Experiments with Lead projectiles of 33 TeV total energy at the CERN SPS
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Our current understanding of the strong interaction, one of the four basic forces of nature, is based on Quantum Chromodynamics (QCD). This theory describes how quarks interact by exchanging gluons, and helps to describe the structures of bound aggregates of two or three quarks that we call hadrons (pions, kaons, protons, neutrons ...) which are held together by the strong force.
QCD is designed in analogy to the well proven theory of elementary electromagnetic interactions: quantum electrodynamics (QED). This theory describes how ordinary charged particles interact by exchanging photons, thus explaining the corresponding bound aggregates which range from atoms and molecules to extended solid materials, and are held together by the electromagnetic force. Our familiar, cold world consists of such objects, with basic QED structure. An interesting further realm of QED structures, beyond cold objects, is presented by the electromagnetic plasma states governing stellar interiors at high energy density. With increasing temperature, the medium becomes more and more ionized: electrons lose their binding to individual atoms or molecules. In the extreme case, of a white dwarf burned-out star, the electrons completely decouple from the atomic nuclei forming a continuous Fermi liquid. The properties of white dwarfs are governed by this electron plasma which is a perfect conductor even though the initial constituents of the star were perfectly neutral atoms. In modern terminology, the electrons are completely deconfined in this plasma from their low temperature atomic bound states.
It is one of the remaining open problems of strong interaction QCD to clarify whether the formal analogy between QED and QCD, proven to exist as far as the corresponding cold objects (atoms and hadrons) are concerned, extends to finite temperature behaviour. Does a deconfined (colour conducting) continuous QCD plasma state exist? This is not merely a formality - assessing behaviour fully analogous to QED would satisfy the desire to show that such gauge theories are, universally, the last word concerning the fundamental interactions. Additional emphasis is placed on the demonstration of plasma existence, stemming from two further considerations:
In the terrestrial laboratory the only conceivable avenue toward creating a volume of sufficient size, energy density and life time could be to bombard heavy nuclei head-on at ultra-relativistic energy. This violent collision might create, for a short instant of time, a "fireball" reaction volume that allows to generate the plasma state . To this end, Lead nuclei (208Pb) are accelerated at CERN since 1994 to an energy of 158 GeV per nucleon in the projectile, i.e. to about 33 TeV total energy. This involved a newly built special injector accelerator along with the synchrotron-chain BOOSTER, PS, SPS. At the Brookhaven AGS, similarly heavy Gold projectiles are available since 1993 but it is the special point of the ten times higher CERN energy that one can be confident here (based on former experiments with lighter nuclear projectiles) to reach the necessary energy density, of several GeV per cubic fermi of reaction volume, where we expect from QCD to create a plasma.
How do we imagine the dynamical evolution of a high energy, head-on collision of heavy nuclei ?
In the centre of mass frame, two Lorenz-contracted discs filled with nucleons interpenetrate. In the initial phase of microscopic nucleon collisions, these QCD bound states (of three valence quarks, virtual "sea" quark-antiquark pairs, and gluons) get disrupted and spread out over phase space. These processes create a pre-equilibrium state of partons which still possess a preferential momentum orientation along the beam direction. Partonic rescattering will, in the next stage, bring the reaction volume closer to a thermal distribution, with an approximate equipartition of momentum orientation, and also approaching a flavour equipartition among the three light quark species, up, down and strange. The energy density being high enough, this state may settle into the equilibrium plasma state of quarks and gluons which is the desired object: a deconfined, continuous QCD high temperature state, if this state really exists! At an energy density of several GeV per cubic fermi, this phase will expand and cool until the phase transition point is reached where the partons recombine into hadrons. It would be nice now to see all newly born hadrons going directly into the detector from the phase transition point. However, nature has put an obstacle into the way of such a direct observation of the phase transition products: the spatial hadron density is very high still, the new hadrons will keep interacting until further expansion dilution finally sets them free to escape into empty space. The interesting parts of this overall evolution, i.e. the passage from a hypothetical plasma state to its characteristic phase transition into hadrons, is thus shrouded both by a preceding incomprehensible pre-equilibrium partonic phase, and by a subsequent hadronic rescattering phase.
