![]() Individual quark numbers are conserved in strong and e-m inter- actions, but can be changed by ☑ in weak interactions. Looking more closely we see that the individual quark numbers, U (up) and D (down) are also conserved, with the usual convention that an antiquark has a value – 1 : on each side U = 4 and D = 2. Looking again at our ‘impossible reaction’ and assigning a quark number of -1 to antiquarks, we can tally the quarks as follows: Left-hand side: Q = 3 + ( 1 – 1 ) = 3 Right-hand side: Q = 3 + 3 = 6 On the other hand the Q totals for the p + p p + n + π + are the same on both sides. Why not? Because the total baryon number on the left is 1 and on the right it is 2! The conservation of baryon number is really a special case of the conservation of quark number, Q. The following reaction could not happen: p + π – n + n even if the p and π – had enough kinetic energy and even though it conserves charge and lepton number. Again, baryon number is always conserved. proton, has B = 1 antibaryons, have B = – 1 leptons and mesons have B = 0. (b) Conservation of baryon and quark numbers Similarly to lepton number we define a baryon number, B. Explain which conservation law would be violated. (c) An isolated proton cannot decay into a neutron in this way. (b) State which of the interactions controls this decay. (a) Identify X and justify your choice in terms of relevant conservation laws. The half life of the decay is 10.1 minutes. The equation is: p n + e + + X where X is an unidentified particle. ![]() The radioactive isotope 13 7 N decays by the emission of a positron, when one of the protons in its nucleus transforms into a neutron. Calculate the mass of a neutron (a) in kg, (b) in MeV / c 2. The masses of atomic particles are often expressed in terms of the atomic mass unit, u which has a value of 1.660 × 10 –27 kg. State the increase in kinetic energy of each (a) in eV and (b) in J. A proton, an electron and a helium nucleus are each accelerated through a potential difference of 500 V.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |