Search Results (18 total)

We present a measurement of the longitudinal spin cross section asymmetry for deep-inelastic muon-nucleon interactions with two high transverse momentum hadrons in the final state. Two methods of event classification are used to increase the contribution of the photon-gluon fusion process to above 30%. The most effective one, based on a neural network approach, provides the asymmetries A_p^lN→lhhX=0.030±0.057(stat)±0.010(syst) and A_d^lN→lhhX=0.070±0.076(stat)±0.010(syst). From these values we derive an averaged gluon polarization ΔG/G=-0.20±0.28(stat)±0.10(syst) at an average fraction of nucleon momentum carried by gluons 〈η〉=0.07.

We present the results of the spin asymmetries A₁ of the proton and the deuteron in the kinematic region extending down to x=6×10^-5 and Q²=0.01 GeV². The data were taken with a dedicated low x trigger, which required hadron detection in addition to the scattered muon, so as to reduce the background at low x. The results complement our previous measurements and the two sets are consistent in the overlap region. No significant spin effects are found in the newly explored region.

We present a next-to-leading order QCD analysis of the presently available data on the spin structure function g₁ including the final data from the Spin Muon Collaboration. We present results for the first moments of the proton, deuteron, and neutron structure functions, and determine singlet and nonsinglet parton distributions in two factorization schemes. We also test the Bjorken sum rule and find agreement with the theoretical prediction at the level of 10%.

We present the final results of the spin asymmetries A₁ and the spin structure functions g₁ of the proton and the deuteron in the kinematic range 0.0008<x<0.7 and 0.2<Q²<100 GeV². For the determination of A₁, in addition to the usual method which employs inclusive scattering events and includes a large radiative background at low x, we use a new method which minimizes the radiative background by selecting events with at least one hadron as well as a muon in the final state. We find that this hadron method gives smaller errors for x<0.02, so it is combined with the usual method to provide the optimal set of results.

We have measured the spin-dependent structure function g₁^p in inclusive deep-inelastic scattering of polarized muons off polarized protons, in the kinematic range 0.003<x<0.7 and 1 GeV²<Q²<60 GeV². A next-to-leading order QCD analysis is used to evolve the measured g₁^p(x,Q²) to a fixed Q₀². The first moment of g₁^p at Q₀²=10 GeV² is Γ₁^p=0.136±0.013 (stat) ±0.009 (syst) ±0.005 (evol). This result is below the prediction of the Ellis-Jaffe sum rule by more than two standard deviations. The singlet axial charge a₀ is found to be 0.28±0.16. In the Adler-Bardeen factorization scheme, Δg≃2 is required to bring ΔΣ in agreement with the quark-parton model. A combined analysis of all available proton, deuteron, and ³He data confirms the Bjorken sum rule.

The Mark J Collaboration at the DESY e⁺e^- collider PETRA presents results on the electroweak reactions e⁺e^-→μ⁺μ^-τ⁺τ^-,μ⁺μ^-γ, and e⁺e^-μ⁺μ^-. The c.m. energy range is 12 to 46.78 GeV. In the μ⁺μ^- and τ⁺τ^- channels the total cross sections and the forward-backward asymmetries are reported and compared with other experiments. The results are in excellent agreement with the standard model. The weak-neutral-current vector and axial-vector coupling constants are determined. The values for muons and τ’s are compatible with universality and with the predictions of the standard model. In the μ⁺μ^-γ channel, all measured distributions, including the forward-backward muon asymmetry, are in excellent agreement with the electroweak theory. Our data on the two-photon process, e⁺e^-μ⁺μ^-, agrees with QED to order α⁴ over the entire energy range and the Q² range from 0.7 to 166 GeV².

We use the reaction e⁺e^-→hadrons, in the Mark J detector at the DESY electron-positron collider PETRA, to determine the hadronic cross section up to 46.78 GeV. The production of a top quark with a charge equal to (2/3) is excluded up to 46.6 GeV with 95% C.L. The observed rise in the cross section at higher energies is consistent with the electroweak prediction for a Z⁰ mass of 93 GeV. We describe some unusual muon inclusive events.

