4,681 research outputs found
The deconfinement phase transition, hadronization and the NJL model
One of the confident predictions of QCD is that at sufficiently high
temperature and/or density, hadronic matter should undergo a thermodynamic
phase transition to a colour deconfined state of matter - popularly called the
Quark-Gluon Plasma (QGP). In low energy effective theories of Quantum
Chromodynamics (QCD), one usually talks of the chiral transition for which a
well defined order parameter exists. We investigate the dissociation of pions
and kaons in a medium of hot quark matter decsribed by the Nambu - Jona Lasinio
(NJL) model. The decay widths of pion and kaon are found to be large but finite
at temperature much higher than the critical temperature for the chiral (or
deconfinement) transition, the kaon decay width being much larger. Thus pions
and even kaons (with a lower density compared to pions) may coexist with quarks
and gluons at such high temperatures. On the basis of such premises, we
investigate the process of hadronization in quark-gluon plasma with special
emphasis on whether such processes shed any light on acceptable low energy
effective theories of QCD.Comment: Invited talk at Hadrons'99, Coimbra, Portugal. To appear in the
Proceedings "Hadron Physics : Effetive Theories of Low Energy QCD" (Ed. A. H.
Blin), American Institute of Physics. 7 pages RevTeX, with 4 embedded
postscript figure
Revisiting Synthesis for One-Counter Automata
We study the (parameter) synthesis problem for one-counter automata with
parameters. One-counter automata are obtained by extending classical
finite-state automata with a counter whose value can range over non-negative
integers and be tested for zero. The updates and tests applicable to the
counter can further be made parametric by introducing a set of integer-valued
variables called parameters. The synthesis problem for such automata asks
whether there exists a valuation of the parameters such that all infinite runs
of the automaton satisfy some omega-regular property. Lechner showed that (the
complement of) the problem can be encoded in a restricted one-alternation
fragment of Presburger arithmetic with divisibility. In this work (i) we argue
that said fragment, called AERPADPLUS, is unfortunately undecidable.
Nevertheless, by a careful re-encoding of the problem into a decidable
restriction of AERPADPLUS, (ii) we prove that the synthesis problem is
decidable in general and in N2EXP for several fixed omega-regular properties.
Finally, (iii) we give a polynomial-space algorithm for the special case of the
problem where parameters can only be used in tests, and not updates, of the
counter
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