The Quark-Gluon Plasma Paradox July 9, 2007Posted by dorigo in physics.
Lazily browsing the web in search of something that could divert my attention from the endless morning talks of the CMS-italy meeting, which is taking place at the 5-star Romano Palace Hotel in Catania, I found myself reading a hep-ph preprint on the “Quark-gluon plasma paradox”, by Dariusz Miskowiec.
The word “paradox” triggers my interest like few other things. The paper in question is a pleasantly easy read, and is very qualitative in style. I have no clue on whether the ideas contained therein are meaningful or crackpotty, but the reasoning is simple enough that it can be summarized here in a simple way. In fact, after a bunch of reports from PASCOS 2007, which I unfortunately wrote in a way hard to understand for whomever lacks a PhD in theoretical particle physics (me included), I made the resolution of keeping the next ten posts in the Physics category at a level simple enough that my 8-years-old son can understand. Explaining a quark-gluon plasma paradox, however, does require some initial pedantic definitions, so bear with me if you will.
A quark-gluon plasma is a state of matter believed to form when the density of hadrons (or their temperature, which is in some sense equivalent: the two quantities are in fact connected by the equation of state of the system) exceeds a certain critical level. Hadrons are particles composed of quarks. Mesons are hadrons composed of quark-antiquark pairs, and baryons are hadrons composed of three quarks. Both have zero net color charge (they are “colorless”), and have integer baryon number B, while quarks and gluons are colored and have fractional (quarks) or zero (gluons) baryon number. To exemplify, you can make a B=0 meson by combining a B=-1/3 anti-red antiquark with a B=1/3 red quark. Or you can make a B=1 baryon by combining three B=1/3 quarks of the three different “primary” colors red, green, and blue.
These particles, mesons and baryons, may lose their individuality if you bring some of them together at a high enough temperature. They “melt” in the resulting plasma, composed of quarks and gluons that move around freely, “deconfined”, i.e. not bounded inside an enclosing volume of zero net color and integer baryon number. Quarks and gluons, which are usually sources of the QCD (Quantum ChromoDynamic) color field, are free from the QCD force which usually binds them because they are tightly encircled by other charges, whose effect is to screen their own charge.
A quark-gluon plasma is believed to be present inside neutron stars, and to have constituted the bulk of matter in the initial microsecond after the big bang: two reasons why studying it is of great interest to physicists, astrophysicists, and cosmologists. However, creating a plasma of quarks and gluons is not easy. It is the goal of the highest energy heavy ion collisions that are being produced at the RHIC and that will be studied by the ALICE experiment at the LHC.
By colliding two heavy nuclei at high enough energy, physicists believe that during the brief instants of time when the nuclei overlap, their quark matter will have a temperature high enough to form a plasma. The creation of this funny state of matter can then be detected by observing its decay signatures, which involve a number of peculiar characteristics.
In the paper by Miskowiec these experimental details are not discussed. Instead, one is asked to perform a gedanken experiment by imagining a chunk of plasma extended into a thin, enormous ring, with a 1000 light years diameter. Let us forget the easy objection that producing such a thing is not a piece of cake, because the paradox obviously requires a good dose of fantasy and just obeying to physical laws.
If you were to cut the ring at one position, Miskowiec argues, the plasma would start to create hadrons – the bound states of the plasma constituents – at the two loose ends, continuing to do so until the ring would completely disappear into a finite number of ordinary particles. But if you were to cut the ring simultaneously at two different ends, separated by light years of distance – “not casually connected”, you might have the plasma hadronize unti the remaining bits have fractional baryon number or non-neutral color charge! (see picture below).
By this reasoning, one is led to believe that baryon number and color charge quantum numbers of hadrons may retain some meaning even inside a plasma of quarks and gluons – that is to say, that the deconfinement phase is not describable as a sea of independent quarks and gluons uncorrelated and free. Otherwise, one would need to buy into some sort of mechanism a’la Einstein-Podolsen-Rosen, whereby the thousand-light-year ring behaves in some coherent way, such that cutting it at one edge collapses its wavefunction everywhere at once. But that seems to allow for superluminal transfer of information, which is hard to acknowledge. Another possibility involves some restraining assumptions on the way the plasma hadronizes, which appear at odds with the current models of the first instants after the big bang.
One nice feature of the paper is that it makes definite predictions, for a change: quoting from the concluding section,
“… the concept of QGP, state of matter with uncorrelated quarks, antiquarks, and gluons, leads to isolated objects with fractional baryon numbers, unless supernuminal signalling is allowed, or, by some mechanism, the hadronization is restricted to the surface of the QGP volume, meaning that e.g. the hadronization in the Early Universe took at least minutes rather than a couple of microseconds. The third, obvious, way of avoiding the paradox is to declare the uncorrelated QGP as non-existent, and to replace it by a state consisting of quark clusters with integer baryon numbers (resonance matter). Both the surface-hadronization and the resonance matter options result in a liquid- rather than a gas-like structure of the matter. This agrees with the hydrodynamical character of the matter created in nuclear collisions at RHIC and, at the same time, indicates that this character will be preserved at higher temperatures.”
As I noted at the start, I am in no position to decide without further study (which I have no time to embark on) on the soundness of the reasoning illustrated in the paper by Miskowiec. Anybody here willing to comment ?