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Quantum Chromodynamics (QCD) QCD Is the Theory That Describes the Action of the Strong Force

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Quantum Chromodynamics (QCD) QCD Is the Theory That Describes the Action of the Strong Force Quantum chromodynamics (QCD) QCD is the theory that describes the action of the strong force. QCD was constructed in analogy to quantum electrodynamics (QED), the quantum field theory of the electromagnetic force. In QED the electromagnetic interactions of charged particles are described through the emission and subsequent absorption of massless photons (force carriers of QED); such interactions are not possible between uncharged, electrically neutral particles. By analogy with QED, quantum chromodynamics predicts the existence of gluons, which transmit the strong force between particles of matter that carry color, a strong charge. The color charge was introduced in 1964 by Greenberg to resolve spin-statistics contradictions in hadron spectroscopy. In 1965 Nambu and Han introduced the octet of gluons. In 1972, Gell-Mann and Fritzsch, coined the term quantum chromodynamics as the gauge theory of the strong interaction. In particular, they employed the general field theory developed in the 1950s by Yang and Mills, in which the carrier particles of a force can themselves radiate further carrier particles. (This is different from QED, where the photons that carry the electromagnetic force do not radiate further photons.) First, QED Lagrangian… µ ! # 1 µν LQED = ψeiγ "∂µ +ieAµ $ψe − meψeψe − Fµν F 4 µν µ ν ν µ Einstein notation: • F =∂ A −∂ A EM field tensor when an index variable µ • A four potential of the photon field appears twice in a single term, it implies summation µ •γ Dirac 4x4 matrices of that term over all the values of the index •ψe Dirac four-spinor of the electron field • e = 4πα, 1/α ≈ 137, ! = c =1 µ µ electromagnetic four-current density of the electron: J = e ψeγ ψe From the Euler–Lagrange equation of motion for a field, we get: µ µ (iγ ∂µ − me )ψe = eγ Aµψe Dirac equation for the electron □Aµ = e ψ γ µψ Maxwell equations for the EM fields e e in the Lorentz gauge µ d'Alembert operator □ =∂ ∂µ Vacuum polarization in QED force between two electrons + (in natural units) + + p - e = 4⇡↵ - - + - + - + - - + - + + QED vacuum is dielectric: electric charge is screened; interaction becomes weak at large distances In QED the presence of an electric charge eo polarizes the vacuum and the charge that is observed at a large distance differs from eo and is given by e = eo/ε with ε>1 the dielectric constant of the vacuum. QED vacuum Let us consider empty space. In a quantum field theory, we cannot just say that the ground state of the empty space is the state with no quanta - we have to solve the proper field equations, with proper boundary conditions, and determine what is the state of the field. Such a state may or may not contain quanta. In particular, whenever the space has a boundary, the ground state of the field does contain quanta - this fact is called the vacuum polarization effect. In QED, this is a very well known, and experimentally verified effect. For example, two conducting parallel plates attract each other, even if they are not charged and placed in otherwise empty space (this is called the Casimir effect). Here, the vacuum fluctuations of the electron field may create in an empty space virtual electron-positron pairs. These charged particles induce virtual polarization charges in the conducting plates (it means virtual photons are created, travel to plates, and reflect from them). Hence, the plates become virtually charged, and attract one another during a short time when the existence of the virtual charges, and virtual photons, is allowed by the Heisenberg principle. All in all, a net attractive force between plates appears. d − distance between the plates Also: Lamb shift QCD Lagrangian 1 L = ψ iγ µ !δ ∂ +ig Gαt $ψ − m ψ ψ − Gα Gµν QCD ∑( qi "# ij µ ( µ α )ij %& qj q qi qi ) 4 µν α q µ ! # 1 µν LQED = ψeiγ "∂µ +ieAµ $ψe − meψeψe − Fµν F 4 µν µ ν ν µ αβγ µ ν •Gα =∂ Gα −∂ Gα − gf Gβ Gγ color fields tensor µ four potential of the gluon fields (a=1,..8) •Gα • tα 3x3 Gell-Mann matrices; generators of the SU(3) color group •f αβγ structure constants of the SU(3) color group •ψi Dirac spinor of the quark field (i represents color) • g = 4παs (! = c =1) color charge (strong coupling constant) • The quarks have three basic color-charge states, which can be labeled as i=red, green, and blue. Three color states form a basis in a 3-dimensional vector space. A general color state of a quark is then a vector in this space. The color state can be rotated by 3 × 3 unitary matrices. All such unitary transformations with unit determinant form a Lie group SU(3). • A crucial difference between the QED and QCD is that the gluon field tensors contain the additional term representing