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Does annihilation always produce new particles?

  1. May 14, 2014 #1
    I have some basic confusion on the annihilation, please help me to understand these:

    1 Does antiparticles all have negative energy? And the energy we talking about here is the rest mass energy? If it is, does it means antiparticles have negative mass since E=mc^2 ?

    My understanding on annihilation is simply two particles (one normal, one anti) collide, vanish and form new particles (photon or other bosons) to conserve momentum and energy.

    2 If the answer to question 1 is yes and yes, then I assume the rest mass energy are cancelled, the new particles only have the kinetic energy of the old ones? And what if the kinetic energy of the old particles are not enough to form a new pair?

    Please correct me if there is any mistake, thanks
     
  2. jcsd
  3. May 14, 2014 #2

    Nugatory

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    Staff: Mentor

    No, antiparticles do not have negative energy and negative mass.
     
  4. May 14, 2014 #3

    Bill_K

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    All particles, both "normal" and "anti", have positive energy. The masses of the particle and its antiparticle are always the same. (And before you ask, no, antiparticles do not travel backwards in time. :smile:)

    Some particles, like the photon, are their own antiparticle. Others, like the gluon, are distinct from their antiparticle but you can't say which is normal and which is anti. (The antiparticle of a gluon is a gluon of a different color.)

    The terminology is not always consistent. For example, the reaction e+ + e- → ν + ν-bar is also sometimes called annihilation. And W+ + W- → (whatever) is usually referred to as vector boson fusion.
     
  5. May 17, 2014 #4
    This is a very common misconception. Anti particles actually have masses identical to their particles (the mass of a positron is identical to the mass of an electron, for instance). In fact there is no particularly important reason to call a particle matter and the other one antimatter. It's just a convention and the reverse convention would be just as good.
     
  6. May 19, 2014 #5
    Antiparticles do, in fact, have a positive mass-energy. It's just their charge which is negative.

    An annihilation is when a particle collides with its antiparticle. This collision turns the entire mass-energy of both the particles into energy. That is, all the rest mass, all the kinetic energy, everything, is converted into energy, which will then be released carried by a boson or bosons (we'll say photons).
    These photons will have a total energy equal to the total energy of the particles and antiparticles (including the energy of its rest mass given by Einstein's formula of relativity, kinetic energy, and any other energy it possesses. I believe this would include the energy of the strong force between quarks if the annihilation was of a hadron).
    Their total momentum will also be equal to the momentum of the particles which were annihilated.
     
    Last edited: May 19, 2014
  7. May 20, 2014 #6

    mfb

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    Inverted, not negative. And that is true for all sorts of charges, not just the electric one.
    Electrons are counted as matter and have a negative (electric) charge, while positrons (their antiparticles) have a positive charge.

    Or other particles.

    Sure.
    Hadron annihilations usually produce a few pions and sometimes other particles.
     
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