Hint: the mass of the electron and positron are the same. Just use E=Mc^2. Express
final answer in MeV or KeV. Hint2: Answer between 250KeV and 1.2MeV.
Yes isn't that the equivalence principle .Now may I ask this question, "Is the local physics of a mass being accelerated by an outside source the same as the local physics of a mass being accelerated by an engine attached to the mass that is being accelerated?"
Yes isn't that the equivalence principle .
Initial angular momentum isn't zero, obviously.you can produce 3 or more photons K^2 , interesting , How does conservation of spin work in that case? I'm just wondering
Initial angular momentum isn't zero, obviously.
Basically, before annihilation, e-p pair briefly forms a bound state known as a positronium atom. Except for the fact that it can completely decay via annihilation, it behaves exactly like a hydrogen atom with reduced mass. If the annihilation happens from 1s state, only an even number of photons can be produced to conserve angular momentum. Rule of thumb in particle decay reactions is that the one with smallest number of products has the highest branching fraction. So you'd usually get 2 photons, but 4, 6, et cetera are also possible.
Of course, when the two random particles meet, their initial angular momentum is not generally zero. So when positronium forms, it is not in the 1s state. There are two things that can happen. Most common is that the positronium first decays to 1s state, emitting some low energy photons, just like hydrogen atom would. From there, above scenario takes place. Another possibility is that annihilation occurs before it decays to 1s. Say, it happens in 2p. Here, the angular momentum is one, so only an odd number of photons can be produced to conserve angular momentum. However, you can't produce just one, because that does not conserve translational momentum. So 3 is the lowest number you can get, and that's the most likely outcome. But again, 5, 7, and so on are also possible.
Notice that if we ignore angular momentum conservation, probability of 3 photon emission is much lower than probability of 2 photon emission. This means that a half-life of positronium in 2p is much higher than in 1s. And that's what usually gives it time to decay to 1s and annihilate into 2 photons.
Hm? What do you mean? There are two more charged particles in the lepton family, namely the muon and tau particles, and you could think of these as heavy versions of electron, since pretty much all other properties are identical, but an e-p pair does not have enough energy to produce a muon, let alone tau. Muon, on the other hand, can and does decay into an electron and a couple of neutrinos. Muon-anti-muon reaction, therefore, can have far more possible outcomes than e-p annihilation.
I am not aware of a single reaction where electron would become a heavier lepton. Muons and Tau are usually created in pair production reactions or in decay of mesons and baryons.
For example, a π+ meson may decay into a μ+ and νμ via emission of W+ bozon. In fact, that's almost always how it decays. That makes sense, since π+ has energy of 139.6 MeV, and μ+ is 105.7 MeV. The difference can be absorbed into kinetic energy of the muon and muon neutrino in proportion that would allow conservation of momentum.
Electrons and positrons, on the other hand, are only 512 keV. So there is not nearly enough energy to get a muon out of it. Of course, the muon wave function will still contribute due to tunneling effect and flavor oscillation, but with this energy gap, the contribution is so tiny as to be absolutely irrelevant.
A neutron does decay into proton, electron, and a neutrino. A positron can decay into a neutron and a positron if the conditions are favorable for such decay. (E.g. in a heavy nucleus, resulting in beta-bar decay, or in a neutron star.)
But yes, there is a difference between quarks in the baryon (protons, neutrons, ...) and in the meson (pions, rho mesons, ...) The difference is color charge. Both baryons and mesons are color "neutral", but while the color charge of meson is completely canceled, the color charge of baryons is merely balanced.
Unlike electric charges, which there are just two of, there are 6 possible color charges. There are 3 colors and 3 anti-colors. Red, green, blue, anti-red, anti-green, anti-blue. The names have nothing to do with visible spectrum, but rather with concept of color neutrality.
Every quark has a color. Every anti-quark has an anti-color. Color is a conserved quantity. So when you create a red charge, you must also create an anti-red charge. Gluons, which are carriers of strong nuclear force carry a color and an anti-color. And that's what results in color-neutrality.
Suppose a red quark emits a gluon. That gluon must carry a color, and the only available option is red. So it takes a red charge away from the gluon. It also needs an anti-color, and the two available options are anti-green or anti-blue. If it takes anti-green, green charge must also be created and deposited on the quark. It becomes green. If it takes anti-blue, quark becomes blue.
Now, what quark can absorb this gluon? Since it caries red, it can be absorbed by anti-red. In that case, the color of anti-red gluon changes to anti- of whatever the emitting gluon has become. So red-anti-red becomes blue-anti-blue or green-anti-green. The other option is to have the same color that the emitting quark. So red-blue would become blue-red and red-green would become green-red.
It is the nature of strong nuclear interaction that if the possible emitted gluons don't have anything to interact with, it will result in pair-production for the interaction to occur. So you cannot have an isolated quark. Furthermore, you cannot have a group of quarks that are not color-neutral. If you have a red quark, you must either have an anti-red to go with it or both blue and green quarks.
Particles consisting of a quark with color and quark with anti-color are called mesons. Because they contain a quark and an anti-quark they can spontaneously annihilate, sometimes, producing electrons or positrons as a byproduct.
Particles consisting of three quarks, one of each color, are called baryons. Particles consisting of three anti-colors are their anti-particles. A baryon does not contain anti-quarks. It cannot annihilate spontaneously without violating color conservation. It must meet a particle containing 3 anti-colors, which would have to be an anti-particle baryon. It does not have to be its own anti-particle, however. A neutron will annihilate with an anti-proton just fine, resulting in production of an electron. This process is a little bit more complicated, however, since it involves weak interactions.