![]() Therefore, such flavour quantum numbers are not of great use. These are conserved in strong and electromagnetic interactions, but violated by weak interactions. Leptons may be assigned the six flavour quantum numbers: electron number, muon number, tau number, and corresponding numbers for the neutrinos. Weak isospin and weak hypercharge are gauged in the Standard Model. In addition, one defines a quantum number called weak hypercharge, Y W, which is −1 for all left-handed leptons. Each doublet of a charged lepton and a neutrino consisting of opposite T 3 are said to constitute one generation of leptons. electron, muon and tau) and + 1 / 2 for the three associated neutrinos. In addition, leptons carry weak isospin, T 3, which is − 1 / 2 for the three charged leptons (i.e. In the electroweak theory, on the other hand, this symmetry is broken, and flavour changing processes exist, such as quark decay or neutrino oscillations.įlavour quantum numbers Leptons Īll leptons carry a lepton number L = 1. In quantum chromodynamics, flavour is a conserved global symmetry. ![]() Such matrices form a Lie group called SU(2) (see special unitary group). In other words, the theory possesses symmetry transformations such as M ( u d ), where u and d are the two fields (representing the various generations of leptons and quarks, see below), and M is any 2 × 2 unitary matrix with a unit determinant. ![]() Any (complex) linear combination of these two particles give the same physics, as long as the combinations are orthogonal, or perpendicular, to each other. If there are two or more particles which have identical interactions, then they may be interchanged without affecting the physics. Strong interactions conserve all flavours, but all flavour quantum numbers (other than B and L) are violated (changed, non-conserved) by electroweak interactions. In some theories, such as the grand unified theory, the individual baryon and lepton number conservation can be violated, if the difference between them ( B − L) is conserved (see Chiral anomaly). Thus, the eigenvalues of the various charge operators are conserved.Ībsolutely conserved flavour quantum numbers in the Standard Model are: A hadron's overall flavour quantum numbers depend on the numbers of constituent quarks of each particular flavour.Īll of the various charges discussed above are conserved by the fact that the corresponding charge operators can be understood as generators of symmetries that commute with the Hamiltonian. Analogously, the five flavour quantum numbers ( isospin, strangeness, charm, bottomness or topness) can characterize the quantum state of quarks, by the degree to which it exhibits six distinct flavours (u, d, s, c, b, t).Ĭomposite particles can be created from multiple quarks, forming hadrons, such as mesons and baryons, each possessing unique aggregate characteristics, such as different masses, electric charges, and decay modes. In atomic physics the principal quantum number of an electron specifies the electron shell in which it resides, which determines the energy level of the whole atom. Due to their quantum description, flavour states may also undergo quantum superposition. This is known as a flavour change, or flavour transmutation. In particular, the action of the weak force is such that it allows the conversion of quantum numbers describing mass and electric charge of both quarks and leptons from one discrete type to another. Quantum field theory, however, allows interactions that can alter other facets of a particle's nature described by non dynamical, discrete quantum numbers. In classical mechanics, a force acting on a point-like particle can only alter the particle's dynamical state, i.e., its momentum, angular momentum, etc. They can also be described by some of the family symmetries proposed for the quark-lepton generations. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles. The Standard Model counts six flavours of quarks and six flavours of leptons. In particle physics, flavour or flavor refers to the species of an elementary particle.
0 Comments
Leave a Reply. |