Johann Rafelski, a theoretical physicist, is Professor of Physics at the University of Arizona (Tucson) and a Member of the Arizona Theoretical Astrophysics Program. He is a Visiting Scientist at CERN, a senior visiting scientist at LULI-Ecole Polytechnique Palaiseau in France and Adjunct Professor at the International Centre for Relativistic Astrophysics Network (ICRANet) at the University of Rome and Pescara. Prof. Rafelski is married to Victoria Grossack who has co-authored with `Alice' Underwood a series of novels inspired by ancient
Greek mythology of the Bronze Age: The Tapestry of Bronze
Johann’s research interests cover a broad range. To ALICE he is known mainly for his study of strange and charmed particle
signatures of deconfined quark-gluon plasma formed in relativistic heavy ion collisions. This relates to his work on matter
formation in quark-gluon plasma hadronization processes, both in the laboratory, and in the early universe.
We met Prof. Rafelski during the 4th International Conference on Particle and Fundamental Physics in Space (SpacePart12) that took place at CERN in November 2012 and discussed with him the links between Heavy Ions physics, the study of QGP at LHC and its cosmological implications.
A.M How does “heavy ion physics” relate to an international conference on “Particle and Fundamental Physics in Space”?
There are various overlaps between the two fields. Let me mention two examples. The AntiMatter in Space (AMS) spectrometer on board of the International Space Station (ISS) has as an important experimental program component the search for antimatter and strangelets, that is drops of cold quark matter consisting of about equal number of up, down and strange quarks. Antimatter and strange quark matter may also be formed in relativistic heavy ion collisions. The other example: it has been recently discovered that the very highest, ultra-high energy elementary cosmic particles are very likely iron nuclei. The recent studies of high energy heavy ion collisions at LHC provide a basis for simulating the interactions of these heavy nuclei in the Earth’s atmosphere. The forthcoming proton-nucleus collision program will give us opportunity to improve model implementation and simulations and hopefully lead to a better understanding of these ultra high energy iron interactions and particle cascades in the atmosphere. These are just two examples of the many ways in which the study of heavy ion physics is related to particle and fundamental physics in space. These connections exist since among the pillars of the relativistic heavy ion program is the understanding of the properties of the early universe and of extreme states of elementary matter (unlike solid state matter, our matter is made of elementary particles).
A.M. Which has been your personal interest/contribution in this meeting?
The heavy-ion program is based on three foundational pillars: the formation of a new state of matter, namely the quark-gluon plasma (QGP), the study of (de)confinement mechanism through which matter acquires its inertial mass, and finally, a better understanding of the origin and evolution of our universe the topic I reported on at the meeting.
The study of relativistic heavy ion collisions provide us with an understanding of how matter surrounding us must have been formed, which directly relates to how matter was formed in the early universe, that is how the QGP turns to a large number of particles of normal matter which then evolve, annihilate and continue being cooked until they finally create the universe that is as we know it today. During this process we might encounter production of objects which could be accessible to experimental search or the production of other structure effects e.g. strangelets, antimatter, remnants of the early hadronization period in the evolution of the universe.
Fig.1: Particle energy-temperature as function of time (bottom) and equivalent density (top), the arrowed domains describe experimental methods
addressing this stage in the evolution of the early universe.
A.M Can we say that our universe was once dominated by this new phase of matter that we now study at the LHC?
The laws of nature suggest that for about 20 μsec after the Big Bang a very high temperatures of several hundred MeV was present; this was a stage during which free quarks and gluons existed. This was a hot particle soup that already contained many of the building blocks of the usual matter that today surrounds us: up and down quarks now hidden in protons and neutrons, electrons, neutrinos, some heavy leptons, strange and charm quarks and also gluons. This was a state of quark-gluon plasma matter filling the universe. This view of the early universe is the product of over thirty years of continuous research in the fields of cosmology, heavy ion collisions, and particle physics. I think it is not possible to imagine a different way in which the early universe might have evolved considering the present day knowledge and the current LHC work.
Of course this view of the early universe wasn’t always like this. When I first arrived at CERN, in the mid 70’s there were many ideas about a proton-antiproton universe, yet quarks which have been around for a few years were not entering in that description. When we proposed the heavy-ion physics program at CERN in the Fall of 1980, one of the justifications was the opportunity to recreate the form of matter that should have been present in the early quark-gluon universe. Following almost thirty years of hard work we are now convinced that a hot soup of quarks and gluons must have existed and dominated the early universe properties. And before the QGP phase of matter, the universe was in an electro-weak phase of matter, present when temperatures were higher by a factor thousand.
The study of relativistic heavy ion collisions provide us with an understanding of how matter surrounding us must have been formed, which directly relates to how matter was formed in the early universe, that is how the QGP turns to a large number of particles of normal matter which then evolve, annihilate and continue being cooked until they finally create the universe that is as we know it today.
