{"query": "How does the amount of energy affect how stuff scatters?", "pos": "it has been suggested that one may construct a lorentz - invariant noncommutative field theory by extending the coordinate algebra to additional, fictitious coordinates that transform nontrivially under the lorentz group. integration over these coordinates in the action produces a four - dimensional effective theory with lorentz invariance intact. previous applications of this approach, in particular to a specific construction of noncommutative qed, have been studied only in a low - momentum approximation. here we discuss lorentz - invariant field theories in which the relevant physics can be studied without requiring an expansion in the inverse scale of noncommutativity. qualitatively, we find that tree - level scattering cross sections are dramatically suppressed as the center - of - mass energy exceeds the scale of noncommutativity, that cross sections that are isotropic in the commutative limit can develop a pronounced angular dependence, and that nonrelativistic potentials (for example, the coloumb potential) become nonsingular at the origin. we consider a number of processes in noncommutative qed that may be studied at a future linear collider. we also give an example of scattering via a four - fermion operator in which the noncommutative modifications of the interaction can unitarize the tree - level amplitude, without requiring any other new physics in the ultraviolet."}
{"query": "Why go for canonical instead of grand-canonical?", "pos": "the production of hadrons in relativistic heavy ion collisions is studied using a statistical ensemble with thermal and chemical equilibrium. special attention is given to exact conservation laws, i.e. certain charges are treated canonically instead of using the usual grand canonical approach. for small systems, the exact conservation of baryon number, strangeness and electric charge is to be taken into account. we have derived compact, analytical expressions for particle abundances in such ensemble. as an application, the change in @xmath0 ratios in ags experiments with different interaction system sizes is well reproduced. the canonical treatment of three charges becomes impractical very quickly with increasing system size. thus, we draw our attention to exact conservation of strangeness, and treat baryon number and electric charge grand canonically. we present expressions for particle abundances in such ensemble as well, and apply them to reproduce the large variety of particle ratios in gsi sis 2 a gev ni  ni experiments. at the energies considered here, the exact strangeness conservation fully accounts for strange particle suppression, and no extra chemical factor is needed.    [ on the exact conservation laws in thermal models ]"}
{"query": "How good is the mean-field approximation at guessing the ground state features of molecular stuff?", "pos": "we present a model for molecular materials made up of polar and polarizable molecular units. a simple two state model is adopted for each molecular site and only classical intermolecular interactions are accounted for, neglecting any intermolecular overlap. the complex and interesting physics driven by interactions among polar and polarizable molecules becomes fairly transparent in the adopted model. collective effects are recognized in the large variation of the molecular polarity with supramolecular interactions, and cooperative behavior shows up with the appearance, in attractive lattices, of discontinuous charge crossovers. the mf approximation proves fairly accurate in the description of the gs properties of mm, including static linear and non - linear optical susceptibilities, apart from the region in the close proximity of the discontinuous charge crossover. sizeable deviations from the excitonic description are recognized both in the excitation spectrum and in linear and non - linear optical responses. new and interesting phenomena are recognized near the discontinuous charge crossover for non - centrosymmetric clusters, where the primary photoexcitation event corresponds to a multielectron transfer."}
