Hyo-sung Ahn

Measurement of charm production in two-photon processes using inclusive lepton events at TRISTAN

Abstract Measurements of open charm production in photon-photon collisions made with the AMY detector at TRISTAN are reported. Charmed hadrons were identified by detecting the high momentum muons or electrons from their semileptonic decays. The data sample corresponds to an integrated luminosity of 275 pb −1 at an average center of mass energy of 58 GeV. Results are presented in the form of cross sections of inclusive leptons from charm for both muons and electrons. The measured cross section is 1.8 standard deviations higher than theoretical predictions based on the direct and photon-gluon fusion process, where the mass of charm quark is assumed to be 1.6 GeV.

A measurement of the photon structure function F2γ at Q2 = 6.8 GeV2

The photon structure function F-2(gamma) has been measured at an average Q(2) value of 6.8 GeV2 using data collected by the AMY detector at the TRISTAN e(+)e(-) collider. The measured F-2(gamma) is compared with several QCD-based parton density models

New limits on the masses of the selectron and photino

Abstract A study of e + e − annihilations into final states containing a single energetic photon with no accompanying particles is made at a center of mass energy of 57.8 GeV. The measured cross section is consistent with expectations from standard model processes and is used to set limits on the masses of the scalar electron and photino particles predicted by supersymmetry theories. If the photino is assumed to be massless, the 90% confidence level lower limit on the mass of the degenerate scalar electron is 65.5 GeV. If the results of all the single photon experiments are combined, this lower limit increases to 79.3 GeV.

Measurement of D-*+/- production in two-photon processes at TRISTAN

The inclusive cross section for the production of charmed D*(+/-) mesons in two-photon processes is measured with the AMY detector at the TRISTAN e(+)e(-) collider. D*(+/-) mesons ate identified from the distribution of charged-particle transverse momenta relative to the jet axis. A data sample corresponding to an integrated luminosity of 176 pb(-1) at a center-of-mass energy of 58 GeV is used to determine a cross section sigma(e(+)e(-) --> e(+)e(-)D*(+/-)X) = 270 +/- 49(stat) +/- 38(syst) pb. The results are compared with theoretical expectations based on the Vector Meson Dominance, direct quark-parton model, and resolved photon processes.

Observation of exclusive ηc production in two photon interactions at TRISTAN

Abstract We observe η c production in the two photon process e + e − → e + e − η c at an e + e − center-of-mass energy of 58 GeV. We determine the radiative decay width Γ γγ ( η c ) to be 27±16(sta)±10(sys) keV.

Observation of the color coherence effect in sub-jet multiplicity of three-jet and four-jet events in e+e− annihilations

We have measured the ratio of the average sub-jet multiplicity in 3-jet to 2-jet events, R(23), and 4-jet to 2-jet events, R(24), using the e(+)e(-) annihilation data at root s = 58 GeV obtained from the AMY detector. We compare the measured values of R(23) and R(24) With the predictions of coherent and incoherent Monte Carlo models. R(23) is also compared with the NLLA calculations to all-orders. The color coherence effect has been observed to contribute to the suppression of R(23) and R(24) Also, the difference of the behavior between gluon jets in 3-jet and 4-jet events has been studied.

Measurement of the forward-backward asymmetry in e(+)e(-)->b(b)over-bar and the b-quark branching ratio to muons at TRISTAN using neural networks

The forward-backward asymmetry in e(+)e(-) --> b (b) over bar at root s = 57.9 GeV and the b-quark branching ratio to muons have been measured using neural networks. Unlike previous methods for measuring the b (b) over bar forward-backward asymmetry where the estimated background from c-quark decays and other sources are subtracted, here events are categorized as either b (b) over bar or non-b (b) over bar events by neural networks based on event-by-event characteristics. The determined asymmetry is -0.429 +/- 0.044(stat) +/- 0.047(sys) and is consistent with the prediction of the standard model. The measured B (B) over bar mixing parameter is 0.136 +/- 0.037(stat) +/- 0.040(sys) +/- 0.002(model) and the measured b-quark branching ratio to muons is 0.122 +/- 0.006(stat) +/- 0.007(sys).

An experimental study of the process e+e−→e+e−μ+μ−

Abstract We present the results of a study of e + e − μ + μ − final states produced in e + e − collisions at a center-of-mass energy of 58 GeV using the AMY detector at TRISTAN. The data correspond to a total integrated luminosity of 292.8 pb −1 . The results show good agreement with order α 4 QED predictions.

Measurement of α s using NLLA+O( α S 2 ) in e + e − annihilation at s=58.0 GeV

Abstract A measurement of the strong coupling constant α S is presented using hadronic events produced in e + e − annihilations at s =58.0 GeV from the AMY detector at TRISTAN. The measurement is based on comparisons of the distributions of thrust, heavy jet mass, total jet broadening, wide jet broadening, and energy-energy correlations with QCD calculations resummed up to next-to-leading-logarithms matched with the O ( α S 2 ) perturbative calculation. Combining the results of the individual evaluations, we find α S (58 GeV )=0.132±0.006 .