interaction between color-charged gluons. • While sources of the electromagnetic field depend on currents that involve a small parameter, gluons are sources of the color field without any small parameter. Gluons are not only color-charged, but they also produce very strong color fields. –15– Quark masses PDG, 2010 Figure 3.Thevaluesofeachquarkmassparametertaken fromt mass: the Data 172.44 Listings. Points ± 0.13 from (stat) papers reporting± 0.47 no (syst error ) bars are colored grey. Arrows indicate limits reported. The grey regions indicate values excluded by our evaluations; some regions were determined in part though examination of Fig. 2. July 30, 2010 14:34 Vacuum polarization in QCD The left diagram is shared by QED and QCD which renders the interaction stronger at shorter distance (screening). The second diagram arising from the nonlinear interaction between gluons in QCD has the antiscreening effect, which makes the coupling weaker at short distance. • Color is anti-screened • Color builds up away from a source • Interaction becomes strong at large distances (low momenta) • Confinement of quarks; quarks are not observed as isolated particles g2 Strong coupling constant α = s 4π In quantum field theory, the coupling constant is an effective constant, which depends on four-momentum Q2 transferred. For strong interactions, the Q2 dependence is very strong (gluons - as the field quanta - carry color and they can couple to other gluons). A first- order perturbative QCD calculation (valid at very large Q2) gives: 2 12π running coupling α Q = s ( ) 2 2 constant! (22 − 2n f )⋅ ln(Q / ΛQCD ) nf =6 − number of quark flavors LQCD − a parameter in QCD (~0.22 GeV), an infrared cutoff The spatial separation between quarks goes as " ! = Q2 Therefore, for very small distances and high values of Q2, the inter- quark coupling decreases, vanishing asymptotically. In the limit of very large Q2, quarks can be considered to be “free” (asymptotic freedom). On the other hand, at large distances, the inter-quark coupling increases so it is impossible to detach individual quarks from hadrons (confinement). Asymptotic freedom was described in 1973 by Gross, Wilczek, and Politzer (Nobel Prize 2004). It is customary to quote as at the 91 GeV energy scale (the mass of the Z boson) In QED, coupling increases with increasing energy. At low energies, the fine structure constant a≈ 1/137, whereas at the scale of the Z boson, about 90 GeV, one gets a ≈ 1/127. Chiral symmetry For massless quarks, QCD Lagrangian preserves helicity. Indeed, since a massless quark travels at the speed of light, the handedness or chirality of the quark is independent of any Lorentz frame from which the observation is made. L = L (ψ )+L (ψ ) the QCD interaction does not couple the QCD QCD L QCD R left and right-handed quarks The mass term explicitly breaks the chiral symmetry as: mqψqψq = mqψqLψqR + mqψqRψqL The main origin of the chiral symmetry breaking, however, may be described in terms of the fermion condensate (vacuum condensate of bilinear expressions involving the quarks in the QCD vacuum) formed through nonperturbative action of QCD gluons. Spontaneous symmetry breaking due to the strong low-energy QCD dynamics, which rearranges the QCD vacuum: 3 ψqLψqR ∝ ΛQCD ≠ 0 QCD vacuum The QCD vacuum polarization effect is extremely strong, and the empty space is not empty at all - it must contain a soup of spontaneously appearing, interacting, and disappearing gluons. Moreover, in the soup there also must be pairs of virtual quark-antiquark pairs that are also color-charged, and emit and absorb more virtual gluons. It turns out that the QCD ground state of an “empty” space is extremely complicated. At present, we do not have any glimpse of a possibility to find the vacuum wave function analytically. Some ideas of what happens are provided by the QCD lattice calculations, in which the gluon and quark fields are discretized on a four-dimensional lattice of space-time points, and the differential field equations are transformed into finite-difference equations solvable on a computer. http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/index.html The typical four-dimensional structure of gluon-field configurations averaged over in describing the vacuum properties of QCD. The volume of the box is 2.4 by 2.4 by 3.6 fm, big enough to hold a couple of protons. Color, Gluons Gluons are the exchange particles which couple to the color charge . They carry simultaneously color and anticolor. red antigreen antiblue blue green antired http://www.particleadventure.org Gluons What is the total number of gluons? According to SU3, 3x3 color combinations form a singlet and an octet. The octet states form a basis from which all other color states can be constructed. The way in which these eight states are constructed from colors and anticolors is a matter of convention.
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