A.M. When does the QGP phase transition take place in the evolution of the early universe? Are there any problems understaning its consequences?
There are various stages in the evolution of the early universe that relate to what we can check in experiments today. In the beginning we had the period in which (as just noted above) the Higgs vacuum is melted and hence everything, all particles were massless. These ideas are the basis of the Standard Model and the search for the Higgs particle completes our understanding of this era in the universe evolution.
As the universe cools and expands we go through the "Higgs transition" during which all fundamental particles in the premordial soup acquire masses. All quarks, leptons but also the particles mediating the forces (W, Z, gluons and even free Higgs particles) are present during this period. This is already the state of quark gluon plasma as we have all the appropriate building blocks but it is a bit different from the current laboratory state as it contains all six quarks in a large number. As the temperature drops towards the range we explore in LHC experiments, the heavy particles disappear – at microsecond time scale the quark-universe is nearly just like we find it in the experiment.
In quantitative detail, the 70% of all stuff in the early universe we recreate in micro-bang at LHC, the other 30% we do not, there is not enough time and size to thermalize and keep neutrinos, and even electron pairs and photons escape without participating in the micro-bang. We have formed QGP plasma consisting of free quarks and gluons that ever more strongly interact and out of this plasma by the time about 30 microseconds elapsed in the universe, and just a tiny fraction of this at LHC, we enter into the matter phase that is everywhere around us. We call this hadronic gas, for it is dominated by strongly interacting particles, hadrons, such as protons, antiprotons, mesons and many more. The important point is that at the beginning when the hadron matter phase emerges from the QGP we have huge amounts of both matter and antimatter present but as temperature drops, in the early universe there is a lot of annihilation between matter and antimater – conversly, in laboratory micro-bang one of the interesting outcomes is that we produce antimatter abundantly.
Fig.2: Hadronization of the universe (right) fills space with abundant mesons and nucleons-antinucleon pairs, which rapidly annihilate. As
the universe expands (left) the residual 10-10 fraction of baryons remaining - the matter around us- ultimately dominates the fate
of the universe, with dark matter and later dark energy emerging as important components in very recent period of time at the very
left invisible edge of the figure.
A.M. How does the Higgs mechanism and the hadronization process of the QGP contribute to the matter and mass of the visible universe?
Today, the Higgs mechanism contributes very little to the visible mass, maybe less than 0.1%. Even so, it is fundamentally associated with the mass of the matter. It is very probable but actually not yet proven that the electron acquires its mass through the Higgs mechanism. In fact what we know is that gauge mesons, W,Z, top and bottom quark and the heaviest tau-lepton derive much if not all of their mass from the Higgs vacuum properties. By extension it is believed that also light quarks and leptons derive their mass from the Higgs vacuum. Even if this is the case very little of the mass of matter around us is due to Higgs vacuum properties.
The dominant mass-giving mechanism about which there are few if any doubts is quark confinement. Protons derive their mass from the interaction of quarks with the strong QCD vacuum. To understand this mechanism imagine a domain of deconfined space-time kept from collapse by light quarks: these quarks in this space domain have zero point energy which is nearly a hundred times bigger than their mass and that’s how a cluster of few quarks we call a hadron (which includes protons and neutrons) acquires mass. You can see that a proton acquires less than 1 percentile of its mass from the Higgs mechanism providing for the quark mass and 99% from the interaction of quarks with the confining vacuum. The understanding of the origin of hadronic mass is outcome of research work on quark confinement that was initiated in the early ‘70s by one of the first directors of CERN, Victor Weisskopf.
Fig.3: From QGP to Hadrons by way of Statistical Hadronization Model: recombinant quark hadronization, main consequence in laboratory experiments:
strangeness enhancement leads to enhancement of multi ‘heavy’ flavored (strange, charm, bottom) antibaryons progressing with ‘heavy flavor content; Rafelski's signature of QGP formation in laboratory experiments.
A.M. Can we use the models describing the properties of the QGP produced at LHC in order to describe the whole visible universe? Which are the assumptions needed?
We are observing a relatively highly homogeneous universe measuring the cosmic microwave background (CMB) and the photons we see are decendents of matter and antimatter from a time period close to the hadronization phase of the QGP. This is the period of matter-antimatter annihilation. I must admit that it is hard for me to see that we get out of this era that lasted up to a good fraction of a second, a photon filled universe that seems to ignore completely this very special matter-antimatter period in universe history. We may need still to learn how to look for signatures of this period; signatures that are imprinted on the cosmic microwave background at this late stage. Among experimental methods that are developed today at LHC is the study of particle-particle correlations. These offer perhaps a path to learn how we can look back at the early matter-antimatter universe in the study of CMB photons.
