Johann Rafelski
Department of Physics, University of Arizona
RESEARCH INTERESTS
Vacuum Structure, Strong Fields
Comments on the Vacuum and its exploration
Among the most far-reaching developments of the five decades of research into the consequences of
fundamental interactions is the recognition that the true physical vacuum is a state of
complex and physically significant dynamical structure. While the vacuum by its
definition is empty, devoid of real matter, its quantum wave function can be highly non-trivial,
deviating considerably from the non-interacting Fock space wave function to which the perturbative
expansion of interactions in quantum field theory refers. As an example one finds that the true vacuum
of strong interactions comprises glue and quark condensates.
This structure of the vacuum has been considered an important component for the understanding
of strong interaction physics and of the quark confinement, and thus of 99% of the mass of matter
we see. Add to this that the current paradigm
of particle physics is that the electroweak vacuum Higgs field is the origin of all elementary
particle masses, and it is clear that no effort should be spared to further the understanding
of the vacuum. In essence this is an effort aiming at the understanding of the origin of visible
matter particle masses, and more generally the true nature of physics laws.
Einstein, before the development of quantum mechanics, realized this as well. He noted that
a vacuum state which characterizes, and indeed defines, the laws of physics must exist. He
called the "vacuum" at this early time by its later much abused historical name "ether". In a letter
to Lorentz in 1919 Einstein writes: "It would have been more
correct if I had limited myself, in my earlier publications, to emphasizing only the non-existence
of the ether velocity, instead of arguing the total non-existence of the ether, for I can see that with
the word ether we say nothing else than that space has to be viewed as a carrier of physical qualities.
In his paper, "Ether and the Theory of Relativity"originally published by Julius Springer
(Berlin, 1920) he continues in summary: "Recapitulating, we may say that according to the general theory of
relativity space is endowed with physical qualities; in this sense,
therefore, there exists an ether. According to the general theory
of relativity space without ether is unthinkable; for in such space
there not only would be no propagation of light, but also no possibility
of existence for standards of space and time (measuring-rods and
clocks), nor therefore any space-time intervals in the physical
sense. But this ether may not be thought of as endowed with the
quality characteristic of ponderable media, as consisting of parts
which may be tracked through time. The idea of motion may not be
applied to it." in collected papers of Albert Einstein, Volume 7,
1918-21, Michel Janssen et. al., editors,
(pp. 305...309; 321) Princeton University Press 2002.
Oddly enough, 91 years after these words have been written, and 77 years after Heisenberg, Weisskopf made
the first computations of quantum vacuum structure within the newly conceived Quantum Electrodynamics (QED),
we still remain in dark about how to bring together these two roots of the
structured vacuum = Einstein's ether. Intrinsically, both theoretical frameworks (general relativity and QED, i.e.
more generally, Quantum Field Theory) remain
inconsistent with each other. Efforts to create a viable common theoretical framework are
not convincing. In such a situation, work on development of novel tools to explore the vacuum maybe
the most promising next step.
Strong (laser) fields, particle production and vacuum structure
Intense crossed laser fields can be arranged to result in either electrical or
magnetic field being dominant. From the perspective of elementary particle physics,
a laser field is practically constant in space and time, considering the length scale of
the visible light wavelength.
Sufficiently strong electrical fields lead to particle production, due to vacuum
instability first noted by Schwinger.
For a (slowly changing) strong electrical field, the critical Schwinger
field of quantum electrodynamics, the ratio of electron mass squared to electric charge
is Ec=1.3255 10^18 V/m.
This field must be established over a spatial and temporal volume in excess of the electron Compton wave
length (L=m/hbar c =386 fm), such that an effective potential well is formed well in excess of twice the mass of
an electron
(see e.g. Quantum Electrodynamics of Strong Fields, Greiner-Muller-Rafelski, Springer 1985).
Above Ec and for spatially extended fields, even the back-reaction of the nonlinear
vacuum polarization due to particle pair fluctuations
is of the same magnitude as is the field itself. We have developed a phase-space method
that allows to address this situation
(Phys. Rev. D 44 p1825 (1991)).
Further development and application of this theoretical framework should lead to better understanding of the
vacuum collapse of QED in supercritical external fields.
A fundamental result, required by symmetry and relativistic invariance, and derived in a non-trivial fashion by
Schwinger was that a single coherent (laser) beam propagates in the physical vacuum without
non-linearities, it literally slips through, with the vacuum disturbance it creates self-restoring.
However, slightest deviation from translational invariance, such as a light beam arising from several laser
beams, crates a preferred frame of reference, and will inflict a lasting "damage"
to the QED vacuum, with the final state having in particular a high particle content.
