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## Order-disorder
In quantum field theory and statistical mechanics in the thermodynamic limit, a system with a global symmetry can have more than one phase. For parameters where the symmetry is spontaneously broken, the system is said to be The simplest example is the freezing of a liquid into a solid. The translational and rotational symmetries are broken in the ordered solid phase, and they are restored in the disordered liquid phase. But there are many other examples. In high energy physics, the formation of the chiral condensate in quantum chromodynamics is an ordering transition. ## Additional recommended knowledge
## Superselection SectorsA large physical system with many degrees of freedom does not always visit every possible state, even if it has enough energy. If a magnet is magnetized in a certain direction, each spin will fluctuate at any temperature, but the net magnetization will never change. The reason is that it is infinitely improbable that all the infinitely many spins at each different position will all fluctuate together in the same way. A bottle sitting on a table will never move, assuming no external forces act. This is true even though the energy of the bottle at any position would be exactly the same. A microscopic bottle would eventually occupy every position with equal probability. But because the bottle is large, the probability of it ever moving is zero. No local fluctuation of the atoms will ever come together to move it any distance. The bottle is not only motionless on a macroscopic scale. Its center of mass is motionless microscopically. The fluctuations of the atoms define an average position for the cork which is defined to within less than a nuclear radius. This is because the average of N independent fluctuations scales to zero as . Assuming only that the atomic fluctuations do not conspire over long distances, the suppression factor can be enormous. A big system often has There are quantum fluctuations, superpositions arising from different configurations of a phase-type path integral, and statistical fluctuations from a Boltzmann type path integral. Both of these path integrals have the property that large changes in an effectively infinite system require an improbable conspiracy between the fluctuations. So there are both statistical mechanical and quantum mechanical superselection rules. In a theory where the vacuum is invariant under a symmetry, the conserved charge leads to superselection sectors in the case that the charge is conserved. Electric charge is conserved in our universe, so it seems at first like a trivial example. But when a superconductor fills space, or equivalently in a Higgs phase, electric charge is still globally conserved but no longer defines the superselection sectors. The sloshing of the superconductor can bring charges into any volume at very little cost. In this case, the superselection sectors of the vacuum are labeled by the direction of the Higgs field. Since different Higgs directions are related by an exact symmetry, they are all exactly equivalent. This suggests a deep relationship between symmetry breaking directions and conserved charges. ## Discrete SymmetryIn the 2D Ising model, at low temperatures, there are two distinct pure states, one with the average spin pointing up and the other with the average spin pointing down. This is the ordered phase. At high temperatures, there is only one pure state with an average spin of zero. This is the disordered phase. At the phase transition between the two, the symmetry between spin up and spin down is broken. Below the phase transition temperature, an infinite ising model can be in either the mostly-plus or the mostly-minus configuration. If it starts in the mostly-plus phase, it will never reach the mostly-minus, even though flipping all the spins will give the same energy. By changing the temperature, the system acquired a new superselection rule--- the average spin. There are two superselection sectors--- mostly minus and mostly plus. When a new superselection rule appears, the system has For any finite system, the superselection is imperfect. An Ising model on a finite lattice will eventually fluctuate from the mostly plus to the mostly minus at any nonzero temperature, but it takes a very long time. The amount of time is exponentially small in the size of the system measured in correlation lengths, so for all practical purposes the flip never happens even in systems only a few times larger than the correlation length. ## Continuous SymmetriesIf a statistical or quantum field has three real valued scalar fields φ and define the action in a quantum field context or free energy in the statistical context. There are two phases. When t is large, the potential tends to move the average φ to zero. For t large and negative, the quadratic potential pushes φ out, but the quartic potential prevents it from becoming infinite. If this is done in a quantum path integral, this is a So as t moves toward more negative values in either context, the field has to choose some direction to point. Once it does this, it cannot change its mind. The system has In the disordered phase, the superselection sectors are described by the representation of SO(3) under which a given configuration transforms globally. Because the SO(3) is unbroken, different representations will not mix with each other. No local fluctuation will ever bring in nontrivial SO(3) configurations from infinity. A local configuration is entirely defined by its representation. There is a mass gap, or a correlation length, which separates configurations with a nontrivial SO(3) transformations from the rotationally invariant vacuum. This is true until the critical point in t where the mass gap disappears and the correlation length is infinite. The vanishing gap is a sign that the fluctuations in the SO(3) field are about to condense. In the ordered region, there are field configurations which can carry topological charge. These are labeled by elements of the second homotopy group . Each of these describe a different field configuration which at large distances from the origin is a winding configuration. Although each such isolated configuration has infinite energy, it labels superselection sectors where the difference in energy between two states is finite. In addition, pairs of winding configurations with opposite topological charge can be produced copiously as the transition is approached from below. When the winding number is zero, so that the field everywhere points in the same direction, there is an additional infinity of superselection sectors, each labelled by a different value of the unbroken SO(2) charge. In the ordered state, there is a mass gap for the superselection sectors labeled by a nonzero integer, because the topological solitons are massive even infinitely massive. But there is no mass gap for the all the superselection sectors labeled by zero because there are massless Goldstone bosons describing fluctuations in the direction of the condensate. If the field values are identified under a It is interesting that O(3) charges only make sense in the disordered phase and not at all in the ordered phase. This is because when the symmetry is broken there is a condensate which is charged, which is not invariant under the symmetry group. Conversely, the topological charge only makes sense in the ordered phase and not at all in the disordered phase, because in some hand-waving way there is a "topological condensate" in the disordered phase which randomizes the field from point to point. The randomizing can be thought of as crossing many condensed topological winding boundaries. The very question of what charges are meaningful depends very much on the phase. Approaching the phase transition from the disordered side, the mass of the charges particles approaches zero. Approaching it from the ordered side, the mass gap associated with fluctuations of the topological solitions approaches zero. ## Examples in Particle Physics- Higgs Mechanism
In the standard model of particle physics, in the electroweak sector, the low energy model is SU(2) and U(1) broken to U(1) by a Higgs doublet. The only superselection rule determining the configuration is the total electric charge. If there are monopoles, then the monopole charge must be included. If the Higgs t parameter is varied so that it does not acquire a vacuum expectation value, the universe is now symmetric under an unbroken SU(2) and U(1) gauge group. If the SU(2) has infinitesimally weak couplings, so that it only confines at enormous distances, then the representation of the SU(2) group and the U(1) charge both are superselection rules. But if the SU(2) has a nonzero coupling then the superselection sectors are separated by infinite mass because the mass of any state in a nontrivial representation is infinite. By changing the temperature, the Higgs fluctuations can zero out the expectation value at a finite temperature. Above this temperature, the SU(2) and U(1) quantum numbers describe the superselection sectors. Below the phase transition, only electric charge defines the superselection sector. - Chiral Quark Condensate
Consider the global flavour symmetry of QCD in the chiral limit where the masses of the quarks are zero. This is not exactly the universe in which we live, where the up and down quarks have a tiny but nonzero mass, but it is a very good approximation, to the extent that isospin is conserved. Below a certain temperature which is the symmetry restoration temperature, the phase is ordered.
The chiral condensate forms, and pions of small mass are produced. The SU(N It is an open question whether the deconfinement temperature of QCD is also the temperature at which the chiral condensate melts. |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Order-disorder". A list of authors is available in Wikipedia. |