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+1 vote
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in Physics by (71.5k points)

A neutral K meson decays in flight via K0 → π+π. If the negative pion is produced at rest, calculate the kinetic energy of the positive pion. 

[Mass of K0 is 498 MeV/c2; that of π± is 140 MeV/c2]

1 Answer

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The only way π is emitted at rest in the lab system is when it is emitted at θ1= 180 in the CMS (rest frame of K) with with the same speed as K in the lab system. In that case π will be emitted at θ2 = 0 in the CMS. 

The energy released 

Q = 498 − 2 × 140 = 218MeV

As the product particles are identical, each pion carries half of the enrgy, 109 MeV

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