It is of considerable interest to
explore how the QED vacuum interacts with such more complex laser pulses,
when the Schwinger field strength Ec is exceeded. Extrapolating
our earlier work, we see good reasons to expect in a suitable environment
a highly effective conversion of laser energy into a large number of electron-positron pairs.
Another interesting question is if we could generate a "photonium“ state,
i.e. a domain of a self-stabilized field configuration comprising two potential wells,
originating in both the laser field and
the vacuum e+e- pairs, a strongly polarized vacuum state.
Separation of virtual electron and positron pairs in suitably prepared laser-electric field profiles,
may result in highly efficient conversion of laser energy into
electron-positron pairs. Indeed it seems possible that a large fraction of the 60 000 GeV energy contained in
such laser pulses could be converted to e+e- pairs. In passing we note that this energy content is higher
than today reached in central collisions of heaviest nuclei at RHIC-BNL.
In such situation, the pulse would become
opaque and one can expect formation of a cold QED plasma comprising temperatures which in near future
could reach T=20MeV. This would result in truly remarkable rate of positron production, perhaps as
many as several million in each such laser pulse event. While the confirmation of the Schwinger positron
production and study of the Schwinger tunneling effect is of considerable fundamental interest,
the formation of positron beams of high intensity could have unforeseen
practical implications. Moreover, it turns out that this environment
could be also a source of muons, which in the present conditions are produced
much more rarely, but that could rapidly change as more extreme laser configurations become
available.
Compared to quark-deconfined Quark-Gluon Plasma (QGP),
this laser generated QED plasma is, at first today relatively "cold". Even so one can expect
significant interference of QED and QCD phenomena already at this temperature scale. It turns out
that neutral pions would be, after e+e- pairs, the most frequently produced particles in cold plasma.
They will serve as diagnostic tools of the cold plasma, as well as probes of the QCD vacuum structure at
these temperatures, well below the deconfinement phase transition. A particularly interesting aspect of the
range T= 1-20 MeV is that this is the domain in which current quark thresholds for u, d quark pairs are
overcome. While free current quarks cannot exist at this temperature range, the presence of
virtual quark pair excitations may considerably impact the properties of the vacuum.
At this point we recall that quarks play an active role in shaping the QCD vacuum structure.
Being dual carriers of both `color' and `electric' charges they also respond
to externally applied electromagnetic fields. Thus, in principle, the vacuum
of strong interactions influences higher order QED processes such as
photon-photon scattering, and conversely, a QED cold plasma must be affecting the QCD vacuum properties.
The key to the above developments are the recent advances of laser technology.
The peta- and exawatt lasers under current development should open a novel path in the investigation
of the QED and QCD vacuum structure. Crossed laser beams allow to form intense electric and magnetic
configurations which are capable to alter the equilibrium quark condensate configuration.
Aside of resolving the issue of fast dynamics, the novel element is that on scale of hadronic
interactions the domain of strong electromagnetic fields formed is expected to be of considerably
greater spatial and temporal extent.
The direct focused exawatt laser will reach about 1% of this critical field strength over optical
wavelengths (micrometer). However, harmonic focusing should allow the petawatt system to greatly
exceed the QED critical field over distances which are a 1000 times greater than the Compton wavelength.
For an exawatt laser focused to 3.10^34 W/cm2 one finds a field of 5.10^20 V/m over a relatively
large (30 fm) scale, which amounts to local field energy density of about (34 MeV)^4.
A harmonically focused petawatt laser thus has the potential for a in-depth study the QED
vacuum structure at the interface to QCD vacuum structure.
Quark-Gluon Plasma and heavy ion collisions
Free quarks have never been observed. Quarks are confined, the vacuum abhors free
color. However, at sufficiently high temperature the structure of the vacuum melts,
quarks are set free, a deconfined QGP state of matter should form.
It is widely believe that a new state of matter has been formed
in relativistic nuclear collisions both at RHIC and SPS-CERN.
The case made is that in particular at RHIC this is the expected, interacting plasma of
quarks and gluons: In a domain much larger than normal hadron size
color-charged quarks and gluons are
propagating, constrained by an external
`frozen vacuum', which abhors color.
There should be a pronounced boundary in temperature and/or baryon density
between confined and deconfined phases of matter, irrespective
of the question if there is, or is not, a true phase transition under the
conditions created in these collisions.
Unlike the field of strong laser fields which is today in its early beginning phase,
the melting of the QCD vacuum state has been researched theoretically for 30 years and
experimentally for 115 years. This is a mature field where a key issue is to reconcile
theoretical predictions with experimental observations.
We develop tools for, and perform analysis of,
strange and heavy flavor hadron production in
relativistic heavy ion collisions.
Insights into the nature of the resulting hot hadronic matter fireball
are gained from interpretation of an array of particle
yields, particle spectra and yield fluctuations.
