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Summary of the plenary session E
Nuclear Physics A478 (1988) 183c-185c North-Holland, Amsterdam
183c
SUMMARY OF THE PLENARY SESSION E J .W. NEGELE
Department
of Physics, Massachusetts Inst. of Technology, Cambridge, Massachusetts 02139,
USA
One of the fundamental problems in contemporary nuclear physics is to understand the phases of hadronic matter and the transitions between them. To place the problem in perspective, it is useful to contrast the study of hadronic matter with that of ordinary condensed matter systems, to recall the full range of phases and transitions of physical interest, and to highlight some of the salient open problems. Compared to nuclear physicists, condensed matter physicists enjoy luxurious lives . They have at their disposal bulk matter under an extremely wide range of controllable external conditions . Their microscopic understanding may be formulated in terms of interactions via static potentials using non-relativistic quantum mechanics . In nuclear physics, controlled experiments on bulk matter are replaced either by experiments on small clusters of at most a few hundred particles or by totally uncontrollable and only partially observable astrophysical events. And for us, the simplichly of non-relativistic quantum mechanics with static interactions is replaced by the complexity of quantum chromodynamics. To appreciate the restrictiveness of being denied access to bulk systems, imaging trying to discern the rich superfluid phase structure of liquid helium if one were. limited to observation of helium drops composed of a few hundred atoms . This example is particularly appropriate to the contribution in this session of Professor Shapiro, who has considered the possibility of triplet pairing in nuclear matter which is analogous to the more general pairing states observed in liquid helium. The range of physical systems in which to study the phase of hadronic matter is far too broad to addre.^^-_ in a single session . One system is ordinary nuclear matter, composed of neutrons and protons at the density observed in the interior of heavy nuclei . A somewhat different system is the charge-neutral matter occurring in supernova collapse and in neutron stars . Yet another system, which is the principal focus of the invited talks in this session, is the hadronic matter at zero net baryon density which is believed to be formed in the central region of very high energy collisions and can be studied in lattice QCD at zero chemical potential. This range of systems sustains a rich phase structure which, depending upon the density and temperature, can involve nucleonic, mesonic, and quark and gluon degrees of freedom. For self-bound saturating nuclear matter, there is the standard liquid-gas phase transition. In neutron star matter, as the baryon density is increased past nuclear matter density, there is a phase transition in which the clustered phase at 0375-9474/88/$03 .50 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
184c
J. W. Negele / Summary of the Plenary Session E
low density is replaced by a uniform liquid pha;;a at higher density. The liquid g%ase of nuclear and neutron star matter, in turn, possessess- and p-wave superfluid phases as well as the possibility of pion and perhaps even kaon condensed phases. Of specific interest in this session are the deconfinement transition, in which the quark clusters we identify as hadrons are replaced by quark matter or aquark-gluon plasma, and the chiral symmetry breaking transition. Thus, the problem of phase transitions in nuclear matter is far broader than can be addressed in this single session, and several other sessions are directly relevant to this topic, including the subsequent plenary session on the interplay between astrophysics, cosmology, and particle and nuclear physics, and the parallel sessions on high energy heavy ion physics, multiparticle dynamics andextreme states, and astrophysics and cosmology. The invited lectures in this session summarize two major experimental and theoretical initiatives beingundertaken to explore the transition from nuclearmatter to a new deconfined phase of hadronic matter. Ultrarelativistic heavy ion collisions have entered an important new stage with the recent collisions of oxygen beams with heavy targets at the CERN SPS. At this stage, since the data has yet to be fully analyzed, these experiments serve more to whet our appetites and to focus our thinking on the observables which are accessible and their ambiguities, rather than to answer ourfundamental questions concerning the phases of hadronic matter. To complement theextremelyoptimistic view Dr. Willis presented, suggesting that there may already be experimental evidence for a phase transition, it is useful to mention several concerns arisingfrom the theoretical perspective. One major concern is the extreme dependence of current analyses upon comparisons with uncertain theories . To the extent to which the presently discussed signatures are dependent upon model-dependentcalculations of transverse momentum distributions, flavor production, or dilepton pair production, fundamental improvements will be required in our theoretical treatment of hadronic many-body theory and quantitative QCD calculations to make these signatures credible. A second reservation concerns the sizes of present projectiles . Underthe best of circumstances with the largestnuclear targets and projectiles, one may already question whether the time scale will be long enough and spatial extent largeenough to produce a reasonable approximation to bulk equilibrated matter, and for the extreme case of nucleon-nucleon collisions, great optimism indeed is required. The anticipated progression to heavier nuclei is thus crucial in two respects, both in providing the most optimal circumstances for production of a quark-gluon plasma and in providing systematic A-dependence which couldprovide anon-trivial test of themodels used to define presentsignatures of a phase transition. Substantial progress has been made in lattice QCD calculations in recent years, and is reviewed in detail in the paper by Professor Ukawa. From this work, many aspects of the phase transition at zero baryon density are becoming clear. Despite remaining questions concerning the continuum limit and problems associated with describing twoflavorsof quarks on alattice, thecurrentevidence suggests afirst-order
