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Homework Help: Relativistic particle decay

  1. Sep 3, 2016 #1
    1. The problem statement, all variables and given/known data
    Unstable particles cannot live very long. Their mean life time t is defined by N(t) = N0e−t/τ , i.e., after a time of t, the number of particles left is N0/e. (For muons, τ=2.2µs.) Due to time dilation and length contraction, unstable particles can still travel far if their speeds are high enough

    Problem 4
    a) For musons at rest, traveling at 0.6c, 0.8c, 0.9999c, plot the N(t)/N0 ratio as a function of time.

    b) For muons traveling at 0.8c, derive N(L)/N0 as a function of L, where L is the distance travelled, measured in the earth RF. You don’t need to plot these functions.

    c) Assume you are designing an experiment using muons beams that are directed at a target in a neighboring city 50km away. You want to have at least half of the muons reach the target without decaying. What is the minimum speed of the muons you must have?

    2. Relevant equations
    N(t) = N0e−t/τ

    t_e = t_m *γ

    L_m = L_e/γ

    t_e & L_e is the time and length measured in the earth frame of reference

    and t_m and L_m is the time and length measured in the muon's frame of reference

    I did all the parts but I feel pretty unsure about it. I was hoping you guys could take a look and let me know if it seems ok. Thanks in advance!
    3. The attempt at a solution
    Parts a.) and b.) are in the attached image

    c.) N(L_e) = N_0 * e^(-L_e/v*τ_e)

    after some algebra I end up with the expression

    v = [ (τ_m*ln(2)/L_e)^2 + 1/c^2]^-1/2

    τ_m = 2.2 * 10^-6 s
    L_e = 50*10^3 m

    after plugging in these values I get

    v = 299,987,443.2 m/s

    Attached Files:

  2. jcsd
  3. Sep 3, 2016 #2


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    Your work looks correct to me. You could express the answer for part (c) in terms of the speed of light c.
  4. Sep 3, 2016 #3
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