You can see that a proton acquires less than 1 percentile of its mass from the Higgs mechanism providing for the quark mass and 99% from the interaction of quarks with the confining vacuum. The understanding of the origin of hadronic mass is outcome of research work on quark confinement that was initiated in the early ‘70s by one of the first directors of CERN, Victor Weisskopf.
A.M. Can the study of QGP give us information that would explain the observed particle abundances - one of the key facts/observables for modern cosmology?
Once the early universe hadronized, there were 1010 times more protons, antiprotons and other particles compared to those observed today. Most of particles today are the cosmic background photons and each of these photons was once another particle, probably a hadron, and the universe was much smaller with densities way above nuclear densities. You can imagine that the entire universe was just one big gigantic hot superdense elementary matter with properties similar to those observed in LHC heavy-ion collisions. At LHC the size of the QGP system is comparable to diameter of an atomic nucleus, perhaps 10-14 m which corresponds to a volume of 10-51 km3. This is a very tiny fraction of the 10’s km in size universe that was then the origin of our present day visible universe. So micro-bang is really a tiny model which we recreate in order to study all these exciting early universe phenomena. In most branches of science the models that we built are bigger than 10-56, which is my best estimate of model to early universe size ratio! Therefore it is a big question if from this very tiny `micro-bang’ model of the early universe we can, or can’t learn everything we need to know about the early universe big-bang! The good news is that we also know the laws of physics, have big computers and above all, unlike the big-bang which happened only once -for us?- we can observe many, many micro-bangs, varying conditions and thus testing our understanding.
Fig.4:Micro-Bang model recreating the early universe Big-Bang in the laboratory: the two main differences are the different time scales and
different baryon asymmetry.
The key property that we need to take into account in our models of the big bang is the different time scale. The tiny model of the universe has a correspondingly tiny time scale which is sufficient to study only strong interaction acting in the micro-bang. We miss in the micro-bang the electromagnetic and weak particles which were equilibrated in the early universe, so that the abundances of these particles corresponded to prevailing temperatures. However, these particles are not present in heavy-ion collisions generated short lived micro-bang. We do not have abundances of electromagnetic and weak interacting particles at all and this part is added by hand in our calculations of the early universe properties.
At LHC the size of the QGP system is comparable to diameter of an atomic nucleus, perhaps 10-14 m which corresponds to a volume of 10-51 km3. This is a very tiny fraction of the 10’s km in size universe that was then the origin of our present day visible universe. So micro-bang is really a tiny model which we recreate in order to study all these exciting early universe phenomena. In most branches of science the models that we built are bigger than 10-56, which is my best estimate of model to early universe size ratio!
A.M. How do we connect present day world to the QGP in the early universe?
Let’s think for a moment about a broader view of our universe. Today we can only look back in the history of the universe by detecting photons to a temperature of 0.26 eV (about 3000 K) when electrons and nuclei recombined. This is our firrst picture taken when the universe was about 1000 times smaller in size. Then, the next snapshot comes from the abundances of light isotopes made in the period of the big-bang nucleosynthesis (BBN) when temperatures are another factor 10000 times greater, at T=30 keV and more. Looking beyond this we can’t see anything until we get to the QGP created at the LHC micro-bang, corresponding to an era when the universe was another factor 10000 hotter and smaller in size at T=300 MeV. From these three snapshots we must learn everything and interpret all the phenomena present in the universe considered at large.
This third picture view is just a tiny piece of the entire universe but it is created in the laboratory and we can consider this micro-bang to be a model of the universe which we then generalize, so as to be able to use it to understand the universe when it was 1-100 micro seconds old.
A.M. Do we understand the asymmetry between matter-antimatter?
The matter-antimatter asymmetry that we currently observe in our universe remains a great mystery. I am not convinced that the matter-antimatter asymmetry was already pre-established in the earlier times immediately following the Big Bang. We have also to allow the possibility of a mechanism that could have been operating during the first `long’ second after the big bang – a period in which the universe evolved through a stage when there were large abundances of hadronic species.
It remains a mystery how the relatively small elementary standard model CP violation could have led to the observed matter antimatter asymmetry. We still have to discover what exactly produced matter-antimatter asymmetry which is of the order of 10-10 compared to primordial particle number. There is a difference of many orders of magnitude between what is described as the CP breaking of nature’s fundamental laws, and what we observe in the universe. One possible explanation might be related to the great number of hadron reactions that are taking place in quark-hadron universe mimicking the famous Urca process in astrophysics. Here we need to keep in mind that a late matter-antimatter asymmetry formation requires mechanism of baryon number non-conservation. A probable alternative is the separation of matter and antimatter which motivates the AntiMatter in Space (AMS) experiment running at the International Space Station (ISS) and which is one of the key projects addressed at this conference.