A decade of intense theoretical work has been devoted to the question of how to define suitable observable features that might pin down the transient plasma, and its phase transition by hadronization. Two major avenues of experimental approach have emerged:
This hadron thermometer idea resembles the experience with our air that are commonplace to us: at room temperature the air consists mostly of molecular O2 and H2 with very little monoatomic O and N. The population ratios [O]:[O2] and [N]:[N2] are similar thermometer variables: they increase with the energy density or temperature. At the SPS we use the [Kaon]:[pion] or the [Omega]:[pion] ratios, instead.
A further analogy to commonplace classical physics is employed by the hadron spectrometers to gain information about the nature of the parton-hadron phase transformation (is it first or second order, does it involve a large latent heat jump, etc.). In explosives one distinguishes different types of phase transition, deflagration, detonation etc., by the patterns in which they emit the explosion gases into space - either uniformly or in "blast waves" or in directed shock fronts. The QCD analogies of such emission patterns are looked at in so-called "4pi" hadron spectrometers which cover simultaneously all directions of hadron emission from the fireball to pin down the complete picture of the explosive phase transition. It consists of the individual trajectories of several thousand emitted hadrons: a formidable instrumental task.
No single, universal experiment could be constructed with state of the art detector technology that would cover all of the potential signals of plasma creation, enumerated above. For example, the detection of photons and of J/Psi mesons each require a highly specialized experiment technique and layout, preventing simultaneous observation of pion correlation or strange particle production because the corresponding instrumentation would be, logically and topographically, in the way of the instrumentation required for the former observables. This feature of mutually contradictory optimal layouts is characteristic of all such "fixed target" experiments, conducted with extracted synchrotron beams hitting a stationary target. We note in passing that universal, comprehensive experiments can be constructed with considerably higher compatibility at colliding beam facilities such as RHIC at BNL (starting up in 2000) and LHC at CERN (planned for 2005).
It is, therefore, not surprising to see seven major experiments (and a multitude of smaller experiments employing nuclear emulsion track recording techniques) devoted to investigate the various physics quantities, of interest in the analysis of ultra-relativistic Lead on Lead-collisions. These major experiments are, at first sight, complementary to each other but they still provide for a certain overlap in their coverage of relevant observables, such that most of the expected results can be cross-chequed. Furthermore all experiments provide for instrumentation capable of selecting central, head-on collisions of Pb projectiles which obviously offer the most favourable conditions to create a high energy density "fireball". All experiments will thus report results referring to a common class of collision events.
For the sake of orientation of the outside observer a brief description follows of the major experiments. Note that the CERN code names for the experiments refer to the two major external beam systems of the SPS, the West Area (Meyrin) and the North Area (Prevessin). "NA49" thus refers to the 49th experiment conducted in the North Area since its construction in 1976, "WA98" indicating the 98th experiment conducted in the West Area. Note also that this numbering convention does not imply that of the order of 150 independent, newly constructed experiments have by now been conducted with external SPS beams. Many experiments were re-coded in the passage of time according to their stepwise evolution toward redefined physics goals. In line with such adaptation/recycling strategies of rejuvenating experimental gear toward new purposes, we encounter several key experiments in the Pb beam programme that have seen a previous trajectory in elementary particle physics, whereas other experiments were newly constructed specifically for nuclear beam physics.
At the Quark Matter '99 Conference in Torino (Italy) the scientific community reflected upon the status of the search for quark matter. The outcome can be briefly summarized as follows:
Recent lattice QCD calculation predict the phase transformation between partons and hadrons to occur at an energy density of about 1-1.5 GeV per cubic Fermi. Experimental estimates from calorimetric study of bulk transverse energy production and from hadron yield measurement in full phase space indicate that this density is reached in sulphur induced reactions, and far surpassed in central Pb+Pb collisions [NA49, WA98].
