B^-/τ_{B̅ ⁰}  =  1.091±0.023(stat)±0.014(syst).

Using the Belle detector operating at the KEKB e⁺e^- storage ring, we have measured the mean multiplicity and momentum spectrum of neutral pions from the decays of the Υ(4S) resonance. We measure a mean of 4.70±0.04±0.22 neutral pions per Υ(4S) decay.

We present a measurement of the standard model CP violation parameter sin2φ₁ based on a 29.1 fb^-1 data sample collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. One neutral B meson is fully reconstructed as a J/ψK_S, ψ(2S)K_S, χ_c1K_S, η_cK_S, J/ψK_L, or J/ψK^*0 decay and the flavor of the accompanying B meson is identified from its decay products. From the asymmetry in the distribution of the time intervals between the two B meson decay points, we determine sin2φ₁  =  0.99±0.14(stat)±0.06(syst). We conclude that we have observed CP violation in the neutral B meson system.

We report a determination of the B_d⁰-B̅ _d⁰ mixing parameter Δm_d based on the time evolution of dilepton yields in ϒ(4S) decays. The measurement is based on a 5.9 fb^-1 data sample collected by the Belle detector at KEKB. The proper-time difference distributions for same-sign and opposite-sign dilepton events are simultaneously fitted to an expression containing Δm_d as a free parameter. Using both muons and electrons, we obtain Δm_d  =  0.463±0.008 (stat)±0.016 (syst) ps^-1. This is the first determination of Δm_d from time evolution measurements at the ϒ(4S). We also place limits on possible CPT violations.

We present a measurement of the standard model CP violation parameter sin2φ₁ (also known as sin2β) based on a 10.5 fb^-1 data sample collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric e⁺e^- collider. One neutral B meson is reconstructed in the J/ψK_S, ψ(2S)K_S, χ_c1K_S, η_cK_S, J/ψK_L, or J/ψπ⁰ CP-eigenstate decay channel and the flavor of the accompanying B meson is identified from its charged particle decay products. From the asymmetry in the distribution of the time interval between the two B-meson decay points, we determine sin2φ₁  =  0.58_-0.34^+0.32(stat)_-0.10^+0.09(syst).

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)].