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Evidence for the 2π Decay of the Meson

This seminar presents experimental evidence for the 2π decay of the meson, including the experiment layout, analysis methods, and conclusions.

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Evidence for the 2π Decay of the Meson

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  1. Sokendai seminar 2nd[ EVIDENCE FOR THE 2πDECAY OF THE MESON ] 2004.5.31 Y.Uchida

  2. 1.Experiment layout • mesons produced at Brookhaven AGS(Alternating Gradient Synchrotron) • An internal beryllium (Be) target bombarded by 30GeV proton • Neutral beam: 30 degrees relative to the circulating protons. Selected by Pb 1st collimator. • Charged particle: removed by strong deflection magnet

  3. 1.Experiment layout • Pb→1st Collimator(average distance of 14.5ft from the internal target)→ Sweeping magnet (at ~20ft, 512kG-in) →2nd Collimator(at 55ft) → Helium Bag • Pb: attenuate the gamma ray in the beam (1.5-in thickness) • Sweeping magnet: remove electron and positron • Helium Bag: restrain reaction of meson in beam • The detector for the decay products (two spectrometers) • Spark chamber : • Track delineation separated by magnetic field of 178kG-in. • Triggered on a coincidence between water cherenkov. • Water cherenkov & scintillation counter: • Select particle 75% above light velocity.

  4. 2-1. Analysis (compute) • Analysis program computed (assuming each charged particle had the mass of the charged pion) • Vector momentum • Invariant mass • Vector sum and the angle , between it and the direction of the beam were determined. • : two-body decay , : three-body decays • equal mass is not preferred result when the three-body decays analyzed in this way.

  5. 2-2. Analysis (An important calibration of the apparatus & data reduction ) • There was afforded by observing the decays of mesons produced by coherent regeneration in tungsten on decay field. • Regenerator was successively placed at intervals of 11in.along the region of the beam sensed by the detector to approximate the spatial distribution of the ‘s. • regeneration coherent have same momentum and direction beam ,then the decay simulates the direct decay of the into two pions.

  6. 2-3. Analysis • decays in the He gas • Experimental distribution in → Fig.2(a) • Compared in the figure with the result of a Monte Carlo calculation which takes into account the nature of the interaction and the form factors involved in the decay,coupled with the detection efficiency of the apparatus. • Form factor ratio ,and relative abundance 0.47,0.37 • The scalar interaction has been computed as well as the vector interaction with a form-factor ratio . • Distribution in for those events　 → Fig.2(b) • Fall in the mass range from 490 to 510 MeV together with the corresponding result for the Monte Carlo calculation. • Those two analyses are the same within the respective revolution.

  7. 2-3. Analysis • The angular distribution from three mass range (results from the more accurate measuring machine) → Fig.3 • The average of the distribution of masses of those events in Fig.3 with is found to be MeV . • A corresponding calculation has been made for the tungsten data resulting in a mean mass of . • Take the mass of the to be known and compute the mass of the secondaries for two-body decay. • Assuming one of the secondaries to be a pion,the mass of the other particle is determined . • Obtain events in the forward peak after subtraction of background out of a total corrected sample of 22700 decays.

  8. 3.Examination • Data taken with a hydrogen target in the beam • Show evidence of forward peak in the distribution • After subtraction of background without coherent scattering, events are observed in the forward peak at the mass. ~10 events can be expected from coherent regeneration. coherent regeneration. The spatial distribution of the forward events = regular decays.(eliminates the possibility of regeneration having occurred in the collimator.)

  9. 4.Conclusion • decays to two pions • Branching ratio (error is the standard deviation.) • The presence of a two-pion decay mode implies • meson is not a pure eigenstate of CP • are the and mean lives. • : blanching ratio including decay to two • Using ,

  10. Appendix (regeneration of ) • have 1000 times long lifetime,compare with • If progress sec after generate, decay 2πand remain only . • If pass neutral K beam in suitable material, is lost by some reaction,and then is increase relatively. • After pass material ( ) • ingredient: , ingredient: • Beam state Regenerate

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