IGNOU MPH 17 2026 - Consider K meson of mass mK decaying into two pions of mass

Question

Consider K meson of mass m_K decaying into two pions of mass $m_\pi$ each, in the laboratory frame. Show that the energy of each pion is equal to $\frac{m_K}{2}$ and the momenta are $p_\pi = \frac{1}{2}\sqrt{m_K^2 - 4m_\pi^2}$ .

19 Jan 2026

Answer :

Word Count : 240

Consider a K meson at rest in the laboratory frame, decaying into two identical pions, each of mass (m_\pi). By conservation of energy and momentum, the total energy and momentum before and after the decay must be equal. Before decay, the K meson has energy (E_K = m_K c^2) and momentum (p_K = 0). After decay, let the pions have energies (E_\pi) and momenta (p_\pi). Since the pions are identical

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