J. W. Negete / Summary ofthe Plenary Session E
1ß5c
phase transition for the physically relevant case of two flavors of light quarks . Unfortunately, as clearly explained by Ukawa, until someone comes up with a radically newapproach, thereis no immediate prospect of performing an analogous study of the phase transition at finite baryon density. The fundamental appeal of actually solving QCD rather than making simple models which are at best extreme caricatures of it clearly warrants continued effort on both the open conceptual and practical problems . The present discussion of chiral and deconfining phase transitions is presently clouded by the fact that we do not have a strictly defined order parameter in the Landau sense for finite quark masses. The Wilson or Polyakov line is a proper crder parameter forthe deconfinement transition only for mq= co and the chiral order parameter (qq) is strictly applicable only at mq=0, so the questions of whether there are one or two separate transitions at finite mq, and how to characterize them are still open. Anaturalconcern aboutlarge-scale computer calculations is that they mayproduce numbers without producing insight, a fear eloquently expressed in the statement attributed to Picasso: "The problemwith computers is that they only give answers" . Clearly, a key issue is to formulate the right questions to provide real insight into thestructure of the QCDground state anditsexcitations at finite temperature. There are several promising indications that such insight-producing questions are now beingformulated andanswered in lattice calculations . One example wasthecalculation of static screening lengths discussed by Carleton DeTar. By measuring an appropriately defined correlation function at finite temperature, he measured the dispersion relations at finite temperature ofcollective excitations having thequantum number of the pion, nucleon, and otherfamiliar baryons. At one level, the result is intuitively obvious - the pionic collective excitations is massless in the chiral symmetry broken phase and massive in the symmetric phase. However, at another level, it is interesting to ask what the structure is of the collective "pionic" mode in thequark-gluonplasma phase, andwhether it maybe an experimentally significant degree of freedom. Another important set of questions is raised in the abstract of Janos Polonyi. To gain insight into the fundamental excitations and structure of various phases, he has defined on the lattice and calculated the density of color monopoles and is looking for color deformed states analogous to the spatially deformed intrinsic states familiar from deformed nuclei . As in the case of the Kosterlitz-Thouless transition in two-dimensional spin systems, where identification of vortices is the crucial issue in understanding and characterizing the phase transition, such studies in QCD could play an important role in elucidating the essential physics. In conclusion, major experimental and theoretical efforts are now beginning to provide the toolsto explore one of the most challenging and fundamental problems at the interface of contemporary nuclear and particle physics - understanding the phases and phase transitions of hadronic matter.
183c
SUMMARY OF THE PLENARY SESSION E J .W. NEGELE
Department
of Physics, Massachusetts Inst. of Technology, Cambridge, Massachusetts 02139,
USA
One of the fundamental problems in contemporary nuclear physics is to understand the phases of hadronic matter and the transitions between them. To place the problem in perspective, it is useful to contrast the study of hadronic matter with that of ordinary condensed matter systems, to recall the full range of phases and transitions of physical interest, and to highlight some of the salient open problems. Compared to nuclear physicists, condensed matter physicists enjoy luxurious lives . They have at their disposal bulk matter under an extremely wide range of controllable external conditions . Their microscopic understanding may be formulated in terms of interactions via static potentials using non-relativistic quantum mechanics . In nuclear physics, controlled experiments on bulk matter are replaced either by experiments on small clusters of at most a few hundred particles or by totally uncontrollable and only partially observable astrophysical events. And for us, the simplichly of non-relativistic quantum mechanics with static interactions is replaced by the complexity of quantum chromodynamics. To appreciate the restrictiveness of being denied access to bulk systems, imaging trying to discern the rich superfluid phase structure of liquid helium if one were. limited to observation of helium drops composed of a few hundred atoms . This example is particularly appropriate to the contribution in this session of Professor Shapiro, who has considered the possibility of triplet pairing in nuclear matter which is analogous to the more general pairing states observed in liquid helium. The range of physical systems in which to study the phase of hadronic matter is far too broad to addre.^^-_ in a single session . One system is ordinary nuclear matter, composed of neutrons and protons at the density observed in the interior of heavy nuclei . A somewhat different system is the charge-neutral matter occurring in supernova collapse and in neutron stars . Yet another system, which is the principal focus of the invited talks in this session, is the hadronic matter at zero net baryon density which is believed to be formed in the central region of very high energy collisions and can be studied in lattice QCD at zero chemical potential. This range of systems sustains a rich phase structure which, depending upon the density and temperature, can involve nucleonic, mesonic, and quark and gluon degrees of freedom. For self-bound saturating nuclear matter, there is the standard liquid-gas phase transition. In neutron star matter, as the baryon density is increased past nuclear matter density, there is a phase transition in which the clustered phase at 0375-9474/88/$03 .50 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
184c
J. W. Negele / Summary of the Plenary Session E
