Precision measurement of orthopositronium decay rate

• Project/Area Number: 17540236
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Particle/Nuclear/Cosmic ray/Astro physics
• Research Institution: The University of Tokyo
• Principal Investigator: KOBAYASHI Tomio The University of Tokyo, International Center for Elementary Particle Physics, Professor, 素粒子物理国際研究センター, 教授 (50126059)
• Co-Investigator(Kenkyū-buntansha): ASAI Shoji The University of Tokyo, International Center for Elementary Particle Physics, Associate Professor, 素粒子物理国際研究センター, 助教授 (60282505)
• Project Period (FY): 2005 – 2006
• Project Status: Completed (Fiscal Year 2006)
• Budget Amount: ¥3,500,000 (Direct Cost: ¥3,500,000); Fiscal Year 2006: ¥800,000 (Direct Cost: ¥800,000); Fiscal Year 2005: ¥2,700,000 (Direct Cost: ¥2,700,000)
• Keywords: Elementary particle physics / Positronium / QED / higher order correction

Research Abstract

The second order correction of the bound state QED had been calculated in 2000. To test the calculation and the framework of the bound state QED, we have performed the precision measurement of the Orthopositronium decay rate. First of all, we have developed the new setup suitable for the high rate measurement in order to overcome the large statistic error. In the setup, we have adapted the YAP scintillator, which is the fast inorganic scintillator.
We have measured the Orthopositronium decay rate two times in the following conditions.
* RUN I target : SiO_2 aerogel data taking period: 2.3 months
* RUN II target : SiO_2 powder data taking period: 3.1 months
In two measurements, we have changed the target to test the influence of the material on the measurement. The amount of data taken in each measurement corresponds to about 130ppm of the statistic error.
We have adapted the direct pick-off correction method in the measurement. The obtained pick-off ratio, which is the ratio of the annihila tion in the target material, is about 3% for RUN I (SiO_2 aerogel) and about 1% for RUN II (SiO_2 powder). The difference is found to be large. And it is considered to be come from the difference of the fine structure of the material such as the mean distance between the SiO_2 grains.
The obtained decay rates for each run are
* RUN I 7.03876+-0.0009 (stat.)+0.0008-0.0010 (sys.) μs^<-1>
* RUN II 7.04136+-0.0009 (stat.)+0.0007-0.0007 (sys.) μs^<-1>
These are consistent with each other in 1.6σ.
The combined result is
* RUN I, II 7.0401+-0.0006 (stat.)+0.0007-0.0009 (sys.) μs^<-1>
The total error is about 150ppm and the most precious in the world.
The measured value is consistent with the three measurements from 1995 and also agreed with the O( α^2) calculation by the bound state QED. On the other hand, the value differs from the O(α) calculation by about 1.7σ. If we refer to the world average of the measured decay rate, the discrepancy reaches 2.6σ. Therefore, one can conclude the measured decay rate supports the O(α^2) calculation.