We use the reaction e⁺e^-→μ⁺μ^-, in the Mark J detector at the DESY high-energy e⁺e^- collider PETRA, to test the standard electroweak theory and find good agreement. We also set limits on the parameters of several extended gauge theories.

Using the Mark-J detector at the high-energy e⁺e^- collider PETRA, we compare the data from hadron production with the complete second-order QCD calculation over the energy region 22 to 46.78 GeV. We determine the QCD parameter Λ=100±30_-45⁺⁶⁰ MeV which yields the strong-coupling constant α_s=0.12±0.02 for sqrt[s]=44 GeV.

We report mass limits on the Z̃⁰ and W̃^±, supersymmetric partners of the Z⁰ and W^±, respectively, using the MARK-J detector at PETRA. The experimental signatures in both cases are acoplanar lepton pairs with missing energy. For the W̃^±, an additional signature is a single energetic lepton. No evidence is found for either the Z̃⁰(M_Z̃⁰<35GeV) or the W̃^±(M_W̃<25GeV).

We have searched for resonances in the reaction e⁺e^-→hadrons, γγ, μμ, and ee, in the energy range 39.79<sqrt[s]<45.52 GeV, using the Mark J detector at PETRA. We obtain stringent upper limits on the production of toponium and particles postulated to explain Z⁰→lepton pair +γ events observed at the CERN p―p collider. We also set limits on the mass and coupling constant of excited electrons.

The results of a high-statistics study of inclusive muon spectra at PETRA are reported. Improved mass limits have been obtained for heavy quarks, heavy leptons, and charged Higgs particles. It is shown that the fragmentation properties of b quarks and c quarks are different, with the mean fragmentation variables 〈z_b〉=0.75±0.03±0.06, 〈z_c〉=0.46±0.02±0.05 and the average semileptonic branching ratio for the B and C hadrons R (B)=(10.5±1.5±1.3)%, R(C)=(11.5±1.0±1.7)%.

With use of the MARK-J detector at sqrt[s]=34.7 GeV 21 000 e⁺e^-→hadron events have been collected. By measurement of the asymmetry in angular energy correlations the strong coupling constant α_s=0.13±0.01 (statistical)±0.02 (systematic) is determined, in complete second order, and independent of the fragmentation models and QCD cutoff values used.

With a PETRA energy scan in ≤30-MeV steps, the continuum production of open top quark up to 38.54 GeV is excluded. Over regions of energy scan from 29.90 to 38.63 GeV limits are set on the product of hadronic branching ratio and electronic width B_hΓ_ee for toponium to be less than 2.0 keV at the 95% confidence level. By a search for flavor-changing neutral currents in b decay, models without a top quark are excluded.

The measurement of the nonelectromagnetic forward-backward charge asymmetry in the reaction e⁺e^-→μ⁺μ^- at sqrt[s]∼34.6 GeV and in the angular region 0<|cosθ|<0.8 is reported. With a systematic error less than 1%, we observe an asymmetry of (-8.1±2.1)%. This is in agreement with the standard electroweak theory prediction of (-7.6±0.6)%. The weak-current coupling constants are also reported.

The results are reported of a search for excited muons (μ^*) and electrons (e^*) and for a stable charged heavy lepton by measurements of the e⁺e^-→μ⁺μ^-γ, μ⁺μ^-γ, μ⁺μ^-γγ, and γγ final states. Excluded are large regions of the λ (coupling constant), M (mass) planes for spin-½ μ^* and e^*, and a new stable charged heavy lepton with mass < 14 GeV.

A direct test is presented of higher-order QED (α⁴) at large momentum transfers (up to ∼100 GeV²). These tests were carried out with the MARK-J detector at PETRA by comparing the measured cross section for the process e⁺e^-→e⁺e^-μ⁺μ^- with the prediction of QED for 12 GeV<~sqrt[s]<~36.7 GeV. The cross sections and the various kinematic distributions agree with QED.