low density is replaced by a uniform liquid pha;;a at higher density. The liquid g%ase of nuclear and neutron star matter, in turn, possessess- and p-wave superfluid phases as well as the possibility of pion and perhaps even kaon condensed phases. Of specific interest in this session are the deconfinement transition, in which the quark clusters we identify as hadrons are replaced by quark matter or aquark-gluon plasma, and the chiral symmetry breaking transition. Thus, the problem of phase transitions in nuclear matter is far broader than can be addressed in this single session, and several other sessions are directly relevant to this topic, including the subsequent plenary session on the interplay between astrophysics, cosmology, and particle and nuclear physics, and the parallel sessions on high energy heavy ion physics, multiparticle dynamics andextreme states, and astrophysics and cosmology. The invited lectures in this session summarize two major experimental and theoretical initiatives beingundertaken to explore the transition from nuclearmatter to a new deconfined phase of hadronic matter. Ultrarelativistic heavy ion collisions have entered an important new stage with the recent collisions of oxygen beams with heavy targets at the CERN SPS. At this stage, since the data has yet to be fully analyzed, these experiments serve more to whet our appetites and to focus our thinking on the observables which are accessible and their ambiguities, rather than to answer ourfundamental questions concerning the phases of hadronic matter. To complement theextremelyoptimistic view Dr. Willis presented, suggesting that there may already be experimental evidence for a phase transition, it is useful to mention several concerns arisingfrom the theoretical perspective. One major concern is the extreme dependence of current analyses upon comparisons with uncertain theories . To the extent to which the presently discussed signatures are dependent upon model-dependentcalculations of transverse momentum distributions, flavor production, or dilepton pair production, fundamental improvements will be required in our theoretical treatment of hadronic many-body theory and quantitative QCD calculations to make these signatures credible. A second reservation concerns the sizes of present projectiles . Underthe best of circumstances with the largestnuclear targets and projectiles, one may already question whether the time scale will be long enough and spatial extent largeenough to produce a reasonable approximation to bulk equilibrated matter, and for the extreme case of nucleon-nucleon collisions, great optimism indeed is required. The anticipated progression to heavier nuclei is thus crucial in two respects, both in providing the most optimal circumstances for production of a quark-gluon plasma and in providing systematic A-dependence which couldprovide anon-trivial test of themodels used to define presentsignatures of a phase transition. Substantial progress has been made in lattice QCD calculations in recent years, and is reviewed in detail in the paper by Professor Ukawa. From this work, many aspects of the phase transition at zero baryon density are becoming clear. Despite remaining questions concerning the continuum limit and problems associated with describing twoflavorsof quarks on alattice, thecurrentevidence suggests afirst-order
J. W. Negete / Summary ofthe Plenary Session E
1ß5c
phase transition for the physically relevant case of two flavors of light quarks . Unfortunately, as clearly explained by Ukawa, until someone comes up with a radically newapproach, thereis no immediate prospect of performing an analogous study of the phase transition at finite baryon density. The fundamental appeal of actually solving QCD rather than making simple models which are at best extreme caricatures of it clearly warrants continued effort on both the open conceptual and practical problems . The present discussion of chiral and deconfining phase transitions is presently clouded by the fact that we do not have a strictly defined order parameter in the Landau sense for finite quark masses. The Wilson or Polyakov line is a proper crder parameter forthe deconfinement transition only for mq= co and the chiral order parameter (qq) is strictly applicable only at mq=0, so the questions of whether there are one or two separate transitions at finite mq, and how to characterize them are still open. Anaturalconcern aboutlarge-scale computer calculations is that they mayproduce numbers without producing insight, a fear eloquently expressed in the statement attributed to Picasso: "The problemwith computers is that they only give answers" . Clearly, a key issue is to formulate the right questions to provide real insight into thestructure of the QCDground state anditsexcitations at finite temperature. There are several promising indications that such insight-producing questions are now beingformulated andanswered in lattice calculations . One example wasthecalculation of static screening lengths discussed by Carleton DeTar. By measuring an appropriately defined correlation function at finite temperature, he measured the dispersion relations at finite temperature ofcollective excitations having thequantum number of the pion, nucleon, and otherfamiliar baryons. At one level, the result is intuitively obvious - the pionic collective excitations is massless in the chiral symmetry broken phase and massive in the symmetric phase. However, at another level, it is interesting to ask what the structure is of the collective "pionic" mode in thequark-gluonplasma phase, andwhether it maybe an experimentally significant degree of freedom. Another important set of questions is raised in the abstract of Janos Polonyi. To gain insight into the fundamental excitations and structure of various phases, he has defined on the lattice and calculated the density of color monopoles and is looking for color deformed states analogous to the spatially deformed intrinsic states familiar from deformed nuclei . As in the case of the Kosterlitz-Thouless transition in two-dimensional spin systems, where identification of vortices is the crucial issue in understanding and characterizing the phase transition, such studies in QCD could play an important role in elucidating the essential physics. In conclusion, major experimental and theoretical efforts are now beginning to provide the toolsto explore one of the most challenging and fundamental problems at the interface of contemporary nuclear and particle physics - understanding the phases and phase transitions of hadronic matter.