Quite in line with this observation we find the predicted pattern of charmonium suppression, due to a transient partonic medium, to occur here [NA50]. The Psi' yield fades away in central sulphur-nucleus collisions already (which may, however, have reasons other than QCD Debye screening) while the J/Psi yield starts disappearing in central Pb collisions only, in line with the twice smaller radius of the latter state. Actually the J/Psi is harder to break up than most other hadrons, thus offering the most significant signal of a transient partonic phase.
Assuming that a transient partonic phase causes the observed J/Psi breakup it should further manifest itself in the physics observables reflecting its eventual transition back to hadrons. In fact the total strangeness content of the final hadronic phase is consistent with the input into hadronization expected from a partonic phase [WA97, NA49]. Moreover the production ratios of hyperons and anti-hyperons seem to reflect a flavour coalescence mechanism typical of a non QCD perturbative quark-hadron transition.
A further fingerprint [NA44, NA49, WA97] of the hadronization process is present in the overall set of hadronic yield ratios which reveals a characteristic regularity. This observable order among all the produced hadrons serves as a "thermometer" for the phase transition which gives birth to the hadrons, in their various species from pions to Omega hyperons. They are pinned down as far as their various relative contributions are concerned by just a few significant parameters, temperature and energy density, characterizing their environment at birth. The apparent thermal equilibrium of all species can thus be understood to be a consequence of the hadronization phase transformation. From these observations the phase boundary appears to be located at about 180 MeV, corresponding to an energy density of about 1 GeV per cubic fermi in accord with the lattice QCD prediction. We get a first impression of detailed features of the parton to hadron transition from the observation that individual central Pb collision events exhibit no sign of fluctuations in the hadronic population and temperature [NA49], which are prominent in hadronic collisions.
Central Pb+Pb collisions have also facilitated a first precision analysis of two pion Bose-Einstein correlation data and hadronic transverse mass spectra, in terms of collective space-time parameters characterizing the "explosive" expansion dynamics of the initial dense interaction volume. The overall collision time from formation of the "fireball" to decoupling is about 10 fm/c, long in comparison to a typical partonic relaxation time scale, of about 2 fm/c. The expanding system develops strong collective velocity fields, with transverse velocity of ca. 0.6 c, while cooling down to about 120 MeV at freezeout. These findings agree with a picture of an initial partonic cascade ending in a hadronization phase from which hadrons expand in a "blast wave" mode until decoupling [NA44, NA49, WA98].
The features of the initial, dense hadronic phase, right after the hadronization transition, may also set the stage for strong in-medium effects expected to modify vector meson properties such as mass, width, and branching ratios. In fact drastic changes are observed in electron-positron pair mass spectra that reflect the in-medium meson properties [NA34, NA45]. We might see in these data remnants of chiral symmetry restoration which is expected to occur in ultradense matter, concurrent with the QCD deconfinement transition reflected in the other above observables. These observations are being further persued with benefits from improved NA45 experimental facilities.
In summary the SPS experiments have gathered a multitude of crucial observations that together suggest the existence of a bulk deconfined state, of a partonic makeup, governing the early dynamics of the high energy density "fireball" created in central nucleus-nucleus collisions at the SPS energy, sqrt(s)=18-20 GeV. The experimental findings may be summarized by the tentative conclusions that:
The present SPS experiments, and corresponding theory developments will attempt to ascertain these points of view in the upcoming year of fixed target SPS runs, then to merge in confronting the research potential of the "QCD plasma factories" presented by collider experiments at vastly extended energy: RHIC at Brookhaven (startup 2000) and LHC at CERN (startup 2005) with ALICE as dedicated heavy ion experiment.
The Proceedings of the last Quark Matter Conference, held at Torino/Italy in May 99 (Quark Matter99) have appereared in the journal Nuclear Physics Series A, Vol. 661 (1999), Elsevier publ. These proceedings present the detailed material, the highlights of which are tentatively summarized above.