It remains a mystery how the relatively small elementary standard model CP violation could have led to the observed matter antimatter asymmetry. [...] There is a difference of many orders of magnitude between what is described as the CP breaking of nature’s fundamental laws, and what we observe in the universe. One possible explanation might be related to the great number of hadron reactions that are taking place in quark-hadron universe mimicking the famous Urca process in astrophysics.
A.M. Could there be antimatter in the universe? How do you prove that the baryon matter separated during the big-bang QGP phase transition and hadron universe evolution?
The answer is not very simple: if we look billions of years away we really cannot tell if the stars weren’t made of antimatter and conversely we can argue that the closest star to our Sun cannot be made of antimatter, perhaps we can extend this argument to our entire galaxy, that is, it is not made of antimatter, and we should than also argue that the local group of galaxies is made only of matter. But how do you know that across the vastness of the universe, if you move to another cluster of galaxies they are not made of antimatter? Antiprotons flying across the universe do not help us understand the separation of matter-antimatter since antiprotons are relatively easily produced in cosmic particle interaction. However, anti-alpha is difficult to create and hence if you measure a flux of anti-alphas this probably means it has been assembled in an antimatter evironment where there is a soup of antimatter particles.
Fig.5: Strongly interaction particle abundances emerging (right) from Quark universe and evolving (left) towards period of Big-Bang
nucleosynthesis. QGP-universe hadronization created at first a nearly matter-antimatter symmetric state, ensuing matter-antimatter
annihilation yields 10 −10 matter asymmetry, the world around us
A.M. How does the study of QGP help to understand the asymmetry between matter-antimatter?
In QGP transition to hadrons we might have some flaking of matter: both phases at similar entropy, energy density and pressure have a significantly different electrical charge content. A similar contrast of densities applies to other conserved quantities such as baryon number, or nearly conserved strange quark abundance. The difference in charge, baryon, and strangeness between the two phases could and should cause some flaking of baryon number, a density fluctuation similar to what we observe in some chemical reactions. In other words nucleation domains are formed. Such baryon density fluctuation could speed up later the formation of first stars. Moreover, if the effect is strong this can be a mechanism for creating by separation an effective asymmetry of matter-antimatter across macroscopic domains in the universe. This is a scenario in which matter-antimatter asymmetry is not originating in a true initial big-bang asymmetry in the baryon number, or in baryon number violating processes, but arises from dynamical processes triggered by random nucleation processes.
Fig.6:Tracing chemical potentials of quarks, electrons and neutrinos during expansion
of the universe.
This hypothesis can’t be directly experimentally studied at LHC since it requires a much larger QGP domain and large number of baryons, but we can make useful steps in this direction. Imagine that quark-gluon plasma has been produced and converts into the hadron phase. During phase transformation there are as already noted relatively large baryon and strangeness contrast fluctuations which can further grow. We can study this initial nucleation of matter; for example by directly measuring the production of heavy bound states of antimatter such as light anti-nuclei. Study of production of multi strange particles, one of my personal fields of expertise helps here as well.
A.M. Finally, I would like to ask you to summarize the highlights of this conference from your perspective?
I must say that I was most impressed listening to the reports on new instrumental capabilities that we have for the detection of potential big-bang artefacts including the QGP transition in the early universe. For the first time we get the opportunity to use space-based observatories in order to look for exotic objects e.g. strangelets or anti-alphas. The antimatter in space -- AMS experiment offers cold quark matter search, in the early universe, and complements the LHC effort which addresses hot quark matter exploration required for the study of the early universe. The difference is of course that while LHC cannot fail – it has already QGP the Higgs and more, the AMS search has still many challenges, such as attenuation of the flux originating far and away, action of cosmic magnetic fields, to name a few. And, once a signal of interest is found, its interpretation and verification will be a complicated and tedious problem.
The conference of course addressed topics which are more afar from QGP physics and I learned about very interesting recent developments concerning the exploration of gravitational force with LISA-Pathfinder, the advances made in analysis of the frame dragging by Gravity Probe B, the enigmatic properties of gamma ray bursts, GRB, that continue to puzzle, the properties of galactic nuclei. This meeting showed that the universe is filled with unresolved riddles. Combination of methods of Astrophysics and Particle physics may help unravel them faster.
I would like to go back to what I mentioned in the beginning of this interview: There are three pillars of the field of QGP, which also shape its relation to the early universe: the creation of a very dense new phase of matter called QGP; the study of the origin of inertia of matter which is (de)confinement of quarks; and the creation of matter from energy. These different tasks can also be seen as one.
Experiments studying the production of hadrons at the LHC also study the hadronization of the early universe. We explore the matter-antimatter asymmetry, prove that the QCD vacuum has melted and also address the origin of inertia in context of the structure of the vacuum.
The three strands all relate to the early universe,are all connected, while they are also so different. Last but not least at LHC we form QGP with a large strange, charm and even bottom quark content allowing us to address the riddle of flavour.