We explored in depth several promising paths in search of evidence for
the presence of the deconfined QGP state at the time of hadron formation
(hadronization) in relativistic
heavy ion collisions.
We also study quantitatively how matter (protons, neutrons)
formed in the Early Universe during
a period which spanned 10-50mus. In the standard big-bang model, the large primordial baryon and antibaryon
abundance formed at hadronization of the deconfined quark-gluon plasma (QGP)
disappears due to mutual annihilation, exposing a slight net baryon
number observed today. Applying the knowledge of equations
of state of hadronic matter derived from the study of high energy
nuclear collisions we consider quantitatively this
evolution epoch of the early Universe.
The focus of our research work and connection with the experimental work in this very large research area
is on less frequently produced hadronic particles (e.g. strange antibaryons, charmed
and beauty hadrons, massive resonances, charmonium). We have developed a public
analysis tool, SHARE (Statistical HAdronization with REsonance)
which allows a precise model description of experimental
particle yield and fluctuation data. This suite of programs also
allows the evaluation of the bulk source properties including energy,
entropy, strangeness content of all particles produced.
We used this tool to analyze CERN-SPS and RHIC data as function of centrality,
and test the hypothesis of expanding quark-gluon
phase of matter.
Of considerable interest in study of quark deconfinement is also the dynamics of charm and bottom quarks in
a quark-gluon plasma. These heavy quarks provide a
large mass scale which enables contact with
initial production calculable in principle in perturbative QCD.
Our identification of novel combinant formation mechanisms
of charmonium which follow from multiple charm quark
pairs in a region of deconfinement is continued.
We further study dynamics of
hadronization by developing better understanding of hadron spectra emerging
from string fragmentation. We also seek a better understanding
of the manifestation of medium modification of hadron properties.
Among our other recent accomplishments in this field is
the study of the temporal evolution of strangeness to entropy ratio
in an expanding QGP fireball.
An important question in study of QGP dynamics is, how the unknown
initial conditions impact the results. In order to resolve this question, we explore
a wide range of initial gluon (and quark) occupancy, but keep the entropy of
initial state fixed. There is thus a
corresponding variation of initial temperature. We find that
the two effects compensate each other, the ratio of strangeness to entropy is
a robust and initial condition independent observable of QGP.
Ultimately, the expanding QGP fireball falls apart, it hadronizes into
a myriad of particles. How this happens, what are the processes and timescales
involved, is one of major challenges we address in our research.
The totality of experimental results supports a single-freeze-out for the
description of hadron production from an exploding
QGP drop. The particle yields are obtained in the statistical
hadronization model, using near chemical equilibration of quarks and gluons in
the plasma phase, which means that the hadron phase is not in chemical equilibrium.
There are two longstanding riddles in the understanding of relativistic heavy ion
collisions and deconfinement of quarks:
-
what is the mechanism of initial entropy formation in heavy ion collisions, and,
-
what is the mechanism of practically thermal hadronization in elementary
reactions.
The initial entropy formation appears much too fast to be due to
processes involving many individual scatterings. Similarly,
the final thermal spectrum
of particles which emerges in elementary reactions cannot be due
to individual rescattering processes of a few particles produced in e.g. pp reactions.
We believe that these may find a common resolution.
When the process
of particle production is calculated via Schwinger's formula
generalized to consider transverse particle momentum,
the predicted transverse momentum distribution of quarks is
Gaussian, while the thermal spectrum is needed for both initial entropy formation
and the final state observed spectrum of particles. In order to change this result,
we note that the QCD color string is embedded in
the structured QCD vacuum. The quantum character of transverse dynamics of the QCD
string is found responsible for the exponential distribution of the transverse
momentum of a produced quark pair. The quantum physics enters,
since the magnitude of the de Broglie transverse wavelength is
the same as the transverse radius of the QCD string, thus one must expect that
the transverse momentum
acquired in the string breaking is obeying quantum and not classical dynamics. We describe
the resulting dynamical balance between string field lines and the vacuum within a quantum collective dynamical model.
The quantum probability P(E) of finding some value of the string
field E is folded with the Schwinger probability of string
breaking. This is yielding quark pairs at a transverse momentum of
nearly thermal character.
We have published (June 15, 2002)
a survey of soft hadron production phenomena related to QGP formation:
Hadrons and Quark-Gluon Plasma
Jean Letessier and Johann Rafelski
Cambridge University Pres, Cambridge, UK; or
in US:
Cambridge University Pres New York,
A strange quark plasma
Emanuele Quercigh and Johann Rafelski
A feature article on strangeness and QGP has appeared in
PHYSICS WORLD,
Volume 13 Issue 10, October 2000 issue
see also Research Opportunities for Students
Revised July 4, 2017
Melting Hadrons, Boiling Quarks