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Homework Help: Particle Decay Energy

  1. Mar 26, 2017 #1
    1. The problem statement, all variables and given/known data

    A particle detector shows tracks produced by two particles X and Y that were created by the decay of a lambda particle at O.
    Which of the following is a correct statement about energy at the decay?
    The energy of X must be greater than that of Y.
    The combined energy of X and Y must be more than the energy of the lambda particle.
    The mass of the lambda particle must equal the combined energy of X and Y.
    The mass energy of the lambda particle must equal the total energy of X and Y.

    2. Relevant equations

    E = mc^2

    3. The attempt at a solution

    For the first I think we can't tell without values? Or would they be the same?

    For the second, I think energy must be conserved

    For the third it doesn't take into account the energy the lamba particle might have had.

    For the fourth I think it doesn't take into account the mass of X and Y? I'm a bit confused on the difference between total energy and combined energy though
  2. jcsd
  3. Mar 26, 2017 #2


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    I see two curved tracks. Don't know if they are projections on a detector plane or if the tracks are in one and the same plane. Curvature of tracks generally is brought about by a magnetic field and is inversely proportional to momentum ##\perp \vec B##. You are also not given any information on the mass. All you are being told is that it's a Lambda particle, but not which one. Perhaps we can rule out ##\Lambda_c## (three charged particles) and ##\Lambda_b## (even more decay products) and assume we see a ##p^+## and a ##\pi^-##.
    So am I. I don't know what is meant with 'combined energy' in such a situation. To me all four statements seem wrong and the exercise formulation deficient-- but in PF we are not allowed to provide full answers, so it's your choice...
  4. Mar 26, 2017 #3

    Buzz Bloom

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    Gold Member

    Hi Fela:

    I am not an expert, but I will share some thoughts that I hope will be helpful.

    1. You need to guess what is in the teacher's mind, based on previous lectures and demonstrations. Whay comes to my mind is that the traces seen are influenced by an external magnetic field. We see X's path having more curvature than Y's.
    Wikipedia says that there are two common decay possibilities.
    One decay involves charged particles, and the other neutral particles. Do the curved paths suggest which is being shown? Base on that, can you tell which particle is which decay product? Is so, which has more mass?

    2 and 3. You give some interpretation regarding each question, but you need to be clear about whether the statement is TRUE of FALSE.

    4. I am guessing that it is reasonable to assume that the teacher means by this statement that the total mass-energy (including the mass and the mass equivalent of the kinetic energy) of the lambda particle must equal the sum of the mass-energies of the two decay particles. Id this interpretation is correct, is the statement TRUE or FALSE?

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