A test of QCD at low energy

The DIRAC experiment (PS212) aims to measure The lifetime of π+π - atoms using 24GeV proton beamprovided by the CERN PS. the precise measurement of the lifetime enables to determine an -wavepion-pion scattering length in a model independent way. The ππ scattering length已被调用by means of Chiral perturbation theory with a precision of 5% at the starting stage of the DIRACexperiment. Now the precision of the calculation is better than 2.5%. the results are based on theassumption of a spontaneous chiral symmetry breaking due to a strong quark condensation. Therefore，a precise comparison of the theoretical value with the experimental one provides an opportunity toverify the current understanding of chiral symmetry breaking in QCD and to check the magnitude ofthe quark condensation.We analyzed 30% of the collected data and deduced the lifetime as τ =[2.91+0.49-0.62] X 10 <-15> s. This lifetime corresponds to the scattering length |a0·a2| =0.264+0.033 / 0.020m_π <-1>. The errors are mostly due to statistics.In this DIRAC experiment，Japan group has developed a topological trigger device consisting of scintillating-fiberhodoscopes(SFD), peak-sensing circuits（PSC）ADC/TDC hybrid readout circuits(F1) and trigger logic circuits(TLC).他们可以选择out online apair due to a dissociation of π^+π^- atom from a huge amount of background events. Asthe second stage of the DIRAC experiment we measure a lifetime of πK-atoms. For the πK measurementsmore than ten time intense beam than the π^+π^- measurement is required. We developed new high解决方案scintillating fiber hodoscopes(HRH) consisting of 480 column made of 280μm fine fibers30多张照片multipliers with 16 channels.我们发现了一个优秀的人才spatial resolution of 75μm and a time resolution of 0.56ns in rms。

The DIRAC experiment (PS212) aims to measure the lifetime of π+π^- atoms using 24GeV proton beam provided by the CERN PS. The precise measurement of the lifetime enables to determine an s-wave pion-pion scattering length in a model independent way. The ππ scattering length has been calculated by means of Chiral perturbation theory with a precision of 5% at the starting stage of the DIRAC experiment. Now the precision of the calculation is better than 2.5%. The results are based on the assumption of a spontaneous chiral symmetry breaking due to a strong quark condensation. Therefore, a precise comparison of the theoretical value with the experimental one provides an opportunity to verify the current understanding of chiral symmetry breaking in QCD and to check the magnitude of the quark condensation.We analyzed 30% of the collected data and deduced the lifetime as τ = [2.91+0.49-0.62] X 10^<-15> s. This lifetime corresponds to the scattering length |a0・a2| = 0.264+0.033・0.020m_π^<-1>. The errors are mostly due to statistics.In this DIRAC experiment, Japan group has developed a topological trigger device consisting of scintillating-fiber hodoscopes(SFD), peak-sensing circuits(PSC), ADC/TDC hybrid read-out circuits(F1) and trigger logic circuits(TLC). They can select out online a real π^+π^- pair due to a dissociation of π^+π^- atom from a huge amount of background events. As the second stage of the DIRAC experiment we measure a lifetime of πK-atoms. For the πK measurements more than ten time intense beam than the π^+π^- measurement is required. We developed new high resolution scintillating fiber hodoscopes(HRH) consisting of 480 column made of 280μm fine fibers and 30 multi-anode photo multipliers with 16 channels. We found that the HRH has an excellent spatial resolution of 75μm and a time resolution of 0.56ns in rms.