Y. Watanabe

Experimental study of b-quark jets in e+e− annihilation at TRISTAN

Abstract An experimental study of b-quark jets using high-pT electrons was carried out at √s=58 GeV with the TOPAZ detector at the e+e− collider TRISTAN at KEK. The forward-backward charge asymmetry of the b-quark was obtained to be A b b =−0.55±0.27( stat. )±0.07( syst. ) , consistent with the standard model prediction. Also, such jet properties of the b-quark as the average charged multiplicity and the rapidity of charged particles were analyzed. In order to purify the b-quark event samples in this analysis, only events with backward-going electrons or forward-going positrons were used. The energy dependence of these jet properties was studied by making comparisons with the results of the DELCO experiment at the PEP collider (√s=29 GeV) at SLAC.

The silicon vertex detector for the super B factory

The silicon vertex detector (SVD2) of the Belle experiment at KEK (Tsukuba, Japan) is currently close to its limits in terms of the occupancy of the innermost layer and the readout dead time. In order to cope with further increase in luminosity, different levels of upgrades are proposed: small improvements are possible in the short term by tuning the current system at the cost of signal-to-noise and crosstalk. In 2007, the two inner layers of the SVD2 will be replaced by similar detectors with faster readout electronics (SVD2.5) and in the long term, a major upgrade of the full silicon vertex detector (SVD3) is envisaged for the KEK Super B Factory. These upgrade scenarios will be discussed in detail from the hardware point of view along with the options under study.

Charge asymmetry of hadronic events in e+e− annihilation at √s=57.9 GeV

Abstract The charge asymmetry of quark jets produced in e + e − annihilations at 〈√ s 〉=57.9 GeV was measured with the TOPAZ detector at TRISTAN. The observed charge asymmetry is +0.091±0.014(stat.)±0.016(sys.). From the measured differential cross section, the axial vector coupling constant averaged over all quark flavors was determined to be 1.09 −0.21 +0.27 . These values are consistent with the standard model predictions. Possible deviations from the standard model were examined in terms of contact interactions, and the lower limits on the compositeness scale parameters were obtained to be 1.2–7.1 TeV at the 95% confidence level.

A study of pion pair production in the two-photon process

Abstract We studied the e+e−→e+e−π+π− interaction with the untagged two-photon sample obtained by the TOPAZ detector at the TRISTAN e+e− collider. The sample was taken in the energy region s =55.0−60.8 GeV . One of the QED backgrounds, e+e−→e+e−, was clearly removed by the measurement of dE/dx by the TOPAZ-TPC. The π+π− invariant mass spectrum is well reproduced by the f2(1270) resonance, the Born term and their interference. We determined the radiative width of the f2(1270) to be 2.27±0.47(stat.)±0.11(syst.) keV.

A search for charged Higgs bosons at tristan

Abstract We searched for pair production of charged Higgs bosons via e+e− annihilation, using data (23.3 pb−) collected by the TOPAZ detector in the energy range s = 52.0 – 61.4 GeV. We observed no evidence for charged Higgs pair production and extended the excluded region in the MH±BR (hadron) plane. The 95% CL lower mass limit is the most stringent for BR (hadron) = 0.6, giving mH±

Observation of highly virtual photon-photon collisions to hadrons at TRISTAN

We have observed highly virtual (Q2 > 1.05 GeV2) photon-photon collisions to hadronic final states at Se+e−=58 GeV. The integrated luminosity of the data sample was 241 pb−1. Both scattered beam-electrons and scattered beam-positrons were detected using low-angle calorimeters (i.e., both photons were highly virtual, “ouble-tag”); we obtained 115 hadronic events with an estimated background of 10.2 ± 1.1. The cross section obtained was 4.11 ± 0.66 pb in the 2 2 GeV region, while the lowest order quark-parton model predicted 3.00 pb.

Search for pair production of heavy stable particles at TRISTAN

We searched for signatures of new stable particles in e+ e− annihilation using the TOPAZ detector at TRISTAN between √s = 52 and 60 GeV. The search made use of the particle identification by the Time Projection Chamber (TPC). We set new limits on the pair production of new stable charged particles with spins 0 and 12, and charges between 23 and 43. The limits on the production cross section were translated to the lower mass limits of hypothetical new particles, SUSY particles, free quarks and heavy leptons.