W.-S. Hou

Observation of B^+ ¿ ¿K^+

We report the first observation of the charmless hyperonic B decay, B+-->LambdaLambdaK+, using a 140 fb(-1) data sample recorded at the Upsilon(4S) resonance with the Belle detector at the KEKB (e+)(e-) collider. The measured branching fraction is B(B+-->LambdaLambdaK+) = (2.91(+0.90)(-0.70) +/- 0.38) x 10(-6). We also perform a search for the related decay mode B+-->LambdaLambdapi+, but do not find a significant signal. We set a 90% confidence-level upper limit of B(B+-->LambdaLambdapi+) < 2.8 x 10(-6).

Radiation hardness of undoped BGO crystals

Abstract We measured the radiation hardness of undoped BGO crystals from two different manufacturers. Such crystals are proposed to be used in a small-angle calorimeter of the BELLE detector of the KEK B-factory. Transparency and scintillation light output of the crystals were monitored to see the effect of radiation damage. The crystals show considerable radiation hardness up to 10.2 Mrad equivalent dose, which is much higher than the maximum expected dosage of 500 krad per year of running at BELLE.

Survey of the properties of BGO crystals for the Extreme Forward Calorimeter at BELLE

Abstract We check the scintillation light yield of all BGO crystals used for the Extreme Forward Calorimeter (EFC) at BELLE. The percentage spread of the distribution is only 6%. The nonuniformity of the light yields measured lengthwise for the forward EFC is close to 13% and is ∼0% for the backward EFC. The test crystals, two per ingot, from mass-production are subjected to the radiation hardness test. The results obtained at BINP and NTU are in good agreement. Some EFC crystals, selected randomly, show the light yields drop about 25–50% after receiving 1 krad dose and remain stable afterwards. There is little sensitivity to the rate that the 1 krad dose is received, since the recovery process is very slow, in the order of days to weeks.

Performance of undoped BGO crystals under extremely high dose conditions

Abstract We checked the performance of undoped BGO crystals under extremely high dose conditions (dosage up to 88 Mrad). The light outputs of the crystals drop by ∼30% after receiving 10 Mrad of γ-ray dose but remain stable throughout the rest of the test. The self-recovery behavior of BGO under 20 Mrad has also been studied. The reduced variation in light output from pre-radiated BGOs, together with a fixed recovery limit, indicate that a pre-radiation process may be helpful to stabilize the BGO performance in a low dose environment.

Low-dose radiation damage and recovery of undoped BGO crystals

Abstract We measured the radiation damage and its recovery of undoped BGO crystals. A radiation dose of up to 2 Mrad makes the scintillation light output decrease by 20%, but recovers to 100% with a time constant of 1 h. A radiation dose of 10 Mrad makes the scintillation light output decrease by 30%, and recover to 90% with a time constant of 10 h.

Detection of minimum-ionizing particles and nuclear counter effect with pure BGO and BSO crystals with photodiode readout

Abstract Long BGO (Bi 4 Ge 3 O 12 ) and BSO (Bi 4 Si 3 O 12 ) crystals coupled with silicon photodiodes have been used to detect minimum-ionizing particles (MIP). With a low-noise amplifier customized for this purpose, the crystals can detect MIPs with an excellent signal-to-noise ratio. The nuclear counter effect is also clearly observed and measured. Effect of full and partial wrapping of a reflector around the crystal on light collection is also studied.

Measurement of radiation damage on an optical reflector

Abstract We measured the radiation damage on an optical white fluorocarbon reflector called Goretex, which is to be used for aerogel threshold counters and crystal calorimeters of the BELLE detector of the KEK B-factory. Reflectance of the Goretex surface was monitored to see any effect of the radiation damage. Maximum equivalent dose was 8.6 Mrad. No radiation damage is observed within measurement errors.

Measurement of radiation damage on an epoxy-based optical glue

Abstract We measured the radiation damage on an optical glue called Eccobond-24, which is a candidate for CsI and BGO crystal calorimeters of the BELLE detector of the KEK B-factory. Absorption spectrophotometry in the range 300–800 nm was used to monitor the radiation damage. The maximum equivalent dose was 1.64 Mrad. The glue shows effects of damage, but is acceptable for the radiation level in the above-mentioned experiment.

Observation of B{sup 0}{yields}{pi}{sup 0}{pi}{sup 0}

We report the observation of the decay B0-->pi(0)pi(0), using a 253 fb(-1) data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB e(+)e(-) collider. The measured branching fraction is B(B0-->pi(0)pi(0))=(2.3(+0.4+0.2)(-0.5-0.3))x10(-6), with a significance of 5.8 standard deviations including systematic uncertainties. We also make a measurement of the direct CP violating asymmetry in this mode.We report the observation of the decay B{sup 0}{yields}{pi}{sup 0}{pi}{sup 0}, using a 253 fb{sup -1} data sample collected at the {upsilon}(4S) resonance with the Belle detector at the KEKB e{sup +}e{sup -} collider. The measured branching fraction is B(B{sup 0}{yields}{pi}{sup 0}{pi}{sup 0})=(2.3{sub -0.5-0.3}{sup +0.4+0.2})x10{sup -6}, with a significance of 5.8 standard deviations including systematic uncertainties. We also make a measurement of the direct CP violating asymmetry in this mode.

Observation of B{sup 0}{yields}D{sup 0}{eta}{sup '} and B{sup 0}{yields}D*{sup 0}{eta}{sup '}

We report the observation of B0bar --> D0 eta and the first observation of B0bar --> D*0 eta, using 140 fb-1 of data collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric energy e+ e- collider. We find the branching fractions to be B(B0bar --> D0 eta) = [ 1.14 +- 0.20(stat) +0.10-0.13(syst)] x 10^-4 and B(B0bar --> D*0 eta) = [ 1.21 +- 0.34(stat) +- 0.22(syst)] x 10^-4 with significances including systematic uncertainty of 8.9 and 5.3 standard deviations, respectively.